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The development of techniques for the surveillance of LMFBRs
Progress in Nuclear Energy.
T~E D
~
Vol. I, pp. 393 to 408. Pergamon Press 1977. Printed in Great Britain,
OF TECHNIQUES P0R THE ~
~
E
OF LMFBRs
E J Burton~ P G Bentley~ J Bishop t I D Macleod and J A McEnight U.K. Atomic Energy Authority~ Risley Nuclear Power Development Laboratories~ Na~Tington~ England
SUMMARY
The development of surveillance techniques of LMFBRs is determined by the interaction of three factors: the specification of requirements~ improvements in technique and the physical analysis of the processes involved. The specification of requirements, which sets the structure for the discussion~ is mainly concerned with public safety. Two main divisions are identified: those concerned with thermal events in the nuclear core and those concerned directly or indirectly with the mechanical integrity of components. The necessary developments are then discussed in terms of the signal ar~lysis techniques to anticipate various modes of failures. The importance of an adequate understanding of the failure mode is emphasised in optimising the surveillance technique. Core surveillance may be achieved by monitoring individual sub-assemblies or by monitoring bulk conditions. The impOrtant features of sub-assembly monitoring are discussed and the advantages of temperature analysis explained. The spocification of the temperaturemonitoring systems is identified and the conflicting requirements for the reactor sensor discussed t viz adequate band width as against a robust and reliable construction. A theoretical treatment using Monte Carlo techniques allowS a full examination of the choice of method Of temperature analysis. This shows that~ although a filtered rms value has been the preferred choice for detecting either local blockage or sodium boiling~ it may be possible to distinguish the temperature signals of blockages from those of power gradients by an amplitude probability density plot. The advantages of acoustic monitoring using the noise of boiling sodium to detect overheating~ leading to core damage t are eTam4ned. An important consideration is the thermal-~coustic process of sodium boiling t and evidence is submitted from a range of out-of-pile experiments involving local sub-cooled boiling and bulk Boiling in discussing the merits of pulse analysis and pOwer spectral density techniques. An important factor in discriminating background from signal is the extent of cavitation in reactor components. Experiments are mentioned in which pulse techniques have been used to locate boiling sources by spatial correlation. The interpretation of reactor signals requires a detailed knowledge of the transmission of acoustic waves in reactor pools and structures and the effect of gas bubbles. Measurements in PFR and sodium loops have helped to lead to a more quantitative asses~ent of the sensitivity of the acoustic techniques. Structural integrity depends on detecting failure modes~ particularly those arising from crack propagation. Manufacturing defects or pre-existing cracks may be identified by ultrasonic inspection or by stress-wave emission. On-line monitoring for stress-loaded cracks by a stress-wave emission is seen as intrinsically difficult because of low signal strength and high attenuation but initial experiments have indicated possibilities for detecting stresscorrosion cracking. Mechanical failure from fatigue may be anticipated from an understanding of the vibrational modes of the sodium and its coupling with the structure. A one-elghth scale model of a IRFBR design has recently demonstrated the likely vibrational modes. A major handicap in supervising mechanical operation in sodium systems is the o~city of the sodium. Visualisation techniques of the major parts of the core structure are being developed. An important aspect is the study of the information processing required to present an image easy for the reactor operator to unders+~nd. Advances may be made using transform methods to improve object boundaries by modifying the spatial frequencies of the display or record. INTRGD~D TION The development of techniques for the surveillance of fast reactors has the objective of improving their safe and efficient operation t that is~ it is aimed primarily at developing methods for monitoring on-line whilst the reactor is at pOwer to give an early warning of incipient failures. An important difference between the programmes for LMPBRs and for thermal reactors is that the small nt,nber of operating reactors are prototypes~ with the design of demonstration plants to follow. Thus~ although mainter~uce procedures on the more 393
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Burton~ Bentley, Bishop, Macleod and McKn_ight
conventional items of the plant are important, most attention in the UK is being given to safety, particularly to meet the demands for a public assurance that no significant release of fission products will occur. The specification of requirements is an essential precursor to discussing techniques, and the opportunity is taken in the first section of the paper to outline the important features by using a simplified presentation based on probability analysis. In relating this to a particular situation, the physical process involved in the failure must be fully understood; this is being achieved through parallel out-of-pile investigations. In its next two sections the paper concentrates on discussing the techniques required to monitor the nuclear core to ensure its thermal balance and its mechanical integrity. The improvements in detection techniques which can be achieved are constrained by the engineering difficulties which arise from the high power density of the core, the use of sodium~ the inaccessibility of the measuring stations and the need for easy replacement of sensors. Thus~ for thermal monitoring, i% is not practicable to insert instrumentation into the fuel cluster, although measurements m a y b e made at the cluster outlet, and the requirements lead to the use of inferential or diagnostic techniques. For structural monitoring, the requirement is %o detect incipient failures which may occur in the structures over the life of the reactor; hence sensor performance must be maintained, or checked by calibration~ over 20 to 30 years. Because of these two sets of requirements t the reliability of the sensors may be best achieved through robust designs which often conflict with the sensitivity requirements. Thus in general the maximum information is sought from the sensor by analysing the signal using a variety of functions, including power spectral densities and cross-correlations. Obviously modern computers provide economic and elegant methods of performing this analysis, but at present in the DE the introduction of computers to execute safety functions directly (ie to provide trip actuation) is proceeding cautiously. Nevertheless the combined use of sensors and data processing is leading %o the development of a sophisticated sul~eillance system. Aspects of this are discussed, with most emphasis on acoustic techniques as these provide, at least in principle, a measurement system supervising a large volume, but a review is also included of the use of temperature noise to detect local anomalies in an individual sub-assembly.
GENERAL SPECIFICATION OF SURVEILLA/~E SYSTH~ Usually the important surveillance specifications arise from analysis of the fault sequences which could ultimately lead %0 the release of a significant ~mount of re~ioactive fission products. Those which come from the need to manoeuvre and to operate the reactor efficiently at stead~v power are uSually less stringent. There are two broad considerations behind specifications for surveillance systems: first, the general style and likelihood of the fault sequences to be protected against, and secondly, the more detailed mode of failure within each sequence. These are discussed separately below.
Fault A n a ~ s i s The safety analyses have identified three principal categories of faults which could lead to whole core accidents: I.
2.
Loss of flow (LOF) arising from: (a)
loss of power supply to all primary coolant pumps;
(b)
loss of coolant flow to the core through structural failure of the d/agrid or coolant supply pipes.
Transient over power (TOP) arising from: (a)
reactivity addition through uncontrolled withdrawal of absorber rods.
(b)
reactivity addition through structural failure of the core support.
3. A sub-assembly accident arising from local loss of cooling, with the possibility of sudden escalation to a whole-core accident by a pressure pulse caused by interaction between overheated fuel and coolant (FCI). The protection against such accidentsis achieved mainly through engineered safeguards, including instrumentation systems actuating trips for shut-do~q. The specification for the
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instrumentation systems, and therefore for the noise analysis systems, arises from an examination of the role of these safeguards. The precise specification of the instrumentation is a complex process in which the details are most important~ but it is also important to explain the general guidelines which are used. These are valuable in attempting broad comparisons between reactor systems and in evaluating particular instrument techniques. The first step is to consider the role of the engineered safeguards in the safety protection; this is summarised in a simple diagram in Fig ~ based on unpublished work in the UKAEA by Smith and Cowking. This Figure illustrates that there are four main frequency or probability components to any accident sequence which release a substantial amount of radioactivity from the contaimment. First of all the fault must have an initiating cause, there must then be failure to detect and correct it, there must be propagation to a major whole-core accident, and finally this must lead to failure of the containment. The numerical values shown are a present judgement of the situation I but are not definitive or endorsed as an agreed UK position. Without doubt further development will alter best judgement values and narrow down the uncertainties which still remain. Overall the objective is to show that the frequency of major activity release from a plant is not greater than about 10-7 per operating year. Since the values on Fig I are much lower than this~ 10-7 should be a realistic target. Farmer (I), for exampl% would consider that numbers so low as shown are beyond confirmation and unnecessary for safety endorsement, but at the stage where there are still uncertainties it is best to retain a picture formed from best estimates. The value of the analysis to this paper is that it demonstrates the comparative worth of different areas of safety development work. Surveillance development is relevant to all three fault categories and is shown to have a large potential contribution, as discussed below. Protection against LOF and TOP faults arising from malfunctions in the plant rests largely on detecting signals showing deviation from normal plant parameters, for which the use of multiple systems ensures that a high level of protection can be obtained. Structural failure is identified above as a possible contributor to both these faults~ however, and surveillance methods for detecting minor damage or incipient failure would reinforce the safety assurance inherent in good design and manufacturing practices. The values shown on Fig I mainly reflect the more frequent~ highly protected, faults. It will be seen that it is important that the frequency of sudden severe structural failures is not so high as to by-pass this line of protection. The probabilistic analysis of structural failures has received most attention for reactor pressure vessels (Ref. 5) and is now being extended to fast-reactor structures. The analysis shows the extremely high standard of integrity assurance which is sought through the combination of design, construction and instrumentation. Acoustic instrumentation has an important contribution to make through monitoring for flaws and errors during construction and commissioning and for incipient failures, particularly during operation, including mechanical deformation and displacement. Techniques being developed to assist in this are described below, and include acoustic emission and under-sodium viewing. Early detection of localised faults before they develop is judged to be important in ensuring that sub-assembly accidents will not spread to neighbouring sub-assemblies and affect the whole core. It is of interest that world opinion is divided on the risk of the spread of damage and hence on the need to develop techniques for surveillance. Fauske et al. (2) discount the risk and would consequently put less emphasis on the usefulness of surveillance. Smidt and Schleisiek (3), on the other hand, give an analysis similar to that presented here. General discussion of all the techniques for sub-assembly surveillance is outside the scope of this paper*, but two developments are selected of especial interest: first, the use of the analysis of the fluctuations of outlet coolant temperature, and secondly~ the use of acoustic methods to detect the noise of boiling sodium.
Modes of Failure The other general consideration concerns the modes of failure which need to be precisely defined so that the instrument detection system can be optimised. For the sub-assembly accident, the mode of accident progression has received extensive examination~ both through out-of-pile rigs, such as the GfK rig at Karleruhe (Ref. 6) and iD-pile experiments in Scarabee (Ref. 7) and in the USA. The detailed application of the European experiments to temperature noise and acoustic monitoring is discussed below and t also in the companion specialist papers to this Conference (Refs. 9 and 10). A useful discussion is ~iven by Smith (4)
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The examination of modes of failures for the core smfety structures opens up a wider field that has been studied unevenly so far. Perturbing forces which can produce failures include seismic and other external shocks~ internal vibration induced either by flow or by acoustic excitation, thermal shock or melting and material loss by wear or by corrosion. As the UK is in a seismically quiet area~ most effort has been concerned with crack propagation from thermal shock~ corrosion and vibrational fatigue t and aspects of these are discussed below.
BIB-ASSEMBLY MONITORING The concern in the sub-assembly is that the loss of local cooling arising particularly from a blockage may spread~ leading to sodium boiling and eventually a fuel-coolant interaction. Thus an early ~ r n i n g of this potentially propagating event could be given either by detecting the blockage or~ somewhat later~ by detecting boiling of the sodium.
Temperature Noise Sub-assembly faults leading to local overheating and the possibility of escalation to larger accidents are most directly monitored by measuring coolant temperature. However, one of the consequences of high fuel thermal rating in IX~I~BRs is that small local effects give only a small change in average coolant temperature which may be difficult to detect, and yet the local temperatures can be sufficiently high to lead eventually to a major incident. The requirement is therefore for a method of using the temperature information early in a fault sequence so that there is time available to take remedial action before the incident spreads. The techniques of temperature noise analysis offer this possibility, the principles being shown schematically in Fig. 2. In the absence of any malfunction the temperature profile for a central sub-assembly is substantially uniform downstream of the fuel cluster, leading to temperature at the monitoring thermcccuple which is a timeinvariant, constant measure of the mixed mean outlet. A small flow effect within the cluster t for example a local blockage or geometrical distortion~ changes the uniformity of temperatures and leads to a disturbance in exit profile as indicated in the Figure. Here a temperature blip is shown I resulting from a blockage; different local malfunctions disturb the uniform profile in different ways but without affecting the subsequent argument. Coolant flow carries the profile to the monitoring thermocoupl% mixing it by the random motions of turbulence, and converts the non-uniform spatial profile to a time-varying temperature noise. Although small in amplitud% the d~namically varying temperature can be much more sensitively and reliably detected than the corresponding change in mean value. The technique retains the advantages of proven thermccouple sensors of high reliability and offers the opportunity to take remedial action very early in the fault sequence. A further advantage is an inherent low probability of spurious fault indications since the measured parameter is closely linked to the fault condition; for example, it is unlikely that am~ (fairly rapid) change in the shape of temperature exit profile r and hence in temperature noise, could be caused by normal reactor operations or as a result of otherwise benign processes. In practice account must be taken of any temperature distribution at the subassembly exit in its design condition, for example, that arising from a flux gradient. Experimental measurements in an electrically-heated pin bundle in sodium were taken to investigate and calibrate the technique for possible use in the Prototype Fast Reactor (PFR) (Refs. 11 and 12). These showed that the technique was sensitive to small local overheating under non-boiling conditions but that thermocouples available for PE~ did not have sufficiently fast response. At about the same time (1965-67) a prototype monitor was set up on the Dounreay Fast Reactor (DFR) to gain experience in using temperature noise for subassembly fault detection with experimental fuel clusters. Thermocouples were conventional, non-pocket types with a slower response than desirable but sufficient to implement the analysis at detection frequencies of 0.15 and 1.0 Hz. Data shewed a constant background from mixing of the natural f l ~ a d i e n t temperature profile. Some changes in level, measured on different cluster assemblies over one year, were largely correlated with loaown reactor operations. In a few cases, unexplained short transients occurred in the temperature noise which could not be correlated with kno~m sub-assembly faults or fuel damage. Further experimental work is done in co-operation with GfK t Karlsruh% and ECN I Petten~ in sodium rigs with electrically-heated pin clusters simulating both non-boiling and sodium boiling conditions. The technique has also been applied to monitor fuel clusters taken into conditions of abnormally high temperature and sodium boiling in DFR. Onset of boiling was shown by a large increase in measured temperature noise for both blocked and unblocked assemblies (Ref. 13). A theoretical analysis of the generation and magnitude of temperature noise sigzBls resulting from sub-assembly malfunction is being made by statistical simulation of the ~ulrbulent
Development of techniques for the surveillance of LMFBRs
397
coolant flow using a Honte Carlo technique. The theoretical techniques and results of comparison with experimental d~ta are described in Firthts paper to this conference (Ref. lO). The model produces simulated fluctuating signals ~hioh can then be analysed to investigate the optim=n m y of treating the signal. As an interesting example of the value of this capability t Firth shows that the amplitude probability density function can be used to distinguish temperature profiles arising from a blockage from those arising from a power gradient. The theoretical model also allows evaluation of different turbulent mechanisms of tempera%x~re noise production to be compared~ and correctly simulates the effects of transport of temperature fluctuations and the smoothing introduced by a flow mixer. In the case of more advanced sub-~esembly faults, where the incident has proceeded to the point of sodium boiling~ theoretical analysis becomes more difficult and it is necessary to include the transient nature of the boiling process which can lead to an oscillating temperature or "chugging". Current procedures of theoretical az~lysis of temperature noise do not take these boiling effects into consideration, but it is planned to integrate the single-phase model with data from the SABRE code. Thermocouples are well developed for reactor operation and are one of the most reliable and sensitive instruments kno~1 for 12~FBRs. The relatively slow response of conventional thermoeouples is a disadvantage because the analysis frequency is reduced~ leading to problems with increased background noise and s l e ~ r overall response to fault c~adition in the fuel cluster. Special Nco-axial u ,herin.couples have now been developed with a much improved response~ and are suitable for temperature noise monitoring in an operational reactor (Ref. 14). An important feature is their high reliability which is a consequence of the construction. These should enable the techr~iques of temperature noise to be seriously considered in future l ~ R s .
Acoustic Detection of Boilir~ The principal advantage of an acoustic boiling detection system in a reactor is the familiar one that the acoustic signal is transmitted through the medium and so a single detector can monitor a large volume of the core. It is not necessary therefore to mount detectors within the fuel elements, sad a relatively small number of detectors, say 50, would give coverage to the entire core area of a commercial fast reactor with a substantial degree of redundancy. The advantages of the acoustic system therefore would be a much lower capital cost sad, because there are no connections to individual fuel elements, a simpler fuel-chauging prooedl/re. For effective operation the boiling-noise detection system must detect boiling against the reactor background noise. This can be done with greatest sensitivity if there is a difference in character between the signal and background. The acoustic noise from boiling comes mainly f/'om the collapse of vapour bubbles, which in highly subcooled conditions gives rise to sharp high-amplitude pressure pulses. These generate noise with a frequency spectrum extending well beyond lO0 KHz. Unfortunately one potential component of the reactor background, cavitation, produces a very similar signal. Figure 3 shows an extreme example of how the high-frequency noise from a pump cam increase as the suction pressure is reduced to produce cavitation, and incidentally how acoustics can be used to detect the inception of cavitation. In the UK~ therefore~ great came has been taken to reduce cavitation in the pumps and in other reactor components notably the sub-assembly gags. This results in a reactor background noise spectrum which falls off more rapidly at high frequency than the boiling noise signal does. With the background reduced as far as possible there are two techniques which can be used to discriminate between boiling and background. The simpler technique is to look for an increase in the rms level of the signal at high frequencies. Two examples of this technique are shown. First~ l~ig. 4 shows the variation of noise output in a single-chamnel forced-convection boiling experiment performed in the CFNa loop at Grenoble in collaboration with CEA. The variation of the acoustic sigmal as a function of flow is shown in four different one-third octave frequency bands. The variation of the test-section pressure drop with flow is also shown. The Figure shows that there is a clearly detectable increaSe in noise when sub-cooled boiling sTmmt~. As the flow is reduced this signal falls as the result of two factors: first~ the sub--cooling decreases slowing doom the bubble collapse and so reducing the noise source~ and secondly, the increased amount of vapour in the channel increases absorption. Eventually bulk boiling starts and there is a large increase in noise level close to the condition for minimum test-section pressure drop. At still lower flows the acoustic signal drops again because of acoustic absorption by the large amount of vapour present and by gas forced out of solution by the boiling process as described by Macleod et al. (15)
398
Burton, Bentley, Bishop, Macleod and McKnight
The second example of high-frequency detection is taken from the KNS experiment at Karlsruhe which has been described by Huber and Peppler (16). RNL made acoustic measurements~ described by Macleod et al. (17)t in these experiments in collaboration with GfK. In the experiment boiling te a ~ l a c e behind a blockage in a 169-pin simulation of a SNR subassembly in which only 88 central pins are heated. Figure 5 shows the variation of noise as the flow is reduced to produce boiling. The first effect is a reduction in the noise level as cavitation falls~ followed by a large increase as boiling commences. The effect is most noticeable at the highest frequencies. The results shown were obtained with a high-tempera%u~re microphone of the type described by Bishop, et al. (23) mounted on the test section. When the bubble-collapse pulses are well separated, for example in an incipient boiling condition, the contribution to the rms level is low. In these circumstances an amplitudediscrimination technique can be used to give clear detection. Figure 6 shows the data presented in Fig 5 for microphone fl00 analysed using this technique. The number of times the amplitude of the signal exceeds 3.5 times the rms level is plotted against time after a stepwise reduction in flow. An alternative technique is to detect boiling transients by acoustic pulse instrumentation, similar to that mentioned for acoustic emission monitoring of structures. This has the advantage that spurious sources can be excluded by timing location analysis using transducers spaced through the core and primary pool. An application of this method has been ms de for detecting the onset of boiling in DFR where fuel clusters were taken into conditions of abnormally high sodium temperalu~re (Ref. 18). The application of the techniques to the PFR and the experiments performed in the reactor are described in the companion paper (Ref. 8). The results suggest that signifioant boiling will be detectable; a highly sub-cooled boiling source involving 40 kW of thermal energy will be detectable with a margin of 10 dB. In addition an assessment is being made of the methods of detecting large acoustic impulses which might result, for example, from faelcoolant interactions which do not disrupt the sub-assembly wrapper.
STRUCTUP~L INTEGRITY MONITORING The purpose of structural monitoring is to ensure that the constructed components do not contain significant defects, particularly cracks which may grow to their critical length. In addition, a category of instruments is being developed to supervise the mechanical operations of the reactor to prevent malfunction and to detect deformations. A typical example of these instruments is the ultrasonic sweep arm which ensures that components linking the core and the shield are disconnected before the shield is rotated.
Failure Mode Monitorin~ Important structural components, particularly those in the core and primary circuit, are subject to dynsmic stressing from both normal and abnormal reactor operating conditions which must be considered as contributors to failure modes. Known perturbations are studied in the laboratory and on the reactor, and monitoring techniques may be set up to register the extent of damage in borderline cases. Vibrations resulting from normal operating conditions have been measured during PFR commissioning (Ref. 19) , and special attention has been given to vibrations excited by acoustic pressures generated by the primary pumps (Ref. 20). Acoustically excited vibrations are known to be a major contributor to the vibration spectrum in I~IFBRs, and the problems of acoustic-structure coupling have been studied in the laboratory using structural models (Ref. 21) and theoretical analysis ~Ref. 22). In iZFR, sodium-proof acoustic sensors were used for temperatures up to 250~C. Their construction and characteristics are described by Bishop et al. (Ref. 23) and a typical design is shown in Fig 7. Further measurements were made ~----"~accelerometers mounted on the outside of the primary tank and with high-temperature strain ganges. Both accelerometers and strain gauges have continued to operate successfully at the much higher temperatures experienced with the reactor at power and offer the opportunity of some long-term monitoring of vibration levels in the reactor. Temperature cycling both by turbulent mixing of non-unifa/m temperature fields and by transient c h s ~ e s resulting from operational power changes are potential contributors to cyclic stressing and have recently gained some attention. The probability and extent of damage depend strongly on details of structural design and changing operational procedures, so that a full endorsement is unlikely to come from out-of-pile work. In-pile monitoring, perhaps
Development of techniques for the surveillance of LM~BRs
399
integrated ~ith design procedures aimed at providing redundant safety structures, may be needed in future LMFBRs. Acoustic monitoring is a possibility if it can be shown that noises generated by rubbing of broken sections are detectable before structural integrity is impaired. ~ i s will become clearer after current laboratory experiments have identified the scale and nature of the problem in more detail. An intensive investigation has been made into the use of acoustic emission as an early warning indication of structural failure. Although many workers have shown that manufacturing flaws can be discovered by acoustic emission, its use for safety monitoring in L ~ B R s has been found to be subject to two problems. The first is that ductile steels used in its construction are quiet (Ref. 24) so that major propagating defects can be missed even under laboratory conditions (Ref. 25). The second, related point is that Dawson and Ingham (26) have shown that considerable differences exist between test simulations and practical reactor sit~tions, particularly dispersion and attenuation effects, so that optimistic test results are not realisable on full-scale plant. Acoustic emission is therefore not generally recommended al present for monitoring LMI~BR structures. However~ in special cases it can have an application. For example, stress-corrosion cracking has been shown to give large signals which are detectable under plant conditions~ and large-scale simulation of tube plate leaks in sodium water-tubed heat exchangers have shown that the onset and location of tube cracks could be identified by acoustic emission (Ref. 27). The technique gave sufficiently encouraging results for it to be capable of being applied under normal industrial conditions of a full-scale LMI~BR.
Component Ins2ection IA1der Sodium The opacity of sodium means thai direct visualisation of components immersed in it is not practicable. Most indirect visu~lisation techniques rely on ultrasonic pulse echo systems in one form or another. By scanning an object ~ith a pulsed transducer and analysing the returned echoes it is possible to build up an ultrasonic image. The position of the transducer, and the time for the echo to be returned, can be used to locate the object in threedimensional space. The best example of this method applied to liquid sodium is the work at Hanferd (Ref. 28)~ where an array of transducers used simultaneously reduces the total scanning time. The transducers reported for this work have an upper temperature limit of about 250°C. Similar arrangements are being considered for PFR. Development in the UKAEA has tended to concentrate on the needs of operational instruments, eg sweep arms and fuel identification. In these applications temperatures of 550°C or higher are likely; and so the use of waveguides to isolate the transducers from temperature have predominated so far. Details of this approach have been published (Ref. 29). There are two main kinds of waveguide that can be used: liquid-filled tubes or solid rods. Liquid~filled waveguides ideally behave as directly immersed transducers, losses in the waveguide being exceeded by the greater efficiency of low-temperature transducers. In practice, unless reactor sodium can be used in the tube r the sonic path has to be obstructed by liquid retention diaphragms. These introduce further losses I and distort the pulse performance seriously. Solid waveguides distort ultrasonic pulses so badly (because of internal mode conversions) that their use has been restricted to special instances where the waveguide-target distance is already known. However, th~ relative simplicity of installation of solid w~veguides is sufficient incentive for development of computer processes to correct the distortion, or special stranded constructions to eliminate it. An overall objection to waveguides, however, is their inflexibility. At best they are equivalent to a single immersed transducer with limited scanning movements. None the less I useful instruments result, as has been shown by the VISUS system inslalled on the Phenix reactor (Ref.
3o). The development of immersion transducers for under-sodium viewing is hampered by the high operating frequencies (2-5 MHz) involved. This requires an ultrasonic coupling characteristic that can be achieved only by bonding the transducer element to the face of its containment. The bonding has to survive the operating conditions, deal with differential expansion between materials, and provide electrical contact. For ~emperatures below 330°C lead zirconate discs have been successfully bonded into Inconel containments using pure lead. These transducers have been tested for several months of operation up to the melting point of lead. Higher-temperature applications have involved the development of lithium niobate bonding techniques. A number of non-oxidising braze alloys have been investigated, including gold-nickel and silver-copper eutectic. Tests have shown that mechanical and ultrasonic bonds can be formed in this way and pulse echo operation is possible at hig2 temperature. However, further development in this area is necessary to check long-term diffusion effects and to improve the complete wetting of the surface of the crystal. A number of variations in the brazing techniques have been tried; the most recent one gives almost 100% wetting of the crystal/substrate surface, producing excellent mechanical and
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Burton, Bentley, Bishop, Macleod and McKnight
ultrasonic bonds. As an example of the current thinking on the possibilities of extending operational instruments to a more sophisticated r o l % some images obtained on a water model are described. The model simulates the present sweep-arm design for CFRs~ although only a segment of the core is modelled. The displays are in effect sonar pictures obtained in the gap between core and the super-core structure. It is possible to direct the scanning beam so that, in the absence of obstructions~ no echoes can be detected. In Fig. 8a, however~ the beam has been deflected downwards slightly, so that an echo pattern from the core assembly is obtained. Fig. 8b shows a light obstruction in the gap, giving an additional complex echo I and Fig. 8c shows a heavy target, adding a small echo, but casting a shadow on the core assembly. The reactor operation can, therefore~ detect obstructions. He will not be required to analyse the display himself, however. Instead, an on-line processor will recall earlier similar displays~ and deduce what changes have occurred~ whether they are caused by e~Iditions or subtractions from the background pattern, or whether the background pattern itself has changed. Clearly such an approach will also indicate local changes in the core assembly itself. By deflecting the scanning beam upwards an echo pattern from the above-core structure is obtained instead~ and consequently changes in this will also be detected. The quality of image obtained with ultrasonics is ultimately limited by the wavelength (about 0.5 mm in current work), but is likely to be degraded from this by temperature gradients in the sodium. If it were possible to produce an optical image using 0.5 nm radiation it would not appear to be very good because it would be deficient in the higher spatial frequencies to which the eye is sensitive. Electronic display techniques often (and not always intentionally) modify the spatial frequency content of an image to improve its appearance. For visual inspection only many display processes are acceptable. These includ% for exampl% simple digitisation of the grey scale content of the image~ a process which can add higher spatial frequencies to form artificial boundaries in the image. When precise distance measurements are involved, and particularly where one dimension is provided by a pulse-echo time interval, it is important that boundaries are recognised only by the existing spatial frequencies. Their enhancement involves spatial filtering. Both Fourier and Nalsh transforms can be used tO achieve this. Future display systems for LM~BR work will probably include processing which uses them.
CO~L~G
~Ua2KS
Diagnostic techniques are playing an increasingly important role in the development of surveillance techniques for IRI~BRs. The stringent requirements for safety monitoring need to be more precisely defined~ and it is expected that this will lead to a variety of specific applications over a wide field. This is exemplified by the recently expanding R&D programmes for the two main areas considered. For sub-assembly monitoring there is a significant revival of interest in the use of temperature noise analysis. This has been stimulated by the development of robust co-~xial thermocouples, sufficiently reliable for reactor measurements and yet of fast response. In addition, sodium boiling experiments have indicated the promise that the blockages large enough to lead to boiling will give perturbations detectable by temperature noise whilst the onset of boiling also gives large increases in temperature noise. The acoustic detection experiments also indicate that it is likely that boiling in the sub-assembly will be detectable before the fault progresses to involve neighbouring sub-~ssemblies. Significant improvements may prove possible through the use of location techniques. However~ acoustic technology is relatively new in reactor applications t and more detailed work will be required before this conclusion can be substantiated. In particular, assesoments are required of transmission in the reactor environment, including the effect of gas bubbles, and of the stationarity of background noise. For structural monitoring the coverage is more uneven and further elucidation may be expected of the requirements. Vibration monitoring should be developed to study the modes and level of stress but preliminary studies do not suggest that it will be easy to detect flaws early. One reason for this is that the redundant structures adopted often obscure detection of local failures* until total structural failure is imminent. A closer relation between structural design and monitoring is required. The use of acoustic emission to detect cracks is hampered by the low strength of signals from stainless steels and t other than for specific tasks such as stress-corrosion, a more original approach may well be required. One possibility which is being explored is to wind components with a cable which emits stress waves when strained (Ref. 31). An increasing importance may be expected to be attached to methods of monitoring for deformation and displacements using ultrasonics. It may prove to be just as important to develop methods of archiving to record precisely the
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actual constructional installation so that any changes with time ~an be closely monitored. The development of presentational techniques to relieve the operator of the task of interpreting ultrasonic images looks to be a fascinating problem for the next decade.
(I)
F. R. Farmer, Risk quantification and acceptability. Safety, 17, no. 4, 418 (1976).
Letter to the editor, Nuclear
(2)
H. K. Fauske, M. A. Grolmes and S. H. Chart, An assessment of fuel failure propagation in I/~FBRs. International Meeting on Fast Reactor Safety and Related Physics, Chicago, Session 3, paper 4 (1976).
(3)
Smidt and Schleisiek, Fast Breeder safety against propagation of local failures, Nucl.
~ ~s., 40,/(/(1977) (4)
R. D. Smith, Protective Instrumentation for ~ s t (1973).
Heactors.
t~EA,
TRG Report 2397 (R)
(5)
G Jordan and R. OINeill, Safety and reliability requirements for periodic inspection of pressure vessels in the nuclear industry. Institution of Mechanical Engineers Conference paper C48/72 , p. 140, 9-11 May 1972.
(6)
F. Huber, W. Peppler, K. Schleisiek and A. J. Brook, Temperature distribution and local boiling behind a central blockage in a simulated FBR sub-assembly. International Meeting on Fast Reactor Safety and Related Physics, Chicago, Session 17, paper 2 (1976)
(7)
J. Bailly, G. Kessler and H. J. Teague I Essais de suret~ en pile sur combustibles pour reacteurs ~ neutrons rapides, Annals of Nuclear Ener~Tf, 3, 157, Pergamon Press (1976).
(8)
M. A. Grolmes, R. Avery, A. J. Goldman, H. K. Fauske, J. F. Marchaterre t R. D. Rose and A. E. Wright, In.pile experiments and test facilities proposed for fast reactor safety. International Meeting on Fast Reactor Safety and Related Physics, Chicago, Session 6, paper i (1976).
(9)
I. Do Macleod, Eo Catling and C. G. Taylor~ Acoustic detection of boiling in I ~ R s : An estimate of sensitivity derived from experiments during the commissioning of PFR. This Conference.
(lO) D. Firth, A Monte Carlo approach to the theoretical prediction of temperature noise in I~@~BR sub-assemblies. This Conference. (ll) P. G. Bentley, The problems of detection of blockages in the coolant passages of a fast reactor by measurement of coolant temperature, t~AEA, TRG Report 1197 (R)
(1963). (12) Idem, Temperature noise produced by turbulent mixing of coolant from fast reactor subassemblies. UKAEA, TRG Report 2485 (R) (1974). (13) R. Rowley and M. Bayley, private communication (1977). (14) A. Thomson and A. N. Fenton~ High reliability fast response thermocou~les. Proceedings IAEA Specialists Meeting on the In-Core and Primary Circuit Instrumentation of LMFBRs, UKAEA~ Risley, 27-29 Jan 1976. (15) I. D. Macleod, F. G. Latham and C. G. Taylor t Acoustic signals for sodium boiling in an annulus. Fifth Liquid Metal Boiling Working Group, Grenoble, 1974. (16) F. Huber and W. Peppler, Form and development of boiling behind a 49% central blockage in a 169-pin bundle. Seventh Liquid Metal Boiling Working Group, Petten, 1977. (17) I. D. Macleod~ F. G. Latham and J. A. Parker, S ~ e measurements of acoustic noise produced by boiling in a 169-pin cluster in the KNS loop. Ibid., 1977. (18) D. G. Dawson, private co~unication
(1977).
(19) A. W. Nicklin, Internal document, UKAEA, Risley (1975). (20) P. G. Bentley and R. Rowley, ibid. (1974).
402
Burton, Bentley, Bishop, Macleed and McKnight
(21)
P. G. Bentley, D. Firth, R. Rowley and M. J. Beesley, A simple steel/water model for preliminary studies of acoustic vibration in LMI~BR. Fourth International Conference on Structural Mechanics in Reactor Technology, San Francisco, paper F5/7, 15-19 Aug 1977.
(22)
D. Firth, The vibration of a distorted circular cylinder containing liquid. Third International Conference on Structural Mechanics in Reactor Technology, London, paper F2/10, 27-29 Jan 1975.
(23)
J. Bishop, G. H. Broomfield and J. Foley, Acoustic transducers for fast reactors. Proceedings IAEA Specialists Meeting on the Ir.-Core and Primary Circuit Instrumentation of ~ R s , I~LAEA~ Risley, 27-29 Jan 1976.
(24)
T. Ingham etal., Acoustic emission characteristics of steel. Pt I, Int. J. Press. Ves. & Piping, 2, 31-50 (1974). Pt II, ibid., 3, 267-293 (1975).
(25)
Pc G. Bentley, D. G. Dawson, D. J. Hanley and N. Kirby, Acoustic emission test on a 25 mm thick mild steel pressure vessel with inserted defects. Third Conference on Periodic Inspection of Pressurised Components t Institution of Mechanical Engineers, London, paper C209/76, 20-22 Sep 1976.
(26)
D. G. Dawson and T. Ingham, The application of acoustic emission measurements on laboratory testpieces to large scale pressure vessel monitoring. Third International Conference on Structural Mechanics in Reactor Technology, London, paper G3/8, 1-5 Sep 1975.
(27)
D. G. Dawson and P. G. Bentley, Acoustic emission from stress corrosion cracking in 316 stainless steel. European Working Group on Acoustic Emission, Roskilde, Denmark, 15-16 Sep 1976.
(28)
Co K. Day and R. W. Smith, Under sodium viewing. Institution of Electrical Engineers Transactions on Sonics and Ultrasonics, vol SU-21, no. 3, Jul 1974.
(29)
J. A. McKnight, J. R. Fothergill and S. Barnes, The design of ultrasonic viewing systems for CFR operation. BNES Symposium on Reactor Inspection Technology, Bristol, 26 Feb 1975.
(30)
N. Lions et al., Special instrumentation for Phehix. 536, ll-14--'~ 1974.
(31)
E. Duncembe and P. G. Bentley.
BNES Conference, London, 525-
British Patent Application 20121/76 (1976).
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OF TECHNIQUES P0R THE ~
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E J Burton~ P G Bentley~ J Bishop t I D Macleod and J A McEnight U.K. Atomic Energy Authority~ Risley Nuclear Power Development Laboratories~ Na~Tington~ England
SUMMARY
The development of surveillance techniques of LMFBRs is determined by the interaction of three factors: the specification of requirements~ improvements in technique and the physical analysis of the processes involved. The specification of requirements, which sets the structure for the discussion~ is mainly concerned with public safety. Two main divisions are identified: those concerned with thermal events in the nuclear core and those concerned directly or indirectly with the mechanical integrity of components. The necessary developments are then discussed in terms of the signal ar~lysis techniques to anticipate various modes of failures. The importance of an adequate understanding of the failure mode is emphasised in optimising the surveillance technique. Core surveillance may be achieved by monitoring individual sub-assemblies or by monitoring bulk conditions. The impOrtant features of sub-assembly monitoring are discussed and the advantages of temperature analysis explained. The spocification of the temperaturemonitoring systems is identified and the conflicting requirements for the reactor sensor discussed t viz adequate band width as against a robust and reliable construction. A theoretical treatment using Monte Carlo techniques allowS a full examination of the choice of method Of temperature analysis. This shows that~ although a filtered rms value has been the preferred choice for detecting either local blockage or sodium boiling~ it may be possible to distinguish the temperature signals of blockages from those of power gradients by an amplitude probability density plot. The advantages of acoustic monitoring using the noise of boiling sodium to detect overheating~ leading to core damage t are eTam4ned. An important consideration is the thermal-~coustic process of sodium boiling t and evidence is submitted from a range of out-of-pile experiments involving local sub-cooled boiling and bulk Boiling in discussing the merits of pulse analysis and pOwer spectral density techniques. An important factor in discriminating background from signal is the extent of cavitation in reactor components. Experiments are mentioned in which pulse techniques have been used to locate boiling sources by spatial correlation. The interpretation of reactor signals requires a detailed knowledge of the transmission of acoustic waves in reactor pools and structures and the effect of gas bubbles. Measurements in PFR and sodium loops have helped to lead to a more quantitative asses~ent of the sensitivity of the acoustic techniques. Structural integrity depends on detecting failure modes~ particularly those arising from crack propagation. Manufacturing defects or pre-existing cracks may be identified by ultrasonic inspection or by stress-wave emission. On-line monitoring for stress-loaded cracks by a stress-wave emission is seen as intrinsically difficult because of low signal strength and high attenuation but initial experiments have indicated possibilities for detecting stresscorrosion cracking. Mechanical failure from fatigue may be anticipated from an understanding of the vibrational modes of the sodium and its coupling with the structure. A one-elghth scale model of a IRFBR design has recently demonstrated the likely vibrational modes. A major handicap in supervising mechanical operation in sodium systems is the o~city of the sodium. Visualisation techniques of the major parts of the core structure are being developed. An important aspect is the study of the information processing required to present an image easy for the reactor operator to unders+~nd. Advances may be made using transform methods to improve object boundaries by modifying the spatial frequencies of the display or record. INTRGD~D TION The development of techniques for the surveillance of fast reactors has the objective of improving their safe and efficient operation t that is~ it is aimed primarily at developing methods for monitoring on-line whilst the reactor is at pOwer to give an early warning of incipient failures. An important difference between the programmes for LMPBRs and for thermal reactors is that the small nt,nber of operating reactors are prototypes~ with the design of demonstration plants to follow. Thus~ although mainter~uce procedures on the more 393
394
Burton~ Bentley, Bishop, Macleod and McKn_ight
conventional items of the plant are important, most attention in the UK is being given to safety, particularly to meet the demands for a public assurance that no significant release of fission products will occur. The specification of requirements is an essential precursor to discussing techniques, and the opportunity is taken in the first section of the paper to outline the important features by using a simplified presentation based on probability analysis. In relating this to a particular situation, the physical process involved in the failure must be fully understood; this is being achieved through parallel out-of-pile investigations. In its next two sections the paper concentrates on discussing the techniques required to monitor the nuclear core to ensure its thermal balance and its mechanical integrity. The improvements in detection techniques which can be achieved are constrained by the engineering difficulties which arise from the high power density of the core, the use of sodium~ the inaccessibility of the measuring stations and the need for easy replacement of sensors. Thus~ for thermal monitoring, i% is not practicable to insert instrumentation into the fuel cluster, although measurements m a y b e made at the cluster outlet, and the requirements lead to the use of inferential or diagnostic techniques. For structural monitoring, the requirement is %o detect incipient failures which may occur in the structures over the life of the reactor; hence sensor performance must be maintained, or checked by calibration~ over 20 to 30 years. Because of these two sets of requirements t the reliability of the sensors may be best achieved through robust designs which often conflict with the sensitivity requirements. Thus in general the maximum information is sought from the sensor by analysing the signal using a variety of functions, including power spectral densities and cross-correlations. Obviously modern computers provide economic and elegant methods of performing this analysis, but at present in the DE the introduction of computers to execute safety functions directly (ie to provide trip actuation) is proceeding cautiously. Nevertheless the combined use of sensors and data processing is leading %o the development of a sophisticated sul~eillance system. Aspects of this are discussed, with most emphasis on acoustic techniques as these provide, at least in principle, a measurement system supervising a large volume, but a review is also included of the use of temperature noise to detect local anomalies in an individual sub-assembly.
GENERAL SPECIFICATION OF SURVEILLA/~E SYSTH~ Usually the important surveillance specifications arise from analysis of the fault sequences which could ultimately lead %0 the release of a significant ~mount of re~ioactive fission products. Those which come from the need to manoeuvre and to operate the reactor efficiently at stead~v power are uSually less stringent. There are two broad considerations behind specifications for surveillance systems: first, the general style and likelihood of the fault sequences to be protected against, and secondly, the more detailed mode of failure within each sequence. These are discussed separately below.
Fault A n a ~ s i s The safety analyses have identified three principal categories of faults which could lead to whole core accidents: I.
2.
Loss of flow (LOF) arising from: (a)
loss of power supply to all primary coolant pumps;
(b)
loss of coolant flow to the core through structural failure of the d/agrid or coolant supply pipes.
Transient over power (TOP) arising from: (a)
reactivity addition through uncontrolled withdrawal of absorber rods.
(b)
reactivity addition through structural failure of the core support.
3. A sub-assembly accident arising from local loss of cooling, with the possibility of sudden escalation to a whole-core accident by a pressure pulse caused by interaction between overheated fuel and coolant (FCI). The protection against such accidentsis achieved mainly through engineered safeguards, including instrumentation systems actuating trips for shut-do~q. The specification for the
Development of techniques for the surveillance of L ~ R s
]95
instrumentation systems, and therefore for the noise analysis systems, arises from an examination of the role of these safeguards. The precise specification of the instrumentation is a complex process in which the details are most important~ but it is also important to explain the general guidelines which are used. These are valuable in attempting broad comparisons between reactor systems and in evaluating particular instrument techniques. The first step is to consider the role of the engineered safeguards in the safety protection; this is summarised in a simple diagram in Fig ~ based on unpublished work in the UKAEA by Smith and Cowking. This Figure illustrates that there are four main frequency or probability components to any accident sequence which release a substantial amount of radioactivity from the contaimment. First of all the fault must have an initiating cause, there must then be failure to detect and correct it, there must be propagation to a major whole-core accident, and finally this must lead to failure of the containment. The numerical values shown are a present judgement of the situation I but are not definitive or endorsed as an agreed UK position. Without doubt further development will alter best judgement values and narrow down the uncertainties which still remain. Overall the objective is to show that the frequency of major activity release from a plant is not greater than about 10-7 per operating year. Since the values on Fig I are much lower than this~ 10-7 should be a realistic target. Farmer (I), for exampl% would consider that numbers so low as shown are beyond confirmation and unnecessary for safety endorsement, but at the stage where there are still uncertainties it is best to retain a picture formed from best estimates. The value of the analysis to this paper is that it demonstrates the comparative worth of different areas of safety development work. Surveillance development is relevant to all three fault categories and is shown to have a large potential contribution, as discussed below. Protection against LOF and TOP faults arising from malfunctions in the plant rests largely on detecting signals showing deviation from normal plant parameters, for which the use of multiple systems ensures that a high level of protection can be obtained. Structural failure is identified above as a possible contributor to both these faults~ however, and surveillance methods for detecting minor damage or incipient failure would reinforce the safety assurance inherent in good design and manufacturing practices. The values shown on Fig I mainly reflect the more frequent~ highly protected, faults. It will be seen that it is important that the frequency of sudden severe structural failures is not so high as to by-pass this line of protection. The probabilistic analysis of structural failures has received most attention for reactor pressure vessels (Ref. 5) and is now being extended to fast-reactor structures. The analysis shows the extremely high standard of integrity assurance which is sought through the combination of design, construction and instrumentation. Acoustic instrumentation has an important contribution to make through monitoring for flaws and errors during construction and commissioning and for incipient failures, particularly during operation, including mechanical deformation and displacement. Techniques being developed to assist in this are described below, and include acoustic emission and under-sodium viewing. Early detection of localised faults before they develop is judged to be important in ensuring that sub-assembly accidents will not spread to neighbouring sub-assemblies and affect the whole core. It is of interest that world opinion is divided on the risk of the spread of damage and hence on the need to develop techniques for surveillance. Fauske et al. (2) discount the risk and would consequently put less emphasis on the usefulness of surveillance. Smidt and Schleisiek (3), on the other hand, give an analysis similar to that presented here. General discussion of all the techniques for sub-assembly surveillance is outside the scope of this paper*, but two developments are selected of especial interest: first, the use of the analysis of the fluctuations of outlet coolant temperature, and secondly~ the use of acoustic methods to detect the noise of boiling sodium.
Modes of Failure The other general consideration concerns the modes of failure which need to be precisely defined so that the instrument detection system can be optimised. For the sub-assembly accident, the mode of accident progression has received extensive examination~ both through out-of-pile rigs, such as the GfK rig at Karleruhe (Ref. 6) and iD-pile experiments in Scarabee (Ref. 7) and in the USA. The detailed application of the European experiments to temperature noise and acoustic monitoring is discussed below and t also in the companion specialist papers to this Conference (Refs. 9 and 10). A useful discussion is ~iven by Smith (4)
396
Burton, Bentley, Bishop, Macleod and McKnight
The examination of modes of failures for the core smfety structures opens up a wider field that has been studied unevenly so far. Perturbing forces which can produce failures include seismic and other external shocks~ internal vibration induced either by flow or by acoustic excitation, thermal shock or melting and material loss by wear or by corrosion. As the UK is in a seismically quiet area~ most effort has been concerned with crack propagation from thermal shock~ corrosion and vibrational fatigue t and aspects of these are discussed below.
BIB-ASSEMBLY MONITORING The concern in the sub-assembly is that the loss of local cooling arising particularly from a blockage may spread~ leading to sodium boiling and eventually a fuel-coolant interaction. Thus an early ~ r n i n g of this potentially propagating event could be given either by detecting the blockage or~ somewhat later~ by detecting boiling of the sodium.
Temperature Noise Sub-assembly faults leading to local overheating and the possibility of escalation to larger accidents are most directly monitored by measuring coolant temperature. However, one of the consequences of high fuel thermal rating in IX~I~BRs is that small local effects give only a small change in average coolant temperature which may be difficult to detect, and yet the local temperatures can be sufficiently high to lead eventually to a major incident. The requirement is therefore for a method of using the temperature information early in a fault sequence so that there is time available to take remedial action before the incident spreads. The techniques of temperature noise analysis offer this possibility, the principles being shown schematically in Fig. 2. In the absence of any malfunction the temperature profile for a central sub-assembly is substantially uniform downstream of the fuel cluster, leading to temperature at the monitoring thermcccuple which is a timeinvariant, constant measure of the mixed mean outlet. A small flow effect within the cluster t for example a local blockage or geometrical distortion~ changes the uniformity of temperatures and leads to a disturbance in exit profile as indicated in the Figure. Here a temperature blip is shown I resulting from a blockage; different local malfunctions disturb the uniform profile in different ways but without affecting the subsequent argument. Coolant flow carries the profile to the monitoring thermocoupl% mixing it by the random motions of turbulence, and converts the non-uniform spatial profile to a time-varying temperature noise. Although small in amplitud% the d~namically varying temperature can be much more sensitively and reliably detected than the corresponding change in mean value. The technique retains the advantages of proven thermccouple sensors of high reliability and offers the opportunity to take remedial action very early in the fault sequence. A further advantage is an inherent low probability of spurious fault indications since the measured parameter is closely linked to the fault condition; for example, it is unlikely that am~ (fairly rapid) change in the shape of temperature exit profile r and hence in temperature noise, could be caused by normal reactor operations or as a result of otherwise benign processes. In practice account must be taken of any temperature distribution at the subassembly exit in its design condition, for example, that arising from a flux gradient. Experimental measurements in an electrically-heated pin bundle in sodium were taken to investigate and calibrate the technique for possible use in the Prototype Fast Reactor (PFR) (Refs. 11 and 12). These showed that the technique was sensitive to small local overheating under non-boiling conditions but that thermocouples available for PE~ did not have sufficiently fast response. At about the same time (1965-67) a prototype monitor was set up on the Dounreay Fast Reactor (DFR) to gain experience in using temperature noise for subassembly fault detection with experimental fuel clusters. Thermocouples were conventional, non-pocket types with a slower response than desirable but sufficient to implement the analysis at detection frequencies of 0.15 and 1.0 Hz. Data shewed a constant background from mixing of the natural f l ~ a d i e n t temperature profile. Some changes in level, measured on different cluster assemblies over one year, were largely correlated with loaown reactor operations. In a few cases, unexplained short transients occurred in the temperature noise which could not be correlated with kno~m sub-assembly faults or fuel damage. Further experimental work is done in co-operation with GfK t Karlsruh% and ECN I Petten~ in sodium rigs with electrically-heated pin clusters simulating both non-boiling and sodium boiling conditions. The technique has also been applied to monitor fuel clusters taken into conditions of abnormally high temperature and sodium boiling in DFR. Onset of boiling was shown by a large increase in measured temperature noise for both blocked and unblocked assemblies (Ref. 13). A theoretical analysis of the generation and magnitude of temperature noise sigzBls resulting from sub-assembly malfunction is being made by statistical simulation of the ~ulrbulent
Development of techniques for the surveillance of LMFBRs
397
coolant flow using a Honte Carlo technique. The theoretical techniques and results of comparison with experimental d~ta are described in Firthts paper to this conference (Ref. lO). The model produces simulated fluctuating signals ~hioh can then be analysed to investigate the optim=n m y of treating the signal. As an interesting example of the value of this capability t Firth shows that the amplitude probability density function can be used to distinguish temperature profiles arising from a blockage from those arising from a power gradient. The theoretical model also allows evaluation of different turbulent mechanisms of tempera%x~re noise production to be compared~ and correctly simulates the effects of transport of temperature fluctuations and the smoothing introduced by a flow mixer. In the case of more advanced sub-~esembly faults, where the incident has proceeded to the point of sodium boiling~ theoretical analysis becomes more difficult and it is necessary to include the transient nature of the boiling process which can lead to an oscillating temperature or "chugging". Current procedures of theoretical az~lysis of temperature noise do not take these boiling effects into consideration, but it is planned to integrate the single-phase model with data from the SABRE code. Thermocouples are well developed for reactor operation and are one of the most reliable and sensitive instruments kno~1 for 12~FBRs. The relatively slow response of conventional thermoeouples is a disadvantage because the analysis frequency is reduced~ leading to problems with increased background noise and s l e ~ r overall response to fault c~adition in the fuel cluster. Special Nco-axial u ,herin.couples have now been developed with a much improved response~ and are suitable for temperature noise monitoring in an operational reactor (Ref. 14). An important feature is their high reliability which is a consequence of the construction. These should enable the techr~iques of temperature noise to be seriously considered in future l ~ R s .
Acoustic Detection of Boilir~ The principal advantage of an acoustic boiling detection system in a reactor is the familiar one that the acoustic signal is transmitted through the medium and so a single detector can monitor a large volume of the core. It is not necessary therefore to mount detectors within the fuel elements, sad a relatively small number of detectors, say 50, would give coverage to the entire core area of a commercial fast reactor with a substantial degree of redundancy. The advantages of the acoustic system therefore would be a much lower capital cost sad, because there are no connections to individual fuel elements, a simpler fuel-chauging prooedl/re. For effective operation the boiling-noise detection system must detect boiling against the reactor background noise. This can be done with greatest sensitivity if there is a difference in character between the signal and background. The acoustic noise from boiling comes mainly f/'om the collapse of vapour bubbles, which in highly subcooled conditions gives rise to sharp high-amplitude pressure pulses. These generate noise with a frequency spectrum extending well beyond lO0 KHz. Unfortunately one potential component of the reactor background, cavitation, produces a very similar signal. Figure 3 shows an extreme example of how the high-frequency noise from a pump cam increase as the suction pressure is reduced to produce cavitation, and incidentally how acoustics can be used to detect the inception of cavitation. In the UK~ therefore~ great came has been taken to reduce cavitation in the pumps and in other reactor components notably the sub-assembly gags. This results in a reactor background noise spectrum which falls off more rapidly at high frequency than the boiling noise signal does. With the background reduced as far as possible there are two techniques which can be used to discriminate between boiling and background. The simpler technique is to look for an increase in the rms level of the signal at high frequencies. Two examples of this technique are shown. First~ l~ig. 4 shows the variation of noise output in a single-chamnel forced-convection boiling experiment performed in the CFNa loop at Grenoble in collaboration with CEA. The variation of the acoustic sigmal as a function of flow is shown in four different one-third octave frequency bands. The variation of the test-section pressure drop with flow is also shown. The Figure shows that there is a clearly detectable increaSe in noise when sub-cooled boiling sTmmt~. As the flow is reduced this signal falls as the result of two factors: first~ the sub--cooling decreases slowing doom the bubble collapse and so reducing the noise source~ and secondly, the increased amount of vapour in the channel increases absorption. Eventually bulk boiling starts and there is a large increase in noise level close to the condition for minimum test-section pressure drop. At still lower flows the acoustic signal drops again because of acoustic absorption by the large amount of vapour present and by gas forced out of solution by the boiling process as described by Macleod et al. (15)
398
Burton, Bentley, Bishop, Macleod and McKnight
The second example of high-frequency detection is taken from the KNS experiment at Karlsruhe which has been described by Huber and Peppler (16). RNL made acoustic measurements~ described by Macleod et al. (17)t in these experiments in collaboration with GfK. In the experiment boiling te a ~ l a c e behind a blockage in a 169-pin simulation of a SNR subassembly in which only 88 central pins are heated. Figure 5 shows the variation of noise as the flow is reduced to produce boiling. The first effect is a reduction in the noise level as cavitation falls~ followed by a large increase as boiling commences. The effect is most noticeable at the highest frequencies. The results shown were obtained with a high-tempera%u~re microphone of the type described by Bishop, et al. (23) mounted on the test section. When the bubble-collapse pulses are well separated, for example in an incipient boiling condition, the contribution to the rms level is low. In these circumstances an amplitudediscrimination technique can be used to give clear detection. Figure 6 shows the data presented in Fig 5 for microphone fl00 analysed using this technique. The number of times the amplitude of the signal exceeds 3.5 times the rms level is plotted against time after a stepwise reduction in flow. An alternative technique is to detect boiling transients by acoustic pulse instrumentation, similar to that mentioned for acoustic emission monitoring of structures. This has the advantage that spurious sources can be excluded by timing location analysis using transducers spaced through the core and primary pool. An application of this method has been ms de for detecting the onset of boiling in DFR where fuel clusters were taken into conditions of abnormally high sodium temperalu~re (Ref. 18). The application of the techniques to the PFR and the experiments performed in the reactor are described in the companion paper (Ref. 8). The results suggest that signifioant boiling will be detectable; a highly sub-cooled boiling source involving 40 kW of thermal energy will be detectable with a margin of 10 dB. In addition an assessment is being made of the methods of detecting large acoustic impulses which might result, for example, from faelcoolant interactions which do not disrupt the sub-assembly wrapper.
STRUCTUP~L INTEGRITY MONITORING The purpose of structural monitoring is to ensure that the constructed components do not contain significant defects, particularly cracks which may grow to their critical length. In addition, a category of instruments is being developed to supervise the mechanical operations of the reactor to prevent malfunction and to detect deformations. A typical example of these instruments is the ultrasonic sweep arm which ensures that components linking the core and the shield are disconnected before the shield is rotated.
Failure Mode Monitorin~ Important structural components, particularly those in the core and primary circuit, are subject to dynsmic stressing from both normal and abnormal reactor operating conditions which must be considered as contributors to failure modes. Known perturbations are studied in the laboratory and on the reactor, and monitoring techniques may be set up to register the extent of damage in borderline cases. Vibrations resulting from normal operating conditions have been measured during PFR commissioning (Ref. 19) , and special attention has been given to vibrations excited by acoustic pressures generated by the primary pumps (Ref. 20). Acoustically excited vibrations are known to be a major contributor to the vibration spectrum in I~IFBRs, and the problems of acoustic-structure coupling have been studied in the laboratory using structural models (Ref. 21) and theoretical analysis ~Ref. 22). In iZFR, sodium-proof acoustic sensors were used for temperatures up to 250~C. Their construction and characteristics are described by Bishop et al. (Ref. 23) and a typical design is shown in Fig 7. Further measurements were made ~----"~accelerometers mounted on the outside of the primary tank and with high-temperature strain ganges. Both accelerometers and strain gauges have continued to operate successfully at the much higher temperatures experienced with the reactor at power and offer the opportunity of some long-term monitoring of vibration levels in the reactor. Temperature cycling both by turbulent mixing of non-unifa/m temperature fields and by transient c h s ~ e s resulting from operational power changes are potential contributors to cyclic stressing and have recently gained some attention. The probability and extent of damage depend strongly on details of structural design and changing operational procedures, so that a full endorsement is unlikely to come from out-of-pile work. In-pile monitoring, perhaps
Development of techniques for the surveillance of LM~BRs
399
integrated ~ith design procedures aimed at providing redundant safety structures, may be needed in future LMFBRs. Acoustic monitoring is a possibility if it can be shown that noises generated by rubbing of broken sections are detectable before structural integrity is impaired. ~ i s will become clearer after current laboratory experiments have identified the scale and nature of the problem in more detail. An intensive investigation has been made into the use of acoustic emission as an early warning indication of structural failure. Although many workers have shown that manufacturing flaws can be discovered by acoustic emission, its use for safety monitoring in L ~ B R s has been found to be subject to two problems. The first is that ductile steels used in its construction are quiet (Ref. 24) so that major propagating defects can be missed even under laboratory conditions (Ref. 25). The second, related point is that Dawson and Ingham (26) have shown that considerable differences exist between test simulations and practical reactor sit~tions, particularly dispersion and attenuation effects, so that optimistic test results are not realisable on full-scale plant. Acoustic emission is therefore not generally recommended al present for monitoring LMI~BR structures. However~ in special cases it can have an application. For example, stress-corrosion cracking has been shown to give large signals which are detectable under plant conditions~ and large-scale simulation of tube plate leaks in sodium water-tubed heat exchangers have shown that the onset and location of tube cracks could be identified by acoustic emission (Ref. 27). The technique gave sufficiently encouraging results for it to be capable of being applied under normal industrial conditions of a full-scale LMI~BR.
Component Ins2ection IA1der Sodium The opacity of sodium means thai direct visualisation of components immersed in it is not practicable. Most indirect visu~lisation techniques rely on ultrasonic pulse echo systems in one form or another. By scanning an object ~ith a pulsed transducer and analysing the returned echoes it is possible to build up an ultrasonic image. The position of the transducer, and the time for the echo to be returned, can be used to locate the object in threedimensional space. The best example of this method applied to liquid sodium is the work at Hanferd (Ref. 28)~ where an array of transducers used simultaneously reduces the total scanning time. The transducers reported for this work have an upper temperature limit of about 250°C. Similar arrangements are being considered for PFR. Development in the UKAEA has tended to concentrate on the needs of operational instruments, eg sweep arms and fuel identification. In these applications temperatures of 550°C or higher are likely; and so the use of waveguides to isolate the transducers from temperature have predominated so far. Details of this approach have been published (Ref. 29). There are two main kinds of waveguide that can be used: liquid-filled tubes or solid rods. Liquid~filled waveguides ideally behave as directly immersed transducers, losses in the waveguide being exceeded by the greater efficiency of low-temperature transducers. In practice, unless reactor sodium can be used in the tube r the sonic path has to be obstructed by liquid retention diaphragms. These introduce further losses I and distort the pulse performance seriously. Solid waveguides distort ultrasonic pulses so badly (because of internal mode conversions) that their use has been restricted to special instances where the waveguide-target distance is already known. However, th~ relative simplicity of installation of solid w~veguides is sufficient incentive for development of computer processes to correct the distortion, or special stranded constructions to eliminate it. An overall objection to waveguides, however, is their inflexibility. At best they are equivalent to a single immersed transducer with limited scanning movements. None the less I useful instruments result, as has been shown by the VISUS system inslalled on the Phenix reactor (Ref.
3o). The development of immersion transducers for under-sodium viewing is hampered by the high operating frequencies (2-5 MHz) involved. This requires an ultrasonic coupling characteristic that can be achieved only by bonding the transducer element to the face of its containment. The bonding has to survive the operating conditions, deal with differential expansion between materials, and provide electrical contact. For ~emperatures below 330°C lead zirconate discs have been successfully bonded into Inconel containments using pure lead. These transducers have been tested for several months of operation up to the melting point of lead. Higher-temperature applications have involved the development of lithium niobate bonding techniques. A number of non-oxidising braze alloys have been investigated, including gold-nickel and silver-copper eutectic. Tests have shown that mechanical and ultrasonic bonds can be formed in this way and pulse echo operation is possible at hig2 temperature. However, further development in this area is necessary to check long-term diffusion effects and to improve the complete wetting of the surface of the crystal. A number of variations in the brazing techniques have been tried; the most recent one gives almost 100% wetting of the crystal/substrate surface, producing excellent mechanical and
400
Burton, Bentley, Bishop, Macleod and McKnight
ultrasonic bonds. As an example of the current thinking on the possibilities of extending operational instruments to a more sophisticated r o l % some images obtained on a water model are described. The model simulates the present sweep-arm design for CFRs~ although only a segment of the core is modelled. The displays are in effect sonar pictures obtained in the gap between core and the super-core structure. It is possible to direct the scanning beam so that, in the absence of obstructions~ no echoes can be detected. In Fig. 8a, however~ the beam has been deflected downwards slightly, so that an echo pattern from the core assembly is obtained. Fig. 8b shows a light obstruction in the gap, giving an additional complex echo I and Fig. 8c shows a heavy target, adding a small echo, but casting a shadow on the core assembly. The reactor operation can, therefore~ detect obstructions. He will not be required to analyse the display himself, however. Instead, an on-line processor will recall earlier similar displays~ and deduce what changes have occurred~ whether they are caused by e~Iditions or subtractions from the background pattern, or whether the background pattern itself has changed. Clearly such an approach will also indicate local changes in the core assembly itself. By deflecting the scanning beam upwards an echo pattern from the above-core structure is obtained instead~ and consequently changes in this will also be detected. The quality of image obtained with ultrasonics is ultimately limited by the wavelength (about 0.5 mm in current work), but is likely to be degraded from this by temperature gradients in the sodium. If it were possible to produce an optical image using 0.5 nm radiation it would not appear to be very good because it would be deficient in the higher spatial frequencies to which the eye is sensitive. Electronic display techniques often (and not always intentionally) modify the spatial frequency content of an image to improve its appearance. For visual inspection only many display processes are acceptable. These includ% for exampl% simple digitisation of the grey scale content of the image~ a process which can add higher spatial frequencies to form artificial boundaries in the image. When precise distance measurements are involved, and particularly where one dimension is provided by a pulse-echo time interval, it is important that boundaries are recognised only by the existing spatial frequencies. Their enhancement involves spatial filtering. Both Fourier and Nalsh transforms can be used tO achieve this. Future display systems for LM~BR work will probably include processing which uses them.
CO~L~G
~Ua2KS
Diagnostic techniques are playing an increasingly important role in the development of surveillance techniques for IRI~BRs. The stringent requirements for safety monitoring need to be more precisely defined~ and it is expected that this will lead to a variety of specific applications over a wide field. This is exemplified by the recently expanding R&D programmes for the two main areas considered. For sub-assembly monitoring there is a significant revival of interest in the use of temperature noise analysis. This has been stimulated by the development of robust co-~xial thermocouples, sufficiently reliable for reactor measurements and yet of fast response. In addition, sodium boiling experiments have indicated the promise that the blockages large enough to lead to boiling will give perturbations detectable by temperature noise whilst the onset of boiling also gives large increases in temperature noise. The acoustic detection experiments also indicate that it is likely that boiling in the sub-assembly will be detectable before the fault progresses to involve neighbouring sub-~ssemblies. Significant improvements may prove possible through the use of location techniques. However~ acoustic technology is relatively new in reactor applications t and more detailed work will be required before this conclusion can be substantiated. In particular, assesoments are required of transmission in the reactor environment, including the effect of gas bubbles, and of the stationarity of background noise. For structural monitoring the coverage is more uneven and further elucidation may be expected of the requirements. Vibration monitoring should be developed to study the modes and level of stress but preliminary studies do not suggest that it will be easy to detect flaws early. One reason for this is that the redundant structures adopted often obscure detection of local failures* until total structural failure is imminent. A closer relation between structural design and monitoring is required. The use of acoustic emission to detect cracks is hampered by the low strength of signals from stainless steels and t other than for specific tasks such as stress-corrosion, a more original approach may well be required. One possibility which is being explored is to wind components with a cable which emits stress waves when strained (Ref. 31). An increasing importance may be expected to be attached to methods of monitoring for deformation and displacements using ultrasonics. It may prove to be just as important to develop methods of archiving to record precisely the
Development of techniques for the surveillance of I ~ R s
401
actual constructional installation so that any changes with time ~an be closely monitored. The development of presentational techniques to relieve the operator of the task of interpreting ultrasonic images looks to be a fascinating problem for the next decade.
(I)
F. R. Farmer, Risk quantification and acceptability. Safety, 17, no. 4, 418 (1976).
Letter to the editor, Nuclear
(2)
H. K. Fauske, M. A. Grolmes and S. H. Chart, An assessment of fuel failure propagation in I/~FBRs. International Meeting on Fast Reactor Safety and Related Physics, Chicago, Session 3, paper 4 (1976).
(3)
Smidt and Schleisiek, Fast Breeder safety against propagation of local failures, Nucl.
~ ~s., 40,/(/(1977) (4)
R. D. Smith, Protective Instrumentation for ~ s t (1973).
Heactors.
t~EA,
TRG Report 2397 (R)
(5)
G Jordan and R. OINeill, Safety and reliability requirements for periodic inspection of pressure vessels in the nuclear industry. Institution of Mechanical Engineers Conference paper C48/72 , p. 140, 9-11 May 1972.
(6)
F. Huber, W. Peppler, K. Schleisiek and A. J. Brook, Temperature distribution and local boiling behind a central blockage in a simulated FBR sub-assembly. International Meeting on Fast Reactor Safety and Related Physics, Chicago, Session 17, paper 2 (1976)
(7)
J. Bailly, G. Kessler and H. J. Teague I Essais de suret~ en pile sur combustibles pour reacteurs ~ neutrons rapides, Annals of Nuclear Ener~Tf, 3, 157, Pergamon Press (1976).
(8)
M. A. Grolmes, R. Avery, A. J. Goldman, H. K. Fauske, J. F. Marchaterre t R. D. Rose and A. E. Wright, In.pile experiments and test facilities proposed for fast reactor safety. International Meeting on Fast Reactor Safety and Related Physics, Chicago, Session 6, paper i (1976).
(9)
I. Do Macleod, Eo Catling and C. G. Taylor~ Acoustic detection of boiling in I ~ R s : An estimate of sensitivity derived from experiments during the commissioning of PFR. This Conference.
(lO) D. Firth, A Monte Carlo approach to the theoretical prediction of temperature noise in I~@~BR sub-assemblies. This Conference. (ll) P. G. Bentley, The problems of detection of blockages in the coolant passages of a fast reactor by measurement of coolant temperature, t~AEA, TRG Report 1197 (R)
(1963). (12) Idem, Temperature noise produced by turbulent mixing of coolant from fast reactor subassemblies. UKAEA, TRG Report 2485 (R) (1974). (13) R. Rowley and M. Bayley, private communication (1977). (14) A. Thomson and A. N. Fenton~ High reliability fast response thermocou~les. Proceedings IAEA Specialists Meeting on the In-Core and Primary Circuit Instrumentation of LMFBRs, UKAEA~ Risley, 27-29 Jan 1976. (15) I. D. Macleod, F. G. Latham and C. G. Taylor t Acoustic signals for sodium boiling in an annulus. Fifth Liquid Metal Boiling Working Group, Grenoble, 1974. (16) F. Huber and W. Peppler, Form and development of boiling behind a 49% central blockage in a 169-pin bundle. Seventh Liquid Metal Boiling Working Group, Petten, 1977. (17) I. D. Macleod~ F. G. Latham and J. A. Parker, S ~ e measurements of acoustic noise produced by boiling in a 169-pin cluster in the KNS loop. Ibid., 1977. (18) D. G. Dawson, private co~unication
(1977).
(19) A. W. Nicklin, Internal document, UKAEA, Risley (1975). (20) P. G. Bentley and R. Rowley, ibid. (1974).
402
Burton, Bentley, Bishop, Macleed and McKnight
(21)
P. G. Bentley, D. Firth, R. Rowley and M. J. Beesley, A simple steel/water model for preliminary studies of acoustic vibration in LMI~BR. Fourth International Conference on Structural Mechanics in Reactor Technology, San Francisco, paper F5/7, 15-19 Aug 1977.
(22)
D. Firth, The vibration of a distorted circular cylinder containing liquid. Third International Conference on Structural Mechanics in Reactor Technology, London, paper F2/10, 27-29 Jan 1975.
(23)
J. Bishop, G. H. Broomfield and J. Foley, Acoustic transducers for fast reactors. Proceedings IAEA Specialists Meeting on the Ir.-Core and Primary Circuit Instrumentation of ~ R s , I~LAEA~ Risley, 27-29 Jan 1976.
(24)
T. Ingham etal., Acoustic emission characteristics of steel. Pt I, Int. J. Press. Ves. & Piping, 2, 31-50 (1974). Pt II, ibid., 3, 267-293 (1975).
(25)
Pc G. Bentley, D. G. Dawson, D. J. Hanley and N. Kirby, Acoustic emission test on a 25 mm thick mild steel pressure vessel with inserted defects. Third Conference on Periodic Inspection of Pressurised Components t Institution of Mechanical Engineers, London, paper C209/76, 20-22 Sep 1976.
(26)
D. G. Dawson and T. Ingham, The application of acoustic emission measurements on laboratory testpieces to large scale pressure vessel monitoring. Third International Conference on Structural Mechanics in Reactor Technology, London, paper G3/8, 1-5 Sep 1975.
(27)
D. G. Dawson and P. G. Bentley, Acoustic emission from stress corrosion cracking in 316 stainless steel. European Working Group on Acoustic Emission, Roskilde, Denmark, 15-16 Sep 1976.
(28)
Co K. Day and R. W. Smith, Under sodium viewing. Institution of Electrical Engineers Transactions on Sonics and Ultrasonics, vol SU-21, no. 3, Jul 1974.
(29)
J. A. McKnight, J. R. Fothergill and S. Barnes, The design of ultrasonic viewing systems for CFR operation. BNES Symposium on Reactor Inspection Technology, Bristol, 26 Feb 1975.
(30)
N. Lions et al., Special instrumentation for Phehix. 536, ll-14--'~ 1974.
(31)
E. Duncembe and P. G. Bentley.
BNES Conference, London, 525-
British Patent Application 20121/76 (1976).
403
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