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Results of experiments at the AVR reactor
Nuclear Engineering and Design 121 (1990) 143-153 North-Holland
143
R E S U L T S O F E X P E R I M E N T S A T T H E AVR R E A C T O R H. G O T T A U T Forschungszentrum Ji~fich GmbH, Ji~lich, Fed. Rep. Germany and K. K R U G E R Arbeitsgemeinschaft Versuchsreaktor GmbH, Jiilich, Fed. Rep Germany Received December 1989
The most important experiments at the AVR reactor and their results will be discussed. This choice illustrates the significance of the "AVR experiment" for high-temperature reactor development in Germany and for the construction and operation of future HTR projects.
1. Introduction
The AVR reactor has been used for quite a number of experiments and tests carried out in the course of its 21 years of operation. Long-time operation of this firstof-its-kind reactor has also provided the possibility of using the plant as a test bed for a variety of experiments aimed at deepening the understanding of the operational, physical and chemical processes involved and improving methods and procedural approaches. It should be noted, however, that the AVR was not designed as an experimental reactor. The primary goal was to demonstrate the feasibility and safe operation of the HTR concept using spherical fuel elements and to help gather operating experience. The main emphasis during the first years of operation was therefore placed on demonstrating the reliability and availability of the overall plant, and the results obtained have been convincing [1]. The operation was accompanied by a series of measurements, including relevant experiments on core physics and plant behaviour which have led to valuable findings. However, prolonged experience increasingly suggested that the plant should be additionally used as an experimental facility. This led to the successive installation and operation of a number of important experiments. After a critical review of the results achieved and an evaluation of the possibilities for further experiments to supplement and complete these findings as well as to derive additional benefit for future HTR projects, an AVR experimental programme
was defined in 1985 for the remaining time of operation until the end of 1988. This programme combined the projects already under way and was specifically supplemented by additional experiments of particular relevance for issues of HTR safety [2]. The programme has been carried out in close cooperation between AVR GmbH, the Jiilich Research Centre and the reactor companies HRB and Interatom. Certain projects additionally involved relevant institutions abroad, such as the Japanese institute JAERI, the US research centre ORNL and General Atomics, as well as the Austrian research centre Seibersdorf and the British UKAEAWinfrith.
2. Experimental results at the AVR
2.1. Reactor physics and plant behaviour of the A VR are comprehensively understood and covered by model calculations," the conversion from H E U to L E U fuel did not pose any problems Since the beginning of operation, the AVR has also served for measurements and calculations accompanying reactor operation. The first aim was, of course, the adjustment and control of the loading condition and reactivity inventory. It had to be ensured that the reactor could be selectively operated with respect to power and coolant gas temperature and could be safely shut down and kept subcritical at any time. This was not a
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144
H. Gottaut, K. Kriiger / Results of experiments at the A VR reactor
simple task in view of the addition of large quantities of highly different fuel elements with varying fissile material and heavy metal contents carried out in the course of operation, and in view of the increase in the average outlet temperature to 9 5 0 °C in February 1974, the continuous conversion from H E U to LEU since mid-1982, and increasing demands due to entirely different experiments, especially in the last years of operation. Based on the results from reactivity measurements during downtimes, on temperature measurements in the bottom and side reflectors and in the graphite noses of the shutdown rods as well as measurements of neutron flux and bum-up, a procedure was developed meeting all requirements by selective charging, addition and withdrawal of fuel elements [3]. AVR operation and specific experiments on core physics and thermohydraulics also made it possible, in particular, to verify theoretical models and test relevant computer codes describing the long-term behaviour and the dynamic short-term behaviour of pebble bed HTRs. In this connection, quite a number of experiments were carried out investigating, for example, the flow of spheres to complement the measurements of neutronphysically and thermohydraulically relevant operating parameters, and also including transient tests to study the impact of short-term changes in the coolant flow or neutron economy as well as the influence of time-dependent xenon poisoning of the reactor core after step load changes and compare the results with computer predictions. This is discussed in more detail elsewhere I4]. 2.2. The radial distribution of maximum hot gas temperatures in the core was determined by monitor elements," gas temperatures of 1050°C to beyond 1280°C were observed An average coolant gas core outlet temperature is generally specified as the reference parameter for the hot coolant temperature of the AVR. It is calculated from the core power, coolant gas inlet temperature and helium mass flow. Direct measurement of the gas temperature in the core is not possible. An experiment served to determine the radial distribution of the maximum hot gas temperature in the core [5]. For this purpose, labelled monitor spheres were selectively loaded via the fuelling machine. Each monitor sphere contained 20 melt wires with different melting points from 655°C to 1280°C. After discharge and post-examination, the maximum temperature detected by each sphere was determined from X-ray melting patterns gained by X-ray examination. The radial temperature distribution was then obtained using data from the flow
Maximum Melting Temperature 1280°C
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Fig. 1. Frequency distribution of maximum temperatures detected by temperature monitor spheres passing the AVR core. of spheres in the core. Fig. 1 shows the frequency distribution of the temperatures determined as a result of the experiment at full power and an average hot gas temperature of 950°C. A number of spheres detected temperatures of at least 1280°C. These spheres have passed through the core zone between the graphite noses where a locally elevated power density prevails. 2.3. The passive safety of small high-temperature reactors was demonstrated at the A VR for two relevant accidents: failure of the active cooling system with blocked shutdown rods and loss of coolant In September 1970, the experiment was carried out which demonstrated safe AVR shutdown in the event of any active cooling system failing and simultaneous blocking of the four shutdown rods. For this experiment, the two coolant gas blowers of the AVR were stopped with the reactor operating at full power and the main circuit valves closed to suppress natural convection. None of the shutdown rods was inserted [6]. Fig. 2 shows the power variation observed together with the curves for the temperatures measured in the reflector graphite. The power immediately decreased sharply as a consequence of the negative temperature coefficient and increased again approaching 2 MW after approx. 23 h due to xenon decay and balancing out at about 300 kW. The temperature variations are completely undramatic. While the hot reactor zones, represented by the measuring point in the upper reflector nose, cooled down, the temperatures in the colder zones increased by max. 250°C. Self-stabilization of the H T R core due to physical laws was again convincingly demonstrated by this experiment. A further safety-relevant large-scale experiment comprises the loss-of-coolant accident simulation tests car-
145
H. Gottaut, K. Kriiger / Results of experiments at the A VR reactor
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ried out towards the e n d of the operating phase. These experiments have d e m o n s t r a t e d safe decay heat removal for a real p l a n t in the event of depressurization w i t h o u t resorting to active cooling measures. T h e m e a s u r e d d a t a o b t a i n e d were also used to v a h d a t e the c o m p u t e r prog r a m s e n a b l i n g transfer to other H T R plants. This project has b e e n very successful. After p l a n t s h u t d o w n a n d depressurization, the reactor was o p e r a t e d at 4 M W to establish a steady initial state whose t e m p e r a t u r e profile was approximately identical to that d u r i n g full power operation. T h e actual experiment was initiated b y s t o p p i n g the blowers. T h e afterheat, meanwhile decayed, was simulated b y fission power as if the accident were to occur d u r i n g full power o p e r a t i o n [7]. A total of four e x p e r i m e n t s were carried out: two with closed a n d two with o p e n m a i n circuit valves. T h e r m o h y d r a u l i c b e h a v i o u r typical of a n H T R was
especially d e m o n s t r a t e d b y the two e x p e r i m e n t s cond u c t e d for a period of m o r e t h a n 100 h in O c t o b e r 1988. T h e m e a s u r i n g points next to the core centre in the graphite noses at core m i d - h e i g h t showed a m a x i m u m t e m p e r a t u r e increase of 300 K established 13 h after experiment initiation (fig. 3). A relatively r a p i d temperature decrease is to b e o b s e r v e d in the u p p e r core region, whereas a slow increase in t e m p e r a t u r e is f o u n d in the b o t t o m reflector. This is caused b y convective heat t r a n s p o r t from the core surface to the steam generator which c o n t i n u e d to operate and, o n the o t h e r h a n d , b y heat a b s o r p t i o n in the initially cold lower core region. A c o m p u t e r simulation of this t e m p e r a t u r e red i s t r i b u t i o n is s h o w n in fig. 4. T h e e x p e r i m e n t a l results with o p e n a n d closed m a i n circuit valves are almost congruent, which shows t h a t n a t u r a l c o n v e c t i o n t h r o u g h cold gas recycling does n o t c o n t r i b u t e t o w a r d s heat removal in a depressurized reactor. Prior to the experi-
146
H. Gottaut, K. Kriiger / Results of experiments at the A VR reactor
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ments in October 1988, monitor spheres had again been added for temperature measurement, cf. section 2.2, this time only to the inner core. At the time of the experiment, they were in the region of the highest temperatures. The majority of the monitor spheres, meanwhile discharged, detected a temperature between 1070 and 1085 C.
2.4. Mass tests o f fuel elements in the A V R have demonstrated the excellent quality o f modern spherical f u e l elements; specific activities in the primary coolant gas are low
The AVR was also used as a test bed for testing spherical fuel elements. A total of 14 different fuel
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H. Gottaut, K. Kriiger / Results of experiments at the A VR reactor
element types have been tested under operating conditions involving large piece numbers. This process, as well as the measuring and test results obtained, reflects the development of the spherical fuel element and also demonstrates the excellent status reached to date. Milestones in this development (table 1) comprised the use of pressed fuel elements of German fabrication since April 1969 after the initial core had been loaded with American elements. Approx. 150000 pressed fuel elements with BISO particles have been used since 1969, including 20 000 from the THTR production batch. The third phase is finally characterized by changing over to fuel elements with TRISO fuel particles containing a dense SiC layer between the inner and outer PyC coating to further reduce fission product release. Since 1982, the main emphasis has also been placed on testing low-enriched LEU fuel elements. This modification only concerned the particle fuel kernel for which low-enriched UO 2 was used instead of U / T h mixed oxides. These LEU fuel elements achieved a maximum bum-up of 11% FIMA in the AVR. They have reached the target values of the HTR-MODUL with regard to bum-up, irradiation time and fast neutron dose.
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Table 1 Qualification of HTR fuel dements at AVR, principle lines • High enriched Uranium fuel elements for AVR and THTR (HEU-BISO particles) since 1969: carbidic fuel, (U, Th)C2 since 1971: oxidic fuel, (U, Th)O2 since 1974: elements taken from THTR production line • Low enriched Uranium fuel elements for THTR succeeding plants (LEU-TRISO particles) since 1982: UO2 fuel elements
The TRISO-LEU elements have proved to be an excellent product, which has also been confirmed by annealing tests so far carded out at temperatures of 1600°C and above. It has not been possible, however, to carry out such tests on elements with maximum bum-up. The statements on the quality of the fuel elements used are based on the results from two different types of investigation: - i n v e s t i g a t i o n s concerning the mechanical stability even after high fluences and concerning corrosion; investigations into fission product release. -
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Fig. 5. AVR facilities for primary coolant gas extraction to investigate activity and dust bebaviour.
148
H. Gottaut, K. Kriiger / R e s u l t s of experiments at the A VR reactor
Studies concerning the mechanical stability comprised measurements of the sphere diameter of discharged fuel elements and of the required crushing force as a function of fast neutron fluence as well as investigations into corrosion [8]. It may be stated that all fuel elements have been successfully tested at high burn-up and also at an average hot gas temperature of 950°C. Of particular significance for safety are the studies concerning fission product release from the fuel elements and behaviour of the activity released in the primary system. A series of relevant tests were carried out in several different test loops which, in part, served additional or even specific purposes; see also section 2.5 below. Fig. 5 gives a survey of these experiments. Common to all is the possibility of determining the activities contained in the primary gas by primary gas sampling. They differ with respect to the sampling location and primary flow rate, but also with regard to their configuration and the method used. Two categories are differentiated for the location of sampling, i.e. hot primary gas sampling in the experiments VAMPYR I and II and cold primary gas sampling in the other experiments. Post-examination results from these experiments, operational experience and computer assessment lead to the following statements. Values as specified in table 2 have been determined for the specific activities in hot coolant gas. These are typical data measured in the period 1984 to 1987 with the reactor operated at a thermal power of 46 MW and an average hot gas temperature of 950 ° C. The integral value specified for fission noble gases was derived from measuring results in the cold region, similar to the
Table 2 Specific activities of the primary coolant gas [Bq/m3 (ISA)] EFission noble gas Tritium C 14 Cs-137 1-131 Ag-ll0m Sr-90 Co-60
4.6 × 108 3.7 × 107 1.9 × 107 3.0 × 102 5.2 × 102 4.9 × 101 2.0 × 1 0 2 1.0 × 101
values for tritium and 14C, corrections having been made for the decay of short-lived fission gases. The values for solid fission products are based on measuring results from the VAMPYR I experiment. This is an appropriate basis for estimating the fission gas inventory of the primary loop. Fig. 6 shows the fission gas inventory variations in the primary loop since 1973 compared with the adjusted average hot gas temperature [9]. It is striking to note that the values have been relatively low since 1983 at a hot gas temperature of 950°C, whereas they significantly increased between 1975 and 1981. The values from 1975 to 1977 can be correlated fairly well with the use of fuel elements in which defective particles were found later on. The elevated values since 1979 may be attributed to high H20 partial pressures in the primary gas as a consequence of the water ingress in 1978. The low fission gas inventory since 1983 is attributable almost exclusively
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H. Gottaut, K. Kriiger / Results of experiments at the A VR reactor
fission product release from modem fuel elements. This release is only attributable to fuel element uranium contamination outside the particle coating due to manufacture. The successful reduction of this contamination to a current fraction of 10-5 relative to the fuel element forms the basis of the good release behaviour of modern fuel elements. This has also been convincingly demonstrated by a number of heating tests on fuel elements discharged from the AVR to temperatures > 1600°C and by investigations of the activity profiles in the fuel-free shell of the fuel elements [10].
to uranium contamination of the fuel elements due to manufacture. This example also shows that good control of the integral release from the core is possible and conclusions can thus be drawn for the release behaviour of the fuel elements used under the operational boundary conditions involved by monitoring the fission gas concentration in the coolant. Activity inventories can also be assessed for solid fission products by calculation from the time integral of the source rates of the core taking account of decay. The source rates were determined from the specific concentrations measured in the hot gas and the corresponding values in the cold gas for which satisfactory agreement was found in the cold gas filter and dust experiment measurements. The inventories of solid fission products are thus accumulated values, in contrast to the fission gas data. Part of the fission gases is regularly removed from the primary system by gas purification. Fig. 7 shows the inventory progression determined for the most important solid fission products outside the core since 1973. The increased rise observed in 1975 is correlated with the results for fission gas activity and caused by the fuel elements with defective particles. The increase in the silver inventory in 1983 is attributed to the addition of LEU elements since 1982. The iodine inventory curve has been constant on average during the past few years. Variations in time are governed by reactor power and average coolant gas temperature. Measurements of the primary gas activity performed in the last years of operation confirm the relatively low
2.5. The deposition of sofid fission products in the primary loop is also determined to a significant extent by dust; a high fine dust fraction with grain sizes in the range of 1 pm has been ascertained; this is the principal carrier of mobilizable activity The safety of HTR plants from a radiological point of view cannot be evaluated by using the fission product source term and accumulated activity inventory in the primary loop alone, but is also based on knowledge concerning the transport and deposition behaviour of solid fission and activation products and the role played by dust. These data determine the possibilities for inspection, maintenance and repair and, consequently, plant safety during normal operation and in the event of accidents. The transport of the activity of solid fission and activation products entrained by the coolant gas into the primary loop and the deposition of these products on the surfaces and sinks of the primary loop are
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H. Gottaut, K. Kriiger / R e s u l t s of experiments at the A VR reactor
complex processes. They are governed by the fact that two fractions are involved. On the one hand, there is the free activity of atomic and molecular nuclides transported with the coolant gas and passed into the coolant gas flow as a function of the local boundary conditions in the core. On the other hand, there is the dust produced in the core by fuel element circulation and contaminated by concurrent abrasion. The transport and deposition of both fractions depend on specific parameters (material, surface density and temperature, partial pressure, as well as grain spectrum, particle size and state of agglomeration) and on the nature of species, flow conditions and surface condition. Furthermore, deposition is also essentially governed by the interaction between these two fractions. In order to clarify these problems, a number of experiments have been carried out with the experimental facilities already mentioned (fig. 5), in particular with VAMPYR I, VAMPYR II and the dust experiment. These tests were also used for the verification of computer codes describing the processes involved in model terms and for the derivation of appropriate model data. The VAMPYR I experiment served in the main for determining the specific activities in hot gas, since it was not possible to use relevant HTR materials as realistic test material for deposition studies due to the activation in the hot front region of the discharge tube. Deposition profiles have been measured, however, in the cold back region and were successfully recalculated
101
using the PATRAS code [11]. Fig. 8 shows the result of such investigations for the year 1974. AVR tests conducted at that time did not reveal any particular contribution by dust. The data set used for computation had been derived from other reactor experiments. In early 1987 the VAMPYR II experiment became operational. It served to study HTR-relevant materials due to its configuration avoiding disturbing activation. Difficulties were encountered in the evaluation of the first experiment. They are related to the unexpectedly high amount of fine dust found in different experiments since the end of 1986. Fine dust exhibits grain sizes mainly < 1 ~m. Until the AVR was shut down, three further experiments were carried out under a modified programme, the last two involving the use of special fine dust filters to study the influence of dust. The post-examinations of these experiments have not yet been completed so that no final statement can be made at present. The dust experiment installed in the cold gas region of the AVR [12] served to study the interaction between nuclides and dust particles and to determine the dustborne and free activity fractions in the primary gas. A two-train experimental arrangement with different filter design is used largely to separate free activity from dust in one train and compare the results with the integral result of the second train equipped with normal filters. A number of experiments were carded out varying the gas temperature and test duration. These studies have not been completed either. The overall evaluation will
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151
H. Gottaut, K. Kriiger / Results of experiments at the A VR reactor
provide information about the validity of a model extension of the PATRAS code, including the interaction with dust. The dust inventory of the AVR was estimated to be approx. 60 kg by the end of 1988, the average annual production rate amounting to approx. 3 kg. Dust is deposited primarily on the surfaces or in the wake flow areas of the primary loop under steady-state operating conditions. Under such conditions, the dust concentration measured in the primary gas amounts to only about 5 ILg/m3 (ISA) [9]. Since 1986, the grain size distribution of dust samples from different experiments has been determined using improved analysis techniques. Fig. 9 shows a typical result. The distribution maximum is below the grain size of 1 #m and thus lower than previously expected. This result is typical of the AVR and independent of the sampling location according to the state of the art [13]. Samples of dust were taken and their activity measured at different times during operation and at different locations in the primary circuit. The activity load varies significantly depending on the location and time of sampling and is also different depending on the
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Table 3 Specific activities on dust [106 Bq/g]. The variation range results from measurements at different sampling locations and different times Cs-137 Cs-134 1-131 Ag-ll0m Sr-89 Sr-90 Co-60
2 0.70 0.10.619 0.2 -
96 27 3 43 42 363 8
nuclide considered. This reflects the complex correlations involved in transport and deposition. Table 3 specifies the ranges in which specific dust activities were measured for some important nuclides. The dust is highly contaminated. Although the dust inventory in the primary circuit is relatively low, the dust-bound activity was found to be fairly high. In contrast, the activity inventory in the primary gas is low, according to table 2, for a total gas volume of approx. 1600 m3 (ISA). Dust deposition is therefore of great significance for safety evaluations in the event of dust being released from the primary loop during a depressurization accident. In this case, it is important to know whether and how deposited activities and dusts can be remobilized. Deposited particles are expected to be remobilized wherever boundary conditions arise in the loop due to sudden changes in flow, leading to an abrupt increase in shear forces acting on the particles as compared with normal conditions. Experiments in recent years have shown that additional influences can play a role in practical operation. These tests involved a sudden increase in coolant circulator speed leading to a higher flow rate of the primary gas. However, this is not likely to have caused the increase in shear forces discussed above. The experiments were carried out with the reactor shut down [13]. Rapid increases in speed from approx. 1500 to a maximum of approx. 4000 r e v / m i n were adjusted as compared to approx. 3500 r e v / m i n during normal operation. The dust concentration was determined at one train of the dust experiment in comparison with the activity variation measured over time. Fig. 10 shows the result for two different blower speeds. The remobilized dust was collected simultaneously in the experimental facility of the dust experiment, VAMPYR I, and in the cold gas filter, followed by post-examination. The dust concentrations determined were higher by several orders of magnitude as compared with normal operation. Depending on the speed, the concentrations vary and decrease again with
152
H. Gottaut, K. Kriiger / Results of experiments at the A VR reactor
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different half-lives. This is indicative of a different composition of the dust. The low deposition rate observed for the remobilized dust can be explained by the fact that particularly fine dust is involved. However, it is also important to note that the volume of dust thus remobilized is only a few percent of the dust inventory. The findings and results obtained in a number of tests comprising different experimental arrangements at the AVR still need to be finally evaluated, compared with each other, and assessed within an overall evaluation procedure. 2.6. Adoanced measuring techniques were successfully tested at the A VR under realistic reactor operating conditions The AVR was also an ideal tool for testing advanced techniques for measuring in the region near the core or for primary gas purification under realistic reactor operating conditions. These studies have been primarily conducted within the scope of development work for a nuclear process heat facility with HTR, but they are nevertheless of general significance for a variety of applications. One example is the combined thermocouple/noise thermometer developed by KFA and Interatom for reliable and accurate high-temperature measurements of long-term stability in the region near the core. The noise thermometer makes use of the temperature dependence of the thermal noise of selected resistance materials and can be used separately or in combination with thermocouples for measurements involving electronic expenditure. A combined system has been in operation since 1984 in the AVR top reflector and was successfully
tested with remote signal transmission after having been automated in 1987. Due to its proven accuracy and stability, this system can be used for the in-situ calibration of thermocouples [14]. Successful system tests have also been carried out with Japanese neutron flux detectors. These are fission chambers which can be used at temperatures up to more than 800°C. These chambers have been tested at different locations in a temperature range up to 500°C and over the entire power range from start-up to full power operation. The measuring systems have functioned perfectly [15] so that fission chambers are now also available for HTR application in addition to the SPND detectors developed at the KFA. The neutron flux values measured in the region of the steam generator are of additional significance for a comparison of the results from core physics computation and for planning postexaminations at the decommissioned AVR reactor. Control of the tritium concentration in the primary gas and steam cycle as well as procedures for adequately removing tritium from the primary system are of high significance for product gas contamination in a process heat facility. Two procedures were tested at the AVR under realistic operating conditions, after their basic suitability had been demonstrated on a laboratory scale, with the aim of reducing the tritium concentration in the primary gas to a tolerable level. One of these processes involves a filter concept making use of the transfer of tritium and hydrogen through highly permeable walls into a secondary system doped with water and conversion into non-permeable tritiated water which can then be rinsed out [16]. A prototype of this filter acting selectively for tritium was installed at the AVR. It has not been possible, however, to satisfactorily test this filter prior to AVR shutdown due to measuring and experimental problems. Post-examinations of the test components are expected to elucidate these problems. The second process involves gas purification by gettering tritium and hydrogen as well as most of the other impurities in the primary gas in cerium mixedmetal beds. However, the efficiency of this process is restricted by possible passivation of the gettering material due to the high concentrations of CO impurities. Two getter beds have been tested at the AVR. Valuable findings have been derived for the design and operation of such beds [17], although planned long-time testing has not been satisfactorily completed. Promising results were obtained despite many initial problems by the AVR test of newly developed detector systems for continuous tritium control in secondary and primary loops. The suitability of the measurement pro-
H. Gottaut, K. Krfiger / Results of experiments at the A VR reactor
cedure developed for the steam cycle has been demonstrated [18]. Measurements in primary gas must satisfy particularly high requirements due to the low tritium concentration of approx. 105 B q / m 3 to be detected in the presence of a fission gas concentration higher by about two orders of magnitude. A measurement procedure involving sophisticated measuring and computer techniques was tested in two experiments at the AVR. The results can be correlated with those of measurements performed during operation, although the evaluation has not yet been completed [19].
3. Summary During 21 years of A V R operation the reactor has been a valuable tool for a number of experiments on operating behaviour, plant safety, H T R fuel element testing and testing of HTR-relevant measuring techniques. The experiments at the A V R have also significantly contributed towards improving and qualifying the computer codes used for studies on core physics, thermohydraulics and fission product behaviour in HTRs. Some of the experiments have successfully demonstrated the special safety characteristics of small HTRs. Certain more extensive plans have not been feasible. The most important project involved the conversion of the A V R into a nuclear process heat facility for demonstrating the safe extraction and use of H T R heat for coal-refining processes [20]. In-depth investigations concerning the condition of the A V R were carried out for this project. They proved that all plant components were in a good state and suitable for long-term further operation after almost 17 years of operation. The results and experience available provide a fundamental contribution towards the development, planning, construction and operation of future H T R plants. They will be rounded off and complemented by the final evaluation results of certain experiments and, in particular, by the planned post-examination programme at the decommissioned plant. A summarizing documentation of the results of the A V R experimental programme is being prepared.
References [1] E. Ziermann, Review of 21 years of power operation at the AVR experimental nuclear power station in Jiilich, Nucl. Engrg. Des. 121 (1990) 135-142, in this issue.
153
[2] K. Kriiger and M. Wimmers, Test Program of the AVR Exp. Nuclear Power Plant Jiihch, AVR Report (Nov. 1987). [3] M. Wimmers and A. Bergerfurth, Die Physik des AVRReaktors. Tagung AVR - 20 Jahre Betrieb, Aachen, VDI Report 729 (May 1989) 131-145. [4] L. Wolf et al., High-temperature reactor core physics and reactor dynamics, Nucl. Engrg. Des. 121 (1990) 227-240, in this issue. [5] H. Derz, H. Gottaut et ai., Die Bestimmung der maximalen Kiihlgastemperaturen im AVR-Core mittels Monitorkugeln, Under preparation. [6] H. Dering, Kurzbericht zum Vierstab-Klemmversuch am 22./23, AVR-AN (Nov. 1970). [7] K. Kri~ger, Experimentelle Simulation eines Kiihlmittelverlustst0rfalls mit dem AVR-ReaverluststSrfalls mit dem AVR-Reaktor. JOL-2297 (Aug. 1989). [8] G. Ivens and M. Wimmers, Der AVR als Testbett fiir Brennelemente. Tagung AVR - 20 Jahre Betrieb, Aachen, VDI Report 729 (May 1989) 161-175. [9] C.B.v.d. Decken and U. Wawrzik, Staub- und Aktivitatsverhalten. Tagung AVR - 20 Jahre Betrieb, Aachen, VDI Report 729 (May 1989) pp. 239-253. [10] H. Nabielek, W. Schenk et al., Development of advanced HTR fuel elements, Nucl. Engrg. Des. 121 (1990) 199-210, in this issue. [11] N. Iniotakis, J. Malinowski et al., Results from Plate-Out Investigations. IAEA-Specialists Meeting on Coolant Chemistry, Plate-Out and Decontamination in Gas-Cooled Reactors, Jiihch, Summary Report (Dec. 1980) 35-43. [12] N. Ihiotakis et al., Das Staubexperiment am AVR, Under preparation. [13] U. Wawrzik, P. Biedermann and H.F. Oetjen, Staub im AVR-Reaktor, Verhalten bei transienten Stri~mungsbedingungen. Jahrestagung Kerntechnik, Travemiinde (May 1988). [14] H. Brixy, H. Hecker et al., Temperaturmessung im Decken-reflektor des AVR-Reaktors mit kombinierten Thermoelement-Rauschthermometern. Jahrestagung Kerntechnik, Travemiinde (May 1988). [15] H.J. Hantke, E. Wahlen and H. Brixy, Einbau und Erprobung von Zusatzinstrumentierung am AVR-Reakfor, Under preparation. [16] N. Iniotakis and J. Hinssen, Tritiumfilterexperiment am AVR, KFA-IRB/Internal Report IB-9-86. [17] K.H. Hammelmann, W. Str~iter and W. Fr~hling, Getterung von Tritium mit Hilfe von Cer-Mischmetall am Kugelhaufenreaktor AVR. Jahrestagung Kerntechnik, Diisseldorf (May 1989). [18] F. Haase, Tritiummessung im Wasser, HTA-16. HRB Report AVRV-6470-BE-GHRA 006059 (Dec. 1988). [19] B. Heinrich, Tritiummessung im Gas, HTA-16. HRB Report AVRV-6470-BE-GHRA 006404 (July 1989). [20] Umbau des AVR-Reaktors zu einer Prozessw~'meanlage, Ergebnisse der Vorplanungsphase, KFA-HTA Project Report (June 1985).
143
R E S U L T S O F E X P E R I M E N T S A T T H E AVR R E A C T O R H. G O T T A U T Forschungszentrum Ji~fich GmbH, Ji~lich, Fed. Rep. Germany and K. K R U G E R Arbeitsgemeinschaft Versuchsreaktor GmbH, Jiilich, Fed. Rep Germany Received December 1989
The most important experiments at the AVR reactor and their results will be discussed. This choice illustrates the significance of the "AVR experiment" for high-temperature reactor development in Germany and for the construction and operation of future HTR projects.
1. Introduction
The AVR reactor has been used for quite a number of experiments and tests carried out in the course of its 21 years of operation. Long-time operation of this firstof-its-kind reactor has also provided the possibility of using the plant as a test bed for a variety of experiments aimed at deepening the understanding of the operational, physical and chemical processes involved and improving methods and procedural approaches. It should be noted, however, that the AVR was not designed as an experimental reactor. The primary goal was to demonstrate the feasibility and safe operation of the HTR concept using spherical fuel elements and to help gather operating experience. The main emphasis during the first years of operation was therefore placed on demonstrating the reliability and availability of the overall plant, and the results obtained have been convincing [1]. The operation was accompanied by a series of measurements, including relevant experiments on core physics and plant behaviour which have led to valuable findings. However, prolonged experience increasingly suggested that the plant should be additionally used as an experimental facility. This led to the successive installation and operation of a number of important experiments. After a critical review of the results achieved and an evaluation of the possibilities for further experiments to supplement and complete these findings as well as to derive additional benefit for future HTR projects, an AVR experimental programme
was defined in 1985 for the remaining time of operation until the end of 1988. This programme combined the projects already under way and was specifically supplemented by additional experiments of particular relevance for issues of HTR safety [2]. The programme has been carried out in close cooperation between AVR GmbH, the Jiilich Research Centre and the reactor companies HRB and Interatom. Certain projects additionally involved relevant institutions abroad, such as the Japanese institute JAERI, the US research centre ORNL and General Atomics, as well as the Austrian research centre Seibersdorf and the British UKAEAWinfrith.
2. Experimental results at the AVR
2.1. Reactor physics and plant behaviour of the A VR are comprehensively understood and covered by model calculations," the conversion from H E U to L E U fuel did not pose any problems Since the beginning of operation, the AVR has also served for measurements and calculations accompanying reactor operation. The first aim was, of course, the adjustment and control of the loading condition and reactivity inventory. It had to be ensured that the reactor could be selectively operated with respect to power and coolant gas temperature and could be safely shut down and kept subcritical at any time. This was not a
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H. Gottaut, K. Kriiger / Results of experiments at the A VR reactor
simple task in view of the addition of large quantities of highly different fuel elements with varying fissile material and heavy metal contents carried out in the course of operation, and in view of the increase in the average outlet temperature to 9 5 0 °C in February 1974, the continuous conversion from H E U to LEU since mid-1982, and increasing demands due to entirely different experiments, especially in the last years of operation. Based on the results from reactivity measurements during downtimes, on temperature measurements in the bottom and side reflectors and in the graphite noses of the shutdown rods as well as measurements of neutron flux and bum-up, a procedure was developed meeting all requirements by selective charging, addition and withdrawal of fuel elements [3]. AVR operation and specific experiments on core physics and thermohydraulics also made it possible, in particular, to verify theoretical models and test relevant computer codes describing the long-term behaviour and the dynamic short-term behaviour of pebble bed HTRs. In this connection, quite a number of experiments were carried out investigating, for example, the flow of spheres to complement the measurements of neutronphysically and thermohydraulically relevant operating parameters, and also including transient tests to study the impact of short-term changes in the coolant flow or neutron economy as well as the influence of time-dependent xenon poisoning of the reactor core after step load changes and compare the results with computer predictions. This is discussed in more detail elsewhere I4]. 2.2. The radial distribution of maximum hot gas temperatures in the core was determined by monitor elements," gas temperatures of 1050°C to beyond 1280°C were observed An average coolant gas core outlet temperature is generally specified as the reference parameter for the hot coolant temperature of the AVR. It is calculated from the core power, coolant gas inlet temperature and helium mass flow. Direct measurement of the gas temperature in the core is not possible. An experiment served to determine the radial distribution of the maximum hot gas temperature in the core [5]. For this purpose, labelled monitor spheres were selectively loaded via the fuelling machine. Each monitor sphere contained 20 melt wires with different melting points from 655°C to 1280°C. After discharge and post-examination, the maximum temperature detected by each sphere was determined from X-ray melting patterns gained by X-ray examination. The radial temperature distribution was then obtained using data from the flow
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145
H. Gottaut, K. Kriiger / Results of experiments at the A VR reactor
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ried out towards the e n d of the operating phase. These experiments have d e m o n s t r a t e d safe decay heat removal for a real p l a n t in the event of depressurization w i t h o u t resorting to active cooling measures. T h e m e a s u r e d d a t a o b t a i n e d were also used to v a h d a t e the c o m p u t e r prog r a m s e n a b l i n g transfer to other H T R plants. This project has b e e n very successful. After p l a n t s h u t d o w n a n d depressurization, the reactor was o p e r a t e d at 4 M W to establish a steady initial state whose t e m p e r a t u r e profile was approximately identical to that d u r i n g full power operation. T h e actual experiment was initiated b y s t o p p i n g the blowers. T h e afterheat, meanwhile decayed, was simulated b y fission power as if the accident were to occur d u r i n g full power o p e r a t i o n [7]. A total of four e x p e r i m e n t s were carried out: two with closed a n d two with o p e n m a i n circuit valves. T h e r m o h y d r a u l i c b e h a v i o u r typical of a n H T R was
especially d e m o n s t r a t e d b y the two e x p e r i m e n t s cond u c t e d for a period of m o r e t h a n 100 h in O c t o b e r 1988. T h e m e a s u r i n g points next to the core centre in the graphite noses at core m i d - h e i g h t showed a m a x i m u m t e m p e r a t u r e increase of 300 K established 13 h after experiment initiation (fig. 3). A relatively r a p i d temperature decrease is to b e o b s e r v e d in the u p p e r core region, whereas a slow increase in t e m p e r a t u r e is f o u n d in the b o t t o m reflector. This is caused b y convective heat t r a n s p o r t from the core surface to the steam generator which c o n t i n u e d to operate and, o n the o t h e r h a n d , b y heat a b s o r p t i o n in the initially cold lower core region. A c o m p u t e r simulation of this t e m p e r a t u r e red i s t r i b u t i o n is s h o w n in fig. 4. T h e e x p e r i m e n t a l results with o p e n a n d closed m a i n circuit valves are almost congruent, which shows t h a t n a t u r a l c o n v e c t i o n t h r o u g h cold gas recycling does n o t c o n t r i b u t e t o w a r d s heat removal in a depressurized reactor. Prior to the experi-
146
H. Gottaut, K. Kriiger / Results of experiments at the A VR reactor
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ments in October 1988, monitor spheres had again been added for temperature measurement, cf. section 2.2, this time only to the inner core. At the time of the experiment, they were in the region of the highest temperatures. The majority of the monitor spheres, meanwhile discharged, detected a temperature between 1070 and 1085 C.
2.4. Mass tests o f fuel elements in the A V R have demonstrated the excellent quality o f modern spherical f u e l elements; specific activities in the primary coolant gas are low
The AVR was also used as a test bed for testing spherical fuel elements. A total of 14 different fuel
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H. Gottaut, K. Kriiger / Results of experiments at the A VR reactor
element types have been tested under operating conditions involving large piece numbers. This process, as well as the measuring and test results obtained, reflects the development of the spherical fuel element and also demonstrates the excellent status reached to date. Milestones in this development (table 1) comprised the use of pressed fuel elements of German fabrication since April 1969 after the initial core had been loaded with American elements. Approx. 150000 pressed fuel elements with BISO particles have been used since 1969, including 20 000 from the THTR production batch. The third phase is finally characterized by changing over to fuel elements with TRISO fuel particles containing a dense SiC layer between the inner and outer PyC coating to further reduce fission product release. Since 1982, the main emphasis has also been placed on testing low-enriched LEU fuel elements. This modification only concerned the particle fuel kernel for which low-enriched UO 2 was used instead of U / T h mixed oxides. These LEU fuel elements achieved a maximum bum-up of 11% FIMA in the AVR. They have reached the target values of the HTR-MODUL with regard to bum-up, irradiation time and fast neutron dose.
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The TRISO-LEU elements have proved to be an excellent product, which has also been confirmed by annealing tests so far carded out at temperatures of 1600°C and above. It has not been possible, however, to carry out such tests on elements with maximum bum-up. The statements on the quality of the fuel elements used are based on the results from two different types of investigation: - i n v e s t i g a t i o n s concerning the mechanical stability even after high fluences and concerning corrosion; investigations into fission product release. -
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148
H. Gottaut, K. Kriiger / R e s u l t s of experiments at the A VR reactor
Studies concerning the mechanical stability comprised measurements of the sphere diameter of discharged fuel elements and of the required crushing force as a function of fast neutron fluence as well as investigations into corrosion [8]. It may be stated that all fuel elements have been successfully tested at high burn-up and also at an average hot gas temperature of 950°C. Of particular significance for safety are the studies concerning fission product release from the fuel elements and behaviour of the activity released in the primary system. A series of relevant tests were carried out in several different test loops which, in part, served additional or even specific purposes; see also section 2.5 below. Fig. 5 gives a survey of these experiments. Common to all is the possibility of determining the activities contained in the primary gas by primary gas sampling. They differ with respect to the sampling location and primary flow rate, but also with regard to their configuration and the method used. Two categories are differentiated for the location of sampling, i.e. hot primary gas sampling in the experiments VAMPYR I and II and cold primary gas sampling in the other experiments. Post-examination results from these experiments, operational experience and computer assessment lead to the following statements. Values as specified in table 2 have been determined for the specific activities in hot coolant gas. These are typical data measured in the period 1984 to 1987 with the reactor operated at a thermal power of 46 MW and an average hot gas temperature of 950 ° C. The integral value specified for fission noble gases was derived from measuring results in the cold region, similar to the
Table 2 Specific activities of the primary coolant gas [Bq/m3 (ISA)] EFission noble gas Tritium C 14 Cs-137 1-131 Ag-ll0m Sr-90 Co-60
4.6 × 108 3.7 × 107 1.9 × 107 3.0 × 102 5.2 × 102 4.9 × 101 2.0 × 1 0 2 1.0 × 101
values for tritium and 14C, corrections having been made for the decay of short-lived fission gases. The values for solid fission products are based on measuring results from the VAMPYR I experiment. This is an appropriate basis for estimating the fission gas inventory of the primary loop. Fig. 6 shows the fission gas inventory variations in the primary loop since 1973 compared with the adjusted average hot gas temperature [9]. It is striking to note that the values have been relatively low since 1983 at a hot gas temperature of 950°C, whereas they significantly increased between 1975 and 1981. The values from 1975 to 1977 can be correlated fairly well with the use of fuel elements in which defective particles were found later on. The elevated values since 1979 may be attributed to high H20 partial pressures in the primary gas as a consequence of the water ingress in 1978. The low fission gas inventory since 1983 is attributable almost exclusively
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H. Gottaut, K. Kriiger / Results of experiments at the A VR reactor
fission product release from modem fuel elements. This release is only attributable to fuel element uranium contamination outside the particle coating due to manufacture. The successful reduction of this contamination to a current fraction of 10-5 relative to the fuel element forms the basis of the good release behaviour of modern fuel elements. This has also been convincingly demonstrated by a number of heating tests on fuel elements discharged from the AVR to temperatures > 1600°C and by investigations of the activity profiles in the fuel-free shell of the fuel elements [10].
to uranium contamination of the fuel elements due to manufacture. This example also shows that good control of the integral release from the core is possible and conclusions can thus be drawn for the release behaviour of the fuel elements used under the operational boundary conditions involved by monitoring the fission gas concentration in the coolant. Activity inventories can also be assessed for solid fission products by calculation from the time integral of the source rates of the core taking account of decay. The source rates were determined from the specific concentrations measured in the hot gas and the corresponding values in the cold gas for which satisfactory agreement was found in the cold gas filter and dust experiment measurements. The inventories of solid fission products are thus accumulated values, in contrast to the fission gas data. Part of the fission gases is regularly removed from the primary system by gas purification. Fig. 7 shows the inventory progression determined for the most important solid fission products outside the core since 1973. The increased rise observed in 1975 is correlated with the results for fission gas activity and caused by the fuel elements with defective particles. The increase in the silver inventory in 1983 is attributed to the addition of LEU elements since 1982. The iodine inventory curve has been constant on average during the past few years. Variations in time are governed by reactor power and average coolant gas temperature. Measurements of the primary gas activity performed in the last years of operation confirm the relatively low
2.5. The deposition of sofid fission products in the primary loop is also determined to a significant extent by dust; a high fine dust fraction with grain sizes in the range of 1 pm has been ascertained; this is the principal carrier of mobilizable activity The safety of HTR plants from a radiological point of view cannot be evaluated by using the fission product source term and accumulated activity inventory in the primary loop alone, but is also based on knowledge concerning the transport and deposition behaviour of solid fission and activation products and the role played by dust. These data determine the possibilities for inspection, maintenance and repair and, consequently, plant safety during normal operation and in the event of accidents. The transport of the activity of solid fission and activation products entrained by the coolant gas into the primary loop and the deposition of these products on the surfaces and sinks of the primary loop are
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H. Gottaut, K. Kriiger / R e s u l t s of experiments at the A VR reactor
complex processes. They are governed by the fact that two fractions are involved. On the one hand, there is the free activity of atomic and molecular nuclides transported with the coolant gas and passed into the coolant gas flow as a function of the local boundary conditions in the core. On the other hand, there is the dust produced in the core by fuel element circulation and contaminated by concurrent abrasion. The transport and deposition of both fractions depend on specific parameters (material, surface density and temperature, partial pressure, as well as grain spectrum, particle size and state of agglomeration) and on the nature of species, flow conditions and surface condition. Furthermore, deposition is also essentially governed by the interaction between these two fractions. In order to clarify these problems, a number of experiments have been carried out with the experimental facilities already mentioned (fig. 5), in particular with VAMPYR I, VAMPYR II and the dust experiment. These tests were also used for the verification of computer codes describing the processes involved in model terms and for the derivation of appropriate model data. The VAMPYR I experiment served in the main for determining the specific activities in hot gas, since it was not possible to use relevant HTR materials as realistic test material for deposition studies due to the activation in the hot front region of the discharge tube. Deposition profiles have been measured, however, in the cold back region and were successfully recalculated
101
using the PATRAS code [11]. Fig. 8 shows the result of such investigations for the year 1974. AVR tests conducted at that time did not reveal any particular contribution by dust. The data set used for computation had been derived from other reactor experiments. In early 1987 the VAMPYR II experiment became operational. It served to study HTR-relevant materials due to its configuration avoiding disturbing activation. Difficulties were encountered in the evaluation of the first experiment. They are related to the unexpectedly high amount of fine dust found in different experiments since the end of 1986. Fine dust exhibits grain sizes mainly < 1 ~m. Until the AVR was shut down, three further experiments were carried out under a modified programme, the last two involving the use of special fine dust filters to study the influence of dust. The post-examinations of these experiments have not yet been completed so that no final statement can be made at present. The dust experiment installed in the cold gas region of the AVR [12] served to study the interaction between nuclides and dust particles and to determine the dustborne and free activity fractions in the primary gas. A two-train experimental arrangement with different filter design is used largely to separate free activity from dust in one train and compare the results with the integral result of the second train equipped with normal filters. A number of experiments were carded out varying the gas temperature and test duration. These studies have not been completed either. The overall evaluation will
M [!uCi/cm 2]
T [°C]
......
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800
'" , .
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"700 "'"- - .. " "-.
.~T,
. . . . Tempera ture Theory . . . . . . . . p Experiment
-600 "500
~
"400
o ~ o j 1-[3 = 1,2%o [ ~ T i ]
o
"300
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.200
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,100
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2'3'4'5'6'7'8'9'10'11'12'13'14'15'16'17'18'1
Fig. 8. Deposition of Ag-110m along the titanium tube of the VAMPYR-12 experiment, measured and calculated.
151
H. Gottaut, K. Kriiger / Results of experiments at the A VR reactor
provide information about the validity of a model extension of the PATRAS code, including the interaction with dust. The dust inventory of the AVR was estimated to be approx. 60 kg by the end of 1988, the average annual production rate amounting to approx. 3 kg. Dust is deposited primarily on the surfaces or in the wake flow areas of the primary loop under steady-state operating conditions. Under such conditions, the dust concentration measured in the primary gas amounts to only about 5 ILg/m3 (ISA) [9]. Since 1986, the grain size distribution of dust samples from different experiments has been determined using improved analysis techniques. Fig. 9 shows a typical result. The distribution maximum is below the grain size of 1 #m and thus lower than previously expected. This result is typical of the AVR and independent of the sampling location according to the state of the art [13]. Samples of dust were taken and their activity measured at different times during operation and at different locations in the primary circuit. The activity load varies significantly depending on the location and time of sampling and is also different depending on the
2,0' 1 ~m
1,5
qo
t 1,0
1,0
0,5
0,5 I
QO
0,0
-0,0 1,0 /am 10,0 ----- X,X Fig. 9. Typical grain size distribution of fine AVR dust. 0,1
Table 3 Specific activities on dust [106 Bq/g]. The variation range results from measurements at different sampling locations and different times Cs-137 Cs-134 1-131 Ag-ll0m Sr-89 Sr-90 Co-60
2 0.70 0.10.619 0.2 -
96 27 3 43 42 363 8
nuclide considered. This reflects the complex correlations involved in transport and deposition. Table 3 specifies the ranges in which specific dust activities were measured for some important nuclides. The dust is highly contaminated. Although the dust inventory in the primary circuit is relatively low, the dust-bound activity was found to be fairly high. In contrast, the activity inventory in the primary gas is low, according to table 2, for a total gas volume of approx. 1600 m3 (ISA). Dust deposition is therefore of great significance for safety evaluations in the event of dust being released from the primary loop during a depressurization accident. In this case, it is important to know whether and how deposited activities and dusts can be remobilized. Deposited particles are expected to be remobilized wherever boundary conditions arise in the loop due to sudden changes in flow, leading to an abrupt increase in shear forces acting on the particles as compared with normal conditions. Experiments in recent years have shown that additional influences can play a role in practical operation. These tests involved a sudden increase in coolant circulator speed leading to a higher flow rate of the primary gas. However, this is not likely to have caused the increase in shear forces discussed above. The experiments were carried out with the reactor shut down [13]. Rapid increases in speed from approx. 1500 to a maximum of approx. 4000 r e v / m i n were adjusted as compared to approx. 3500 r e v / m i n during normal operation. The dust concentration was determined at one train of the dust experiment in comparison with the activity variation measured over time. Fig. 10 shows the result for two different blower speeds. The remobilized dust was collected simultaneously in the experimental facility of the dust experiment, VAMPYR I, and in the cold gas filter, followed by post-examination. The dust concentrations determined were higher by several orders of magnitude as compared with normal operation. Depending on the speed, the concentrations vary and decrease again with
152
H. Gottaut, K. Kriiger / Results of experiments at the A VR reactor
1200
~/
n o = 15oo rnin "1
k
m31000
800"
T1, 2
=
80 rain
600
400' ~ / T I , 2 = 31 rnin n = 4000 rain"I
200 ~,...~n =3000 rain "1
= ~ = ~ n n m ~a= '
0 0
1
2
3 •- - ~
4
h
5
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Fig. 10. Curves of dust concentration in the primary gas after a flow increase caused by a blower transient.
different half-lives. This is indicative of a different composition of the dust. The low deposition rate observed for the remobilized dust can be explained by the fact that particularly fine dust is involved. However, it is also important to note that the volume of dust thus remobilized is only a few percent of the dust inventory. The findings and results obtained in a number of tests comprising different experimental arrangements at the AVR still need to be finally evaluated, compared with each other, and assessed within an overall evaluation procedure. 2.6. Adoanced measuring techniques were successfully tested at the A VR under realistic reactor operating conditions The AVR was also an ideal tool for testing advanced techniques for measuring in the region near the core or for primary gas purification under realistic reactor operating conditions. These studies have been primarily conducted within the scope of development work for a nuclear process heat facility with HTR, but they are nevertheless of general significance for a variety of applications. One example is the combined thermocouple/noise thermometer developed by KFA and Interatom for reliable and accurate high-temperature measurements of long-term stability in the region near the core. The noise thermometer makes use of the temperature dependence of the thermal noise of selected resistance materials and can be used separately or in combination with thermocouples for measurements involving electronic expenditure. A combined system has been in operation since 1984 in the AVR top reflector and was successfully
tested with remote signal transmission after having been automated in 1987. Due to its proven accuracy and stability, this system can be used for the in-situ calibration of thermocouples [14]. Successful system tests have also been carried out with Japanese neutron flux detectors. These are fission chambers which can be used at temperatures up to more than 800°C. These chambers have been tested at different locations in a temperature range up to 500°C and over the entire power range from start-up to full power operation. The measuring systems have functioned perfectly [15] so that fission chambers are now also available for HTR application in addition to the SPND detectors developed at the KFA. The neutron flux values measured in the region of the steam generator are of additional significance for a comparison of the results from core physics computation and for planning postexaminations at the decommissioned AVR reactor. Control of the tritium concentration in the primary gas and steam cycle as well as procedures for adequately removing tritium from the primary system are of high significance for product gas contamination in a process heat facility. Two procedures were tested at the AVR under realistic operating conditions, after their basic suitability had been demonstrated on a laboratory scale, with the aim of reducing the tritium concentration in the primary gas to a tolerable level. One of these processes involves a filter concept making use of the transfer of tritium and hydrogen through highly permeable walls into a secondary system doped with water and conversion into non-permeable tritiated water which can then be rinsed out [16]. A prototype of this filter acting selectively for tritium was installed at the AVR. It has not been possible, however, to satisfactorily test this filter prior to AVR shutdown due to measuring and experimental problems. Post-examinations of the test components are expected to elucidate these problems. The second process involves gas purification by gettering tritium and hydrogen as well as most of the other impurities in the primary gas in cerium mixedmetal beds. However, the efficiency of this process is restricted by possible passivation of the gettering material due to the high concentrations of CO impurities. Two getter beds have been tested at the AVR. Valuable findings have been derived for the design and operation of such beds [17], although planned long-time testing has not been satisfactorily completed. Promising results were obtained despite many initial problems by the AVR test of newly developed detector systems for continuous tritium control in secondary and primary loops. The suitability of the measurement pro-
H. Gottaut, K. Krfiger / Results of experiments at the A VR reactor
cedure developed for the steam cycle has been demonstrated [18]. Measurements in primary gas must satisfy particularly high requirements due to the low tritium concentration of approx. 105 B q / m 3 to be detected in the presence of a fission gas concentration higher by about two orders of magnitude. A measurement procedure involving sophisticated measuring and computer techniques was tested in two experiments at the AVR. The results can be correlated with those of measurements performed during operation, although the evaluation has not yet been completed [19].
3. Summary During 21 years of A V R operation the reactor has been a valuable tool for a number of experiments on operating behaviour, plant safety, H T R fuel element testing and testing of HTR-relevant measuring techniques. The experiments at the A V R have also significantly contributed towards improving and qualifying the computer codes used for studies on core physics, thermohydraulics and fission product behaviour in HTRs. Some of the experiments have successfully demonstrated the special safety characteristics of small HTRs. Certain more extensive plans have not been feasible. The most important project involved the conversion of the A V R into a nuclear process heat facility for demonstrating the safe extraction and use of H T R heat for coal-refining processes [20]. In-depth investigations concerning the condition of the A V R were carried out for this project. They proved that all plant components were in a good state and suitable for long-term further operation after almost 17 years of operation. The results and experience available provide a fundamental contribution towards the development, planning, construction and operation of future H T R plants. They will be rounded off and complemented by the final evaluation results of certain experiments and, in particular, by the planned post-examination programme at the decommissioned plant. A summarizing documentation of the results of the A V R experimental programme is being prepared.
References [1] E. Ziermann, Review of 21 years of power operation at the AVR experimental nuclear power station in Jiilich, Nucl. Engrg. Des. 121 (1990) 135-142, in this issue.
153
[2] K. Kriiger and M. Wimmers, Test Program of the AVR Exp. Nuclear Power Plant Jiihch, AVR Report (Nov. 1987). [3] M. Wimmers and A. Bergerfurth, Die Physik des AVRReaktors. Tagung AVR - 20 Jahre Betrieb, Aachen, VDI Report 729 (May 1989) 131-145. [4] L. Wolf et al., High-temperature reactor core physics and reactor dynamics, Nucl. Engrg. Des. 121 (1990) 227-240, in this issue. [5] H. Derz, H. Gottaut et ai., Die Bestimmung der maximalen Kiihlgastemperaturen im AVR-Core mittels Monitorkugeln, Under preparation. [6] H. Dering, Kurzbericht zum Vierstab-Klemmversuch am 22./23, AVR-AN (Nov. 1970). [7] K. Kri~ger, Experimentelle Simulation eines Kiihlmittelverlustst0rfalls mit dem AVR-ReaverluststSrfalls mit dem AVR-Reaktor. JOL-2297 (Aug. 1989). [8] G. Ivens and M. Wimmers, Der AVR als Testbett fiir Brennelemente. Tagung AVR - 20 Jahre Betrieb, Aachen, VDI Report 729 (May 1989) 161-175. [9] C.B.v.d. Decken and U. Wawrzik, Staub- und Aktivitatsverhalten. Tagung AVR - 20 Jahre Betrieb, Aachen, VDI Report 729 (May 1989) pp. 239-253. [10] H. Nabielek, W. Schenk et al., Development of advanced HTR fuel elements, Nucl. Engrg. Des. 121 (1990) 199-210, in this issue. [11] N. Iniotakis, J. Malinowski et al., Results from Plate-Out Investigations. IAEA-Specialists Meeting on Coolant Chemistry, Plate-Out and Decontamination in Gas-Cooled Reactors, Jiihch, Summary Report (Dec. 1980) 35-43. [12] N. Ihiotakis et al., Das Staubexperiment am AVR, Under preparation. [13] U. Wawrzik, P. Biedermann and H.F. Oetjen, Staub im AVR-Reaktor, Verhalten bei transienten Stri~mungsbedingungen. Jahrestagung Kerntechnik, Travemiinde (May 1988). [14] H. Brixy, H. Hecker et al., Temperaturmessung im Decken-reflektor des AVR-Reaktors mit kombinierten Thermoelement-Rauschthermometern. Jahrestagung Kerntechnik, Travemiinde (May 1988). [15] H.J. Hantke, E. Wahlen and H. Brixy, Einbau und Erprobung von Zusatzinstrumentierung am AVR-Reakfor, Under preparation. [16] N. Iniotakis and J. Hinssen, Tritiumfilterexperiment am AVR, KFA-IRB/Internal Report IB-9-86. [17] K.H. Hammelmann, W. Str~iter and W. Fr~hling, Getterung von Tritium mit Hilfe von Cer-Mischmetall am Kugelhaufenreaktor AVR. Jahrestagung Kerntechnik, Diisseldorf (May 1989). [18] F. Haase, Tritiummessung im Wasser, HTA-16. HRB Report AVRV-6470-BE-GHRA 006059 (Dec. 1988). [19] B. Heinrich, Tritiummessung im Gas, HTA-16. HRB Report AVRV-6470-BE-GHRA 006404 (July 1989). [20] Umbau des AVR-Reaktors zu einer Prozessw~'meanlage, Ergebnisse der Vorplanungsphase, KFA-HTA Project Report (June 1985).