Transient response of self-powered neutron detectors

N U C L E A R INSTRUMENTS AND METHODS

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TRANSIENT RESPONSE OF SELF-POWERED NEUTRON DETECTORS H. BI3CK,

Atominstitut Vienna, Austria P. G E B U R E C K and D. S T E G E M A N N

Technische Universitiit Hannover, Germany Received 12 September 1974 Tile behaviour of self-powered neutron detectors with Co, Er, H f and Pt emitters was investigated during reactor square wave and pulse operation. The detector's response was compared with the current of an excore ionization chamber. Characteris-

tical deviations from linearity were observed with all detectors at fast reactor periods. The exact cause of these deviations is not yet fully understood but several possibilities for the non-linear behaviour of self powered neutron detectors are outlined.

1. Introduction

The use of self-powered detectors for instance in a reactor safety system provides a prompt signal behaviour during the whole irradiation period in the reactor core. Theoretical and experimental investigations on this problem have been performed in refs. 17, 18. For a detailed investigation of the transient behaviour of Co-detectors as well as erbium-, hafnium- and platinum-detectors, which were developed and fabricated at the Institut fiir Kerntechnik, T.U. Hannover, a set of these devices had been tested during square wave and pulsed reactor operation in the TRIGAReactor of the Atominstitut der 0sterreichischen Hochschulen, Vienna.

For the control of local neutron flux and power distributions in power reactors several methods are being applied, as for example the TIP (Traverse Incore Probe)-System in Boiling Water Reactors or the Aero Ball System in Pressurized Water Reactors ~,2). Moreover, since a few years self powered neutron detectors are also used for power distribution measurements3'4). Because of their low burnup wmadium detectors have been preferredS), but the great disadvantage of this type and also of the wellknown rhodium detector is a poor time behaviour under transient conditions due to the half-life of the corresponding//-decays. However, this can be improved somewhat by an electronic circuit using inverse amplifiers6). The total current of these detectors has only a small prompt responding component, which has been analysed in ref. 7. For fast time response a more suitable type is the cobalt detector, which initially responds promptly to changes in neutron flux. The delayed background current increases slowly because of the build up of long lived 6°Co and 61Co isotopes in the emitter material. Its fraction exceeds 10% of the total signal after an irradiation time of 1.5 y in a flux of 1 X 1013 n cm -2 s -~, referring to theoretical s) and experimental investigations 9' t 0). Besides the just mentioned emitter materials Rh, V, and Co, a number of new materials has been investigated and tested11-14). The aim is the development of prompt responding self powered detectors with high neutron- but negligible gamma-sensitivity, low burnup rate and small delayed signal component. With respect to these requirements investigations are in progress, concerning optimum detector parameters and special geometries12'15), as e.g. the construction of a two emitter detector with gamma compensation for sodium-cooled fast breeder reactors16).

2. Measuring devices Fast reactor periods were obtained by the operation modes "square wave" and "pulse" in the T R I G A reactor, Vienna. A power ramp, e.g. "square wave", will be started at a level of 1 kW. This power is adjusted by two control rods. The third rod then can be shot out of the core by air pressure within 10 ms. The height of the power ramp may be determined by adjustment of a shock-absorber, which stops the socalled "pulse-rod" in any axial position. Immediately after shooting the pulse rod a control system, developed at the Atominstitut19), starts to maintain a constant power level. An optimum adjustment of the shock-absorber and the two control rods limits the over-shoot of the power to a few percent. The maximum power during square wave operation is limited to about 500 kW because of the large negative temperature coefficient of reactivity. If the power maximum exceeds this value, the strong temperature increase in the fuel leads to a steady decrease in power although all the rods are moved out. For pulse operation the initial power was kept on I0-100 W. After shooting the pulse rod out of the

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H. BOCK et al.

critical core, the reactor power is only affected now by the fuel temperature and the resultant temperature coefficient. The pulse height may be adjusted within certain limits (max. 250 MW) by positioning the shock-absorber. Two detectors were installed at the same time in the central irradiation channel of the Triga reactor and comparison was made between their signals and those of an uncompensated outcore boron ionization chamber. A sufficient distance between the detectors, mounted in an aluminium sample holder, should avoid any mutual influence by local flux depression. Recording of the rapidly changing detector currents

was performed by an automatic ranging dc-amplifier, developed at the Atominstitut2°), and an H & B Lumiscript, fig. 1. The dc-amplifier has 5 electronically switched decades and an output voltage of 0-1 V. Since the input is free selectable between 1 0 - 9 A to 10 -11 A, currents up to 10 -4 A can be measured. The amplifier has a symmetrical FET-input. The inverting input serves for compensation of any perturbation on the measuring signal. For good linearity an exact adjustment of the switching hysteresis by means of a sine generator (30 Hz) is necessary (figs. 2 and 3). The output of the amplifier has been adapted carefully to the corresponding loop of the Lumiscript to avoid re-

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Fig. I. Experimental arrangement. (1) Reactor core, (2) reactor tank, (3) self-powered neutron detectors, (4) ionisation chamber, (5) automatic range selector amplifier, (6) Lumiscript recorder, (7) ampere meter.

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SELF-POWERED

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flexions and other perturbations during switching. A typical record of the Lumiscript is shown in fig. 4. It consists of the recorded measuring signal and the decade identification, which indicates every switching to the 2nd, 3rd, and 4th decade.

TABLE 1 Detector sensitivity. Detector

3. Investigations performed 3.1.

Sinegenerator

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(A/R h-1 cm)

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M E A S U R E M E N T OF L I N E A R I T Y

The self-powered detector signals have been compared with the current of an ionization chamber (Type RC6EB) during increasing the reactor power in steps from 1 W up to 250 kW. This corresponds to a neutron flux range from 4 x 107 up to 1 x 101an c m - 2 s -1. Pig. 5 shows the results of these measurements. For better comparison the current of the ionization chamber differs in scale. Linear behaviour of the SPN-detectors could be observed at neutron fluxes higher than about 1 x 1011 n c m - 2 s- 1. The cable compensation currents were measured at 1 x 1013n c m - 2 s- 1and are shown in fig. 5, too. Their portions relative to the total signals are rather small, because only about 30 cm of each cable had been within the core. The sensitivities of the detectors could be estimated by the help of absolute thermal neutron flux calibration with gold foils and dose rate measurements with CaF 2 and LiF dosimeters (table 1). A repetition of linearity measurements after the series of square wave and pulse operations proved no

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DETECTORS

changes in detector currents larger than the statistical deviations. Deviations indeed were not expected, since the neutron dose absorbed during these investigations had been too low for the production of essential quantities of activated nuclei in the emitter material. The isolation resistances of all the detectors can be seen in table 2. The values had been taken 5 min after applying the voltage, because the current decreases continuously for a certain time due to polarization effects in the A120 3. For comparison of the measured values a reference point (5 minutes) had been chosen. 3.2. SQUAREWAVEOPERATION The self powered detectors were tested as described in chapter 2. A certain variation in the height of the power ramps required the normalization of the increasing part and the decrease during the plateau to the

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TABLE 2 Detector resistance before and after irradiation. E = emitter, C = c o m p e n s a t o r , G = ground. Detector

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Resistance (~ ) E-+C

before i r r a d i a t i o n E--+G

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power maximum ( = 100%), fig. 6. With this normalized curves it is possible to compare the time behaviour of the four detectors. The measurements demonstrate that the different detectors are reaching the maximum nearly at the same time. A small deviation from linearity compared with the ionization chamber could be observed for all the detectors. Also after reaching the plateau the detector signals decrease with some delay. Changes in power and consequently in the detector currents are extended to some seconds during this

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operation mode. Any deviation from linearity will be generally originated by superposition of several current components, if exclusion of temperature effects is assumed. Usually, linearity measurements are performed at different stationary power levels and the activation of any short lived nuclei will yield a constant signal component. If the reactor power is changing quickly, saturation activity would not be obtained. This will produce some non-linearity in the detector signals, which is expected to change somehow with increasing

SELF-POWERED NEUTRON DETECTORS

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The automatic ranging DC-amplifier was found to be very advantageous for reactor pulse measurements with pulse widths of about 40 ms.

3.3. PULSE OPERATION Following the investigations with square wave operation the four SPN-detectors were exposed to several reactor pulses with power peaks up to 250 MW. The corresponding maximum neutron flux was 1 x 1016 n c m - E s -1 and the gamma dose rate about 1.2x 1011 Rh -1.

Each detector was subjected to eight reactor pulses with different amplitudes. For better evaluation and comparison of the different detector types normalization was made to the pulse peaks. Fig. 7 shows the results averaged from eight measurements. The absolute currents range from 1 x 10- 5 A (Pt-detector) up to 3 x 10- 4 A for the ionization chamber.

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Obviously, all the neutron detectors are reaching the maximum at the same time with regard to the measuring accuracy. A different behaviour concerning the linearity of the increasing and decreasing detector current can be observed. The damped slope of all the detectors after the pulse peak and the deviation from linear decrease is due to the large amount of delayed neutrons causing a negative reactor period of about 80 seconds. Only before and immediately after the pulse the reactor behaves promptly.

TABLE 3 Ratio of ionization current to detector current in dependence of reactor operation mode. The ratio was calculated at a neutron flux of 1 × 1013 n cm -2 s -1 for steady state operation, at the plateau centre for square wave operation and at the pulse m a x i m u m for pulse operation. Steady state operation

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4. Discussion of the results

As a result of the investigations during medium-fast (seconds-range) and fast (milliseconds-range) changes of reactor power it can be stated that self powered detectors are comparable to ionization chambers with respect to their time behaviour. The spread of the current peaks at power pulses is less than 10 ms. With respect to the linearity of the detector signals, deviations were found during reactor square wave as well as pulse operation. An explanation could not yet be given. The detectors actually had no burn-up, so that any influence referring to this can be excluded. Different changes of the neutron spectra in the incore position of the SPN-detectors and the out-core position of the ionization chamber during a pulse would be of importance only if there are essential epithermal resonances in the emitter material. In this case a hardening of the spectrum during the reactor pulse would yield additional capture-gamma-reactions in the detector which produce time-dependent current contributions. The first resonances of the used emitter materials are as follows: erbium hafnium

0.46 eV, 0.94 eV,

cobalt platinum

135 eV, 12 eV.

The effective neutron temperature increases during a pulse of 250 MW peak power about 45.5°C at the T R I G A reactor Vienna21). That leads to a change in the mean neutron energy of 3.95 x 10 -3 eV. Obviously, this small energy increase cannot generate any additional current contributions by the resonances. Other perturbations of the detector signal might be produced by temperature effects in the mineral insulated cables during the pulse. The SPN-detectors have been in direct contact with the coolant so that the temperature at the detector surface is assumed to be equal to the coolant temperature (30°C). During the reactor pulse the fuel temperature increases immediately and ~s reaching about 350°C within 5 s. Since the detectors

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do not contain any fissionable material and have small diameters ( ~ 3.5 cm) compared with the ones of the fuel elements, the temperature change during a short reactor pulse will be much lower than during a power square wave. For exact temperature measurements a special detector containing a thermocouple would be useful. Investigations on mineral insulated cables without the influence of nuclear irradiation, performed at the Atominstitut, showed a number of temperatureinduced effects resulting in background cable currents. However, one cannot exclude temperature effects as a potential origin of the linearity deviations. In this case the effect should be approximately the same for all detectors, whilst the experiments yield some different behaviours. For continuation of the investigations on temperature effects, including the influence of the reactor radiation field, the development of a heated in-core capsule for installation of several detectors is in progress. Another process that may produce an over-proportionality in the detector current is the excitation of short lived isomeric nuclear levels during the pulse, by which a contribution of short lived current components especially before the pulse peak, will be obtained. The different behaviour of the four detectors could eventually be explained by such effects. In table 3 the ratio of the ionization chamber current to the detector current has been calculated for stationary operation, square wave and pulse operation. As can be seen, this ratio decreases for Co and Hf, or in other words, the signals increase over-proportional. The Er-detector shows nearly a linear and the Ptdetector a less-than-proportional behaviour.

SELF-POWERED NEUTRON DETECTORS

5. Conclusions The a i m o f the investigations was to test self p o w e r e d detectors with different emitter materials together with a n i o n i z a t i o n c h a m b e r in a series o f m e d i u m - f a s t a n d fast changes o f r e a c t o r p o w e r a n d to c o m p a r e the detector currents. The detectors were exposed to p o w e r - c h a n g e s which d o n o t occur in n o r m a l reactor operation. Especially changes in n e u t r o n flux o f several decades within a few milliseconds are nonrealistic o p e r a t i o n modes. But t h r o u g h the experiments a n u m b e r o f questions were raised that should be clarified by further investigations. F o r this p u r p o s e it is p l a n n e d to c o m p a r e the detector currents with the signal o f a miniaturefission-chamber which will be installed in the same incore position. In this way a n y p e r t u r b a t i o n caused by different local p o s i t i o n s a n d m o u n t i n g s will be avoided. F u r t h e r m o r e the time b e h a v i o u r o f S P N - d e t e c t o r s with high b u r n u p is o f special interest because this b e h a v i o u r will be affected essentially by the build-up o f higher isotopes in the e m i t t e r material. The a u t h o r s gratefully a c k n o w l e d g e the help o f

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M r F. Richter in s u p p o r t i n g the experiments a n d the evaluation o f the measurements.

References 1) A. Goodings, Nucl. Engng. Intern. 15 (1970) 599. ~) O. Strindehag et al., AE-102 (1969). 3) O. Strindehag et al., AE-440 (1971). 4) G. Bachmann et al., ATW 16 (1971) 600. 5) j. Anderson et al., AE-359 (1969). 6) W. Johnstone et al., Nuclex •972, Techn. Meeting No. 10/15. 7) W. Seifritz, Nucl. Sci. Engng. 49 (1972) 358. 8) j. A. Sovka, AECL-3368 (1969). 9) O. Strindehag et al., AE-455 (1972). 10) R. Verdont, CEA-R-4411 (1973). al) C. N. Jackson, BNWL-395 (1967). 12) p. Gebureck et al., Proc IAEA Syrup. on Nuclear power plant control and instrumentation (Prague, 1973), IAEA-SM168/G-8, p. 783. la) G. Ramirez et al., Nucl. Instr. and Meth. 85 (1970) 279. 14) K. Emelchnov et al., At. Ener. (USSR) 34 (1973) 203. 15) p. Goldstein, Trans. Am. Nucl. Soc. 15 (1972) 981. 16) K. Mochizuki et al., Proc. IAEA Symp. on Nuclear power plant control and instrumentation (Prague, 1973), IAEASM-168/G-6, p. 757. 17) W. Jaschik et al., Nucl. Sci. Engng. 53 (1974) 61. 18) H. B/Sck et al., Nucl. Instr. and Meth. 87 (1970) 299. 19) B. Frey, Dissertation (TH Wien, 1968). 2o) O. Lasser et al., Nucl. Instr. and Meth. 101 (1972) 527. 21) C. M. Fleck et al., Acta Phys. Austr. 29 (1969) 152.