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Development and applications of LPRM detectors
Nuclear Instruments and Methods in Physics Research A 418 (1998) 420—428
Development and applications of LPRM detectors K.R. Prasad *, S.P. Chaganty , V. Balagi , E. Unnikrishnan, K. Venkateswara Rao Electronics Division, BARC, Bombay 400 085, India Control Systems Group, Electronics Corporation of India Limited, Hyderabad 500 762, India Received 6 April 1998
Abstract Miniature fission chambers with integral coaxial mineral insulated cable assembly have been developed for Local Power Range Monitoring in the BWR power station at Tarapur. The detectors measure neutron flux at fixed locations within the core in the power range of reactor operation and provide alarm in case of power distribution anomaly. Performance tests in DC mode have shown that the detectors have a neutron sensitivity of 1;10\ A/nv and a gamma sensitivity of 2;10\ A/(R/h). A gradual burnup due to neutron flux limits the useful life of these devices to about 2 yr. However, by operating the detector in Campbelling mode, its operational life can be extended to six years. 1998 Elsevier Science B.V. All rights reserved.
1. Introduction In the core of a reactor, the neutron flux distribution is directly related to the thermal power distribution. In-core detectors can provide an alarm in case of any power distribution anomaly. Fission chambers are most commonly used as in-core neutron-flux sensors in Boiling Water Reactors (BWRs). Compared to neutron detectors that depend on the B(n, a) reaction, fission detectors undergo relatively slow burn up of the uranium coating and provide satisfactory operation in the pulse-counting, mean-square-voltage and current modes of signal processing. Electronics Corporation of India Limited has developed in-core fission detectors with integral
* Corresponding author.
mineral insulated cable assemblies for use at the 235 MWe BWRs at the Tarapur Atomic Power Station (TAPS). As a part of quality assurance performance tests have been carried out on these devices at the Electronics Division in Bhabha Atomic Research Centre (BARC). The present paper describes the development, testing and performance of some of these devices.
2. Principle of operation The neutron monitoring systems in a BWR make use of miniature fission detectors which are incorporated in three subsystems for the Source Range Monitor (SRM), Intermediate Range Monitor (IRM) and the Local Power Range Monitor (LPRM), covering a dynamic range of more than 10 decades in neutron flux (Fig. 1). In addition,
0168-9002/98/$19.00 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 0 6 7 0 - 6
K.R. Prasad et al./Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 420—428
421
Fig. 1. Schematic diagram of Tarapur reactor core.
Traversing In-core Probe (TIP) detectors are used to calibrate the power range detectors. Details of BWR in-core instrumentation systems have been described in literature [1]. In these detectors thermal neutrons produce fission events in the uranium coating and the argon gas is ionised. Collection of the charge by an applied voltage gives rise to a signal in the external circuit. To increase the neutron sensitivity and to enhance the signal-to-noise ratio, uranium enriched to greater than 90% in U is used as the neutron sensing material. Since gamma radiation is always present along with neutrons, the output signal cannot be related to the neutron flux unambiguously. This is because a significant fraction of the gamma-induced signal is not proportional to the instantaneous reactor power.
3. Local power range monitoring At power level, fission chambers can be used in DC mode over a flux range of 10—10 nv. These detectors constitute the Local Power Range Monitoring system in which they are used as fixed sensors to measure neutron flux at four axial locations about 1 m apart. The reactor coolant is in direct contact with the detectors. A total of 52 detectors are permanently installed at specific locations and the signals are continuously monitored. Since the signal induced by neutrons is much stron-
ger than the perturbing gamma signal, compensation for the gamma effect is not necessary until burn up is significant. A DC amplifier processes the signals and an alarm is provided whenever the flux from any detector reads too high.
4. Mechanical construction Electronics Corporation of India Limited took up the development programme of LPRM, SRM, IRM and TIP detectors for use in TAPS for the past few years. The schematic diagram of an in-core fission detector is shown in Fig. 2. The components are made of SS304. The electrode gap is 0.25 mm and the sensitive length is 25 mm. The anode diameter is 3.7 mm and the cathode OD and ID are 5.4 and 4.2 mm, respectively. The sensitive volume is filled with argon at atmospheric pressure. Argon is chosen because it is chemically inert even in the presence of intense radiation fields. It has good thermal conductivity that removes heat developed by the signal generating process. Its low thermal neutron cross-section prevents depletion by nuclear transformation and it has suitable ionisation properties. All the in-core fission detectors have an integral coaxial mineral insulated cables that can withstand high neutron and gamma radiation, as well as high ambient temperature and pressure. All the joints in
422
K.R. Prasad et al./Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 420—428
Fig. 2. Schematic diagram of in-core fission detector.
the detector capsule as well as those with the cable are laser-welded. The detector assembly is subjected to hydrostatic pressure tests up to 2050 psi pressure. The high purity alumina insulated SS sheathed cables are heated for 72 h at 150°C until the insulation improves to 10 ohms. The cold end of the cable is then sealed by laser welding a ceramic-metal seal. The detector capsule is out-gassed before gas filling. All welds are helium leak tested to ensure leak-rates lower than 10\ std.cc/s.
5. Tests and results During the development various mechanical tests are carried out on the detector capsule and the detector-cable assembly. The design was validated by carrying out vibration tests with 1 g acceleration at 60 Hz in the horizontal direction for 2 h and at 3 g acceleration at 20 Hz for 6 h. The assembly was also tested in an autoclave at 2000 psi pressure and 360°C for 16 h to establish the mechanical integrity of the detector. The insulation resistance and current sensitivity were found to be unchanged after these tests. The fabrication and processing technology have been improved over a period of time as a result of the feedback from performance tests at BARC and TAPS. Neutron sensitivity tests are carried out on every one of the LPRM detectors at the swimming pool type research reactor Apsara at BARC before they are accepted for installation at TAPS. The detectors are tested either in the Thermal Column or the dry tube location in the core and the neutron flux is estimated by foil activation method by the Radiation Standards and Instrumentation Division, BARC. »/I saturation characteristics are plotted at various power levels and the
Fig. 3. »/I characteristics of LPRM detector at Apsara thermal column.
linearity of the signal output with respect to neutron flux is checked. The saturation characteristics are shown in Fig. 3. The operating voltage is 100 V DC. The gamma sensitivity is estimated in a field of about 200 kR/h at the cobalt-60 irradiation facility in the Food Technology Division at BARC. A variation not more than $20% in the nominal sensitivity (1;10\ A/nv and 2;10\ A/R/h) is acceptable.
6. Extrapolation of performance at high flux The ion collection efficiency of an ion chamber depends upon the electrode spacing for a given gasfill, applied voltage and ionisation rate. To estimate the neutron sensitivity, LPRM detectors are usually tested in a thermal flux of 10—10 nv although they are intended for operation at 10 nv. The performance at lower fluxes can be used as the basis for a calculation to ensure adequate ion collection efficiency at a higher flux.
K.R. Prasad et al./Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 420—428
According to Boag [2] the collection efficiency f at an applied voltage of » is given by 1 f" , 1#(m/6) a d(q and m"( , » ek k where k and k are the ionic mobilities, e is the electronic charge (1.6;10\ C), a the recombination coefficient (cm\), d the equivalent electrode separation (0.03 cm) and q is the ionisation rate (esu/s). The value of m, considered a constant for a given gas at a given pressure, varies slightly from chamber to chamber and hence is usually estimated for a particular chamber. The value of m for argon was obtained by taking the value of q (6.25 esu/sec) from the observed saturation characteristics at 10 nv flux. By taking the value of » for 90% collection efficiency » from Fig. 3 (12 V), m works out to be 24.3 and this value can be used to predict the saturation voltage at higher fluxes. Ion chambers are normally operated at a voltage"2» for saturation. Substituting the values for m, d and q the values for » for higher fluxes are calculated (Table 1). They compare well with the measured m"m
Table 1 Flux (nv)
Voltage for 90% saturation (calculated)
Voltage for 90% saturation (measured)
10 1.3;10 4.8;10 10
23 52 76 115
— 48 70 —
Fig. 4. »/I characteristics of LPRM detector at TAPS.
423
values of » obtained from saturation curves of LPRM detectors at 163 MW power level at TAPS (Fig. 4).
7. Extension of life Accumulated neutron irradiation or fluence causes burn up of uranium in LPRM detectors resulting in a decrease of neutron sensitivity. Since the gamma sensitivity is not affected by burn up, the ratio of neutron to gamma-induced current decreases over a period of time due to burn up. For PRM detectors the typical neutron and gamma sensitivities are 1;10\ A/nv and 2;10\ A/R/h, respectively. At full power, the neutron and gamma intensities in TAPS core are given by 6.6;10 nv and 8;10 R/h. Therefore, in the case of a fresh detector I /I equals 41.25 where I and I refer to A A the neutron and gamma induced current, respectively. The detectors are replaced when the current due to delayed gamma rays rises to 4% of the total current. Since 20% of the gamma radiation is due to delayed gamma rays, the end-of-life is reached when I "5I . That is, the LPRM detector is re A placed when I falls from 660 to 80 lA. Since the current from the detector is directly proportional to the number of U atoms, the useful life ¹ can be calculated from the equation N"N e\N(2. Substituting N/N "80/660, p" 532;10\ cm and "6.6;10 nv, ¹ works out to be 1.74 yr. Thus, in DC mode, the useful life of a LPRM detector is limited to less than 2 yr. By employing Campbelling or mean-square voltage (MSV) mode technique of signal processing, it is possible to extend the useful life of LPRM detectors. The method [3] consists of measuring the variance in the detector signal as an indicator of neutron flux (Fig. 5). The variance is proportional to the square of the charge liberated in the detector, whereas the mean DC signal is proportional to the charge. Thus, by measuring the white noise component of the detector current, very high neutron-to-gamma signal ratio is achieved. In this technique, the additional advantage is that high insulation requirement is not very stringent. This is because leakage currents do not contribute much
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K.R. Prasad et al./Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 420—428
Fig. 5. Campbell (MSV) method.
as the charge liberated by fission fragments is much greater than that due to leakage current. Thus detectors that have lower insulation due to ageing can continue to be used in Campbell technique. In MSV mode operation, the response of the circuit for N number of pulses is given by
[E(t)!E(t)]"N
[ f (t)] dt
and
E(t)"N
f (t) dt,
where f (t)"response of the circuit for each pulse, N"Average no. of pulses generated, E(t)"response of the circuit for N no. of pulses, q e\R 0! , f (t)" C
(1)
NqR E(t)"NqR[E(t)!E(t)]" . 2C
(2)
Eq. (1) gives an expression for the DC signal while Eq. (2) gives that of the MSV signal. The neutronto-gamma signal ratio can be calculated by assuming the MSV sensitivity of a miniature fission chamber of the type NA27 made by General Electric. Its DC sensitivity is equal to that of NA100 detector made by the same company for LPRM applications. In the MSV mode its neutron sensitivity"4;10\ A/Hz/nv and gamma sensitivity"1.5;10\ A/Hz/R/h. For typical neutron
and gamma intensities in the reactor, the ratio of Campbell signals induced by neutrons and gamma rays for a fresh detector would be 22 000. It would take 6.74 yr for this ratio to reduce to 5. This extends the life of the detector by 5 yr (Fig. 6). Since the AC content of the signal is processed, a fall in the insulation resistance of the detector-cable assembly does not affect the measurement. Tests were carried out in Apsara reactor on an LPRM detector made by ECIL and the DC and Campbell signals were recorded at various power levels (Fig. 7). The fall in signal after a reactor scram (Fig. 8) shows that MSV mode has a better gamma discrimination. Above 500 Hz frequency the Campbell signal is comparable to white noise. The upper cutoff frequency is limited by the cable capacitance and input termination. Power spectral density measurements carried out at TAPS showed that the !3 db cutoff is about 5 kHz. The measured power spectral density matched well with the expected value. The ideal bandwidth was found to be 1—2 kHz with an average 1 s time constant for converting AC signal to DC output. Since a large number of old LPRM detectors in TAPS reactor began to approach end-of-life and/or showed low insulation, it was decided to apply the Campbell technique and operate these detectors in the mean-square voltage mode. Electronic Division developed the required instrumentation and retrofitted four new modules in place of the old DC channels. These channels can be operated in DC and MSV modes simultaneously. Detectors with an insulation resistance of 250 k) have shown improved performance. The modified channels will
K.R. Prasad et al./Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 420—428
Fig. 6. Operational life of LPRM detectors in DC and Campbell mode.
Fig. 7. Linearity of MSV and DC signal w.r.t. reactor power.
425
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K.R. Prasad et al./Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 420—428
Fig. 8. Change in MSV and DC signal after reactor scram.
be used on all the LPRM detectors after further studies [4].
8. Reactor noise analysis The signal from a detector exposed to neutron flux from a reactor is composed of a direct current component and a very small fluctuating noise component. Analysis of neutron noise from an in-core neutron detector has been used to monitor the condition of the in-core components of a reactor [5]. Signals from LPRM detectors installed in TAPS reactors have been used to estimate coolant transit times [6]. In BARC, LPRM detectors made by ECIL were used to measure in-core neutron flux in Dhruva, the 100 MW heavy water moderated,
heavy water cooled research reactor and correlate the noise signal with local perturbations. Three LPRM detectors made by ECIL and two cobalt self-powered detectors developed in Electronics Division were used for in-core flux measurement. The information was compared with noise signals obtained from out-of-core detectors, which indicated the global average perturbations [7]. The DC component of the signal is plotted as a measure of the axial flux pattern along the reactor core in Fig. 9. The AC component of the signal was electronically separated and recorded simultaneously with vibration signals mounted on extension of the core structures. Figs. 10 and 11 show the correspondence between the neutron noise signal peaks and the mechanical vibration peaks at 8 and 13.5 Hz.
K.R. Prasad et al./Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 420—428
427
9. LPRMs with U In view of the difficulty experienced in obtaining U isotope enriched to 90% or more, Electronics Division has taken up the development of pulse fission detectors with U [8]. U has a fission cross-section comparable to that of U but the alpha activity from U is much greater. The spurious count rate due to alpha pile-up places an upper limit on the amount of U that can be used in a pulse fission counter. However, the background current due to alpha activity will not pose any problem in using U fission chambers as LPRM detectors. This is because to obtain the required current sensitivity of 1;10\ A/nv, about 3 mg of U will have to be used in the fission chamber that has an electrode spacing of 0.3 mm. This will result in a background current less than 2 nA which is negligible compared to a signal current of 1 lA at 1% of full power reactor operation. Fig. 9. Axial flux pattern.
10. Use of self-powered detectors as LPRM detectors
Fig. 10. Power spectrum of in-core neutron noise.
Self-powered detectors [9] are simple devices that measure in-core neutron flux and do not require any voltage for operation. Various types of self-powered detectors are in use in reactors for in-core flux mapping and other applications [10]. A vanadium self-powered detector (SPD) made by ECIL is presently installed in TAPS core location and is working satisfactorily. It has a current sensitivity of 9;10\ A/nv, which is much lower compared to that of LPRM detectors. Further tests need to be carried out before self-powered detectors can be used as an alternative to LPRMs.
References
Fig. 11. Coherence of in-core neutron noise and mechanical vibration.
[1] J. Forster, Boiling water reactor instrumentation systems, in: J.M. Harrer, J.G. Beckerley (Eds.), Nuclear Power Reactor Instrumentation Systems Handbook, TID 25952-P2, Ch. 16. [2] J.W. Boag, in: F.H. Attix, W.C. Roesch (Eds.), Radiation Dosimetry, vol. II, Academic Press, New York, 1966, Ch. 9, pp. 1—72.
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K.R. Prasad et al./Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 420—428
[3] R.A. DuBridge, IEEE Trans. Nucl. Sci. NS-14 (1) (1967) 241. [4] S.P. Chaganty, B.R. Bairi, IAEA Specialists Meeting on Experience in Ageing, Maintenance and Modernisation of Instrumentation and Control Systems for Improving Nuclear Power Plant Availability, NUREG/CP-0134, Rockville, Maryland, USA, 1993. [5] L.A. Banda, IEEE Trans. Nucl. Sci. NS-26 (1) (1979) 910. [6] S.S. Joshi, K.K. Arora, R.V. Koparde, Progr. Nucl. Energy 15 (1985) 921.
[7] K. Anantharaman, S.K. Sinha, R.I.K. Moorthy, IAEA Int. Seminar on Enhancement of Research Reactor Utilisation, Bombay, 1996. [8] K.R. Prasad, V. Balagi, Rev. Sci. Instrum. 67 (6) (1996) 2197. [9] J.W. Hilborn, Nucleonics 22 (1964) 69. [10] N.P. Goldstein, W.H. Todt, IEEE Trans. Nucl. Sci. NS269 (1) (1979) 916.
Development and applications of LPRM detectors K.R. Prasad *, S.P. Chaganty , V. Balagi , E. Unnikrishnan, K. Venkateswara Rao Electronics Division, BARC, Bombay 400 085, India Control Systems Group, Electronics Corporation of India Limited, Hyderabad 500 762, India Received 6 April 1998
Abstract Miniature fission chambers with integral coaxial mineral insulated cable assembly have been developed for Local Power Range Monitoring in the BWR power station at Tarapur. The detectors measure neutron flux at fixed locations within the core in the power range of reactor operation and provide alarm in case of power distribution anomaly. Performance tests in DC mode have shown that the detectors have a neutron sensitivity of 1;10\ A/nv and a gamma sensitivity of 2;10\ A/(R/h). A gradual burnup due to neutron flux limits the useful life of these devices to about 2 yr. However, by operating the detector in Campbelling mode, its operational life can be extended to six years. 1998 Elsevier Science B.V. All rights reserved.
1. Introduction In the core of a reactor, the neutron flux distribution is directly related to the thermal power distribution. In-core detectors can provide an alarm in case of any power distribution anomaly. Fission chambers are most commonly used as in-core neutron-flux sensors in Boiling Water Reactors (BWRs). Compared to neutron detectors that depend on the B(n, a) reaction, fission detectors undergo relatively slow burn up of the uranium coating and provide satisfactory operation in the pulse-counting, mean-square-voltage and current modes of signal processing. Electronics Corporation of India Limited has developed in-core fission detectors with integral
* Corresponding author.
mineral insulated cable assemblies for use at the 235 MWe BWRs at the Tarapur Atomic Power Station (TAPS). As a part of quality assurance performance tests have been carried out on these devices at the Electronics Division in Bhabha Atomic Research Centre (BARC). The present paper describes the development, testing and performance of some of these devices.
2. Principle of operation The neutron monitoring systems in a BWR make use of miniature fission detectors which are incorporated in three subsystems for the Source Range Monitor (SRM), Intermediate Range Monitor (IRM) and the Local Power Range Monitor (LPRM), covering a dynamic range of more than 10 decades in neutron flux (Fig. 1). In addition,
0168-9002/98/$19.00 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 0 6 7 0 - 6
K.R. Prasad et al./Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 420—428
421
Fig. 1. Schematic diagram of Tarapur reactor core.
Traversing In-core Probe (TIP) detectors are used to calibrate the power range detectors. Details of BWR in-core instrumentation systems have been described in literature [1]. In these detectors thermal neutrons produce fission events in the uranium coating and the argon gas is ionised. Collection of the charge by an applied voltage gives rise to a signal in the external circuit. To increase the neutron sensitivity and to enhance the signal-to-noise ratio, uranium enriched to greater than 90% in U is used as the neutron sensing material. Since gamma radiation is always present along with neutrons, the output signal cannot be related to the neutron flux unambiguously. This is because a significant fraction of the gamma-induced signal is not proportional to the instantaneous reactor power.
3. Local power range monitoring At power level, fission chambers can be used in DC mode over a flux range of 10—10 nv. These detectors constitute the Local Power Range Monitoring system in which they are used as fixed sensors to measure neutron flux at four axial locations about 1 m apart. The reactor coolant is in direct contact with the detectors. A total of 52 detectors are permanently installed at specific locations and the signals are continuously monitored. Since the signal induced by neutrons is much stron-
ger than the perturbing gamma signal, compensation for the gamma effect is not necessary until burn up is significant. A DC amplifier processes the signals and an alarm is provided whenever the flux from any detector reads too high.
4. Mechanical construction Electronics Corporation of India Limited took up the development programme of LPRM, SRM, IRM and TIP detectors for use in TAPS for the past few years. The schematic diagram of an in-core fission detector is shown in Fig. 2. The components are made of SS304. The electrode gap is 0.25 mm and the sensitive length is 25 mm. The anode diameter is 3.7 mm and the cathode OD and ID are 5.4 and 4.2 mm, respectively. The sensitive volume is filled with argon at atmospheric pressure. Argon is chosen because it is chemically inert even in the presence of intense radiation fields. It has good thermal conductivity that removes heat developed by the signal generating process. Its low thermal neutron cross-section prevents depletion by nuclear transformation and it has suitable ionisation properties. All the in-core fission detectors have an integral coaxial mineral insulated cables that can withstand high neutron and gamma radiation, as well as high ambient temperature and pressure. All the joints in
422
K.R. Prasad et al./Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 420—428
Fig. 2. Schematic diagram of in-core fission detector.
the detector capsule as well as those with the cable are laser-welded. The detector assembly is subjected to hydrostatic pressure tests up to 2050 psi pressure. The high purity alumina insulated SS sheathed cables are heated for 72 h at 150°C until the insulation improves to 10 ohms. The cold end of the cable is then sealed by laser welding a ceramic-metal seal. The detector capsule is out-gassed before gas filling. All welds are helium leak tested to ensure leak-rates lower than 10\ std.cc/s.
5. Tests and results During the development various mechanical tests are carried out on the detector capsule and the detector-cable assembly. The design was validated by carrying out vibration tests with 1 g acceleration at 60 Hz in the horizontal direction for 2 h and at 3 g acceleration at 20 Hz for 6 h. The assembly was also tested in an autoclave at 2000 psi pressure and 360°C for 16 h to establish the mechanical integrity of the detector. The insulation resistance and current sensitivity were found to be unchanged after these tests. The fabrication and processing technology have been improved over a period of time as a result of the feedback from performance tests at BARC and TAPS. Neutron sensitivity tests are carried out on every one of the LPRM detectors at the swimming pool type research reactor Apsara at BARC before they are accepted for installation at TAPS. The detectors are tested either in the Thermal Column or the dry tube location in the core and the neutron flux is estimated by foil activation method by the Radiation Standards and Instrumentation Division, BARC. »/I saturation characteristics are plotted at various power levels and the
Fig. 3. »/I characteristics of LPRM detector at Apsara thermal column.
linearity of the signal output with respect to neutron flux is checked. The saturation characteristics are shown in Fig. 3. The operating voltage is 100 V DC. The gamma sensitivity is estimated in a field of about 200 kR/h at the cobalt-60 irradiation facility in the Food Technology Division at BARC. A variation not more than $20% in the nominal sensitivity (1;10\ A/nv and 2;10\ A/R/h) is acceptable.
6. Extrapolation of performance at high flux The ion collection efficiency of an ion chamber depends upon the electrode spacing for a given gasfill, applied voltage and ionisation rate. To estimate the neutron sensitivity, LPRM detectors are usually tested in a thermal flux of 10—10 nv although they are intended for operation at 10 nv. The performance at lower fluxes can be used as the basis for a calculation to ensure adequate ion collection efficiency at a higher flux.
K.R. Prasad et al./Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 420—428
According to Boag [2] the collection efficiency f at an applied voltage of » is given by 1 f" , 1#(m/6) a d(q and m"( , » ek k where k and k are the ionic mobilities, e is the electronic charge (1.6;10\ C), a the recombination coefficient (cm\), d the equivalent electrode separation (0.03 cm) and q is the ionisation rate (esu/s). The value of m, considered a constant for a given gas at a given pressure, varies slightly from chamber to chamber and hence is usually estimated for a particular chamber. The value of m for argon was obtained by taking the value of q (6.25 esu/sec) from the observed saturation characteristics at 10 nv flux. By taking the value of » for 90% collection efficiency » from Fig. 3 (12 V), m works out to be 24.3 and this value can be used to predict the saturation voltage at higher fluxes. Ion chambers are normally operated at a voltage"2» for saturation. Substituting the values for m, d and q the values for » for higher fluxes are calculated (Table 1). They compare well with the measured m"m
Table 1 Flux (nv)
Voltage for 90% saturation (calculated)
Voltage for 90% saturation (measured)
10 1.3;10 4.8;10 10
23 52 76 115
— 48 70 —
Fig. 4. »/I characteristics of LPRM detector at TAPS.
423
values of » obtained from saturation curves of LPRM detectors at 163 MW power level at TAPS (Fig. 4).
7. Extension of life Accumulated neutron irradiation or fluence causes burn up of uranium in LPRM detectors resulting in a decrease of neutron sensitivity. Since the gamma sensitivity is not affected by burn up, the ratio of neutron to gamma-induced current decreases over a period of time due to burn up. For PRM detectors the typical neutron and gamma sensitivities are 1;10\ A/nv and 2;10\ A/R/h, respectively. At full power, the neutron and gamma intensities in TAPS core are given by 6.6;10 nv and 8;10 R/h. Therefore, in the case of a fresh detector I /I equals 41.25 where I and I refer to A A the neutron and gamma induced current, respectively. The detectors are replaced when the current due to delayed gamma rays rises to 4% of the total current. Since 20% of the gamma radiation is due to delayed gamma rays, the end-of-life is reached when I "5I . That is, the LPRM detector is re A placed when I falls from 660 to 80 lA. Since the current from the detector is directly proportional to the number of U atoms, the useful life ¹ can be calculated from the equation N"N e\N(2. Substituting N/N "80/660, p" 532;10\ cm and "6.6;10 nv, ¹ works out to be 1.74 yr. Thus, in DC mode, the useful life of a LPRM detector is limited to less than 2 yr. By employing Campbelling or mean-square voltage (MSV) mode technique of signal processing, it is possible to extend the useful life of LPRM detectors. The method [3] consists of measuring the variance in the detector signal as an indicator of neutron flux (Fig. 5). The variance is proportional to the square of the charge liberated in the detector, whereas the mean DC signal is proportional to the charge. Thus, by measuring the white noise component of the detector current, very high neutron-to-gamma signal ratio is achieved. In this technique, the additional advantage is that high insulation requirement is not very stringent. This is because leakage currents do not contribute much
424
K.R. Prasad et al./Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 420—428
Fig. 5. Campbell (MSV) method.
as the charge liberated by fission fragments is much greater than that due to leakage current. Thus detectors that have lower insulation due to ageing can continue to be used in Campbell technique. In MSV mode operation, the response of the circuit for N number of pulses is given by
[E(t)!E(t)]"N
[ f (t)] dt
and
E(t)"N
f (t) dt,
where f (t)"response of the circuit for each pulse, N"Average no. of pulses generated, E(t)"response of the circuit for N no. of pulses, q e\R 0! , f (t)" C
(1)
NqR E(t)"NqR[E(t)!E(t)]" . 2C
(2)
Eq. (1) gives an expression for the DC signal while Eq. (2) gives that of the MSV signal. The neutronto-gamma signal ratio can be calculated by assuming the MSV sensitivity of a miniature fission chamber of the type NA27 made by General Electric. Its DC sensitivity is equal to that of NA100 detector made by the same company for LPRM applications. In the MSV mode its neutron sensitivity"4;10\ A/Hz/nv and gamma sensitivity"1.5;10\ A/Hz/R/h. For typical neutron
and gamma intensities in the reactor, the ratio of Campbell signals induced by neutrons and gamma rays for a fresh detector would be 22 000. It would take 6.74 yr for this ratio to reduce to 5. This extends the life of the detector by 5 yr (Fig. 6). Since the AC content of the signal is processed, a fall in the insulation resistance of the detector-cable assembly does not affect the measurement. Tests were carried out in Apsara reactor on an LPRM detector made by ECIL and the DC and Campbell signals were recorded at various power levels (Fig. 7). The fall in signal after a reactor scram (Fig. 8) shows that MSV mode has a better gamma discrimination. Above 500 Hz frequency the Campbell signal is comparable to white noise. The upper cutoff frequency is limited by the cable capacitance and input termination. Power spectral density measurements carried out at TAPS showed that the !3 db cutoff is about 5 kHz. The measured power spectral density matched well with the expected value. The ideal bandwidth was found to be 1—2 kHz with an average 1 s time constant for converting AC signal to DC output. Since a large number of old LPRM detectors in TAPS reactor began to approach end-of-life and/or showed low insulation, it was decided to apply the Campbell technique and operate these detectors in the mean-square voltage mode. Electronic Division developed the required instrumentation and retrofitted four new modules in place of the old DC channels. These channels can be operated in DC and MSV modes simultaneously. Detectors with an insulation resistance of 250 k) have shown improved performance. The modified channels will
K.R. Prasad et al./Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 420—428
Fig. 6. Operational life of LPRM detectors in DC and Campbell mode.
Fig. 7. Linearity of MSV and DC signal w.r.t. reactor power.
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K.R. Prasad et al./Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 420—428
Fig. 8. Change in MSV and DC signal after reactor scram.
be used on all the LPRM detectors after further studies [4].
8. Reactor noise analysis The signal from a detector exposed to neutron flux from a reactor is composed of a direct current component and a very small fluctuating noise component. Analysis of neutron noise from an in-core neutron detector has been used to monitor the condition of the in-core components of a reactor [5]. Signals from LPRM detectors installed in TAPS reactors have been used to estimate coolant transit times [6]. In BARC, LPRM detectors made by ECIL were used to measure in-core neutron flux in Dhruva, the 100 MW heavy water moderated,
heavy water cooled research reactor and correlate the noise signal with local perturbations. Three LPRM detectors made by ECIL and two cobalt self-powered detectors developed in Electronics Division were used for in-core flux measurement. The information was compared with noise signals obtained from out-of-core detectors, which indicated the global average perturbations [7]. The DC component of the signal is plotted as a measure of the axial flux pattern along the reactor core in Fig. 9. The AC component of the signal was electronically separated and recorded simultaneously with vibration signals mounted on extension of the core structures. Figs. 10 and 11 show the correspondence between the neutron noise signal peaks and the mechanical vibration peaks at 8 and 13.5 Hz.
K.R. Prasad et al./Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 420—428
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9. LPRMs with U In view of the difficulty experienced in obtaining U isotope enriched to 90% or more, Electronics Division has taken up the development of pulse fission detectors with U [8]. U has a fission cross-section comparable to that of U but the alpha activity from U is much greater. The spurious count rate due to alpha pile-up places an upper limit on the amount of U that can be used in a pulse fission counter. However, the background current due to alpha activity will not pose any problem in using U fission chambers as LPRM detectors. This is because to obtain the required current sensitivity of 1;10\ A/nv, about 3 mg of U will have to be used in the fission chamber that has an electrode spacing of 0.3 mm. This will result in a background current less than 2 nA which is negligible compared to a signal current of 1 lA at 1% of full power reactor operation. Fig. 9. Axial flux pattern.
10. Use of self-powered detectors as LPRM detectors
Fig. 10. Power spectrum of in-core neutron noise.
Self-powered detectors [9] are simple devices that measure in-core neutron flux and do not require any voltage for operation. Various types of self-powered detectors are in use in reactors for in-core flux mapping and other applications [10]. A vanadium self-powered detector (SPD) made by ECIL is presently installed in TAPS core location and is working satisfactorily. It has a current sensitivity of 9;10\ A/nv, which is much lower compared to that of LPRM detectors. Further tests need to be carried out before self-powered detectors can be used as an alternative to LPRMs.
References
Fig. 11. Coherence of in-core neutron noise and mechanical vibration.
[1] J. Forster, Boiling water reactor instrumentation systems, in: J.M. Harrer, J.G. Beckerley (Eds.), Nuclear Power Reactor Instrumentation Systems Handbook, TID 25952-P2, Ch. 16. [2] J.W. Boag, in: F.H. Attix, W.C. Roesch (Eds.), Radiation Dosimetry, vol. II, Academic Press, New York, 1966, Ch. 9, pp. 1—72.
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[3] R.A. DuBridge, IEEE Trans. Nucl. Sci. NS-14 (1) (1967) 241. [4] S.P. Chaganty, B.R. Bairi, IAEA Specialists Meeting on Experience in Ageing, Maintenance and Modernisation of Instrumentation and Control Systems for Improving Nuclear Power Plant Availability, NUREG/CP-0134, Rockville, Maryland, USA, 1993. [5] L.A. Banda, IEEE Trans. Nucl. Sci. NS-26 (1) (1979) 910. [6] S.S. Joshi, K.K. Arora, R.V. Koparde, Progr. Nucl. Energy 15 (1985) 921.
[7] K. Anantharaman, S.K. Sinha, R.I.K. Moorthy, IAEA Int. Seminar on Enhancement of Research Reactor Utilisation, Bombay, 1996. [8] K.R. Prasad, V. Balagi, Rev. Sci. Instrum. 67 (6) (1996) 2197. [9] J.W. Hilborn, Nucleonics 22 (1964) 69. [10] N.P. Goldstein, W.H. Todt, IEEE Trans. Nucl. Sci. NS269 (1) (1979) 916.