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Development of “Fission Chamber Free” Fork Detector (FDET) for Safeguards Measures on LWR Spent Fuel Assemblies

Young-Gil Lee*, Stefan Jagenbrein*, Solomon Amos Kamya*

Seung-Ho Ahn#, Ki-Hyun Kim#, Jae-Beom Park#, Na-Young Lee#

* IAEA (International Atomic Energy Agency)

# KINAC (Korea Institute of Nuclear Nonproliferation and Control)

Abstract As part of the Agency’s efforts to improve the light water reactor (LWR) spent fuel verification technology, the IAEA developed a Fork Detector (FDET) in which the neutron detectors were changed from fission chambers (FCs) to boron-10 (B-10) lined proportional counters. The purpose of this development was to avoid the difficulties in the transportation and deployment of fission chambers containing fissile material. The validation experiment to investigate the features and performance of a prototype FDET with B-10 counters was carried out at the pressurized water reactor (PWR) spent fuel storage pond of Ulchin nuclear power plant (NPP) in the Republic of Korea in June 2011, within the framework of the Korean Support Programme to the IAEA. The results promised that the FDET with shielded B-10 counters could be used for the neutron detection of PWR spent fuel assemblies cooled over three years. The IAEA is now using the B-10 counter based FDET(B10) in the implementation of safeguards measures, specifically on the LWR spent fuel assemblies during the transfer campaign prior to loading them into the transfer cask to move from wet to dry storage at commercial NPPs. The traditional fission chamber based FDET(FC) continues to be used as a safeguards measure to resolve core fuel anomalies because the extremely high gamma dose of freshly discharged core fuel assemblies exceeds the application limit of FDET(B10). 1. Background

As part of spent fuel verification technology improvement efforts, the Division of Technical and Scientific Services (SGTS) of the IAEA Department of Safeguards developed a Fork Detector (FDET) in which the neutron detectors were changed from fission chambers to boron-10 lined proportional counters as the transportation and deployment of fission chambers containing fissile material are difficult. This modified FDET is named “FDET(B10)”, and the standard FDET with fission chambers is denoted as “FDET(FC)”.

The SGTS was looking for a suitable NPP facility to investigate and validate experimentally the gamma resistance of neutron detectors installed in FDET(B10) while measuring a commercial LWR spent fuel assembly with an extremely high gamma dose strength.

In the letter from Dir/SGTS on 4 March 2011, it was requested that the Member State of the Republic of Korea (ROK) assist the IAEA carry out experimental spent fuel measurements with FDET(B10) at the PWR-NPP facility in the ROK; the Dir/Nuclear Control Div., Nuclear Regulatory Bureau in the Ministry of Education, Science and Technology (MEST) of the ROK accepted the Agency’s request in the reply letter on 3 May 2011.

Based on the ROK Support Programme, the capability and performance test on a prototype of the newly developed FDET(B10) was carried out at the spent fuel storage pond of Ulchin NPP Unit 4 (KOO4) of Korea Hydro and Nuclear Power Co., Ltd. (KHNP) in June 2011.

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2. Description of Prototype FDET(B10)

In the prototype FDET(B10), B-10 lined proportional counters (model: Reuter-Stokes, Inc., RS-P7-0405-201, Dout: 13 mm, Lact.: 127 mm, sensitivity: about 1.3 cps/nv) replaced the fission chambers (sensitivity: about 0.15 cps/nv) for neutron detection (see Fig. 1) because B-10 counters appeared to have the most gamma flux resistance after fission chambers (Refs [1] and [2]). Note that a B-10 enriched boron material is coated on the aluminum foil lined on the inner surface of the B-10 counter housing.

As shown in Fig. 1, the high density polyethylene (PE) main-body of a standard PWR type FDET(FC) was modified to accommodate a set of “bare” (without gamma shield) B-10 counters and a set of “shielded” B-10 counters. Eccentric shape tungsten (W) cylinders (Dout: 35 mm, Teff. shield: 20 mm) were used to shield the B-10 counters.

A dual channel preamp and discriminator (model: Precision Data Technology, Inc., PDT210A) was used for the amplification and transistor-transistor logic (TTL) pulse generation on the signals detected by the B-10 counters. Note that, for the field trial, the existing PDT210A unit characterized on the fission chambers was readjusted to fit the conditions of the B-10 counters. The same type of ion chamber (model: LND, Inc., LND-52113) currently in use for FDET(FC) was used for gamma detection.

Fig. 1. The existing main-body of PWR type FDET(FC) was modified to accommodate ion chamber gamma detectors and B-10 neutron counters (with/without W-shield) for the replacement of fission chambers.

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3. FDET(B10) Validation Experiment Using PWR Spent Fuel Assemblies

A KOO4 facility operator declared that the most recently discharged fuel assemblies had a cooling time (CT) of 1.0 year, and the maximum burn-up (BU) of the fuel assemblies in the pond was 49 GWd/MtU. Thus, seven spent fuel (SF) assemblies ranging between 10 to 1 year CT and 26 to 49 GWd/MtU BU were selected for the measurements. Around the middle zone of the assembly active length, between the 5th and the 6th grids, was selected as the measuring position to give a higher gamma flux environment for the detector. Fig. 2 shows one of the SF assemblies in the measuring position during the FDET(B10) capability and performance test at KOO4.

Two sets of neutron related electronics were operated simultaneously for the B-10 counter plateau measurements; one system with a multichannel analyzer (model: GBS Elektronik GmbH, MCA-166) and a high voltage power supply (model: GBS Elektronik GmbH, NBOX) for the “Shielded B-10 Counters” (Ch-A), and the other system with MCA-166 and NBOX for the “Bare B-10 Counters” (Ch-B). The neutron plateau characteristics measurement of each test assembly was accomplished by elevating the operation voltage of the B-10 counters at regular intervals (10 volts) starting from 400 volts and ending at 700 volts and applying a fixed dwell time (10 seconds) at each voltage.

The ion chambers for the determination of the gamma dose strength of each test assembly were operated at a constant bias voltage (-100 volts) by using a gamma electronics unit (model: D. S. Davision Co. Inc., GRAND3).

Fig. 2. Photo taken during the validation experiment of FDET(B10) at the spent fuel storage pond of Ulchin NPP in Korea. A PWR spent fuel assembly (16x16) is in place of FDET(B10) and the central zone of fuel active length (just above the 5th grid) is positioned at the detector head.

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4. Data Analysis and Evaluation

Fig. 3 shows the plateau characteristics of Ch-A and Ch-B for those assemblies with quite different cooling times of 9-, 5- and 1-year. Ch-A (“shielded” B-10 counters) compared with Ch-B (“bare” B-10 counters) shows much higher gamma resistance due to the W-shields. A blue dotted line in the figure (Ch-A at 670V, Ch-B at 630V) was drawn simply to compare the gamma pileup effects in the neutron detection caused by different gamma flux of test assemblies. The intensities near the green dotted line (at 610V) were used for plateau slope determination. Ch-A-02 and Ch-A-03 show the same slope of 3.8%/10V, but Ch-A-06 of the assembly with a 48 GWd/MtU BU and a 1 year CT shows a slope of 6.0%/10V indicating neutron signals affected by the gamma pileup signals at around 610 volts.

The plateau shape of Ch-B-02 and Ch-B-03 was degraded considerably by the gamma pileup (i.e., a rapid increment), and finally Ch-B-06 shows no noticeable plateau in the figure due to an extremely high gamma dose exposed to the “bare” B-10 counters.

To investigate all of the results together in a graph, the measured intensity was normalized to the average value of 600, 610 and 620 volt intensities. The gamma dose rate (R/h) given in Fig. 4 denotes the exposure at the position of the ion chamber embedded in FDET(B10) while the SF assembly is in the position of the FDET(B10) head.

From the gamma current (rel. unit) measured using GRAND3 electronics, the gamma dose rate (R/h) was calculated by using the following information;

• GRAND3 electronics gamma output = 2.6 x 10-10 A/(rel. unit), where A denotes ampere.

• Ion chamber (LND-52113) sensitivity = 2.5 x 10-11 A/(R/h)

• Measured gamma current from 2 ion chambers (one ion chamber at each fork-tine)

The gamma doses (at the position of the ion chamber in FDET(B10)) of the assemblies measured at KOO4 were in the range from 6.5 x 103 R/h up to 6.1 x 104 R/h. Note that the possible dose rate differences caused by different distances from the assembly surface to the ion chamber, the bare B-10 counter, and the shielded B-10 counter (see Fig. 1) were neglected in this analysis. A minor gamma dose underestimation caused by the gamma shield due to the ion chamber housing material (stainless steel, 1.5 mm thick) was not considered here.

Fig. 4 (Top) shows that no considerable gamma pileup appeared at the 610V plateau region for the assemblies cooled over 3 years. However, gamma pileup signals triggered at around 620V appeared for the assemblies with a 1 year CT. In the case of Fig. 4 (Bottom), even the SF assembly with a 10 year CT shows a serious gamma pileup in the plateau graph. Those fuels with less than a 4 year CT show an exponential-like shape where neutron originated signals were deeply “screened” by strong gamma pileup signals.

For the same 1 year cooled SF assemblies in Fig. 4, a considerable gamma dose difference (46100, 61100 R/h) despite a small 3-cycle sum BU difference (42, 48 GWd/MtU) might be caused by the notably different last cycle BUs (3, 7 GWd/MtU). In the case of a very short cooled SF, gamma activity is more sensitive to the last cycle power history than the summed power of the entire irradiation cycles.

Table 1 shows the neutron detection rates of the SF assemblies with long (10 y), medium (5 y) and short (1 y) cooling times, which were measured with this prototype FDET(B10) in a borated SF pond (boron concentration: about 4100 ppm). Note that the final version of FDET(B10), improved for the safeguards implementation, showed a much better detection efficiency.

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Fig. 3. Comparison of plateau characteristics of the assemblies with notably different cooling times. (Left Column) Ch-A of B-10 counters with a 2 cm thick tungsten shield. (Right Column) Ch-B of B-10 counters without a gamma shield.

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Fig. 4. Normalized intensity versus applied operating voltage to B-10 neutron counters. The gamma dose of each measured assembly was calculated based on the current strength of ion chambers installed in FDET(B10). (Top) Ch-A of B-10 counters with a 2 cm thick W-shield. (Bottom) Ch-B of B-10 counters without a gamma shield.

Table 1. Neutron count rates of PWR (16x16) spent fuel assemblies measured during the FDET(B10) performance test at KOO4.

Type B-10 Lined Proportional Counter

Model RS-P7-0405-201

Outer Diameter 0.5 Inch (13 mm)

Active Length 5 Inches (127 mm)

Detection Sensitivity ~ 1.3 cps/nv

Count Rate per B-10 Counter of Ch-A(Averaged count of 600, 610 and 620 volts)

~ 1000 cps/counter (10 y, 26 GWd/MtU, 2.7 w /o)

~ 6000 cps/counter (5 y, 47 GWd/MtU, 4.2 w /o)

~ 12000 cps/counter (1 y, 48 GWd/MtU, 4.3 w /o)

Prototype FDET(B10)

NeutronCounter

Specif ications

PerformanceTest at

Ulchin NPP(KOO4)

PWR Assembly (16x16)

Pond Water Boron Con.: about 4100 ppm

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From the results of capability and performance test carried out at KOO4, it was proven experimentally that the B-10 counter without a shield was not applicable to measure even an old SF assembly with a 10 year CT. On the other hand, the B-10 counter with a 2 cm thick W-shield survived well at least for those SF assemblies with over 3 year CT.

Fig. 5 shows the comparison of the applicable maximum gamma dose strength between FDET(FC) and FDET(B10) for the neutron detection of LWR SF assemblies.

The Agency has quite a lot of experience in the FDET(FC) operation for SF measurements and it has been established well that the FC coupled with a “low-level-threshold discriminator” works without gamma pileup effects for the SF assembly with a 1 year CT, and the FC with a “high-level-threshold discriminator” functions without gamma pileup effects for the freshly discharged core fuel assembly (Agency experience for the shortest CT is 4 days).

Based on a very positive KOO4 test result of the shielded B-10 counter configuration, the Agency completed the development of the FDET(B10) equipment for safeguards implementation. These “FC free” FDET(B10)s are used widely during SF transfer campaigns at commercial NPPs where the assemblies cooled over 5 years and up to 2 or 3 decades are usually transferred from wet to dry storage worldwide.

Fig. 5. Comparison of the estimated maximum applicable gamma dose strength between FDET(FC) and FDET(B10) for spent fuel assembly measurements. (Left: Orange Color) Created based on the Agency’s over 15 year experience in FDET(FC) measurements for SF verification and open core anomaly resolution. (Right: Green Color) Created based on the results of FDET(B10) performance test carried out at Ulchin NPP Unit 4 (KOO4) in June 2011.

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5. Conclusions

• The following conclusions were drawn from the capability and performance test on a newly developed prototype FDET(B10) conducted using PWR spent fuel assemblies at Ulchin NPP Unit 4 in Korea in June 2011 within the framework of the ROK Support Programme to the IAEA;

◦ Gamma dose strength of the PWR spent fuel assembly with a 10 year CT already exceeded the application limit of the “bare” (without gamma shield) B-10 counter, which has a known maximum applicable gamma dose in the order of 103 R/h.

◦ On the other hand, a B-10 counter with a 2 cm thick tungsten shield survived well and also maintained acceptable plateau characteristics for those PWR spent fuel assemblies cooled over 3 years.

• Based on a very positive result of the shielded B-10 counter configuration, the Agency completed the development of the “fission chamber free” FDET(B10). Both FDET(B10) and FDET(FC) are now being used for safeguards measures, and their characteristics in field application are as follows;

Newly Developed B-10 Counter Based FDET(B10) ◦ It is used for safeguards measures on the LWR spent fuel assemblies during the transfer

campaign, prior to loading them into the transfer cask to move from wet to dry storage.

◦ Easy transportation and deployment is possible because no fissile material or dangerous good is included in the device. It has resulted in cost and time savings.

◦ Field service frequency is about 90% of total FDET use

Traditional Fission Chamber Based FDET(FC) ◦ It is continuously used as a safeguards measure to resolve core fuel anomalies because the

extremely high gamma dose of freshly discharged core fuel assemblies exceeds the application limit of FDET(B10).

◦ Difficulties arise in transportation and deployment due to the fissile materials of the fission chambers. It leads to high costs and is time consuming.

◦ Field service frequency has been reduced from 100% to about 10% after the FDET(B10) development.

6. References

[1] L. Oscar Lindquist and E. J. Dowdy, “Neutron Detector Counting Capabilities for 10B-Lined and 235U Fission Chambers in High Gamma-Ray Fluxes”, LA-10376-MS, March 1985, LANL.

[2] J. R. Phillips, G. E. Bosler, J. K. Halbig, S. F. Klosterbuer, D. M. Lee and H. O. Menlove, “Neutron Measurement Techniques for the Nondestructive Analysis of Irradiated Fuel Assemblies”, LA-9002-MS, November 1981, LANL.

Acknowledgements

The authors are grateful to the state authorities of the Republic of Korea for the acceptance and support of this validation experiment in the framework of the Member State Support Programme to the IAEA. The authors are thankful to the KINAC for the coordination for this task and participation during the field test measurements. Special thanks are due to the Ulchin NPP of KHNP for the perfect and outstanding preparations and support for the successful completion of this performance test.