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Nuclear Instruments and Methods in Physics Research A 619 (2010) 470–474

Contents lists available at ScienceDirect

Nuclear Instruments and Methods inPhysics Research A

0168-90

doi:10.1

� Corr

Nationa

R.O.C. F

E-m

journal homepage: www.elsevier.com/locate/nima

Determination of plutonium content in TRR spent fuel by nondestructiveneutron counting

Yen-Fu Chen a, Rong-Jiun Sheu c, Ling-Huan Chiao d, Ming-Chen Yuan d, Shiang-Huei Jiang a,b,�

a Department of Engineering and System Science, National Tsing Hua University, 101, Sec. 2, Kung Fu Road, Hsinchu 30013, Taiwan, R.O.C.b Institute of Nuclear Engineering and Science, National Tsing Hua University, 101, Sec. 2, Kung Fu Road, Hsinchu 30013, Taiwan, R.O.C.c National Synchrotron Radiation Research Center, 101 Hsin-Ann Road, Hsinchu Science Park, Hsinchu 30076, Taiwan, R.O.C.d Institute of Nuclear Energy Research, 1000, Wenhua Road, Jiaan Village, Longtan Township, Taoyuan County 32546, Taiwan, R.O.C.

a r t i c l e i n f o

Available online 6 February 2010

Keywords:

Spent fuel

Nuclear safeguard

Plutonium assay

Neutron counting

02/$ - see front matter & 2010 Elsevier B.V. A

016/j.nima.2010.01.077

esponding author at: Department of Engin

l Tsing Hua University, 101, Sec. 2, Kung Fu R

ax: +886 3 5725012.

ail address: shjiang@mx.nthu.edu.tw (S.-H. Jia

a b s t r a c t

For the nuclear safeguard purpose, this work aims to nondestructively determine the plutonium

content in the Taiwan Research Reactor (TRR) spent fuel rods in the storage pool before the stabilization

process, which transforms the metal spent fuel rods into oxide powder. A SPent-fuel-Neutron-Counter

(SPNC) system was designed and constructed to carry out underwater scan measurements of neutrons

emitting from the spent fuel rod, from which the 240Pu mass in the fuel rod will be determined. The

SAS2 H control module of the SCALE 5.1 code package was applied to calculate the 240Pu-to-Pu mass

ratio in the TRR spent fuel rod according to the given power history. This paper presents the

methodology and design of our detector system as well as the measurements of four TRR spent fuel rods

in the storage pool and the comparison of the measured results with the facility declared values.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

In the fuel stabilization program of the Institute of NuclearEnergy Research (INER), thirty-five canisters of spent fuels fromthe 40 MW CANDU-type Taiwan Research Reactor, which aremade of natural uranium metal and aluminum cladding and havebeen stored in the storage pool for more than 20 years, will be cutinto pieces, sintered, and then installed in steel storage canistersfor interim storage. It is crucial that the plutonium content in eachTRR spent fuel rod must be nondestructively measured before andafter the stabilization process for a nuclear nonproliferationcontrol purpose. In this study, we developed a method tonondestructively determine the plutonium content in the spentfuel rods in the spent fuel pool before the stabilization process.

The key elements to accurately determine the plutoniumcontent in the nuclear fuel by neutron counting are the absolute240Pu mass, which are the major spontaneous fission sources ofplutonium, and the plutonium isotopic composition for derivingthe total plutonium content from the 240Pu mass. There are twomethods for determining 240Pu mass based on neutron counting:total neutron counting and neutron coincidence counting [1].Some researchers use the neutron coincidence counting toexclude neutron counting contributions from other sources, such

ll rights reserved.

eering and System Science,

oad, Hsinchu 30013, Taiwan,

ng).

as (a,n) and induced fission neutrons, to determine the 240Pu massin the fresh fuel pins of the Fast Breeder Test Reactor(FBTR) [2]and in �1600 spent fuel assemblies of the BN-350 fast breederreactor in Kazakhstan [3]. However, it is not necessary to applyneutron coincidence counting for our measurement because theTRR fuel is metal fuel made of natural uranium. On the otherhand, the neutron source strength in the TRR spent fuel rodscompared with those fuels of the fast breeder reactors is relativelylow and inappropriate for the neutron coincidence countingunderwater, which requires an inconveniently long countingtime. In this work, we apply the much faster total neutroncounting technique to determine the 240Pu mass in the TRR spentfuels.

Regarding the plutonium isotopic composition, Muralidharet al. [2] utilized a high-resolution gamma-ray spectrometry todetermine the plutonium isotopic composition in their measure-ments of FBTR fresh fuel pins. Lee et al. [4] applied calculatedplutonium isotopic compositions in their determination of theplutonium content in TRR spent fuel rods using a spent fuelcoincidence counter. It’s hard to set up a high-resolution gamma-ray spectrometry underwater in the spent fuel pool. Therefore, theplutonium isotopic compositions in the TRR spent fuel rods in thisstudy were determined, too, by the fuel depletion calculation witha documented burnup history.

The Los Alamos National Laboratory (LANL) has been collabor-ating with INER on nuclear safeguard requirement for thestabilization program of TRR spent fuels since 2006. Lee et al.[4] modified the design of the original spent fuel coincidence

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Y.-F. Chen et al. / Nuclear Instruments and Methods in Physics Research A 619 (2010) 470–474 471

counter to fit the requirement of the TRR spent fuel canistermeasurement. The calibration factor of the detector system forcarrying out a 12-step scan measurement of the TRR spent fuelunderwater was determined by the measurement of a fresh TRRfuel rod dipping in a 55 gallon water tank and corrected by aMCNPX simulation. In order to conduct parallel measurements ofthe TRR spent fuels, we designed and constructed a new neutroncounting system with much higher detection efficiency andperformed the detector system calibration in a more rigorousway.

This paper presents the design of our neutron counting system,the procedures of detector calibration, the fuel burnup calcula-tion, and measured results of four TRR spent fuel rods. Themeasured results were compared with the facility declaredvalues. The difference of the comparison and possible causeswere finally analyzed.

2. Materials and methods

A single TRR fuel rod with a diameter of 3.454 cm and a lengthof 304.8 cm weights �54 kg. The range of the burnup of TRR spentfuel rods is from 700 to 1500 MWd/MTU. As a result, thedominant neutron source in the TRR metal spent fuel is thespontaneous fission of 240Pu besides 238U. The specific sponta-neous fission neutron source strengths of 238U and 240Pu are 13.6and 1024 n/s/g, respectively. For the TRR spent fuel with a burnupof 1500 MWd/MTU, 238U spontaneous fission neutrons contribute�14% to the total neutron source strength according to ourSAS2 H [5] calculation. The counts from 238U spontaneous fissionneutrons must be excluded. Since 238U mass remains essentiallyconstant in the TRR spent fuel with a burnup up to 1500 MWd/MTU (less than 0.15% decrease), the neutron counting rate from238U spontaneous fission neutrons can be easily subtracted andthe resultant neutron counting rate can be used to derive themass of 240Pu.

In our nondestructive fuel assay, the plutonium content in aTRR spent fuel rod was determined in two steps: at first, the 240Pumass in the spent fuel rod was derived from the scan measure-ment of a neutron counting system, then the plutonium contentwas determined by multiplying the 240Pu mass by the 240Pu-to-Pumass ratio, which was obtained from the fuel depletion calcula-tion based on a given brunup history.

A hollow-type waterproof SPent-fuel-Neutron-Counter (SPNC)system has been constructed to perform underwater scan of thespent fuel canisters. As shown in Fig. 1, the SPNC embedded withfour high-sensitivity He-3 proportional counters has the overall

Lead

He-3 Proportional Counter

70.5

cm

51.0 cm

15.0 cm

Fig. 1. Cross-sectional and top view of the SPNC system.

dimensions with a diameter of 51.0 cm and a height of 71.5 cm.The proportional counter filled with 4 atm He-3 gas has adiameter of 2.54 cm and its active length is 45.72 cm. Theposition of the neutron counters in the SPNC and the thicknessof the detector lead sleeves for gamma-ray shielding wereoptimized by using MCNP [6] Monte Carlo simulations in termsof achieving the highest efficiency and avoiding gamma-ray pile-up effect. An operation high-voltage of 1710 volts at the countingplateau was determined using a standard Am-Be neutron sourceand also demonstrated free from gamma-ray pile-up effect underan exposure rate of �10�2 Gy/h, which is roughly equivalent tothe gamma-ray field caused by the TRR spent fuel with themaximal burnup of 1500 MWd/MTU.

The aluminum TRR spent fuel canister was loaded into astainless steel transport canister with a length of 360 cm and thentransferred to the scanning measurement frame installed in thespent fuel storage pool and placed horizontally along the centralaxis of the SPNC, which was mounted on a translation stage andcould be driven to move along the rail on the frame. A 12-stepscanning scheme was designed to measure the neutron countingrates along the canister axis with each step of 30 cm, which issmaller than the active length of the He-3 tube. Fig. 2 shows theschematic diagram of the positions of SPNC at each step relativeto the spent fuel canister. The total neutron counting rate of the12-step scan measurement, T, can be related to the total neutronsource strength, S, of the spent fuel rod by the following equation:

T ¼ eMLS ð1Þ

where e is the absolute detection efficiency (counts/n) of the SPNCscan system. ML is the leakage multiplication (dimensionless) totake account of the contribution of induced fissions to the SPNCsystem from the fertile and fissile content in the fuel rod. Thecalibration factor of the SPNC scan system is defined as theproduct of absolute detector efficiency e and leakagemultiplication ML. The neutron source strength of a TRR freshfuel rod made of natural uranium metal is well known. Therefore,the calibration factor of the SPNC scan system can be derived fromthe resultant neutron counts of the fresh fuel measurement. Weexperimentally determined the calibration factor of the SPNCsystem by performing a 12-step scan measurement of thetransport canister containing a TRR fresh fuel rod in a watertrench with dimensions 80 cm wide, 80 cm high, and 500 cm longas shown in Fig. 3.

To determine the total plutonium content in the TRR spentfuel, we need to know the plutonium isotopic composition that isstrongly dependent on fuel burnup history. The SAS2H controlmodule in the SCALE 5.1 code system [7] is originally designed forthe LWR fuel assemblies to generate problem-dependent cross-section data for the following ORIGEN-S code to perform nuclidegeneration and depletion calculation. A modified SAS2H sequencewas adapted to calculate the nuclide inventory of a TRR spent fuelrod. Fig. 4 shows the calculation model with seven regions in aTRR fuel rod. The requested 240Pu-to-Pu mass ratios in a TRR spentfuel rod can then be determined according to its documentedburnup history.

360.0 cm

1 2 3 4 5 6 7 8 9 10 11 12

Step

TRR Spent Fuel RodTransport Canister

Spent Fuel Canister

Fig. 2. Schematic diagram of the 12-step scan measurement of the SPNC system.

ARTICLE IN PRESS

Fuel

Pitch (19.981 cm)

3.454 cm

3.86 cm

4.267 cm

4.474 cm

5.72 cm

5.809 cm

Coolant

Fuel

Cladding

Flow Tube

Air Gap

Calandria Tube

Moderator

A B

A

B

Moderator

Not to Scale

Fig. 3. SAS2H calculational model for a TRR fuel rod.

Fig. 4. Experimental configuration of SPNC calibration with a TRR fresh fuel in a

water trench.

1 2 3 4 5 6 7 8 9 10 11 12Position

0

1

2

3

4

5

6

7

Cou

ntin

g R

ate

(1/s

ec)

Exp.Cal.

Fig. 5. Comparison of the SPNC scan profile of a TRR fresh fuel between

measurement and calculation.

Y.-F. Chen et al. / Nuclear Instruments and Methods in Physics Research A 619 (2010) 470–474472

3. Results and discussion

3.1. Calibration factor

Fig. 5 shows 12-step scan profile of a TRR fresh fuel measuredin the water trench and compared with the simulation result. Themeasured axial profile of neutron counting rate agrees very wellwith that predicted by the MCNP simulation. From the totalcounting rate of the 12-step measurements, and the total neutronsource strength, which is the product of the specific spontaneousfission neutron source strength of 238U and the uranium mass ofthe fuel rod, the calibration factor of the SPNC scan system wasdetermined to be 0.082570.001 counts/n.

A series of sensitivity studies on the calibration factor of theSPNC scan system have been carried out by using MCNPsimulations. The dimensions of the water trench were tested atfirst. It was found that the calibration factor remains essentiallyconstant with the increase of the water trench dimensions, whichimplies the measured calibration factor can be adopted in thescan measurements of the spent fuels in the much deeper andwider spent fuel storage pool. The calibration factor was alsofound to be insensitive to the position of the spent fuel canisterwith respect to the SPNC axis. The effect of neutron sourcedistribution along the fuel rod axis has been investigated foruniform and cosine distributions and for a distribution with twoisolated bumps at the two ends of the rod. It was found that the

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240 P

u-to

-Pu

Mas

sR a

tio0.075

0.070

0.065

0.060

0.055

0.050

0.045

0.040

0.035

0.030600 800 1000 1200 1400

Burnup (MWd/MTU)1600

Fig. 6. Calculated 240Pu-to-Pu mass ratio in a TRR spent fuel as a function of fuel

burnup.

1 2 3 4 5 6 7 8 9 10 11 12Position

0

20

40

60

80

100

120

Cou

ntin

g R

ate

(cou

nts/

sec)

A (1213 MWd/MTU)B (1400 MWd/MTU)C (1249 MWd/MTU)D (1308 MWd/MTU)

Fig. 7. SPNC scan profiles of the four spent fuel canisters.

Table 1Measured results of the plutonium content in four TRR spent fuel rods compared

with facility declared values.

Rod Burnup

(MWd/MTU)

Total counting

rate (c/s)

Plutonium content:

measured/declared

A 1213 380 (0.21%)a 1.288 (0.35%)

B 1400 456 (0.19%) 1.259 (0.29%)

C 1249 613 (0.17%) 2.149 (0.23%)

D 1308 564 (0.17%) 1.786 (0.25%)

a Standard deviation.

Y.-F. Chen et al. / Nuclear Instruments and Methods in Physics Research A 619 (2010) 470–474 473

differences among them are very small, within a few percents. Theeffect of fissile content in the fuel rod on the leakage multi-plication ML was also studied and a negligible influence has beenfound for the fuel rods with a burnup up to 1500 MWd/MTU.

3.2. 240Pu-to-Pu ratio

The plutonium isotopic compositions in each TRR spent fuelrod were obtained by performing the corresponding SAS2Hcalculation. Fig. 6 shows the calculated 240Pu-to-Pu mass ratioin a TRR spent fuel rod as a function of fuel burnup from 700 to1500 MWd/MTU. Concerning the isotopic composition, the totalfuel burnup is the most important parameter among others, suchas the fuel temperature, the specific power, the burn and coolingcycles. However, due to the lack of detailed burnup history foreach TRR spent fuel rod, reasonable assumptions have to be madeto facilitate the calculations; however, they can be sources ofuncertainty on the final result. In our SAS2H calculations, the fueltemperature was assumed to be 700 K and the documentedspecific power in the range from 220 to 330 kW per fuel rod wasused directly. The power history of a TRR spent fuel was roughlyestimated from the documented time steps for fuel rod in coreand out of core. We simplified the power history in the calculationby assuming equally divided burn and cooling time. Severalsensitivity studies on the parameters of fuel temperature, specificpower and cycle histories show that the results in Fig. 6 may have�2% uncertainty due to the incomplete burnup history.

3.3. Spent fuel measurement

Four TRR spent fuel rods with burnups from �1200 to1400 MWd/MTU and cooling times from 30 to 35 years weremeasured by the SPNC scan system in the spent fuel storage pool.Each measurement took about 3 hours to complete the 12-stepscan. The statistic uncertainty of the total counting rate of eachmeasurement was less than 0.3%. Fig. 7 shows the SPNC scanprofiles of these four spent fuel rods along the transport canisteraxis. The scan profiles obviously indicate that all the spent fuelrods remain in good integrity. The plutonium contents in thesefour TRR spent fuel rods were then determined by subtractingthe contribution of 238U spontaneous fission neutrons from themeasured total neutron counting rates and applying themeasured calibration factor and calculated 240Pu-to-Pu mass

ratios based on the burnup history of each spent fuel rod. Table 1lists the total counting rates, burnups, and plutonium contentscompared with facility declared values of the four spent fuel rodsmeasured in this work. As shown in Table 1, the burnup of Rod Bis higher than Rod A as it is true for the total neutron countingrate. Nevertheless, the measured plutonium contents of Rods Aand B are higher than the facility declared values by 25–30%. Thepossible cause of this discrepancy may reasonably be attributed tothe uncertainty associated with the facility declared values, whichwere determined by outdated ORIGEN2 [8] calculations accordingto a burnup history that was simply generated from the averagecore power per rod and the burning durations of each spent fuelrod. Rod C has almost the same burnup as that of Rod A, however,its total neutron counting rate is about 61% higher than that ofRod A and this, in turn, causes the measured plutonium contentabout two times higher than the facility declared value. Theburnup of Rod D is lower than that of Rod B, however, Rod D has atotal neutron counting rate about 24% higher than that of Rod Band hence a measured plutonium content �1.8 times higher thanthe facility declared value. The TRR spent fuel canisters have beenstored in the spent fuel storage pool for more than 30 years. Theuranium metal in the canister may possibly interact withmoisture and become uranium oxide. Because of the presence ofoxygen (a,n) neutrons will be produced by alpha-emitters ofactinides in the fuel rod. Therefore, it is suspected that theabnormally high total neutron counting rate of Rods C and D maybe due to the oxidation of the fuel rod. According to ourcalculation using SAS2H and MCNP, if all the uranium metal ofa TRR spent fuel rod with a burnup of 1300 MWd/MTU hasbecome uranium dioxide, the induced (a,n) neutrons will causethe total neutron counting rate of the SPNC scan measurement toincrease by 71%. The result of this calculation supports our

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Y.-F. Chen et al. / Nuclear Instruments and Methods in Physics Research A 619 (2010) 470–474474

suspicion that Rods C and D have suffered different extents ofoxidation.

4. Summary

In this study, we developed a nondestructive plutonium assayfor the TRR spent fuel stabilization program in the INER before thestabilization process. The high-efficiency SPNC system wasconstructed to perform underwater scan of the TRR spent fuelcanisters. The calibration factor of the SPNC system wasdetermined from a full-scale scan measurement of a TRR freshfuel rod in a water trench. Monte Carlo simulations have beenused extensively to optimize the design of the SPNC and toperform sensitivity studies for relevant parameters of concern. Inaddition to the neutron counting system, the 240Pu-to-Pu massratios as a function of burnup for the TRR spent fuels have beenestimated using the modified SAS2H calculations according to thedocumented burnup histories. The plutonium content in the spentfuel rod was then determined from the measured total neutroncounting rate and the 240Pu-to-Pu mass ratio. The assay methodwas then applied to determinate the plutonium contents in fourTRR spent fuel canisters. The argument between measured anddeclared plutonium in two spent fuel rods was satisfactory within30% taking into account the large uncertainty associated with thedeclared values. The other two rods showed abnormally high totalneutron counting rates and this, in turn, caused measuredplutonium contents to be around two times higher than the

declared values. This unexpected large difference was suspectedto be due to (a,n) neutrons originating from the rod oxidation.

Acknowledgement

We would like to express our thanks to the staff of HealthPhysics and Engineering Divisions of the INER for the help inconducting measurements. This work was funded by the NationalScience Council and Atomic Energy Council of Taiwan, Republic ofChina, under the contract numbers NSC 96-2221-E-007-058-MY3and NSC 97-2623-7-007-001-NU.

Reference

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[2] S. Muralidhar, R. Tripathi, B.S. Tomar, G.K. Gubbi, S.P. Dange, S. Majumdar, S.B.Manohar, Nucl. Instr. and Meth. A 511 (2003) 437.

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[5] O.W. Hermann, C.V. Parks, SAS2H: A Coupled One-Dimensional Depletion andShielding Analysis Module, ORNL/TM-2005/39, Vol. I, Book 3, Section S2, 2000.

[6] J.F. Briesmeister, Ed., MCNP: A General Purpose Monte Carlo N-ParticleTransport Code (version 4C), LA-13709-M, 2000.

[7] SCALE: A Modular Code System for Performing Standardized ComputerAnalyses for Licensing Evaluation (Version 5.1), ORNL/TM-2005/39, 2006.

[8] A.G. Croff, Nucl. Technol. 62 (1983) 335.