INMM 2012, Orlando, FL, 15-19 July, 2012
A New Versatile Safeguards Tool for Verification of PWR Spent Fuel
Y. Ham, S. Sitaraman, P. Kerr, R. Swan and A. Wong
Lawrence Livermore National Laboratory, Livermore, CA
E-mail address of main author: [email protected]
ABSTRACT
A new versatile safeguards tool, Partial Defect Tester (PDET), that can verify various characteristics as
well as the integrity of Pressurized Water Reactor (PWR) spent fuel assemblies (SFAs), has been
developed so that desired safeguards verification can be achieved in situations where conventional
safeguards techniques failed in the past. The major feature of PDET is its capability to detect pin
diversion from PWR spent fuel without requiring any data from facility operators and without
movement of SFAs from their storage positions. Pin diversion detection is achieved by analyzing a
unique set of signatures in PWR SFA based on normalized neutron, gamma, and gamma-to-neutron ratio
signals that are principally dependent on the geometric layout of the guide tubes present in the assembly
where the signal measurements are taken. Diversion of as low as a few percent missing fuel can be
detected using this tool. In addition to the pin diversion detection capability, the data from PDET can be
used to ensure that operator declared data are free from any falsification or fabrication. This article
describes the concept of pin diversion detection, and the methodology for confirming operation
parameters such as burn-up, cooling time and initial enrichment, and how this information is used for
shipper/receiver verification of PWR spent fuel. In addition, the article will also describe the design and
functioning of the PDET hardware.
INTRODUCTION
The issue of potential diversion of spent fuel pins from spent fuel assembly has remained unsolved over
a few decades in the safeguards community. The danger of potential diversion nowadays is even much
higher as fuel vendors are increasingly expected to perform hundreds of fuel repair or reconstitutions
annually around the world, thus, an evolved know-how and sophisticated tools exist to disassemble
irradiated fuel assemblies and reconstitute damaged pins with dummy stainless steel or other type of
rods. Many efforts have been made in the past two decades to develop a technology to identify a
possible diversion of pins and to determine whether some pins are missing or replaced with dummy or
fresh fuel pins [1-3]. Recently, a five-year spent fuel NDA research effort as a part of NGSI (Next
Generation Safeguards Initiative) was initiated in March 2009 to obtain a capability to directly measure
the Pu content in spent nuclear fuel assemblies as well as to detect the diversion of pins from an
assembly [4]. Although the effort is still on-going, none of the 14 exhaustive NDA techniques that were
identified in the effort can independently detect diversion of spent fuel pins from an assembly [5]. To
date, there is no cost-effective instrument that has the capability of performing routine partial defect
testing on spent fuel assemblies.
Lawrence Livermore National Laboratory has successfully developed a novel methodology and
validated the methodology with experiments in detecting removal of spent fuel rods from PWR spent
fuel assemblies without any fuel movement and operator provided information [6-9]. The novel
methodology uses thermal neutron and gamma information obtained by tiny neutron and gamma
detectors inside the guide tubes of PWR spent fuel assemblies. The data obtained in such a manner
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INMM 2012, Orlando, FL, 15-19 July, 2012
provide spatial distribution of neutron and gamma fluxes within a spent fuel assembly that create three
unique profiles or base signatures, normalized neutron, gamma and gamma-to-neutron ratio, when the
data are plotted against detector positions. The base signature of the gamma-to-neutron ratio, in
particular, is relatively invariant to the characteristic variations of spent fuel assemblies such as initial
fuel enrichment, cooling time, and burn-up. Diversion of spent fuel is easily recognized when the
measured data deviate from the base signatures. The methodology has been proven to be effective to
detect as few as 10% missing pins in an assembly, without any fuel movement and operator provided
information.
DESCRIPTION OF THE PDET SYSTEM
The PDET contains an array of 12 fission chambers and 12 ion chambers that are simultaneously to be
inserted into the guide tubes of a 17x17 PWR spent fuel assembly in spent fuel pool. A water-tight
bundle of 24 stainless-steel tubes, which house the set of detectors, is lowered directly into the spent-fuel
assembly only by gravity without requiring a sophisticated positioning instrument. Movement or
disassembly of the spent fuel unit is not required for the insertion. The detector power and signal cables
are routed to a 24-channel data-acquisition system via a water-tight multi-pin feed-through and cable.
The detectors are lowered to a point where the active area of the detectors is 1.25m below the top of the
assembly. This ensures the neutron and gamma-ray flux profiles are largely the same as at the 4m center
of the assembly, and allows for a PDET system size that is easily transportable. The gamma and neutron
data are recorded and analyzed in the data-acquisition system software.
The PDET system consists of the following components:
1.) Support structure for lowering detectors into spent-fuel assembly
2.) Fission chamber neutron detectors and ion chamber gamma-ray detectors
3.) Watertight cabling
4.) Data-acquisition system
5.) Software
Figure 1 PDET Header with SHV
patch panel
Figure 1 PDET Header with one end
of the underwater cable attached
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Figure 2 PDET
Assembly
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PDET Data Acquisition (DAQ) System
The PDET DAQ system is to record the count rate of each of the 12 neutron and 12 gamma-ray detector
individually and to display and store the results for analysis and if necessasry, for comparison to
previous or template measurements. The unique detector configuration and need for a compact system
suitable for field inspector use required the development of custom hardware and software. There are no
COTS systems for this particular application. Two independent data acquisition systems were built at
LLNL: one for portable, and another primarlily for laboratory use. We will limit our description to the
portable one.
The first PDET DAQ system built at LLNL uses pulse processing hardware from the Ortec Fission
Meter and requires 12 channels of fission-chamber detectors to monitor neutrons, and 12 channels of
ion-chamber detectors to monitor gamma rays. The system includes 12 charge-sensitive preamplifiers
for the fission chambers, custom designed current-sensing preamp boards, as well as two “Five-Mode
Counter” multi-channel counting boards. The fission chamber neutron detectors are operated in pulse
mode where individual neutron events can be recorded, and require charge-sensitive preamplifiers.
Figure 4 shows a photograph of the eight-channel Ortec Fission Meter preamp board. One full board and
one half board were used to obtain 12 channels of fission chamber preamps. Gamma detectors, the CPMU-D1 ion chambers from Technical Associates, are operated in current mode, and requires a current
sensing preamplifier capable of sensing currents on the order of tens to hundreds of pA. Because no
commercial preamplifier of this type was available, a custom four-channel board was designed and built.
Figure 5 shows a photograph of the completed four-channel current preamplifier with some components
labled. The SHV connectors are not shown. The output of this preamplifier is a TTL pulse frequency
proportional to a voltage derived after a current-to-voltage conversion. The bias for the 12 ion chambers
is via individual 75V DC button battery stacks.
The PDET DAQ system is controlled with a custom GUI written in C-sharp code. The GUI window is
shown in Figure 7. This figure shows an example of the expected signature in white and the measured
signature in red providing a visual means of detecting a potential diversion. The user can control the
start, stop, and clear of the counter. The run name, comments, and preset time can be set by the user, and
the data can be written out manually or automatically at preset intervals.The GUI is setup to perform two
measurements of the same preset time. A message in between the two measurements instructs the user to
rotate the PDET assembly prior to the second measurement. At the end of the run, the data for all 24
positions of both neutron and gamma-ray detectors are written to a single file.
Figure 4 Eight-channel charge-sensitive preamplifier board for data acquisition of neutron signals.
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Figure 5 LLNL four-channel ion chamber preamplifier board for data acquisition of gamma signals.
Figure 6 Photograph of the completed Fission Meter based PDET data-acquisition system
Figure 7 GUI with Plot of Normalized Neutron Counts displayed
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PDET AS A TOOL FOR SHIPPER/RECEIVER DIFFERENCE VERIFICATION
The Shipper/Receiver Difference (SRD) measurement on spent fuel assemblies has been another
technical challenge for many years for reprocessing facilities. As the shipper's values are based on the
calculated fissile contents of spent-fuel assemblies at the reactor, whereas receiver's values are based on
measurements at the input accountability tank, there exists uncertainty or discrepancy of the reactor
operator calculations of plutonium content up to 5-10 percent. This creates a situation that SRD can be
up to 5-10% even if no material has been lost or diverted. Of course, the discrepancy depends upon the
numbers that operators declare. Verification of shipper’s data such as contents of plutonium or uranium
in a spent fuel assembly is practically impossible as the verification requires verification of fuel
manufacturer’s data, the knowledge of the detailed reactor power operating history as well as the
shipper's isotopic generation and depletion calculation methods. In the case of large discrepancy
between the declaration and measurement data, identifying the source of the discrepancy is nearly
impossible as the solutions in the input accountability tank are obtained from a variety of assemblies
with various burn-ups and cooling times which can come from different shippers. A potential
proliferator who understands this process can exploit this weakness and divert a substantial number of
spent fuel pins prior to the assembly’s arrival at the reprocessing plant. The period of opportunities for
pin diversion could be up to many decades during their storage in the spent fuel pool.
The problem of the SRD can be addressed by applying PDET partial verification methodology as well as
the use of PDET generated data to confirm facility operator’s declaration of burn-ups, cooling time and
initial enrichment. An earlier paper reported that there is a unique relationship between burnup and
neutron count similar to the relationship between burnup and neutron count generated by Fork Detector
as well as a unique relationship between total gamma dose and burnup [10].
The neutron count relationship with burnup can be expressed as
where N is neutron count that is Cm244 cooling time corrected, bu is fuel burnup, c and d are
constants. This relationship is initial enrichment sensitive.
The gamma dose relationship with burnup can be expressed as
where G is gamma dose which is Cs137 cooling time corrected, bu is fuel burnup, a and b are constants.
It is assumed that spent fuel assemblies are at least several years old such that gamma dose is principally
Cs137 dominated.
As PDET data have to satisfy two independent equations above, it would be nearly impossible for
would-be-proliferators to cheat, such as switching spent fuel assemblies by altering both cooling time
and burnup data.
Although the PDET measurement can address SRD issue by confirming that there has been no partial
defect diversion or cheating on the measured SFAs, the methodology does not provide explicit values of
uranium or Pu contents of the assemblies at least for now. How the SRD verification methodology is
utilized in a real environment by IAEA, if the methodology adopted by IAEA, is still to be explored.
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SUMMARY
The PDET was developed to detect diversion of spent fuel pins from pressurized water reactor spent fuel
assemblies. The methodology uses the gamma and neutron flux information inside the guide tube holes
measured by PDET and analyzing the pattern of those data. These data can also be used to confirm the
consistency of the facility operator’s declaration such as burn-up, cooling time and initial enrichment.
Having the features of in-situ measurements and ability to confirm facility operator’s declaration, the
PDET can be a powerful and yet a practical tool to ensure integrity of spent fuel that it encounters.
ACKNOWLEDGEMENT
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore
National Laboratory under Contract DE-AC52-07NA27344. The authors wish to thank the NNSA
Office of Research and Development NA22 and Next Generation Safeguards Initiative Program for their
generous support of this project.
REFERENCES
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