Operator Experience Measuring MOX Scrap at PNC Using the Plutonium Scrap Multiplicity Counter

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Abstract

The Plutonium Scrap Multiplicity Counter (PSMC) is a high efficiency neutron counter designed for measuring impure samples such as mixed-oxide (MOX) scrap materials. The PSMC is able to accurately measure the plutonium content by using the enhanced multiplicity technique. PNC and Canberra installed two PSMCs at PNC's MOX production facility. Calibration tests were performed on these systems by PNC, Canberra and Los Alamos National Laboratory using MOX standards. The materials assayed by the PSMC, results of the calibration tests, and operating experience are reported.

1. Introduction

Neutron counters have been used by safeguard inspectors worldwide for over 20 years to accurately perform Non-Destructive Assay (NDA) of pure samples of plutonium and uranium. These systems have relied on the well-documented neutron coincidence counting technique. Assay of impure samples such as mixed-oxide (MOX) scrap materials present a unique problem. They cannot be accurately measured using the standard coincidence technique because the uncertainties in the chemical form of the plutonium and impurities lead to unpredictable levels of (a,n) neutron emission. For these samples, it is not possible to accurately predict the alpha value used in the multiplication correction. Instead, the multiplicity analysis technique is required because it allows measurement of the alpha value in addition to the mass of ²⁴⁰Pu-effective and multiplication.

Manufacturing processes of nuclear fuel have produced, and will continue to produce a variety of impure materials. PNC chose to apply the enhanced multiplicity technique to provide accurate plutonium masses in these impure samples for their Material Control and Accountancy (MC&A) program. Two Plutonium Scrap Multiplicity Counters (PSMCs) were installed in July 1995 in the PNC MOX production facilities and calibrated using MOX standards. Measurements have been made on impure materials with varying impurities. These measurements have provided the operator experience necessary to determine the applicability of the multiplicity analysis technique for various sample types. The PNC MOX facility, installation of the PSMC, calibration standards and tests, and operating experience measuring MOX scrap material with low and high alpha values in the PSMC are reported.

2. Facility and Waste Description

PNC has two main facilities for MOX fuel production: the PNC Plutonium Fuel Fabrication Facility (PPFF), which includes PFDF and PFFF, and the PNC Plutonium Fuel Production Facility (PFPF). PFFF and PFPF are shown in Figures 1 and 2, respectively. PPFF began producing fuel in January 1972 and has been supplying MOX fuel for the DCA reactor, prototype ATR "FUGEN", and the experimental FBR "JOYO". Initial core fuel assemblies were completed for "JOYO" in 1975 and for "FUGEN" in 1978. Production for "FUGEN" fuel continues today. PFPF was constructed in October 1987 to demonstrate the mass production technology of MOX fuel. PFPF started producing "JOYO" MOX fuel in October 1988. The fabrication of the initial fuel for the FBR "MONJU" was completed in January 1994.

Figure 1.
PFFF MOX fuel production facility.

Figure 2.
PFPF MOX fuel production facility.

Scrap generated from MOX fuel production at PPFF and PFPF can be classified into two waste types: clean scrap that can be recycled as feed materials by dry recovery, and dirty scrap. Currently, dirty scrap cannot be recycled, but the construction of the wet recovery works at PNC will allow recycling in the future. Clean scrap is further divided into several categories, such as pellets and powder, which are separately homogenized by crushing or blending. Prior to use as feed material for MOX fuel, clean scrap is characterized by NDA and DA techniques. Coincidence counting using the known alpha technique is possible for clean scrap because it does not contain impurities. Dirty scrap includes powders stored in the gloveboxes, residues in the conversion process and grinding refuse. It is contaminated with a variety, and large amounts, of impurities. Therefore, normal NDA techniques cannot accurately characterize this waste material. The PSMCs were installed to accurately characterize the dirty scrap. This NDA value combined with DA (e.g. , Isotope Dilution Mass Spectrometry) provide holdup values that can be reported by PNC to the IAEA and the Japanese government for accountancy purposes.

3. PSMC System Description

The PSMC is a commercialized version of a multiplicity counter originally developed by Los Alamos National Laboratory for the IAEA to improve the accuracy of the measurement of MOX scrap material. The PSMC design is optimized for multiplicity counting including:

• High efficiency - In multiplicity counting, the triples rate is proportional to the efficiency cubed. The PSMC has 80 ³He proportional detectors to obtain an efficiency of ~55%.
• Uniform Response- Monte Carlo calculations were performed by LANL to optimize the counter design. As a result of these calculations, the PSMC has four rings of ³He proportional detectors arranged to flatten the energy response, and graphite end plugs to flatten the axial response by reflecting neutrons back into the ³He detection region.
• Small deadtime - The counter has 19 amplifiers to reduce the deadtime.

The PSMC use Canberra’s 2150 multiplicity electronics module, which is an extension of the JSR-12 Neutron Coincidence Counter Electronics (NCCE) module that has been authorized for use by the IAEA. Where the JSR-12 simply sums the R+A and the A gates, the 2150 multiplicity module records the number of times in 256 scalers that a given number of neutrons is detected. This multiplicity distribution is used to determine the singles (S), doubles (D) and triples (T) rates which can be used to solve for three unknowns: the ²⁴⁰Pu-effective mass, sample multiplication and the alpha value.¹ The alpha value is the ratio of the uncorrelated to spontaneous fission neutron events.

Two PSMCs were purchased by PNC and installed in PFFF and PFPF. Both systems are identical to the PSMC installed by LANL for the IAEA. The PSMC in PFPF as shown in Figure 3 is installed under a glovebox. Samples are loaded into the sample well which is positioned around the glovebox drywell. The PSMC in PFFF is a stand-alone system (see Figure 4).

Figure 3.
PSMC installed under glovebox in PFPF MOX facility.

Figure 4.
PSMC installed in PFFF MOX facility.

4. Calibration Procedure

Preliminary calibration parameters were measured at Canberra’s factory using a ²⁵²Cf neutron source. After installation, three standards were provided by PNC to determine adjustments to the calibration parameters for MOX, if required. All three standards listed below were measured for four assays - each assay was composed of 10 runs of 100 second each. In addition to these assays, overnight runs were made (400, 100 second runs) for all three standards. The overnight runs had very good precision, and were also used in the evaluation of the calibration parameters.

The efficiency and gate fractions can be adjusted to minimize the error and to account for minor differences in the response of the counter for plutonium and ²⁵²Cf. Evaluation of the measured data for MOX standards showed that only minor adjustment to the efficiency was required for MOX assays. The gate fractions used are those determined during the F.A.T. The adjusted efficiency and gate fractions are listed in Table 1.

Table 1.
Calibration Parameters based on Measurement of MOX Standards

Once the new parameters were determined, the standards were reanalyzed to verify the results. A summary of the reanalyzes is reported for overnight assays in Table 2. The measured mass agrees for all three standards within 1s.

Table 2.
Measured Plutonium Mass for MOX Standards with New Calibration Parameters

Later the PSMCs were recalibrated by the IAEA, LANL and PNC using additional standards not previously available during the initial installation and slightly modified multiplicity algorithms from LANL, to obtain a single set of calibration parameters for PSMCs at PNC. The recalibration also provided cross references for three PSMCs at PNC, including the IAEA PSMC-1. A list of the final calibration parameters are listed in Table 3. While the new calibration parameters yield results similar to the original calibration parameters they provide the ease of operation since only one calibration set is required for input into the LANL multiplicity software.

Table 3.
Final calibration parameters

5. Operating Experience

Coincidence vs Multiplicity Counting

During the initial installation, tests were performed to evaluate the improvement in the measurement accuracy using multiplicity analysis over coincidence counting for dirty scrap material. Tables 4 and 5 show a partial comparison of the results from a known alpha analysis technique used in standard coincidence counting and a multiplicity analysis technique used in the PSMC. Comparison of the last column in the two tables indicates that the multiplicity analysis provides a clear improvement in the accuracy of the analysis compared to the standard known alpha analysis for dirty scrap material. A more complete comparison of the two techniques has been reported elsewhere¹.

Table 4.
Results of known alpha analysis for MOX scrap material.

Table 5.
Results for multiplicity analysis for MOX scrap material.

Effect of Moisture Content

Three standards were also provided by PNC during the initial installation and calibration, with different amounts of H₂0 to test the effect of moderating material in the sample. The results of the reanalyzes with the adjusted calibration parameters are summarized in Table 6. The results are within 1s of the declared masses with updated isotopics. Assay results demonstrated that concentrations of H₂O up to 1.0 wt% gave no observable differences in the plutonium mass for the multiplicity analysis.

Table 6. Measured Plutonium Mass for MOX Standards containing H₂O

High Alpha Scrap Material

During the recalibration, performance tests were made to determine the applicability of multiplicity counting for MOX scrap material with high alpha values. The value of alpha is impacted by the sample isotopics and chemical composition of the plutonium bearing elements within the sample. For fresh plutonium oxides the value of alpha is generally less than 1. As the ²⁴¹Am content increases or if the material includes nitrates or fluorides the alpha value can exceed 100. With large alpha values the count rates from the neutron counter can easily exceed 1000 kcps. When this happens the error in the triples rates increases and leads to larger errors in the calculated values of mass and alpha.

Test data are listed in Table 7 for PSMC-1, PSMC-a and PSMC-b. PSMC-1 is the IAEA’s counter. PSMC-a and PSMC-b were installed by PNC for dual use accountancy measurements. The samples labeled small are grab samples pulled from the bulk or large samples. After characterization in the PSMC, these grab samples were sent to Siebersdorf in Austria for analysis by the IAEA.

Several conclusions can be drawn from these data. First, there is good agreement in the measured values for the PSMCs. Large - B0115 gives a ²⁴⁰Pu-effective mass of 83.5 g and 83.4 g for PSMC-1 and PSMC-a, respectively. Second, tests varying the assay time indicate that the measurement accuracy is improved for long count times due to improvements in the precision of the triples rate. For example, Large - 6815 gives a ²⁴⁰Pu-effective mass of 38.3 g and 43 g for 600 second and a 10800 second count times, respectively. This test suggests that samples with large alpha values that give poor results in the PSMC in 600 second count times can be re-assayed for longer count times to improve the accuracy of the multiplicity analysis. This measurement also demonstrated that the multiplicity electronics and analysis can provide useful results for alpha values in excess of 10. Similar improvements for longer count times are evident for PSMC-b.

Table 7.
PSMC performance data for impure plutonium samples with high alpha values.

Conclusions

The PSMC has provided significant improvements in assay results over standard coincidence counting for dirty scrap MOX material for input into PNC’s MC&A program. Operator experience has demonstrated that even dirty scrap with high alpha values can be accurately assayed with the PSMC

References

1. H.O. Menlove, et. al., "Plutonium Scrap Multiplicity Counter Operation Manual", Los Alamos National Laboratory Report LA-12479-M, January 1993.

2. D. Davidson and R. McElroy, "Comparison of Neutron Coincidence and Multiplicity Counting Techniques for Safeguards", Proceedings of the 16th Annual Meeting of INMM Japan Chapter, December 7-8, 1995, Tokyo, page 163.

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