Presented at the 41st Annual meeting of the INMM, New Orleans, LA, July 16-20, 2000

ABSTRACT

To meet MPC&A contract terms and programmatic transparency objectives within the Cooperative Threat Reduction Treaty between the US and Russia, it is important to measure the material intended for down blending for its total uranium concentration and for its ²³⁵U enrichment. A two-measurement technique provides the best combination of overall measurement accuracy, flexibility, ease of use, ready commercial availability, and economy of operation. 

Gamma spectrometric systems that can measure the ²³⁵U enrichment are readily available commercially. Active neutron counters for total uranium determinations are also readily available, but generally not in the size required for this program objective. Although there is no commercial off-the-shelf version of such a counter, neutron multiplicity counters are functionally very modular and it is common practice for the commercial companies that build them to produce them in an "as needed" form-factor (within sensitivity parameters). The counter that was found to meet the necessary design criteria is based on a high efficiency passive neutron counter which has been modified to include an excitation source, similar to the one in the Active Well Coincidence Counter (AWCC).

In this paper, we will report on the design and test results of a large scale Hexagonal High Efficiency Multiplicity Counter that was recently built for a mobile nuclear material characterization ISO container for deployment in Russia. 

INTRODUCTION

Since 1994, the Department of Energy has undertaken the mission of upgrading the safeguards and security of Russian nuclear facilities under the Material Protection, Control and Accounting (MPC&A) program. In early 1999, the MPC&A program launched the Material Conversion and Consolidation (MCC) Project initiative with Russia. The mission of MCC Project was to assist in consolidating special nuclear material (SNM) to fewer locations, and to down blend the material with natural or depleted uranium to reduce its attractiveness as a diversion target. However, the process of moving the material from its point of origin to the down blending facility is currently vulnerable to undetected diversion. To eliminate this vulnerability, it is necessary that the material be assayed immediately prior to shipment from the point of origin, and then again immediately upon arrival at the down blending facility. Another reason for such a set of measurements is to assure the shipper facility that its material and its packaging meets the acceptance criteria of the receiver facility, which have been set in the MPC&A program contract terms and programmatic transparency objectives. 

Such measurements, however, present significant procedural challenges arising from calibration, operation, and sensitivity variations between the assay instruments at each end of the transport cycle. One method to overcome these difficulties is to deploy a self-contained instrument van that is transported with the material shipment and is used to make the assays at both ends of the transport cycle. Because the same instruments are used at both ends, the calibration and sensitivity variances are canceled; because the operators of the instruments are different, the MPC&A program can have a high degree of confidence in the validity of the measurement results. 

In order to determine the uranium weight percentage and its enrichment as required it is best to use two measuring techniques. A single measurement technique usually cannot do it alone with sufficient or desired accuracy. A gamma spectroscopic measurement is required to establish the uranium enrichment or the plutonium isotopics. However, a gamma spectrometric technique can not typically establish the weight percentage. Uranium is such a heavy material that the self-attenuation of the characteristic gamma rays prevents a gamma technique from seeing the entire sample. A neutron technique will obtain a measurement result that is directly proportional to an effective mass, which however, is dependent on the uranium enrichment. A neutron technique alone is sufficient if the uranium enrichment is known.

The most common approach, used extensively in various safeguards and waste measurements ^1,2,3,4 , and also the most sensible approach for this project, is not to assume that the uranium enrichment is known, but to use the two techniques in combination to measure the uranium mass in the sample. Combining the mass information with a weight obtained from a simple scale allows the instrument to report weight percentage. 

In order to meet the delivery schedule, it was also imperative that both the gamma and neutron equipment was chosen in such a way that it was either available from inventory or the components were available from inventory. For the gamma part of the system, we decided to use a simplified Segmented Gamma System (SGS) that only uses the MGAU⁵ software for determining the uranium enrichment. The gamma system was also equipped with a scale to establish the weight of the item being measured. If necessary, the system can be converted to a traditional SGS for measuring canisters and drums^6,7 with the addition of a lift mechanism and, if desired, a transmission source and the appropriate software. 

There are a number of neutron counting techniques that can be employed for verification of uranium material. Most neutron counters used for these types of applications use ³He proportional counters as the detector element, although other detector technologies also exist. Both passive and active neutron measurement techniques are used. In the passive mode, neutrons from spontaneously fissioning isotopes, such as ²⁴⁰Pu and ²⁴⁴Cm, are measured. In the active mode, an external source of neutrons (from a neutron generator or isotopic source) first irradiates the sample. Then the neutron signal from fissionable isotopes, such as ²³⁹Pu or ²³⁵U, is measured. Both modes are in common usage in a variety of applications. 

In this case, active neutron analysis is required for ²³⁵U determination. A system favored by the IAEA for such measurements is the Active Well Coincidence Counter (AWCC)⁸. The AWCC uses small Americium-Lithium (Am(Li)) sources to interrogate the uranium material. Typically, AWCCs can also be operated in passive mode by removing the interrogating neutron source. However, standard AWCCs accommodate only smaller cans (typically less than 25 cm in diameter) and were not suited for this application as such. There are published designs of large multiplicity counters^9,10 that have been modified to include an excitation source, similar to the AWCC. Although there was no commercial off-the-shelf version of such a counter, neutron multiplicity counters are functionally very modular and it is common practice for the suppliers of neutron counters to produce them in an "as needed" form-factor (within sensitivity parameters). Therefore, we went ahead and designed a large multiplicity counter for this project using standard components and design techniques. This counter was based upon an existing 3-ring 55% drum multiplicity counter recently installed in the United States. The characteristics and performance of the resulting counter are described below.