Lawrence Livermore National Laboratory (LLNL) has been performing accountability measurements of plutonium (Pu) -bearing items with the 30-gallon drum neutron multiplicity counter (NMC) since August 1998. A previous paper focused on the LLNL experience with Pu-bearing oxide and metal items. This paper expands on the LLNL experience with Pu-bearing salts containing low masses of Pu. All Pu-bearing salts used in this study were measured using calorimetry and gamma isotopic analyses (Cal/Iso) as well as the 30-gallon drum NMC. The Cal/Iso values were treated as being the true measure of Pu content because of the inherent high accuracy of the Cal/Iso technique, even at low masses of Pu, when measured over a sufficient period of time. Unfortunately, the long time period required to achieve high accuracy from Cal/Iso can impact other required accountability measurements. The 30-gallon drum NMC is a much quicker system for making accountability measurements of a Pu-bearing salt and might be a desirable tradeoff. The accuracy of 30-gallon drum NMC measurements of Pu-bearing salts, relative to that of Cal/Iso, is presented in relation to the mass range and alpha associated with each item. Conclusions drawn from the use of the 30-gallon drum NMC for accountability measurements of salts are also included.

In the past several years, several facilities have identified a need for a large multiplicity counter to support safeguards of excess weapons materials and the measurement control and accountability of large, unusual samples. The authors have designed and fabricated two large thermal neutron multiplicity counters to meet this need at two DOE facilities. The first of these counters was built for Rocky Flats Environmental Test Site for use in the initial inventory inspection of excess weapons plutonium offered to International Atomic Energy Agency safeguards. The second counter was built for the Lawrence Livermore National Laboratory (LLNL) to support their material control and accountability program. For the LLNL version of the counter, a removable, fast-neutron interrogation assembly was added for the measurement of large uranium samples. In the passive mode these counters can accommodate samples in containers as large as a 30-gal. drum. This paper reports on the measured performance of these two counters and the data obtained with them.

The pyrochemical multiplicity counter designed and built at Los Alamos has been undergoing tests and evaluation at Lawrence Livermore National Laboratory (LLNL). Measurements have been performed using a variety of plutonium oxide and metal materials. The pyrochemical multiplicity counter uses the information contained in the higher moments of the neutron multiplicity distribution to deduce the three unknowns in the assay problem: 24°Pu-effective mass, ([alpha], n) neutron rate, and self-multiplication. This is an improvement over conventional neutron coincidence counting, which must rely on some estimate of the ([alpha], n) neutron rate or self-multiplication to deduce an assay result. Such conventional techniques are generally unsatisfactory for impure materials for which these quantities are unknown. We present the assay results obtained with the pyro-chemical multiplicity counter and discuss the procedures necessary to produce good assay results. Using these procedures, we have obtained assay accuracies of 1%--2% for oxide materials in 1/2 hour measurement times. We also compare these results to those that would have been obtained using conventional neutron assay techniques and discuss the correlations we have observed between assay results and the ratio of total neutron counts in the different rings of the pyrochemical counter.