Year
1994
Abstract
Determination of the isotopic distribution of plutonium and the americium concentration is required for the assay of nuclear material by calorimetry or neutron coincidence counting. Isotopic information is used in calorimetric assay to compute the effective specific power from the measured isotopic fractions and the known specific power of each isotope. The effective specific power is combined with the heat measurement to obtain the mass of plutonium in the assayed nuclear material. The response of neutron coincidence counters is determined by the 240Pu isotopic fraction with contributions from the other even plutonium isotopes. The effect of the 240Pu isotopic fraction and the other neutron contributing isotopes are combined as 24CPu effective. This is used to calculate the mass of nuclear material from the neutron counting data in a manner analogous to the effective specific power in calorimetry.' A major source of uncertainty for both calorimetric assay and neutron coincidence assay often lies in the determination of the plutonium isotopic distribution as identified by gamma-ray spectroscopy. Thus, the selection of the appropriate isotopic distribution code is of paramount importance to good calorimetric assay and neutron coincidence assay. Three gamma-ray isotopic distribution codes, FRAM (Version 2.1 ),2 MGA (Version 1.03),M and TRIFfD (Version 5.89.1),5 have been compared at the Los Alamos National Laboratoy (LANL) Plutonium Facility under carefully controlled conditions of similar count rates, count times, and 240Pu isotopic fraction.