Comparison of Shuffler and Differential Die-Away Technique Instruments for the Assay of Fissile Materials in 55-Gallon Waste Drums

We compare the features of a die-away technique (DDT) instrument for the assay of 208- L (55-gal.) waste drums and the experimental results obtained using drums with 20 different simulated waste matrices. Active assays of uranium and plutonium were made, along with passive assays of plutonium. The major potential sources of inaccuracy for most wastes are self-shielding and nonuniform distribution of the fissile material throughout a drum's volume. We examined the distribution problem by placing small samples of plutonium and uranium at 15 representative locations within each test matrix. The combined responses from all these locations simulated the case of a uniform distribution. Inaccuracies can grow as the density of moderator or absorber in a matrix is increased, so a wide range of absorber and moderator densities was used in the test drums. Self-shielding and effects due to high-neutron backgrounds were also examined. The instruments' minimum detectable masses for uniform distributions of uranium and plutonium were calculated for the various matrices. 252Cf shuffler and a differential