Neutron Self Multiplication Effects in PDP Items in the Context of Coincidence Counting - 10496

Recently two independent but complementary studies concerning self-multiplication in waste have appeared. In the first by Croft et al a general theoretical approach was presented from which the behaviour could be formulated given knowledge or assumptions about the Pu contaminated waste form. In the second by Stanfield et al an experimental approach was followed based on measurements on a set of Performance Demonstration Plan (PDP) reference items. In the present work we revisit the first principles treatment inserting the PDP characteristics to illustrate how the two approaches are related. Because the PDP items have fixed dimensions and by design are weakly multiplying the first order theoretical approach cast in terms of the Pu density (proportional to mass in this special case) is sufficient to understand the experimental trend. The assay of unknown items must be performed in a carefully prescribed way to ensure quality results. The understanding and treatment of uncertainty in the multiplication is somewhat subjective however since the data needed to interpret an individual drum or waste stream is incomplete or lacking. A general structured approach and understanding can suffice to bound the problem and fortunately self multiplication is typically a minor contributor to the overall uncertainty budget in most waste streams.