Year
1996
Abstract
Self-shielding of interrogating thermal neutrons in “lumps” of fissile material can be a major source of error in transuranic waste assay using the widely employed differential dieaway technique. We are developing a new instrument, the combined thermal/epithermal neutron (CTEN) interrogation instrument to detect the occurrence of self-shielding and mitigate its effects. Neutrons from a pulsed 14- Mev neutron generator are moderated in the graphite walls of the CTEN instrument to provide an interrogating flux of epithermal (60–800 ps) and thermal (800–2800 VS) neutrons. The induced prompt fission neutrons are detected in 4H e proportional counters as a function of time after the generator pulse; these distributions of 4He detector counts differ markedly for plutonium and uranium. We report the results of measurements made with the CTEN instrument, using minimally and highly selfshielding plutonium and uranium sources in 55-gal. drums containing a variety of mock waste matrices. Fissile isotopes and waste forms for which the method is most applicable, and limitations associated with the hydrogen content of the waste package/matrix are described.