Year
1983
Abstract
The Advanced Nuclear Technology Group of the Los Alamos National Laboratory recently designed and built a combined passive- and active-neutron assay system on which group members performed initial characterization measurements. The system is intended to provide sensitive assays at the 100-nCi/g level of both spontaneous fission and fissile transuranic isotopes in large crates of bulk waste. Such crates can be accommodated in the system's assay chamber whose internal dimensions are 64 by 64 by 96 in. Moderated 3He proportional counters provide a measured total 4ir neutron detection efficiency of 13.5 ± 1 % for bareand moderated-fission spectrum sources placed anywhere within the volume of a standard empty crate. Initial passive neutron coincidence measurements indicate a 240pu detection sensitivity of about 10 mg. Enhancement of the fissile-isotope assay sensitivity results from lining the assay chamber walls with a thick layer of graphite and placing specially designed prompt-fission neutron detectors within the walls. Initial pulsed-neutron measurements indicate a fissile assay sensitivity of about 30 mg of 239Pu. After completion of checkout and initial calibration measurements at Los Alamos, the assay system will be installed at a DOE facility for a 1-year test-andevaluation program beginning in July 1983. During the period, actual transuranic waste crates averaging nearly 1000 kg each will be assayed on a routine basis.