Delayed neutron method for measurement of fissile/fertile content of samples ranging from environmental to irradiated fuel

A delayed-neutron assay method is described which is capable of accurately determining the 235U and 238U contents of a wide range of materials. The method consists of irradiation in the Coupled Fast Reactivity Measurement Facility (CFRMF) at the Idaho National Engineering Laboratory (INEL), one position in the thermal driver area for 235U assay, and a fast neutron flux position for 238U. After irradiation, delayed neutron counting is used to measure the uranium content of the sample. By utilizing fast and thermal neutron irradiation and counting cycles, the samples 238U and 235U contents are determined. Minimal sample preparation and handling are required to obtain accurate results. The method has been used successfully on a wide range of samples, including environmental (soil) samples, irradiated and unirradiated fuel, and debris from the Three Mile Island Unit II Reactor core. Measurement precision of up to 0.5% can be achieved; sensitivity for low level samples is 9 counts min-1 nanogram-1 for 235U and 0.02 counts min-1 nanogram-1 for 238U. The fissile and/or fertile contents of samples containing 233U, 239Pu, 240Pu, 237Np, or 232Th could be determined by the same method. Data from representative samples are presented to illustrate the usefulness of these techniques