Nondestructive Assay of Spent Boiling Water Reactor Fuel by Active Neutron interrogation*

Spent boiling water reactor (BWR) fuel from Dresden I was assayed for total fissile mass, using the active neutron interrogation method. The nondestructive assay (NDA) system used has four Sb-Be sources for interrogation of the fuels; the induced fission neutrons from the fuel are counted by four lead-shielded methane-filled proportional counters biased above the energy of the source neutrons. Spent fuel rods containing 9 kg of heavy metal (^1% fissile) were chopped into 5-cm segments and loaded into three 1-liter cans (^3 kg per can). (Because of mechanical constraints in handling the fuel in the reprocessing test facility, the fuel could not be assayed as entire fuel rods.) The three cans were assayed in seven combinations of one, two, or three cans, enabling an evaluation of the precision and accuracy of the NDA system for different amounts of fissile material. The fissile mass in each combination was determined by comparing the induced-fission-neutron counts with the counts obtained from a known standard comprising chopped segments of unirradiated Dresden fuel. These masses were compared to the masses determined by chemical analyses of the spent fuel. The results from the nondestructive assays agreed with results from the chemical analyses to within 2-3%. Similar agreement was obtained when two combinations of canned spent fuel wer