Determination of Pu Content of U/TRU Ingot of Pyroprocessing Using the Passive Neutron Albedo Reactivity Technique

摘要

The major target materials of safeguards for pyroprocessing are spent fuel assemblies, U ingots,U/TRU ingots, and waste materials. Pyroprocessing is known to have a big merit in terms ofproliferation-resistance since Pu is not chemically separated from the other TRU elements. Since~(244)Cm is the most dominant neutron-emitting source among all TRU isotopes, Cm neutronmeasurement can be an indirect evidence for no diversion of Pu in U/TRU ingot. This is afundamental of the Cm balance concept for the nuclear material accountancy of pyroprocessing.Since U/TRU ingot contains almost all Pu of the feed spent fuel, a high accuracy in the assay ofU/TRU ingot is necessary. The neutron-emission rate of 6 kg of U/TRU ingot is about 2.95×10~8neutrons/s from 26.4 g of ~(244)Cm if there is no neutron multiplication. Since the neutron selfmultiplicationof U/TRU ingot is estimated to be over 2 and its neutron-emission rate is very high,there will be significant amount of accidental coincidence in neutron counting and thus it isimpractical to apply the concept of neutron coincidence counting. To accommodate the intensiveneutrons from ~(244)Cm of U/TRU ingot, the neutron detection efficiency of the neutron countershould be kept as low as possible and this could be realized by adopting several numbers of 3Hegas tubes of lower gas pressure covered with neutron absorbing material. Since the coincidencecounting is not available for U/TRU ingot, the Passive Neutron Albedo Reactivity (PNAR)technique is chosen to measure the neutron multiplication of U/TRU ingot, which is dependent onthe fissile contents in U/TRU ingot. The Cm mass and the neutron multiplication of U/TRU ingotare measured by the total neutron counting and ‘Cd Ratio’ of PNAR, respectively. To see thecapability of the PNAR technique for the quantification of the Pu content of U/TRU ingot, a spentfuel library with 81 different cases of various kinds of initial enrichment, burnup, and coolingtime was produced for the MCNPX simulations and the MCNPX simulation results for the CdRatio vs. Pu content of U/TRU ingot show the maximum error in the Pu mass between the linearfit and the real Pu content in U/TRU ingot is 2.14% for 4.5 wt.% initial enrichment cases.