Year
2012
Abstract
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 of proliferation-resistance since Pu is not chemically separated from the other TRU elements. Since 244Cm is the most dominant neutron-emitting source among all TRU isotopes, Cm neutron measurement can be an indirect evidence for no diversion of Pu in U/TRU ingot. This is a fundamental 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 of U/TRU ingot is necessary. The neutron-emission rate of 6 kg of U/TRU ingot is about 2.95x108 neutrons/s from 26.4 g of 244Cm if there is no neutron multiplication. Since the neutron self- multiplication of 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 is impractical to apply the concept of neutron coincidence counting. To accommodate the intensive neutrons from 244Cm of U/TRU ingot, the neutron detection efficiency of the neutron counter should be kept as low as possible and this could be realized by adopting several numbers of 3He gas tubes of lower gas pressure covered with neutron absorbing material. Since the coincidence counting 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 on the fissile contents in U/TRU ingot. The Cm mass and the neutron multiplication of U/TRU ingot are measured by the total neutron counting and ‘Cd Ratio’ of PNAR, respectively. To see the capability of the PNAR technique for the quantification of the Pu content of U/TRU ingot, a spent fuel library with 81 different cases of various kinds of initial enrichment, burnup, and cooling time was produced for the MCNPX simulations and the MCNPX simulation results for the Cd Ratio vs. Pu content of U/TRU ingot show the maximum error in the Pu mass between the linear fit and the real Pu content in U/TRU ingot is 2.14% for 4.5 wt.% initial enrichment cases.