Plutonium Content, Uranium Content, and Subcriticality Data of Spent Fuels •

It is well known that the design concepts of transportation systems, storage facilities, reprocessing processes, etc., of spent nuclear fuels can be economized due to the significant decrease in the reactivity of these fuels as a result of burnup. The fissile isotopic content of spent fuels should be known to estimate the subcriticality of these fuel assemblies in the corresponding systems and processes. However, the isotopic content of plutonium and uranium and the other fission products (FPs) is not systematically known, and this isotopic content depends on the type of reactor (BWR, PWR), bum up history, and the position of fuel rods in the fuel assembly. Furthermore, the accuracy of the calculated results not have been verified in comparison with destructive data, especially since systematic data have not been measured for BWR's and PWR's. At the Japan Atomic Energy Research Institute (JAERI), studies were performed to review burnup data. These studies focused on the isotopic content of plutonium and uranium in spent fuels. The information gained will be used in corresponding subcriticality calculations. In addition, isotopic ratio of 235u to total uranium and 239Pu, 240Pu, 241Pu, 242pu to total plutonium respectively, which depend on the initial 235u enrichment and burnup ratio, was determined for use in the criticality safety evaluation of PWR spent fuels. However, the criticality and subcriticality data of spent fuels are not sufficient to verify criticality calculation results. These data must be systematically measured as a function of the bumup ratio, bumup history, etc. Therefore, an experimental program to measure subcriticality data of spent fuel assemblies in a storage pool has begun at JAERI. The objectives of this program are to apply an exponential experimental technique to obtain benchmark data for verification of criticality calculation codes, to demonstrate this technique as a viable method for the pre-loading burnup measurement, and to present systematic data for fissile isotopic content, burnup history, etc., that are needed for the verification of burn up calculation codes.