Modeling and NDA verification of Mk18A target irradiation campaign at SRS

In 1969 an irradiation campaign began at SRS using targets composed primarily of Pu242. The goal of this campaign was the production of higher actinides, for various applications, including Cf252. This campaign exposed these targets to a very high flux for a period around 18 months using a special core arrangement and replacing driver fuel much more often than was typical. After this the fuel was further irradiated at a lower flux until 1979. This material now contains a high fraction of the world’s supply of Pu244 which is of use to nuclear forensics programs. There is a desire to recover this material for this purpose as well as for the higher actinides it may contain. In order to calculate the radiation field generated by these targets and their actinide inventory, a series of models were created to simulate the irradiation conditions used in this campaign. They included a KENO model of the original core used for the high flux part of the irradiation, a series of MCNP models of later core configurations and several ORIGEN-S runs to deplete the targets using fluxes from the MCNP models. In order to validate the results of this modeling effort, High Purity Germanium spectra were taken of several of these targets stored in L-basin. An air tube was used to displace the water between the surface of the pool and the target assembly. A model of this arrangement was used to calculate the expected peak heights of Cs-137 and Am-241. Comparing this model to the measured spectra showed good agreement. This served as a validation of the reactor model.