Effects of TRU distributions of electron accelerator-driven subcritical core systems on transmutation

Abstract

As part of the effort to investigate the use of an electron accelerator driven system for TRU transmutation, the effects of TRU distributions in the core on transmuter system performance was examined in this paper. The system performance examined includes the transmutation and system power efficiency and changes in power peaking. The transmutation benefits of the system were determined with the introduction of a new parameter, the Transmutation System Effectiveness Parameter (TSEP). TSEP combines the decay heat and radioactivity results into one single parameter that compares the ability of the system to reduce the radioactivity and decay heat of the loaded TRUs. The electron ADS was modeled by using MCNPX and MONTEBURNS as a fast spectrum, Na cooled reactor loosely based on the advanced liquid metal reactor (ALMR) design. NJOY was used to process the cross sections at the desired temperatures. The fuel was a TRU-Zr alloy contained within an HT-9 SS cladding. The subcritical reactor contained four different fuel zones with an equal number of fuel assemblies in each region, each containing one of the four TRU elements: Np, Pu, Cm, Am. Tungsten was used for the target system. The electron ADS was assumed to operate at 500MWth over a 24 month cycle. Results showed that different distribution patterns had a very insignificant effect on the total radioactivity reduction, the total decay heat reduction, and the TRU radiotoxicity reduction. With respect to the TSEP parameter, the calculation results revealed a much stronger dependence on TRU distributions. It seemed that TSEP accurately reflected and penalized the effectiveness of the system for the fission product production. With respect to examining the keff over the cycle, a drastic difference was observed between the cases when Pu is located in the inner most region and the rest of the patterns. The keff for the Pu in the inner most region cases decreased at a much faster rate than did the rest therefore requiring a dramatic increase in beam current over the cycle. The power peaking behavior of the system was found to be dictated by the placement of the Pu material region. The lowest total power peaking as well as the lowest relative assembly power peaking was experienced when the inner most region was filled with Np, followed by Pu, and then by Am and Cm.