Impact of Pu-241 decay on long-life reactor design

The power density of long-life fast reactor cores is significantly lower compared to conventional fast reactors, and thus the decay of 241Pu affects the core performance significantly when transuranic (TRU) fuel is used. This paper investigates the effects of 241Pu decay on the burnup reactivity swing and control requirement of a lead-cooled, small modular fast reactor fueled with TRU recovered from light water reactor (LWR) spent fuel. In particular, the effects of reduced power operation and additional cooling time due to delayed startup from a projected schedule were examined. It was found that the core criticality and reactivity swing is sensitive to the TRU cooling time. For example, a 45 MWt Small Secure Transportable Autonomous Reactor (SSTAR) core designed with TRU fuel of 50 MWd/kgU burnup and 25-year cooling time, whose reactivity swing is 0.85$ over a 20-year lifetime, required an additional excess reactivity of 1.5$ to accommodate a delayed startup up to 5 years and a reduced power operation up to 75% of the rated power. To hold this additional reactivity within the reference core configuration, the 10B enrichment of B4C control rods had to be increased from 19.9% (natural boron) to 60%. These results indicate that TRU-fueled, long-life cores should be designed with sufficient flexibility to achieve the targeted lifetime for all anticipated circumstances deviated from the projected operational plan. (author)