Multi-recycling in the ENHS and the equilibrium core

This work investigates the effect of initial fuel composition, power density and number of recycles on the pitch-to-diameter (P/D) ratio and TRans-Uranium isotopes (TRU) loading required for attaining one of the most important design goals of the Encapsulated Nuclear Heat Source (ENHS) - nearly zero burnup reactivity swing over the ∼20 years of core life. It is found that the required P/D ratio is sensitive to, primarily, the initial concentration of the short-lived isotope 241Pu in the fuel loaded into the first core and to the core power density. The longer is the cooling time of the TRU from LWR spent fuel the smaller becomes the relative 241Pu concentration and the smaller becomes the fraction of 241Pu lost via radioactive decay and, hence, the smaller needs be the conversion ratio required for nearly zero burnup reactivity swing and the larger can be the P/D ratio. Likewise, the higher is the ENHS power density, the smaller becomes the fraction of 241Pu lost via radioactive decay and the larger becomes the P/D required for the first core. The optimal P/D ratio tends to increase with the number of times the fuel is recycled from one ENHS core to the next one. The optimal P/D ratio for the equilibrium composition core is in between 1.53 and 1.59. For a given discharge burnup it tends to somewhat increase with the equilibrium core power density. However, if structural materials will be developed to enable a 20 years core life at elevated power densities, the higher the power density the smaller is the required equilibrium P/D ratio. (author)