Use of minor actinides for reduced power encapsulated nuclear heat source (ENHS) reactor core design

The general objective of this study is to design Encapsulated Nuclear Heat Source (ENHS) reactor cores for half of the reference power level of 125 MWt. A previous study showed that it is impossible to design 62.5 MWt cores for 20 Effective Full Power Years (EFPY) using the reference design values of the average linear heat rate and fuel rod dimensions due to a significant increase of the neutron leakage probability. It was also found that by increasing the fuel volume fraction it is possible to compensate for the increased neutron leakage probability. However, the resulting core requires a very tight lattice of P/D=1.07 when designed to have the specific power of the reference ENHS core. Although this core has a favorable power distribution, coolant void reactivity and reactivity worth of the control elements, its P/D ratio is too small for natural circulation cooling without significant increase in the ENHS module height. The specific objective of this work is to investigate the possibility of increasing the P/D ratio of the 62.5 MWt ENHS cores by using minor actinides (MA) in addition to Pu for the fuel loading, while maintaining the reference core specific power and nearly zero burnup reactivity swing for over 20 EFPY. It is found that the use of MA enables a significant increase of the lattice P/D ratio; the larger the fraction of MA in the fuel, the larger becomes the required P/D ratio. With 50 w/o MA the P/D ratio gets to 1.40 ! - even larger than the P/D=1.36 ratio of the reference ENHS core design. Addition of MA also cause a reduction of the peak fast neutron fluence and, hence, to an increase of the core life and the discharge burnup. However, use of MA leads to degradation of the fuel Doppler and the coolant expansion reactivity coefficients. (author)