The Encapsulated Nuclear Heat Source (ENHS) is a small lead-bismuth-cooled Generation IV reactor designed to have a once-for-life core with a nearly zero burnup reactivity swing and heat removal by natural circulation. All the ENHS cores designed so far have positive coolant void reactivity. This work searches for ENHS core designs having negative coolant void reactivity feedback and quantifies the penalty associated with a design for negative void reactivity. The approaches tried for turning the positive void coefficient negative are as follows: (a) enhancing the neutron leakage probability by reducing the fuel length, introducing neutron absorbers at the core boundary, using a gas-lift pump to introduce gas bubbles throughout the coolant in the core and fission gas plenum regions, and incorporating neutron-streaming channels in and adjacent to the core; (b) introducing into the core materials, such as Ca3N2, having enhanced absorption cross section at high energy; and (c) introducing into the core materials, such as CaH2, that will keep the neutron spectrum softer in the case of coolant voiding.

The preferred negative void reactivity core design consists of 100-cm-long fuel rods, a 20-cm-thick B4C layer below the core, and a voided channel around the core radial boundary. The reactivity effect is negative when the coolant is voided from the entire core and even from only the central region of the core. In order to maintain a nearly zero burnup reactivity swing over at least 20 effective full-power years (EFPY), the core pitch-to-diameter ratio (P/D) has to be reduced from the reference value of 1.36 to 1.20. Correspondingly, for a given ENHS module dimensions the power level that can be removed by natural circulation from this core is ~75% of the reference core. Allowing a burnup reactivity swing of ~0.1% over 20 EFPY enables attaining a negative void reactivity core having P/D of 1.27 that can deliver ~93% of the P/D = 1.36 core power.