Neutronic Analysis of the Submersion‐Subcritical Safe Space (S4) Reactor Using Reduced Enrichment Uranium Fuel

There is a resurgent interest in using space reactors for various space missions. This work models the S 4 (submersion-subcritical safe space) reactor for deploying LEU fuel using Serpent 2. The S 4 reactor is a fast spectrum gas-cooled reactor that ensures subcriticality in case of accident scenarios where the reactor gets submerged in seawater. Increased pressure to curb highly enriched uranium fuel (HEU) in reactor designs has heightened interest in low enriched uranium (LEU). To accomplish this, the S 4 reactor was scaled to multiple sizes: one at twice the diameter and three times the length, another at twice the diameter and length. For the original reactor size, 82 % enrichment possessed a k eff of 1.017 after seven years of operation. For the S 4 reactor scaled to 2X the diameter and 3X the length, initial k eff only attained 0.81 in the case of 20 % enrichment fuel. A 34 % 235 U concentration was estimated to be the lowest enrichment that allowed for criticality after seven years at a more feasible 2X scaling of this reactor design.