Neutronics Analysis and Design Optimization of the Pellet Bed Reactor for Bimodal Applications

Abstract

A three‐dimensional MCNP analysis of the fast spectrum Pellet Bed Reactor (PeBR) is performed to determine reactor core point designs for bimodal system concepts of 10 and 40 kWe. Design requirements include an operation lifetime of ten years in the power mode, and more than 250 hours in the propulsion mode at a specific impulse > 650 seconds and thrust of a few to tens of Newtons, maximum fuel temperature < 1600 K, that is maintained almost constant during power and propulsion modes, a fuel burnup < 2.0 atom%, negative temperature reactivity feedback, passive decay heat removal, redundancy in reactor control, and sub‐criticality during water flooding accidents. The excess reactivity at beginning‐of‐mission (BOM) is as much as $1.5 at 1600 K (keff = 1.012) for a reactor core that measures 0.32 m in diameter and 0.45 m high and is loaded with 6.0 mm, UC, TRISO‐type fuel mini‐spheres. The reactor core point design has a total negative temperature reactivity of $3.0, a desirable operation and safety feature, a total mass of 328 kg and 347 kg for the 10 kWe and 40 kWe system, respectively, and employs two independent control systems with built‐in redundancy in each system: sliding reflector segments and B4C safety rods. The sliding reflector‐safety rods in combination, ensures sub‐criticality during submersion in water or wet sand.