Conceptual Design of an Organic-Cooled Small Nuclear Reactor to Support Energy Demands in Remote Locations in Northern Canada

Abstract

Small nuclear reactors can offer safe, reliable, and long-lasting district heating and electrical power generation to remote locations in Northern Canada. A reactor, cooled and moderated by an organic fluid, based upon the 20 kWth SLOWPOKE-2 research reactor is proposed for potential employment in Northern Canada. To achieve an uprated power of 1 MWth, the conceptual design of the reactor proposes the organic coolant, HB-40, in place of the SLOWPOKE-2’s light water coolant. To increase heat transfer across the reactor’s core, the proposed design incorporates a coolant circulation pump below the reactor. Furthermore, the SLOWPOKE-2’s dimensions are enlarged to house a greater mass of fuel and to provide a larger surface area for heat transfer. Despite these alterations, the reactor has maintained the simplicity of the SLOWPOKE-2 and early indicators suggest the design may be inherently safe. In its present state, the reactor is suitable for district heating of small installations. Operating together, multiple reactors may supply larger heat demands or even electrical power generation while providing added redundancy. Presently, the conceptual design includes the reactor’s core, annular reflector, and initial control system. At full power, the reactor’s coolant circulates unpressurized at a flow rate of 7.63 kg s-1 from an inlet temperature of 290 OC to an outlet temperature of 340 OC. The reactor employs the same 19.89% enriched uranium dioxide as the SLOWPOKE-2 in similar, albeit longer, fuel pins. In the reactor’s core, 660 fuel pins contain 8.610 kg of uranium-235 providing an approximate fuel service life of 11.5 years at full power. The annular reflector of the reactor is composed of beryllium like SLOWPOKE-2, but the annulus splits into 6 identical plates that move away from the core providing long-term regulating control. The reactor’s control system is designed to provide both short-term and long-term regulating control in addition to containing sufficient negative reactivity to achieve shutdown under all circumstances. In addition to the aforementioned reflector plates, the control system is comprised of 25 hafnium control rods and 24 hafnium shutdown rods. An initial investigation of the reactor’s reactivity coefficients has indicated strong potential for inherent safety due to strongly negative correlations between the reactivity and the fuel temperature, the moderator-coolant temperature, and the void fraction. This conceptual design has been developed through simulation of the reactor’s steady-state neutronics using two different computer codes. The first, Los Alamos National Laboratories’ MCNP 6, employs a three-dimensional Monte Carlo-based probabilistic method to infer average particle behaviour from statistical analysis of many neutron histories. The second, Atomic Energy of Canada Limited’s WIMS-AECL, numerically solves the neutron transport equations in two dimensions to determine the average particle behaviour. This initial design phase has yielded promising results that should be explored further with detailed thermal hydraulic modelling of the reactor as well as kinetic modelling of reactor in transient states. To fully achieve inherent safety in the reactor’s design, a containment and shielding system are required to mitigate hazards due to escape of radiation and fission products.