The 25 Mw Super Near Boiling Nuclear Reactor (SNB25) for Supplying Co-Generation Energy to an Arctic Canadian Forces Base

Abstract

Nuclear energy represents a better alternative for the supply of heat and electricity to the Canadian Forces bases in the Arctic (CFS Alert and CFB Nanisivik). In this context, the Super Near-Boiling 25-MWth reactor (SNB25) has been designed as a small unpressurized LWR that displays inherent safety and is intended to run in automatic mode.The reactor employs TRISO fuel particles (20% enrichment) in zirconium-sheathed fuel rods, and is light water cooled and moderated with a normal output temperature is 95oC at atmospheric pressure. Control is via 133 control rods and six adjustable radial reflector plates. The design work used the probabilistic simulation code MCNP 5 and the deterministic code WIMS-AECL Version 3.1, permitting a code-to-code comparison of the results. Inherent safety was confirmed and is mostly due to the large negative void reactivity coefficient of -5.17 mk per % void. A kinetic model that includes thermal-hydraulics calculations was developed to determine the reactor’s behaviour in transient states, and the results further confirm the inherent safety. Large power excursions temperatures that could compromise structural integrity cannot be produced. If the coolant/moderator temperature exceeds the saturation temperature of 100oC, the coolant begins to boil and the large negative void coefficient causes the reactor to become subcritical in 0.84 seconds.The SNB25 reactor’s core life exceeds 12 years between refuellings. A group of 4 SNB25 reactors meets both the heating and electricity requirements of a base like CFB Nanisivik via a hot water network and through an organic Rankine cycle conversion plant.