Design of a guaranteed shutdown state using solid rods for the ACR-1000 reactor

This paper presents the design of a rod-based system for long-term shutdown, known as the guaranteed shutdown state (GSS), of the ACR-1000 ^**TM reactor. Previous CANDU® designs rely on soluble neutron poisons to achieve long-term shutdown. However, due to the operational complexity of using soluble neutron poisons, an alternative shutdown method was desired. The GSS design presented here uses solid boron-carbide rods with a simplified drive system, to provide long-term shutdown. A design-target for subcriticality of -50 mk was chosen in accordance with the IAEA guidelines for long-term fuel storage. An additional -5 mk was added to the criteria to account for calculational uncertainties. To address the possibility of unavailable reactivity devices, the most valuable device was eliminated from the analysis. Also, the sub-criticality requirements assume that the core is cold and that the shortlived fission products have decayed, each of which contributes positive reactivity in the core. WIMS/RFSP/DRAGON [1,2,3] was used as a preliminary tool to determine the most valuable reactivity devices, and the limiting cases were subsequently simulated with the code MCNP5 [4], to determine if the design complied with the sub-critical criteria.