An Innovative Reactivity Control Strategy for Small Modular Reactors

Currently, the main reactivity control methods for small modular reactors (SMR) use control rods as the primary control system, with chemical SHIM or burnable absorbers as secondary systems to control the reactivity excess. However, this kind of control system presents some risks, such as control rod undesired drop or positive moderator coefficient due to the high concentration of boric acid in the moderator. This paper evaluates the possibility of using a displaceable heavy neutron reflector with a neutron absorber (boron) as a secondary reactivity control method. The reference NuScale core geometry has been simulated using the DRAGON5 and DONJON5 deterministic codes, computing the neutron flux and power distribution at each reflector withdrawal step. Two different strategies have been considered: 1) withdrawing the entire movable reflector block towards the upper part of the vessel and 2) separating the reflector block into two equal parts, removing each in different directions from the core equator region. Results indicate that the most suitable reflector withdrawal mechanism is the latter: this solution is promising to replace secondary reactivity control methods in small reactor cores.