Simulation of three-dimensional loss-of-regulation transients in ACR-1000 reactors

The ACR-1000 reactor regulating system (RRS) provides reactivity control to keep the reactor power at a specified level and to compensate for local changes in reactivity that occur due to effects such as refuelling and fuel burnup. In safety analysis a loss of regulation (LOR) accident is postulated where the overall core reactivity increases linearly, due to a malfunction of the RRS. This causes a power excursion which is terminated by either of the two shutdown safety systems for the ACR. The LOR trip coverage assessment required for the two shutdown systems is traditionally performed with a point kinetics model to predict the bulk power increase following a postulated LOR for a range of assumed reactivity rates that conservatively bound the expected possible reactivity rates for device movement due to malfunctions in RRS. To confirm the validity of these simulations and the conservative margins in this type of analysis a more detailed analysis has been performed using the three-dimensional (3D) kinetics module in the RFSP-IST computer code coupled with the CATHENA thermalhydraulic code. In this paper, the modelling and methodology of the 3D loss of reactivity control analysis are discussed and some limited results are presented. (author)