Main heat transport system configuration influence on steam drum level control and safety for a pressure tube type boiling water reactor with multiple interconnected loops

Abstract

NCABWR (Natural-Circulation-Advanced-Boiling-Water-Reactor) is a pressure-tube-type BWR with multiple inter-connected Main-Heat-Transport (MHT) system loops and steam drums (SDs). SD-Level-Control is related to, the MHT coolant inventory and sustained heat removal, hence the overall safety of the Nuclear-Power-Plant (NPP). SD-Level is vital for ensuring presence of adequate coolant-inventory with sufficient and prolonged cooling of fuel, for normal-operation and accidents. In the initial-MHT-configuration of NCABWR the four MHT-loops are, inter-connected at the Common-Reactor-Inlet-Header (CRIH), which influences the SDLC adversely; such occurrences are, not reported in the open literature. Open loop (without SD-level-controllers) step-change response showed that initial MHT-configuration response is stable, non-oscillatory and non-diverging even for a high-magnitude step-change in the feed-flows. Closed-loop (with SD-level-controllers) response, even to minor disturbances leads to diverging SD-Levels due to CRIH-MHT-configuration (Gaikwad, A.J., Vijayan, P., Iyer, K., Bhartiya, S., Kumar, R., Lele, H.G., Ghosh, A.K., Kushwaha, H.S., and Sinha, R.K. (2009). Effect of loop configuration on steam drum level control for a multiple drum interconnected loops pressure tube type boiling water reactor. IEEE Trans. Nucl. Sci. 56: 3712–3725). CRIH is required to overcome Postulated-Initiating-Event (PIE) for single-accumulator-failure-LOCA. A hypothetical MHT-configuration with partitioning of CRIH will isolate each loop from one another and lead to a stable SDLC, but then for the above-mentioned PIE safety is not met. LOCA in one of the four ECCS header-compartments require a mandatory interconnection among, the four MHT-Loops at CRIH. Solution to SDLC issue were explored with changes in MHT-configuration and with innovative solution-schemes for feed-water-system and SDLCs. In the present study the pioneering solution-schemes with individual SD pressure control for SD level control and the Split/Double baffle are proposed, implemented and the performance was tested with simulation are discussed in details with other solution schemes.