(A) study on passively autonomous load-following and frequency regulation operations in a soluble-boron-free SMR

Abstract

In this dissertation, we introduce new innovative operational schemes named the passively autonomous frequency regulation operation (PAFO) and the passively autonomous load-follow operation (PLFO). The new operational schemes are proposed for a soluble-boron-free (SBF) pressurized water reactor (PWR). PAFO and PLFO aim at eliminating the dependence on the control rods and soluble boron in attaining the core power maneuvering. Instead, the core power variation is governed by the passive response of the core to temperature changes in fuel and coolant. This approach is motivated by the strongly negative coolant temperature coefficient (CTC) available in an SBF coolant system. This work is dedicated for studying the feasibility of the newly proposed operational schemes in the ATOM system, which is a 450 MWth SBF small PWR. A preliminary feasibility and scoping studies are made using a lumped PWR system model using a point-kinetics core. Then, we perform detailed 3-D transient multi-physics analyses to further study the feasibility by investigating the variations of the axial shape index and the 3-D power peaking factor during the passive transients. The time-dependent thermal-hydraulic (TH) model comprises core analysis for all fuel assembly channels. The neutronic modeling and analysis are based on the typical two-step procedure for light water reactors. Various group-wise cross-sections and transient parameters are obtained using Monte Carlo simulations using the Serpent-2 code with the ENDF/B-VII7.1 data library. Meanwhile, two different options are implemented for obtaining the 3-D diffusion whole core solution using the nodal methods, one is the nodal expansion method and the other one is the semi-analytical nodal method. The time-dependent neutronic and TH models of the reactor core are coupled with a time-dependent steam generator model to simulate the impacts of power demand variation on the core inlet coolant temperature. A study on coupling a helical coil steam generator (HCSG) model with the core multi-physics is made in this researches. Variation of the feed water flow rate to the HCSG controls the amount of heat extracted from the primary loop to deliver the requested power demand for the PAFO and PLFO in the ATOM system, leading to core inlet coolant temperature and the core power follows the demand variation due to the strongly negative CTC. In addition, the predictor-corrector quasi-static method has been also implemented to reduce computing time of the 3-D transient analyses. It is concluded that the PAFO is well achievable with the conventional dead-bands of the coolant temperature and the axial shape index. We also confirm that a little wider temperature dead-band is required for a successful PLFO.