Design and analysis of procedures for system pressurization and line-switching to once-through mode in plant startup of SCWR

The SCWRs adopt once-through direct steam cycle like modern FPPs (Fossil-fired Power Plants). Plant startup of SCWRs can be divided into several stages. The procedures for system pressurization and line-switching from recirculation mode to once-through mode are proposed in this study. The recirculation system for the system pressurization is designed. It consists of a steam drum, a circulation pump and a heat exchanger. Nuclear heating starts from atmospheric pressure with forced circulation in the recirculation system and the power level is gradually increased. The reactor is pressurized by boiling the coolant in the core as is done in BWRs. After the pressurization to the operating point (25MPa) and raising the steam temperature, the recirculation mode is gradually switched to the once-through mode in order to warm the main steam lines, turbine bypass lines and then the turbines. In order to assess the feasibility of these procedures, the recirculation system is newly modeled in a system transient analysis code developed at the Univ. Tokyo. It is confirmed by a system transient analysis that the reactor can be gradually pressurized to the operating point by nuclear heating like BWRs and then switched to the once-through mode without significant fluctuations of the pressure or flow rate. The fuel cladding temperature and the rate of increase in the coolant temperature are kept below the criteria, respectively, by properly raising the power level with enough coolant flow rate. Using supercritical 'steam' that is generated in the reactor for starting the turbine is a unique method for the SCWRs while turbines of FPPs are started at subcritical pressure using superheated steam that is generated in superheaters. (author)