Safety analysis of a supercritical pressure, light water cooled and moderated reactor with double tube water rods

The supercritical pressure, light water cooled and moderated reactor (SCLWR) has once-through cooling system. All feed water which cools the reactor core flows to the turbines. This paper summarizing the safety analysis of the SCLWR with double tube water rods. The plant system is simple but no natural circulation is established at the loss of feedwater flow. The coolant inventory in the reactor pressure vessel is small. The coolant density coefficient is approximately twice as large as that of the BWR. A computer code (SPRAT) was developed to analyze SCLWR behavior against major accidents and transients at supercritical pressure. In loss of flow events such as loss of off-site power, the flow coast down time should be larger than 10 sec. for avoiding the deterioration in heat transfer. In the flow-excess event such as inadvertent start of the auxiliary feedwater pumps, the power increases approximately 25% by coolant density feedback. In the overpressurization transient such as generator load rejection, the power does not increase even if scram fails. This is because flow stagnantion raises coolant temperature and coolant density change at overpressurization is small in supercritical pressure. The reactivity-induced event such as control rod ejection, is not severe because of the small reactivity ingress. In the loss of coolant accident, the double tube water rods delay the reflood of the core. The core is heated up rapidly because of the small heat capacity and tight lattice pitch of the fuel rods. All analyzed accidents and transients satisfied the criteria, and the feasibility of the reactor was confirmed from the safety point of view.