Physics Characteristics of a Large, Passive, Pressure Tube Light Water Reactor with Voided Calandria

A light water cooled and moderated pressure tube reactor concept has been developed that can survive loss-of-coolant accidents (LOCAs) without scram and without replenishing primary coolant inventory, while maintaining safe temperature limits on the fuel and pressure tube. The reactor employs a solid SiC-coated graphite fuel matrix in the pressure tubes and a calandria tank containing a low-pressure gas, surrounded by a graphite reflector. This normally voided calandria is connected to a light water heat sink. The cover gas displaces light water from the calandria during normal operation, while during LOCAs it allows passive calandria flooding. It is shown that such a system, with high void fraction in the core region, exhibits a high degree of neutron thermalization and a large prompt neutron lifetime, similar to D₂O moderated cores, although light water is used as both coolant and moderator. Moreover, the extremely large neutron migration length results in a strongly coupled core with a flat thermal flux profile and inherent stability against xenon spatial oscillations. The heterogeneous arrangement of the fuel and moderator ensures a negative void coefficient under all circumstances. Flooding of the calandria space with light water results in redundant reactor shutdown. Use of particle fuel allows attainment of high burnups.