Conceptual Design of a Large, Passive Pressure-Tube Light Water Reactor

Abstract

A design for a large, passive, light water reactor has been developed. The proposed concept is a pressure tube reactor of similar design to CANDU reactors, but differing in three key aspects. First, a solid Sic-coated graphite fuel matrix is used in place of pin-rod bundles to enable the dissipation of decay heat from the fuel in the absence of primary coolant. Second, the heavy water coolant in the pressure tubes is replaced by light water, which serves also as the moderator. Finally, the calandria tank, surrounded by a graphite reflector, contains a low pressure gas instead of heavy water moderator, and the normally-voided calandria is connected to a light water heat sink. The cover gas keeps the light water out of the calandria during normal operation, while during loss of coolant or loss of heat sink accidents it allows passive calandria flooding. Calandria flooding also provides redundant and diverse reactor shutdown. The entire primary system is enclosed in a robust, free standing cylindrical steel containment cooled solely by buoyancy-induced air flow, and surrounded by a concrete shield building. It is shown that the proposed reactor can survive loss of coolant accidents without scram and without replenishing primary coolant inventory, while the safe temperature limits on the fuel and pressure tube are not exceeded. It can cope with station blackout and anticipated transients without scram - the major traditional contributors to core damage frequency - without sustaining core damage. The fuel elements can operate under post-CHF conditions even at full power, without exceeding fuel design limits. The heterogeneous arrangement of the fuel and moderator ensures a negative void coefficient under all circumstances. Although light water is used as both coolant and moderator, the reactor exhibits high neutron thermalization and a large prompt neutron lifetime, similar to DgO moderated cores. 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.