Preliminary evaluation of coolant void reactivity of a re-entrant channel pressure-tube supercritical water cooled reactor

The Pressure-Tube Supercritical-Water Cooled Reactor (PT-SCWR) is one of the reactor concepts investigated by the Generation-IV International Forum as a reactor concept capable of achieving an overall nuclear power plant efficiency in the vicinity of 50%, higher than the approximately 35% that characterizes current nuclear power plants. The development process includes studying the neutronic methods that can provide an adequately-accurate modelling of the core. The PT-SCWR evolved from the conventional pressurized heavy-water reactor but presents a more heterogeneous core because of the much higher discharge burnup (∼40 MWd/kg vs. ∼7.5 MWd/kg) and greater variation of coolant density. The latest PT-SCWR concept features re-entrant fuel channels, each consisting of a pressure tube lined by a ceramic thermal insulator, and a central inner flow-tube. The fuel assembly is located in the space between the ceramic insulator and the central flow-tube. The re-entrant channel configuration presents unique coolant voiding characteristics because of the separate central and outer coolant regions. There may be the possibility, under certain postulated accident conditions, that the coolant in the region between the flow-tube and insulator can void before the coolant in the flow-tube. This work presents preliminary calculations of the resulting coolant-void reactivity for the case when only the outer coolant voids as well as for the case when the entire coolant in the channel voids. Lattice as well as full-core calculations are performed. Results show that voiding of only the outer coolant region yields a small, ∼2 mk, positive reactivity, while the subsequent complete coolant voiding induces a large (>20 mk) negative reactivity. (author)