Studies of recriticality transients in BWRS during reflooding: Simulate-3K development and analyses

Recriticality in a BWR has been studied for a total loss of electric power accident scenario. In a BWR, the B₄C control rods may melt and relocate from the core before the fuel during core uncovery and heat-up. If electric power returns during this time-window, unborated water from ECCS systems will start to reflood the partly control rod free core. Recriticality might take place for which the only mitigating mechanisms are the Doppler effect and void formation. Of particular interest are global effects of hydraulic feedback between the coupled reactor core and peripherals system, i.e. non-core components. A study was made using the core kinetics code SIMULATE-3K (S-3K), specially adapted to model the reflooding and recriticality problem. S-3K is a transient version of the advanced 3-D core analysis code SIMULATE-3. S-3K models each fuel channel uniquely, with transient hydraulic and radial fuel pin conduction solutions providing local nuclear feedback for the 3-D neutron spatial kinetics solution. The S-3K peripheral system models provide a representation of such non-core components as the downcomer, lower and upper plenum, and steam separator. The core model couples to the peripherals system model by communicating integral core conditions such as enthalpy rise, exit flow, and core pressure drop. These models and the results of applications of S-3K to reflooding transients in the Swedish advanced BWR, Oskarshamn 3, are the subjects of this paper.