Development of an Evaluation Methodology for the Fuel-Relocation Into the Coolant Plenum in the Core Disruptive Accident of Sodium-Cooled Fast Reactors

In the case of postulated core disruptive accidents (CDAs) of sodium-cooled fast reactors (SFRs), fuel relocation out of the core region decrease the potential for severe power excursions caused by recriticality and control-rod guide tubes (CRGTs) provide an effective path for fuel-relocation. Therefore, a methodology for evaluating molten-fuel relocation through CRGTs is required in order to realistically evaluate sequences and consequences of CDAs. Since the liquid sodium exists at the coolant plenum, pressurizations and coolant void development associated with fuel-coolant interactions (FCIs) are considered to affect fuel-relocation. Therefore, the objective of the present study is to develop the methodology for evaluation of molten-fuel relocation into the coolant plenum with FCIs. In the present study, the SIMMER code which has been developed for CDA analyses was utilized as a technical basis since this code can treat multi-phase, multi-component fluid dynamics with phase-changes supposed to take place in the coolant plenum during fuel-relocation. The evaluation methodology was developed through validations of the SIMMER code using experimental data. A series of fundamental experiments were selected for model validations in which an alloy with low melting temperature and water were used as simulant materials for the fuel and the coolant respectively since the experiments were performed under a simulated CDA condition of SFRs in which a liquid-liquid direct contact was maintained between the melt and water contact surface, and the visual observation on FCI process was effective to validate models based on phenomenological considerations. The code was validated by two steps: In the first step, fundamental validations of melt-discharge into the coolant were performed, namely, momentum exchange functions of flowing-melt both to the wall of relocation-path and to the coolant were validated based on experimental data in which effects of FCIs on melt-discharge into the coolant were eliminated or negligible. In the second step, comprehensive validations of melt-ejection into the coolant were performed, namely, models which affect heat-exchange between the melt and the coolant were validated based on experimental data in which the melt was relocated into the coolant with FCIs. The second step validation required model improvements for suppression of melt-coolant interfacial area based on the results of visual observation in the experiments in order to reproduce the experimental results appropriately. Through the present model validations, the methodology to evaluate molten-fuel relocation into the coolant plenum with FCIs was successfully developed.