抄録
Analyses of severe accidents for sodium-cooled fast reactors have shown that by assuming pessimistic conditions the accident might proceed into a transition phase where a large whole-core-scale pool containing sufficient fuel to exceed prompt criticality by fuel compaction might be formed. Local fuel-coolant interaction (FCI) in the pool is regarded as one of the probable initiators that could lead to such compactive fluid motions. To enhance the evaluation of severe accidents, a three-step research plan has been determined. In Phase 1, a series of simulated experiments, which covers a variety of conditions including much difference in water volume, melt temperature, water subcooling and water release site, was conducted by delivering a given quantity of water into a molten pool formed with a low-melting-point alloy, while in Phase 2, the interaction characteristics (esp. the pressure buildup) was investigated using the SIMMER-III, an advanced fast reactor safety analysis code. It was recognized that the SIMMER-III code, with its existing heat-transfer models, can reasonably reproduce the experimental evidence observed, thereby greatly stimulating us to initiate the Phase 3, in which further numerical analyses using reactor materials are planned. In this work, based on the latest calculations, it is confirmable that similar to the observations in Phases 1 and 2, for a given melt and sodium temperature within the non-film boiling condition, as the volume of sodium entrapped within the pool increases, a limited pressure-buildup is achievable. In addition, the performed analyses also suggest that despite of a comparatively larger temperature range of molten-fuel and sodium possibly varied during reactor accident progression, the isolation effect of vapor bubbles generated at the melt-sodium interface seems to be the unique dominant mechanism that leads to such limited pressurization. Knowledge and fundamental data from this work might be utilized for future empirical-approach studies (e.g. a lookup table for estimating the critical coolant volume required for achieving the saturated pressurization at varied melt and coolant temperatures).
