Evaluation of recriticality behavior in the material-relocation phase for Japan sodium-cooled fast reactor

Suzuki, Toru; Tobita, Yoshiharu; Nakai, Ryodai 

As the most promising concept of sodium-cooled fast reactors, the Japan Atomic Energy Agency (JAEA) has selected the advanced loop-type fast reactor, so-called JSFR. Through the evaluation of event progressions during hypothetical Core Disruptive Accident (CDA) under the design extension condition (DEC), a CDA scenario for JSFR has been evaluated. It has already been demonstrated that In-Vessel Retention (IVR) against CDA could be achieved by taking adequate design measures under best estimate conditions. The whole sequence of CDA scenario for JSFR was categorized into four phases according to the progress of core-disruption status. In the third phase, so-called material-relocation phase, the accident events would progress in the subcritical state. However, if the uncertainties about the molten state of core remnant and their discharge behavior outward from core are conservatively superposed, the disrupted core may lead up to recriticality. In the present study, the factors leading to recriticality in the material-relocation phase were investigated using the phenomenological diagrams, and the recriticality behaviors were evaluated through parametric analyses using SIMMER-III/IV codes. The results of parametric analyses suggested that a significant mechanical energy leading to the boundary failure of reactor vessel would not be released even assuming recriticality due to the uncertainties about molten state and discharge behavior. Through the present evaluation of the hypothetical recriticality event, the CDA scenario for JSFR could obtain further robustness from the viewpoint of achieving IVR.