R and D needs for the evaluation of behavior of radioactive materials of large scale FBRs

The investigation of the source term produced by core disruptive accident (CDA) is strongly required from the viewpoint of the determination of the site suitable source term (SSST). In this study the major sequence of the CDA is extracted for the unprotected loss-of flow (ULOF) of the 1600 MWe class of MOX fueled core, and corresponding transport mechanisms of the fission products (FPs) are then evaluated according to the transport pass within the primary coolant system during the CDA. Dominant consequence of the CDA is induced by the leakage of FPs accumulated in the cover gas due to the blow down of fission gas bubbles out of the core entraining the volatile fission products followed by the energetics which induce the acceleration of the coolant slug of the upper sodium plenum. In addition to the qualitative discussion on the FP transport mechanisms expected in the CDA sequence, experimental examinations are also conducted to evaluate the FPs retention factor of the coolant sodium. The retention factor of aerosols measured in the water experiments is over 100, and evaluated to be about 30 for the real plant condition based on the conservative extrapolation models. (author)