Improvement of Evaluation Method for Initiating-Phase Energetics Based on CABRI-1 In-Pile Experiments

An improved method to evaluate key phenomena in the initiating-phase energetics of unprotected lossof-flow (ULOF) whole-core accidents in liquid-metal fast breeder reactors is presented. Three phenomena, namely, axial fuel expansion, fuel failure, and postfailure fuel motion, have been examined through the CABRI-1 in-pile experiments and analyses with special emphasis on the self-limiting mechanisms of the energetics potential. For the experiment analyses, the SAS3D, PAPAS-2S, and SAS4A computer codes are employed selectively to obtain a detailed investigation of the phenomena and to validate physical models. The improved knowledge obtained through the research efforts in CABRI-1 and relevant safety experiments has been implemented in the revised SAS3D code. This evaluation method, which accounts for the self-limiting mechanisms, has been applied to a reactor analysis of an energetic ULOF sequence. The results of the application study confirm the importance and effectiveness of the method.