An evaluation study on ULOF event sequences in the prototype FBR; An evaluation of CDA reflecting the latest knowledge

Tobita, Yoshiharu; Morita, Koji; Kawada, Kenichi   ; Niwa, Hajime; Nonaka, Nobuyuki

The sequences of ULOF (unprotected loss-of-flow) event in the prototype FBR has been evaluated, as a part of the research and development (R&D) in the reactor safety research, reflecting the latest experimental and analytical knowledge on CDA (core disruptive accident) which has been accumulated at O-arai Engineering Center. In the R&D activity on the FBR reactor safety subject, we have accumulated the experimental knowledge of mitigation mechanism in the energy generating process in CDA, utilizing international in-pile safety experimental programs such as CABRI program, as well as the out-of-pile experiments in Japan and foreign countries. This knowledge has been reflected to the development and validation of the SAS and SIMMER code. The objectives of this study are to apply these new assessment techniques to the prototype FBR and to clarify quantitatively in detail the energy generation process of CDA. In this study, an emphasis is placed on the event sequence of the melt progression phase ("transition phase") which has been recognized as one of the important issues of CDA analysis. The major parameters to be considered in this phase are the change of the mobile molten fuel mass and the history of the fuel motion, and also the relation between these parameters and energy generation mechanism. The following methods and approaches have been taken into account in this evaluation study. (a)The SAS4A code is used for the analysis of the transient behavior in the first Phase driven by core voiding ("initiating phase"), and the SIMMER-III code is used for the latter phases with melt-progression (tansition phase) and also the energy conversion process from the thermal one to the mechanical one. These codes have been developed and validated under the collaboration among PNC, CEA and FZK. (b)The uncertainty band of the void reactivity worth and Doppler coeficient has been reduced through the re-evaluation of the critical experimental data in the neutron physics area. ...