The influence of MFCI modelling on the-predictions of accident codes used in the wac group comparative exercise

This paper is based on work carried out in support of the continuing effort underway within the framework of the Whole Core Accident Codes (WAC) group at Brussels, to evaluate and compare the predictions of different available whole core accident codes for the LMFBR. It is shown that considerable changes in MFCI predictions can result in modest differences to the predictions of energetics in the 1 $s-1 TOP (transient overpower) comparative exercise undertaken by the WAC group. This is reassuring in view of the uncertainties associated with the phenomenon. In particular uncertainties in the sodium EOS at elevated temperatures, although producing a significant effect on two-phase pressures, led to modest changes in the energetics of this particular incident. On the other hand, it is suggested that significant effects can follow from changes in the choice of basic parameters in the Cho-Wright model of MFCI and discussion is sought on the selection of data. The requirement for a treatment of vapour blanketing is indicated and comments on the various possible models sought. Finally the agreement achieved between the various simulations within the restrictive specification of the 1 $s-1 TOP incident are outlined