REASSESSMENT OF A MEDIUM POWER RR SAFETY IN LIGHT OF LESSONS LEARNED FROM FUKUSHIMA ACCIDENT (PART 1)

In the context of the lessons learned from Fukushima Daiichi NPP, the safety of a medium power research reactor under Beyond Design Basis Loss of Flow Accidents (BDB LOFA) has been reassessed. The reactor is MTR pool type with nominal power higher than 20 MW. Two LOFA accidents have been simulated; LOFA with reactor scram and the flapper valves fail to open and LOFA with reactor scram, the flapper valves fail to open and no credit for pump coast down. Full reactor nodalization and input deck with considering all the main reactor components has been carried out using RELAP5/MOD3.3. In the input deck, the core is simulated by two channels; hot channel consisting of one fuel element and average channel containing all the other fuel elements. To calculate the burnout or critical heat fluxes two external correlations applicable for research reactors are considered; Hurtado 1969 and Kaminaga 1998. The results show that, pulsed boiling will occur in the hot channel during the two accidents without excursion in the clad surface temperature. Regarding the safety constraint, while Hurtado correlation predicted a good margin against the burnout phenomenon during the two accidents Kaminaga correlation showed the occurrence of cyclic burnout with short periods in the hot channel during the pulsed boiling. Therefore, from the safety point of view, it is preferable to add additional measures to cope with the opening failure of the flapper valves