Safety analysis of different possibilities for the RA-3 fuel management strategies at 10 MW

Recently, National Atomic Energy Commission (CNEA) has been licensed to operate the RA-3 reactor at 10 MW, after a start up period during which foreseen tests were carried out with increasing power values from 5 MW up to 10 MW. If the reactor starts to operate at 10 MW as its normal power level, and the same fuel loading strategy is maintained, (1 fuel element / cycle), then the cycle (time) length will decrease in around 50% as compared to the corresponding 5 MW cycle length. For this reason, it may be interesting for the operator to load 2 FE / cycle to maintain the cycle length at approximately the same value as that for 5MW. In fact, the loading of two fresh fuel elements (or a fresh one and a slightly depleted one), has already been performed. The purpose of this work is to study the long term core behaviour (with different fuel management strategies) and provide quantitative arguments for a safety assessment under normal operating conditions. Flexibility of the fuel management strategy is analysed with the reactor operating at 10 MW, preserving compliance with the corresponding standards. It is concluded that operation of the reactor at 10 MW with both types of fuel strategies, that is to say, 1 FE / cycle or 2 FE / cycle, is totally compatible with standards in force. Nevertheless, for the case of 2 FE / cycle, the core excess reactivity at the beginning of cycle could be such that criticality might be achieved before the second control rod is completely withdrawn. If such were the case, power start up is disabled due to a condition imposed by the reactor protection system. Although according to Argentinean standards such condition is not mandatory, it is advisable, from the safety point of view, to analyse those actions that could be a consequence of a systematic repetition of this situation. (author)