Core Restraint Analysis for 75 MW Power Ascension Tests of Experimental Fast Reactor 'JOYO'

摘要

In the "JOYO" initial power ascension tests to 75 MW, unexpected large negative power coefficients were observed during ascension to new power levels beyond 50 MW, which results in approximately a 400 permanent loss in reactivity. This anomaly was studied from the viewpoint of radial dimensional change in the core using a three-dimensional structural analysis code along with a whole-core thermal-hydraulic analysis code. Conclusions drawn from the study can be summarized as follows :(1) The absolute value of the power coefficient increases sharply beyond the 50 MW power level due to the outward leaning and thermal bowing of core elements, but the calculated increase in the coefficient is less than 20% of the observed value.(2) At most, an 80 permanent loss in reactivity is caused by this outward leaning, which is 20% of the observed value. (3) These results confirm that the radial dimensional change can not be the main cause for the power coefficient anomaly.