An attempt was made to develop a new model for analysis of the effect of power ramping rate ($\dot{P}$) on probabilities of fuel-cladding failure under power ramping conditions, although magnitude of power increase (ΔP) was generally considered as a main factor that is reponsible for cladding failure. In the present study a model expressed as a function of strain rate that is known as a parameter sensitive indirectly to power ramping rate and that influences susceptibility to stress corrosion cracking(SCC) was combined with a model related to stress.
The results calculated with use of the present model were compared with experimental data(Studsvik Over-Ramp) and those computed with use of SPEAR-BETA model and PROFIT model. As power ramping rate and/or magnitude of power increase increased, failure probability also increased. It was also observed that magnitude of power increase was a primary factor and power ramping rate was a secondary factor that influences fuel-cladding failure. The present results showed a trend that was not observed with use of SPEAR-BETA model but that was similar to the trend obtained with use of PROFIT model for the effects of power ramping rate and magnitude of power increase.
The effect of strain rate that correlates with power ramping rate dominated during holding period after power ramping and failure probability tended to increase during holding period. Hence the effect of strain rate during holding period seems to be correlated with the increase of SCC susceptibility although strain rate decreases during holding period. The effect of strain rate was also considered to reduce delayed failure time that is generally due to delay of iodine release and delay of increase in hoop stress by irradiation creep of Zircaloy cladding.
