Accident-tolerant fuel: quantifying relative risks and timescales in severe accidents
For such severe accident scenarios, a key objective for alternative 'accident tolerant' fuels (or ATFs) is to increase the threshold temperature for possible accelerated hydrogen production and/or fuel damage and core degradation. By reducing hydrogen production, this change to more ATF should also give more time for restoring power and cooling, for mitigating actions like emergency equipment deployment, and consequently effect on- and off-site evacuation decisions. There is always a finite and non-zero probability of core damage due to the likelihood and chance of not restoring power and cooling. Using the Fukushima complete Station Black Out (SBO) and Loss-of-Cooling (LOC) scenario as a starting point, this paper establishes a simple limiting estimate for the potential changes in the timescales available for power and cooling recovery and for emergency evacuation decision-making made possible by using ATFs. The straightforward analysis also provides a basis for estimating the realistic probability of avoiding core damage for a range of assumed ATF failure properties or criteria, and the needed backup cooling requirements. The analysis includes the effects on the recovery probability caused by significant disruption and chaos. The results show that for this accident scenario, there is a relatively small benefit or increase in recovery probability due to only having higher ATF failure limits. The largest relative benefit arises from not only having ATF but also assuring longer cooling by extending the boil off or time before core uncover and damage onset, from somehow adding and/or injecting sufficient make up water, and/or rejecting decay heat, say for five or more days. Obviously, as an added benefit, some ATF concepts avoid or reduce hydrogen production, which improves containment integrity but does not significantly alter the core damage or failure probability.