The maximum operating temperature of the water in PWR (resp. BWR) is 325 °C (resp. 288 °C) and the pressure 155 bar (resp. 72 bar). Because of the high pressure a pipe or vessel rupture may occur and lead to partial or total loss of coolant. In such a case, emergency core-cooling systems would have to come into play. If these fail, the core will melt partially or completely. This is called a core melt accident. The probability of a core melt accident has been calculated to be around 5 x 10~5 per reactor-year for PWRs,[21] and 4 x 10~6 per reactor-year for BWRs [59]. Continuous safety improvements have been made, taking advantage of experience, and recent safety evaluations yield a core melt probability of 10~5 per reactor — year for PWRs [60]. Although core melting induces a large release of radioactivity, the reactor containment structure should prevent significant release to the external atmosphere, as was indeed demonstrated in the Three Mile Island accident. The probability that, despite the containment,^ significant radioactivity would be released to the exterior is one to two orders of magnitude lower than that of core melting. The risk for an individual living in the vicinity of the reactor to die from a cancer induced by accidental radioactivity release is estimated to be around 10~8 for today’s PWRs.

Although the above numbers appear to be small or very small, some prominent experts such as Weinberg [61] have argued that, should the use of nuclear power expand again, a core melt probability of 10~4 (which would lead to one core melt every other year for a 5000 nuclear reactor fleet) would be socially unacceptable. It was, therefore, important to

design deterministically safe reactors. Such is the PIUS [62] design. The PWR reactor is immersed in a huge pool of borated water, and special passive locks ensure that the cooling water and the borated water do not normally mix. If the pressure of the cooling water becomes too high the locks open automatically and the reactor is flooded by the borated water. This would, first, make the chain reaction impossible, and second, ensure residual heat evacuation via natural convection. However PIUS would have a small thermodynamical efficiency and make inspection and maintenance difficult.