PWR and BWR reactors[28]

The maximum working temperature of the water in PWR (resp. BWR) is 325 ${}^{\circ}C$ (resp. 288 ${}^{\circ}C$ ) and the pressure 155 bar (resp. 72 bar). Because of the high pressure pipe, and even vessel rupture may occur and lead to partial or total loss of coolant. In such case, emergency core-cooling systems would come into play. If these fail, the core will melt in part or totality. This is the core melt accident. The probability of core melt accident has been calculated to be around 10^-4 per reactor-year for PWRs, and significantly less for BWRs. Although core melting induces a large release of radioactivity, the reactor countainment should prevent significant release to the external atmosphere, as was, indeed, demonstrated in the TMI accident. The probability that, despite the countainment^4.15, significant radioactivity release to the exterior occurs 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 present PWRs.

Although the above numbers appear to be small or very small, some prominent experts such as A.Weinberg[39] argued that, should nuclear power expand again a core melt probability of 10^-4 which would lead to one core melt every second year for a 5000 nuclear reactor fleet, would be socially unacceptable. It was, therefore, important to design deterministically safe reactors. Such a design is the PIUS[40] design. There the PWR reactor is immersed in a huge pool of borated water,and special, passive, locks assure that the cooling water and the borated water do not mix normally. If the pressure of the cooling water becomes abnormal the locks would open automatically and the reactor would be flooded by the borated water. This would, first, make the chain reaction impossible, and then, assure residual heat evacuation via natural convexion.