Contributions of inherent and passive features of ACP100 at this defense in depth level are as follows: when core uncover is assumed, only for analytic purposes, low heat-up rates of fuel elements in the exposed part of the core are predicted, if the geometry is still intact. The characteristic time of core melting is long, eventually preventing temperature excursion due to a metal-water reaction, which in turn limits the hydrogen generation rate. The hydrogen concentration in the containment is reduced by catalytic recombiners. There is sufficient floor space for cooling of molten debris and extra layers of concrete are used to avoid direct exposure of the containment basement to debris.

18.3.7.4 Level 5: Mitigation of radiological consequences of significant release of radioactive materials

The following passive features of ACP100 make a contribution to this defense in depth level: relatively small fuel inventory, compared with larger NPPs; slower progression of accidents and increased retention of fission products (facilitated by such features as reduced power density, increased thermal inertia, etc.); the containment is located inside the airplane protection concrete and underground building, which reduces the release of fission products due to local deposition. The ACP100 concept provides for extended accident prevention and mitigation by relying on the principles of simplicity, reliability, redundancy and passivity.