Recently, it has been studied to apply the risk information resulting from, for example, Probabilistic Risk Assessment (PRA) to the management of Nuclear Power Plants (NPPs), aiming at the optimization of resources for the improvement of NPP safety. Namely, the bases of PRA including failure databases, safety goals and risk-informed application methods are now being established.

The risk-informed application requires monitoring continuously the risks of core damage and large early release. It is especially important to monitor the risks when the system configuration is changed due to on-line maintenance or out of service caused by any trouble. It must be...