Defence-in-depth and new reactors

Defense-in-Depth (DID) is the structured approach to nuclear reactor safety that is at the basis of the safety features of the current generation of operating plants. This approach developed as a means of compensating for uncertainties in equipment and human performance, and it has evolved since the 1950's from its early use as a reactor safety guiding principle to its current broad, systematic application as an overall safety philosophy incorporating lessons learned from the current generation of operating reactors. The NRC white paper on risk-informed and performance based regulation defines DID as ''...an element of the NRC's Safety Philosophy that employs successive compensatory measures to prevent accidents or mitigate damage if a malfunction, accident, or naturally caused event occurs at a nuclear facility. This philosophy ensures that safety will not be wholly dependent on any single element...The net effect of incorporating defense-in-depth...is that the facility...tends to be more tolerant of failures and external challenges''. In practical terms, DID results from the implementation of multiple measures to prevent and mitigate accidents, to contain their consequences, and to establish an acceptable balance between prevention and mitigation. Its pervasive application in reactor safety design and regulation is translated into many precepts and technical requirements of the current body of regulation. (author)