Overview of Safety of Nuclear Reactors

The utilization of nuclear power development and applications as a reliable source for energy production depends primarily on the ability of nuclear power plant operators and regulators to demonstrate excellence in safety and economic competitiveness. Since the first Enrico Fermi’ nuclear facility, the attention has been focused to manage the power from the nuclei in a controlled way. The possibility of an uncontrolled release of radioactive material leading to environment contamination and radiation exposure must be avoided following any reasonable situation. Excellent safety records have been assured from the operation of, nowadays 440 power production reactors and a much larger number of research and marine propulsion reactors summing up to more than 20000 reactor-year operation, with the noticeable exceptions of Three Mile Island-2 and Chernobyl-4 disasters both due to failure of humans, primarily. This has been possible through devoted research, qualification, discipline and proper management and involving gigantic endeavour from nuclear reactor technologists, designers, operators and safety assessors. Nowadays the human being may benefit of this incommensurable large, safe and non polluting source of energy provided a peaceful use is planned. The synthesis of the most relevant bases for the safety of operation constitutes a challenging effort that may need text-book. In this presentation an attempt is made to highlight sample keywords and to provide examples of results from recent safety analyses. First of all the nuclear safety is neither the privilege nor the ownership of a single Country, but owing to the global impact of any nuclear accident, it belongs to the humanity. The IAEA is a perfect example of international institution spreading and making available the nuclear safety technology to all Countries. The key concepts at the basis of safety classically are summarized as ‘defence-in-depth’, ‘safety-barrier’, and ‘As-Low-As-Reasonably-Achievable’. These concepts are the start of the passive, e.g. the containment, and of the active, e.g. the emergency core cooling systems, features that characterize the design of any nuclear reactor. The examples of results outlined here deal with: a) the safety evaluation of RBMK reactors (i.e. the Chernobyl type); b) the planning of new Accident-Management procedures for improving the safety of current pressurized water reactor, that also show the intrinsic quality of the original design of those systems; c) the results from a safety case, i.e. the double ended guillotine break of the largest piping connected with the pressure vessel of Atucha-2; d) the power oscillation in a Boiling Water Reactor in the ‘extreme’ case when no power limitation system is called in operation. Related to RBMK reactors, constituted by more than 1500 boiling channels, the analyses could demonstrate that the eventual low probability (10-2 – 10-3 per reactor-year) explosion of a single channel does not cause a safety challenge to the remaining intact channels. Related to the Pressurized Water Reactors of current design, it was found that following the complete unavailability of “on site and off site” electrical power (i.e. including the failure of the Diesel engines) the reactor core may survive intact for more than 10 hours only using the water stored in various parts of the plant. Related to the Atucha-2 PHWR (Pressurized Heavy Water Reactor) it was found that the safety aspects do not differ with respect to those characterizing a modern (generation 3) nuclear power plant. Related to the BWR (Boiling Water Reactor), the complex evolution of the core power predicted by advanced computational tools was shown in order to give an idea of the capabilities of those tools. The use of nuclear energy for electricity generation can be considered extremely safe and economic. Every year thousands people die in explosions in coal mines and in gas and oil extraction and transport facilities. Health and environmental impacts arising from fossil fuel use are again, incommensurably higher than in the case of the nuclear energy source.