Release-Category-Oriented Risk Importance Measure in the Frame of Preventive Nuclear Safety Barriers

After the Fukushima accident, the interest on the field of severe accidents has largely increased, both on the management aspects to improve the prevention of severe accident progression and mitigate their consequences, but also in sponsoring research activities focused on reducing the uncertainty still present on physical and chemical phenomena and processes taking place during a postulated severe accident. One of the most relevant and comprehensive approaches to look into the field of severe accidents consists of the Level 2 Probabilistic Risk Assessment (PRA). In order to reveal the relative probabilistic weight each system, structure or component contributes with to the integrated core damage frequency, several consolidated risk measures are available for application, among which Risk Reduction Worth or Fussell-Vesely. This paper discusses the nature of the different approaches underlying the existing nuclear safety barriers and introduces an innovative severe accident risk importance measure. This innovative risk measure takes into account the entire spectrum of accidents leading to radioactive releases rather than only focusing at large and early releases. By applying this tool, the importance is shifted from a consequence-oriented to a frequency-oriented tool where the contribution of the different elements of the plant will be ranked according to their impact on the total radioactive release frequency.