Some Implications of Theoretical Relation between RAW and RRW Measures on Risk Reduction Strategies

In using risk-informed approaches for ensuring safety of operating NPPs(nuclear power plants),risk importance measures obtained from PRAs(probabilistic risk assessments)of the plants are integral elements of consideration in many cases.In PSA models and applications associated with NPPs the risk importance of a particular feature(e.g.function,system,component,failure mode or operator action)can be,most generally,divided into two categories:importance with respect to risk increase potential and importance with respect to risk decrease potential.The representative of the first category,as used for practical purposes,is RAW(risk achievement worth).Representative of the second category,as mentioned in consideration of risk importance,is RRW(risk reduction worth).It can be shown that the two risk importance measures,RAW and RRW,are dependent on each other.The only parameter in this mutual dependency is probability of failure of the considered feature.The paper discusses the relation between RAW and RRW and some of its implications,including those on the general strategies for the reduction of risk imposed for the operation of the considered facility.Two general risk reduction strategies which are considered in the discussion are:a)risk reduction by decreasing the failure probability of the considered feature;and b)risk reduction while keeping the failure probability of the considered feature at the same level.Simple examples are provided to illustrate the differences between two strategies and point to main issues and conclusions.

Upgrade to Nuclear Power Plant Krsko Internal Flooding Probabilistic Safety Analysis

The original internal flooding probabilistic safety analysis （PSA） study of Krsko Nuclear Power Plant （two-loop Pressurized Water Reactor （PWR） plant of Westinghouse design） was performed in mid nineties and limited to reactor core damage risk （Level 1 PSA）. In 2003, it was, together with other safety and hazard analyses, subject to the Periodic Safety Review （PSR）. In the PSR, it was stated that methodological PSA approaches and guidelines have evoluted during the past decade and several observations were provided, concerning the area screening process, residual risk and treatment of plant damage states and risk from radioactivity releases （i.e., Level 2 PSA）. In order to address the PSR observations, upgrade ofKrsko NPP internal flooding PSA was undertaken. The area screening process was revisited in order to cover the areas without automatic reactor trip equipment. The model was extended to Level 2. Residual risk was estimated at both Level 1 and Level 2, in terms of core damage frequency （CDF） and large early release frequency （LERF）, respectively.