The prediction of the time-dependent failure rate has been studied, taking into account the operational history of a component used in applications such as system modeling in a probabilistic safety analysis in order to evaluate the impact of equipment aging and maintenance strategies on the risk measures considered. We have selected a time-dependent model for the failure rate which is based on the Weibull distribution and the principles of proportional age reduction by equipment overhauls. Estimation of the parameters that determine the failure rate is considered, including the definition of the operational history model and likelihood function for the Bayesian analysis of parameters for normally operating repairable components. The operational history is provided as a time axis with defined times of overhauls and failures. An example for demonstration is described with prediction of the future behavior for seven different operational histories.
Nuclear power plants produce electricity with relatively low operational costs and low impacts on the environment, but are also associated with the fear of severe accidents. These are extremely rare, but once they occur, they put an immense pressure on the accident management team. In this paper, we report about an ongoing development of a decision support system called Severa, aimed at supporting the decision-making team during the course of an accident or a training exercise. The software is being developed in the context of the EU H2020 project NARSIS. The system assesses the plant damage state, predicts possible accident progressions and assesses available management actions and their consequences. Two already implemented modules are presented: a monitoring and radioactive-release assessment module. The methodological approach primarily relies on qualitative rule-based multi-criteria models, but also includes other techniques: data analysis, probabilistic safety assessment and eventtree modelling.
Being a facility with potential for radioactive release, any nuclear power plant (NPP) is, over its operating life time, permanently subject to numerous safety reviews with different scopes and objectives. The reviews may be initiated and implemented by various stakeholders, including regulators, utilities or industry. Some of them are, by their nature, general and extensive in terms of different safety areas or safety attributes which are covered. An example of such a review is a Periodic Safety Review (PSR) which is promoted by the International Atomic Energy Agency (IAEA) and a number of national safety authorities in Europe and worldwide. The others may, depending on the objective, be targeted at particular safety area (e.g., ageing management or equipment qualification or safety analyses). Both of the mentioned cases (single general review or multiple targeted reviews over a time period) can generate an inventory of observations (“findings”) or “issues” which need to be addressed but may be very different in their nature and implications, as well as in benefits or resources associated with their resolutions. For some issues a resolution may be straightforward. For others, it may require a feasibility study and identification of options for possible resolution. Also, in some cases the resolution is simply a “must” (e.g., discrepancy from licensing basis) while in some other cases it may be a matter of balance (e.g., effectiveness of maintenance program). Furthermore, while some of the issues may be directly related to operational safety (e.g., non-compliance with single failure criterion or aging-related degradation of safety features), for some others the link to operational safety may not be explicit (e.g., comparison of safety bases against the newly emerging methodologies or issues observed with regard to so called “soft factors”).
The paper discusses types of different observations or issues which may come from general or targeted safety reviews and outlines some basic principles for their comparison and prioritizing with regard to possible safety impacts, which is many times needed for the purpose of developing an action plan for safety improvements.
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