Probabilistic Risk Assessment (PRA), which is also known as Probabilistic Safety Assessment (PSA), is a method to assess and control risks in complicated systems, like nuclear power plants. PRA is used in identification and quantification of possible failures, which helps to improve safety and reliability of a nuclear power plant. The advantage of this method is to identify specially complicated failure combinations that would otherwise be hardly identifiable due to the limitations of human perception.

PRA of nuclear power plants is divided into three levels. Level 1 PRA assesses the frequency of nuclear fuel damage originating from internal and external initiating events. Level 2 PRA assesses the frequency of large and early releases originating from nuclear fuel damage in the reactor core or the spent nuclear fuel pool. Level 3 PRA is used to assess the effects of radionuclide releases in the environment.

Preparation and updating of the PRA must be started at the beginning of a nuclear power plant project because the first plant specific PRA model and documentation will be submitted to STUK for acceptance with the construction license application. In this phase, it is necessary to identify plant specific threshold values for internal and external events. Exceeding these critical thresholds would lead to inevitable consequences. Essentially, the PRA model is important in understanding that the design of the nuclear power plant is justifiable and balanced overall.

What can be done with PRA?

1. Comprehensive risk management: PRA covers the whole nuclear power plant, all its safety significant systems and components. Additionally, the PRA takes into account how nuclear power plant is operated and maintained and how operators are qualified for different tasks. The PRA also considers local external circumstances, and what kind of hazards are possible, like weather conditions.
2. Analyses of components and systems: PRA can be used to model reliability of the systems even for failures of single components, like circuit cards and their effects to safety and operation of the whole nuclear power plant. This helps to identify critical components for safety, which enables building a nuclear power plant where safety is balanced between systems. PRA is used to ensure that safety classification of the systems is in line with safety significance of system.
3. Support in the beginning of the building project: PRA can be integrated in the beginning of a building project, demonstrating the nuclear power plant’s suitability to the Finnish environment and updating external events for selected nuclear power plant site.
4. Assessment of the Operational Limits and Conditions (OLC): PRA can be used to balance and justify the operational limits and conditions, which define limits for safe operation of the nuclear power plant. Also, the maintenance and inspection programs need to be assessed using PRA.
5. Planning of trainings: PRA helps to plan training programs highlighting most significant risks and accident event sequences, which improves the preparedness and actions of the personnel.

The PRA model is continuously updated based on realized construction during the construction phase. Special attention must be given to the identification of common cause failures, where for example the failure of components of the same type could cause simultaneous failure of multiple systems. Additionally, the analysis of human errors needs to be analyzed using methods of human reliability analysis (HRA) and added as part of the PRA model while operating procedures for accidents and operational occurrences will be developed.

At Platom Petri, Simo and Toivo have worked widely within PRA. They have managed PRA programs, served as site experts, and liaised with regulatory bodies. Their experience includes updating reliability data, performing seismic PRA, fire PRA, developing common cause failure models, and studying time-dependent reliability calculations. They have also conducted severe accident analyses using MELCOR code, updated safety analysis reports, and participated in emergency preparedness drills. Additionally, they have been involved in plant modification projects, safety reviews, radiation safety assessments, and the development of licensing documentation. Their responsibilities have included creating full-scale PRA models for spent fuel pools, updating PRA documentation, and managing safety upgrade projects. They can take the results of the PRA, turning it into insight that benefits decision-making.

PRA has been an essential tool for assessing reliability and safety of nuclear power plants for a long time already. PRA can be used to make justified decisions and improve the operation of a nuclear power plant at all levels. Hopefully, this publication will help to spread information about the significance of PRA as well as its applications with new build nuclear power plants in Finland!