PROBABILISTIC SAFETY ANALYSIS

Probabilistic safety analysis is applied at all stages in the lifetime of the NPP (design, operation, and lifetime extension). During the plant’s safety assessment, the probabilistic and deterministic safety analyses are used simultaneously so that to complement each other. The purpose of probabilistic safety analysis is:

• Supporting to provide systematic analysis, to ascertain that the design solutions (systems, procedures, and etc.) are in compliance with the general safety objectives;
• Supporting to demonstrate that a balanced safe design has been achieved, that no single event makes a disproportionately large or significantly uncertain contribution to the overall risk;
• Supporting to identify systems’ procedures and improvements that can reduce the probabilities of severe accidents’ occurrence or mitigate their consequences.

The PSA activities for the ANPP was initiated in 2002. The NRSC also established the Risk Assessment Group (RAG), which plays a key role in the PSA activities for the ANPP. The main activities of RAG are:

• PSA model development,
• PSA model application, and risk-informed decision making,
• time-dependent reliability analysis (ageing PSA),
• development of national regulations in the field of nuclear safety,
• PSA training, and
• international co-operation.

PSA Model Development

The RAG is actively involved in the development of the ANPP Unit 2 level 1 PSA models for the following initiators:

• internal events,
• internal fires, and
• external hazards (excluding seismic events).

Risk profile for ANPP Unit 2

Training course on NPP fire safety visiting worms (Biblis NPP, Germany)

The RAG continuously updates the mentioned PSA models so that to assure their applicability for safety-related decision making.

Special attention is paid to the development of the RAG’s capabilities in the field of fire simulation and fire impact analyses. Currently the NRSC works with two computer codes devoted to fire simulation – CFAST and SYLVIA. CFAST code is used extensively for the fire PSA analysis, whereas the SYLVIA code is currently used for modeling of ventilation systems for critical compartments, as well as for the CFAST results’ verification.

CFAST model of ANPP Unit 2 boron unit

In the field of fire modeling, the professionalism of our employees are developed through local and international trainings, workshops, real analytical practices, investigation of experimental results and other relevant activities.

PSA Model Application and Risk-Informed Decision Making

The PSA models are periodically applied for investigation of specific problems and decision making. The application of plant-specific PSA includes:

• modernization activities’ prioritization,
• PTS investigation (selection of accident scenarios),
• justification of the allowable outage times in the technical specifications, and
• justification of the symptom-based emergency operating procedures (selection of bounding scenarios).

Event tree constructed for selection of critical accident scenarios potentially leading to PTS

Currently, the RAG is in the process of PSA application for the maintenance effectiveness monitoring. From this perspective the PSA is mainly used for the development of indicators and probabilistic criteria for maintenance effectiveness evaluation. In addition, jointly with the ANPP and the IAEA, the NRSC works on the PSA’s application that is meant to identify the combinations of external hazards which are critical for the ANPP. This analysis is performed by the fault sequence analysis’s approach developed and promoted by the IAEA.

Time-Dependent Reliability Analysis (ageing PSA)

According to the PRIS database, most operating reactors have been used for more than 20 years, consequently, the ageing problem gradually becomes more critical. The time-dependence of reliability, on the other hand, is not fully reflected in the current PSA models. Therefore, the NRSC has initiated activities aimed at incorporating ageing in the PSA. In 2006, jointly with the ISTC (project A-1492) and Ageing PSA European Network (EC JRC IET, IRSN, INR, NRI REZ, LEI, and others), the NRSC launched this research project. The purpose of the research is to predict the risk profile behavior in time so that to make the PSA-based decision making process more comprehensive. During this research the NRSC’s achievements were the following:

• collection of wide range statistical data for the VVER-440 safety related equipment;
• implementation of ageing trend analysis for selected components;
• incorporation of analysis’ results in the ANPP PSA model and the development of ageing PSA models which allow to estimate the risk profile evolution for more than 15 years;
• application of the ageing PSA models for risk-informed decision making (feasibility studies were done for risk-informed systems’ ranking and modernization prioritization);
• performance of reliability prediction for safety related pipe segments at the ANPP (by WinPRAISE code).

Increasing trend evaluation for switcher’s failure rate parameter

Research activities are done in close co-operation with Ageing PSA European Network members such are EC JRC IET (Netherlands), IRSN (France), INR (Romania), NRI REZ (Czech Republic), LEI (Lithuania), and others. Together with the APSA Network, the NRSC actively got involved in the review of the PSA topical guidelines and implementation of case studies aimed at verifying the applicability of the ageing PSA, namely:

• review of the qualitative approach for systems’ selection, structures and components for the Ageing PSA (EUR23446EN);
• review of the guidelines for the analysis of data which relates to the NPP’s components’ and systems’ ageing (EUR23954EN);
• case study of the ANPP’s Unit 2 for the APSA component selection;
• data analysis related to the VVER-440 equipment’s agеing (EUR24643EN);
• feasibility study on the applicability of risk-informed decision process of the results that are obtained by using the ageing PSA model (EUR26606EN).

NRSC was involved in the EC JRC publications related PSA topical guidelines and implementation of case studies

Development of National Regulations in the field of Nuclear Safety

In the development of national regulations the RAG is involved in the following fields:

• requirements of the PSA,
• regulatory review guide on the PSA,
• requirements to the format and content of safety assessment report,
• requirements of reactor’s site evaluation,
• requirements of the plant lifetime extension,
• requirements of the maintenance effectiveness evaluation,
• modification implementation procedure, and
• requirements of design safety.

In terms of the national regulation development, the RAG mainly works on the development of probabilistic criteria, on the identification of requirements for systems’ reliability evaluation and performance monitoring, on the development of external and internal hazards’ analysis’ principles, and on other tasks.

PSA Training

The RAG also provides trainings to the NRSC’ newcomers, the Armenian Nuclear Regulatory Authority’s staff, and to university students in the field of probabilistic safety assessment and risk-informed decision making. Moreover, the RAG specialists often receives invitations to take part in international trainings as lecturers. PSA trainings usually cover the following courses (modules):

• Module 1 (the NPP safety and technology)

This section provides the basic information on nuclear safety concepts (safety functions, safety principles, safety assessment, and accidents categorization), and basic information on the NPP technology (plant design, safety features, and plant response logic).

• Module 2 (the introduction to the PSA)

This section provides detailed training on the PSA’s methodology, as well as a brief overview of the PSA’s applications and risk-informed decision making. The PSA methodology training covers all main elements of the PSA (PSA scope, initiating event analysis, accident sequence analysis, system analysis, human reliability analysis, data analysis, and CCF modeling), and the implementation of practical examples for specific PSA tasks. In addition, the 2nd module provides brief information on possible uses of the PSA models, risk-informed decision making principles, and available regulations in this field.

• Module 3 (the advanced PSA information)

This section covers the investigation of the PSA’s specific topics and the familiarization with PSA related codes. Particularly, the trainees deeply investigate the aspects related to the PSA applications and risk-informed decision making, time-dependent reliability modeling, incorporation of ageing aspects in the PSA, fire simulations, and others. The trainees also become familiar with appropriate computer codes such as Risk Spectrum, CFAST, WinBUGS, and others.

NRSC participated in the ANRA staff’s two-stage special PSA training, namely:

• Stage 1 (project AR/RA/03, 2006-2008) – a basic training course on the PSA provided by the NRSC, AVN and GRS.
• Stage 2 (project AR/RA/04, 2008-2010) – a training on the PSA regulatory review and risk-informed decision making provided by NRSC, STUK and UJD.

International Cooperation

In the PSA field, the RAG is deeply involved in several international activities and topical projects such as working group activities, missions, and trainings. The main international activities are described below.

The VVER Regulatory Forum PSA Working Group

To examine the use of the PSA in countries that have the VVER-440 NPPs in operation, the VVER Regulatory Forum established the PSA Working Group (WG) in 2002 (http://wwerforum.org/index.php/en/current-working-groups/probabilistic-safety-assessment). The WG’s aim is to summarize the regulatory use and practices of the PSA, and to compare the PSA’s attributes. In the PSA WG activities the following countries have been involved: Armenia, Bulgaria, Czech Republic, Finland, Hungary, Russia, Slovakia, Ukraine and India. IAEA and GRS have a role of observers in PSA WG. Since the PSA WG’s first mandate in 2005, the RAG has been participating in these events. During this period, the RAG took on the following tasks:

• harmonization of principles on regulatory use of the PSA;
• comparison of the PSA approaches and risk-informed decision making regulations;
• investigation of the national legislation related to the PSA; and
• comparison of particular initiators modeled in the PSA (fires, specific internal events, and so on).

VVER Regulatory Forum PSA WG 4th mandate kick-off meeting (ANRA, June 2014)

For the VVER Regulatory Forum Board, the PSA WG filed a report on the PSA elements’ comparison, legislation analysis, and, also, made recommendations that aim to improve the national practices in the PSA field. Currently, the NRSC oversees the PSA WG 4th mandate’s activities devoted to external hazard analysis, risk-informed regulatory applications, and regulatory perspectives on reliability centered maintenance.

Ageing PSA European Network

In 2005, under the umbrella of European Commission (having European Commission Joint Research Center, Institute of Energy and Transport (EC JRC IET) as the coordinator of the Network), the EC JRC IET established the European Network on Ageing PSA (http://safelife.jrc.ec.europa.eu/apsa/). The APSA Network aims at creating a common platform of interested parties from Europe and beyond to exchange information and latest developments in the area of incorporating ageing effects into PSA, use ageing PSA (APSA) applications, and perform common case studies in close relation to corresponding activities by the OECD and the IAEA. NRSC joined the APSA Network in 2006. During this period the RAG has been involved in the following activities:

• review of the APSA Network guidelines,
• case studies and feasibility studies,
• component reliability data collection and analysis, and
• risk-informed decision making by using the APSA model.

Currently the RAG focuses on time-dependent reliability analysis of passive components.

Cooperation with the IAEA

The cooperation between the RAG and the IAEA PSA team covers the following professional areas:

• The FSA-ANPP project. In cooperation with the ANPP, the RAG works on the IAEA project FSA-ANPP, that aims at applying the fault sequence analysis method (FSA) to the ANPP. The FSA method, which is developed by the IAEA, allows to systematically analyze the combination of external hazards for the particular NPP.
• The IPSART mission. The RAG was involved in the IPSART mission in Kozloduy NPP (KNPP) Unit 5&6. The mission aimed at reviewing the full scope PSA model for the KNPP Unit 5&6.
• The TECDOC development. The RAG is involved in the development of the IAEA TECDOC on integrated risk-informed decision-making. TECDOC presents the framework and main elements of the current approach to integrated risk-informed decision making (IRIDM), which is used in several member states and provides guidance on implementation of the process. Currently TECDOC is in the last stage of development.
• Training activities. The RAG experts were involved as lecturers in the PSA training activities organized by the IAEA. The trainings were mainly devoted to the PSA methodology and practical examples of the PSA applications.

Plant walkdown in the frame of IPSART mission

Cooperation with US NRC/BNL

The RAG cooperates with the US NRC and the BNL mainly in the field of risk-informed applications and in the field of specific PSA issues. This cooperation covers specific project’s implementation and topical workshops. In the field of the PSA, the RAG jointly with the US URC and the BNL are mainly focused on the following aspects:

• risk-informed technical specifications and risk-informed regulatory inspections,
• pipe reliability evaluation that uses WinPRAISE code,
• development of probabilistic criteria for maintenance effectiveness evaluation, and
• NPP long-term operation.

Visit to Oyster Creek NPP in the frame of meeting with BNL/USNRC on risk-informed regulatory inspections