Safety and control area series – Safety analysis

What is safety analysis?

The main purpose of safety analysis is to verify that applicable safety requirements are met in all lifecycle phases of a nuclear power plant (NPP) and to demonstrate that each reactor has adequate safety margins under normal and accident conditions. Safety analysis is a systematic evaluation of the potential hazards associated with the conduct of a proposed activity or facility and considers the effectiveness of preventive measures and strategies in reducing the effects of such hazards.

Safety analysis involves deterministic analysis and probabilistic assessment in support of the siting, design, commissioning, operation or decommissioning of an NPP. The following table provides an overview.

Probabilistic safety assessment

Deterministic safety analysis

Probabilistic safety assessment (PSA) focuses on evaluating the overall risk arising from various events to a nuclear power plant. Often the PSA concept is described as a "triplet" comprising three fundamental questions. 1) What can go wrong? 2) How likely is it? 3) What are its consequences? PSA provides insights into the strengths and weaknesses of the design and operation of a nuclear power plant, as it assists plant management and the regulator to target resources where the largest benefit for plant safety can be obtained. The primary objectives of the PSA are to identify the sequences of events and their probabilities, which lead to challenges to fundamental safety functions, loss of integrity of key structures, release of radionuclides into the environment and public health effects develop a well-balanced NPP design assess the impact of changes to procedures and/or components on the likelihood of core damage

Deterministic safety analysis (DSA) focuses on evaluating the consequences of various events in a broad range of operating conditions to confirm that the design of an NPP meets design and safety requirements derive or confirm operational limits and conditions that are consistent with the design and safety requirements for the NPP assist in establishing and validating accident management procedures and guidelines assist in demonstrating that safety goals, which may be established to limit the risks posed by the NPP, are met A safety analysis program includes interfaces with other programs as necessary to ensure that safety analysis is initiated when needed and that the results of the safety analysis are used appropriately. For example, appropriate safety margins should be applied to address the uncertainties and limitations of PSA.

There are five main CNSC specialist divisions that work within this SCA:

• Probabilistic Safety Assessment and Reliability Division
• Physics and Fuel Division
• Reactor Behaviour Division
• Reactor Thermalhydraulics Division
• Engineering Design Assessment Division

CNSC staff consider how licensees are meeting the regulatory requirements relevant to safety analysis with a focus on the compliance verification of the following specific areas:

• hazard analysis, considering all potential hazards within a facility (such as a fire) and natural hazards (such as an earthquake)
• PSA, looking at the likelihood of an accident occurring
• DSA, looking at outcomes that will happen if an accident occurs
• critical safety, looking at preventing nuclear and radiation accidents from inadvertent, self-sustaining nuclear chain reactions
• severe accident analysis, examining severe accidents involving core melt, containment performance, potential for release of fission products to the environment, and effectiveness of operators’ actions to stop severe accident progression and mitigate the radiological consequences
• management of safety issues (including R&D programs and accident management procedures and guidelines), looking at how the licensee forecasts, plans and implements safety systems and mitigation strategies for any potential consequences resulting from safety issues

Safety analysis quick facts

• Safety analysis tools show that the risk to the public is very minimal.
• DSA (known consequences of accidents) and PSA (the likelihood and potential consequences of accidents) complement each other to create a comprehensive safety risk picture.
• DSA can use either conservative or best-estimate methods. Analyses of initiating events include:
  • internal initiating events (e.g., loss of coolant, moderator events, spent fuel bay events, reduction of primary flow)
  • external initiating events (e.g., fire, seismic, flood, wind/tornado)

Safety analysis trends

Following the events at the Fukushima Daiichi nuclear power plant in 2011, the global nuclear regulatory community decided it was necessary to study more low-probability initiating events, their consequences and mitigation strategies, and ways to incorporate these considerations in severe accident management guidelines for plants and in global and domestic regulatory procedures.

Ongoing efforts and further research are being made to increase our understanding of physical processes and phenomena encountered in reactor accidents, and enhance our modelling capability and the safety analysis methodologies for treatment of uncertainties and quantification of safety margins. Efforts in all of these areas will continue to evolve, to strike the proper balance for public, worker and environmental safety.