Safety analysis
By the term safety analysis an analytical study is meant by which it is demonstrated how safety requirements, such as ensuring the integrity of barriers against radioactive releases and various other requirements, are met for initiating events (both internal events and external hazards that may initiate events) occurring in a broad range of operating conditions, and in other circumstances, such as varying availability of the plant systems. Two properly balanced complementary methods of safety analysis, deterministic and probabilistic, are used jointly in evaluating the safety of an NPP. See SSR 2/1 (Rev. 1), Req. 42
Read more → and SSG-2 (Rev.1) Read more → and SSG-3. Read more →
Probabilistic Safety Analysis
For determination of the full spectrum of challenge mechanisms, useful guidance can be obtained from the Probabilistic Safety Assessment (PSA). A PSA can estimate risk by computing what can go wrong, how likely is it, and what are its consequences:
• PSA level 1 determines the core damage frequency due to a broad spectrum of initiating events which can be internal to the plant (such as equipment failures), external to the plant (seismic events, external flooding events or other natural disasters).
• PSA level 2 determines the progression of core damage events identified in the level 1, the magnitude of challenges to fission product boundaries and the magnitude and timing of releases of radioactivity to the environment.
• For some plants a PSA level 3 is also performed, which analyses the atmospheric dispersion of the releases and their consequences, including health effects and land contamination. These serve as a quantitative input for the on-site and off-site emergency response.
IAEA standards for the conduct of a PSA are described in IAEA → SSG-3 and → SSG-4.
Deterministic Safety Analysis
The objective of deterministic safety analysis for nuclear power plants is to confirm that safety functions can be performed with the necessary reliability and that the necessary structures, systems and components, in combination where relevant with operator actions, are capable and sufficiently effective, with adequate safety margins, to keep the releases of radioactive material from the plant below acceptable limits. Deterministic safety analysis is aimed at demonstrating that barriers to the release of radioactive material from the plant will maintain their integrity to the extent required.
The aim of deterministic safety analyses performed for different plant states is to demonstrate the adequacy of the engineering design, in combination with the envisaged operator actions, by demonstrating compliance with established acceptance criteria.
Deterministic safety analyses predict the response of the plant to postulated initiating events, alone or in combination with additional postulated failures. A set of rules and acceptance criteria specific to each plant state is applied. Typically, these analyses focus on neutronic, thermohydraulic, thermomechanical, structural and radiological aspects, which are analysed with appropriate computational tools. Computational simulations are carried out specifically for predetermined operating modes and plant states.
The results of computations are space and time dependent values of selected physical variables (e.g. neutron flux; thermal power of the reactor; pressures, temperatures, flow rates and velocities of the primary coolant; loads to physical barriers; concentrations of combustible gases; physical and chemical compositions of radionuclides; status of core degradation or containment pressure; and source term for a release to the environment).
Areas of application of DSA
Deterministic safety analysis may be carried out for a number of applications, including:
• Design of nuclear power plants by the designer, or verification of the design by the operating organization. Design analysis is used in the design of a new plant or in modifications to the design of an existing plant, so that the designer can confirm that the design meets the relevant design and safety requirements.
• Safety analysis for licensing purposes (for authorizations), including authorizations for different stages for a new plant. Licensing analysis is used in the design of a new plant, or in modification of the design of an existing plant, to provide evidence to the regulatory body that the design is safe. Regulatory bodies may require new calculations when new evidence arises from research, both theoretical and experimental, or from operational experience at the plant or similar plants.
• Independent verification of the safety analysis by the regulatory body;
• Updating of safety analyses in the context of a periodic safety review to provide assurance that the original assessments and conclusions are still valid;
• Safety analysis of plant modifications;
• Analysis of actual operational events, or of combinations of such events with other hypothetical faults exceeding the limits of normal operation (analysis of near misses);
• Development, verification and validation of emergency operating procedures.
Emergency operating procedures (EOP) define the operator actions during anticipated transients and in accident conditions. Analyses by sophisticated computer codes are used to support the development and verification of EOPs. For validation, where possible, use should be made of plant simulators.
• Development of severe accident management guidelines and emergency planning. Analysis of accidents for supporting accident management describes the plant behaviour in DECs. Operator actions are normally accounted for in the assessment of DECs. The results from analyses of DECs are used to develop operator strategy, the main goals being to prevent severe core damage and to mitigate the consequences of an accident in the event of core damage. Analysis is needed to develop threshold values to initiate SAMG actions, and to develop scenarios for the validation of the SAMG and training for plant staff.
• Demonstration of success criteria and development of accident sequences in Level 1 and Level 2 probabilistic safety assessments.
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