CHAPTER 1: SELECTION OF STRATEGIES

Selection of strategies

In Module 2, an overview of all important severe accident phenomena was given together with their potential challenges to fission product barriers and associated fission product releases. The strategies how to mitigate those challenges were presented as well. For a particular plant, not all those challenges may be relevant, neither the indicated releases, or different challenges may occur, not yet considered in Module 2. Hence, for each particular plant it must be investigated how severe accidents evolve and which fission product barriers (FPB) are challenged / which releases will occur.

For each accident sequence under consideration it should be determined how FPBs are challenged / releases occur, and how severe these challenges/releases are. This process is called search for plant vulnerabilities, see SSG-54, para. 3.11 and 3.12. Read more → The search investigates how specific accidents will challenge critical safety functions and, if these are lost and not restored in due time, how the core will be damaged and how the integrity of other fission product barriers will be challenged. Note that plant vulnerabilities are highly plant-specific and a generic list of vulnerabilities, resulting from a generic approach / reference plant, should not be directly applied to the specific plant under consideration.

A useful tool to support the analysis of plant vulnerabilities is the plant-specific PSA (if it exists). Other approaches can also be used. Guidance on the role of analysis in SAMG development, including its use to search for vulnerabilities, is provided in SSG-54 paragraphs 3.99 to 3.111. Read more → A frequently used approach is documented by the Nuclear Energy Institute (USA), NEI 91-04, rev. 1. contains a set of analyses that can be used alternatively. Read more → A frequently used approach is documented by the Nuclear Energy Institute (USA), NEI 91-04, rev. 1.

Apart from analysis, plant vulnerabilities may also be obtained from engineering judgement, operating experience, insights from related plants, and results from research. In this way, a comprehensive set of insights in the behaviour of the plant during a DEC, without significant fuel degradation or with core melt, should be obtained. It identifies the phenomena that may occur and their expected timing and severity.

Apart from the plant vulnerabilities, it is also important to search for the plant capabilities, i.e. the areas where the plant is 'strong', less or not sensitive to FPB challenges, and having ample capabilities to execute mitigative strategies. This includes the response of the plant to mitigative strategies (for example, flooding the lower head of the reactor pressure vessel to prevent or delay reactor pressure vessel melt-through). See SSG-54, paragraphs 3.13 to 3.19. Read more →

For the severe accident domain, the insights obtained under the search for plant vulnerabilities and plant capabilities should be collected and documented in what is usually called the Technical Basis for severe accident management.

An example of a set of such insights for a BWR is given in the Table 3-1 below. In addition, a widely used Technical Basis is found in the following EPRI reference. Read more →


Table 3-1: Example of the development of severe accident management insights for an NPP, SRS-32.

Negative consequences of strategies

Strategies can also have negative consequences. For example, the use of containment spray system can be useful to decrease containment pressure, but it can also deinert an initially inert containment atmosphere and provoke a hydrogen combustion. Restarting an RCP can result in a large pressure spike in the RCS and cause even a steam generator tube creep rupture. Flooding the debris in the cavity can generate a large pressure pulse in the containment and challenge in this way the containment integrity.

Therefore, each strategy should be investigated for its potential negative consequences, before it is implemented. In the guidelines, such precautions should be included. Where possible, such potential negative consequences should be quantified, so that decision makers are well informed and do not need to estimate such consequences during the evolution of the event on the basis of imperfect knowledge.

It can happen that negative consequences have an impact on the set point of guideline actions. For example, if a set point is set for containment venting, but the containment atmosphere is flammable, the possible pressure spike from the hydrogen combustion should be considered in that set point (which may result in a set point at lower pressure).

A comprehensive list of potential (positive and) negative consequences of strategies (CHLA) is presented in SRS 32, Annex II. Read more →

Plant specific strategy development

On the basis of the vulnerability assessment and identified plant capabilities, as well as the understanding of accident phenomena, severe accident management strategies should be developed for each individual challenge or plant vulnerability. A ''Strategies Report'' or dedicated section of the background material or Executive Volume of the plant-specific severe accident management guidance is a useful way to document this process. Further guidance of aspects to consider when developing plant specific strategies is provided in SSG-54 paragraphs 3.20 to 3.29. Read more →