Nuclear fission

[1]Introduction

The severe accident management guidelines (SAMG) are related to equipment and procedures that should be applied in case of a severe accident. A L2 PSA should model all fuel damage accidents identified by L1 PSA (fuel in reactor cores or storages) and analyze their progression taking into account:

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  the equipment availability and resilience in severe accident conditions,
  
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  the actions (both correct and erroneous) by the response teams in relation with the existing guidance (SAMG),
  
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  the actions specified for the local emergency team as well as the actions specified for national emergency teams,
  
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  the impact of severe accident strategies on physical phenomena, containment failure modes and radioactive release (different options can be studied and their impact on L2 PSA results can be compared).
  

SAM strategies and accordingly L2 PSA should cover the broadest possible scope of severe accidental situations (internal and external initiators, spent fuel pool, multi-units…).

L2 PSAs help ranking NPPs weaknesses or needs for improvement, by means of a suitable metric, in order to target priority efforts (modifications of design, optimization of procedures and guidelines …). They also help identifying some risks that can be reduced by mitigation measures or can be better understood through R&D efforts. An appropriate level of confidence in this ranking process through L2 PSA is definitely needed to be sure that efforts are directed on the relevant issues.

SAM strategies make use of components or systems under severe accident conditions. Note that for any NPP most equipment is not designed for severe accident conditions and that this has to be considered in PSA¹.

For example, the question if the RPV has already failed or not may not be answerable based on the existing measurements. In addition, even if relevant measurements are available, cables and equipment in the containment or nearby buildings, which will experience a beyond design temperature and radiation during a severe accident, may not be available, so that such measurements cannot be used. In addition, many systems and signals have already failed; otherwise there would not be a severe accident. Therefore, especially for Generation 2 plants (for which severe accident conditions are beyond design), the effectiveness of SAMG measures shall be evaluated critically in the L2 PSA.

Generation 3 plants however, have features which allow the identification of plant state during a severe accident using dedicated equipment which is designed for the loads that occur during a severe accident.

It also needs to be mentioned that, whatever the L2 PSA method and objective, the results can be uncertain to a large extent. For example, can L2 PSA be used to identify which part of accident progression is very uncertain and whether some SAM strategies can reduce the uncertainties: the common sense for SAMG is to provide guidance on determining the plant state during a severe accident and to indicate measures to either delay or reduce the release of fission products into the environment.

Also, as mitigative measures may have both positive and negative effects, the goal of SAMG is to provide guidance to the plant staff comparing such risks.

The ASAMPSA2 guidelines [7], [1] discuss in detail how to introduce these issues in a L2 PSA or how to present the results of L2 PSA, but some practical views on SAM strategies verification and improvement based on L2 PSA conclusions are not provided. Verification and improvement of SAM strategies is closely related to the issue of L2 PSA risk metrics to be applied for this task. Since mitigative SAM aims at reducing radioactive release to the environment, a risk measure should be selected which characterizes the radioactive impact outside the plant. Deliverable D30.5 of the ASAMPSA_E project addresses several potential risk measures which might be suitable for this purpose.
The development of the D40.5 deliverable within ASAMPSA_E provides an opportunity to compare experience in the different countries and also objectives that can be associated to this activity.

A content of the D40.5 is given hereafter with the idea to collect the experience of each partner involved in this activity, including potential deficiencies, and then derive some good practices and required progress, addressing:

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  SAM modeling in L2 PSA,
  
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  Positive and negative aspects in present SAM practice,
  
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  Improvement of SAM strategies from L2 PSA experience,
  
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  Verifying the scope of SAMG (all types of accident shall be covered, whatever the initial plant status).
  

It has been decided during the 2015-05-26&27 WP40 technical meeting to not address the SAM verification and improvement for multi-units accident because this issue is considered not yet mature for L2 PSA.

[2]High level objectives of SAM strategies verification and improvement

This chapter compares the objectives formulated in each country in terms of SAM strategies verification and improvement. The objectives can be qualitative (for example, limited off-site countermeasures induced by a severe accident) or quantitative (e.g. frequency of releases exceeding 2x10¹⁵ Bq of I₁₃₁ below 10^-6 /yr as used in Switzerland [3], [14]). They can also be associated to plant safety continuous improvement (e.g. L2 PSA use during periodic safety review) or legal requirement (e.g. demonstrate that strategies are as efficient as reasonable, ALARP concept …).

The objectives may be different for existing or future reactors.

The chapter includes a review of the situation for different NPPs / countries on the following issues:

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  SAM verification and improvement concept (or equivalent);
  

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  objectives to be reached (quantitative or qualitative);
  

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  role of L2 PSA;
  
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  link with PSRs and/or living PSA;
  
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  link with legal requirement.