CHAPTER 2: SEVERE ACCIDENT PHENOMENA AND MITIGATION STRATEGIES

Detailed description of the fission product barrier challenges

The major fission product barriers (FPBs) challenges have been mentioned before. In this chapter each challenge is further described, plus the potential strategies to mitigate it.

In order to be able to develop appropriate strategies, an insight must be obtained in the chronology and severity of the various FPB challenges. Also because mitigating strategies may make use of competing resources, so that is will be possible to assign priorities to the strategies and, hence, to the use of the resources.

For example, RPV failure may occur before containment failure, so strategies to prevent or mitigate RPV failure should be initiated before strategies to prevent or mitigate containment failure.

To determine which strategy is to be initiated, logic diagrams can be useful, from which the actual or potential threat to an FPB can be determined, both in its chronology and its severity.

In order to facilitate the various AM strategies and their field of application, the various stages of core and containment damage are often described in 'plant damage descriptors' (PDCs), each of which gives a specific damage condition of the core and the containment. A well-known used system of PDCs has been formulated by EPRI as follows:

OX = heavily oxidised fuel, but fuel stack still intact

BD = badly damaged core, i.e., core debris formed and relocated to the RPV lower head.

EX = core debris ex-vessel

SFP-OX = Spent fuel is significantly oxidized but intact.

SFP-BD = Spent fuel has undergone significant oxidation and debris formation, and relocation has commenced

CC = containment is closed and cooled

CH = containment integrity challenged

B = containment bypassed (e.g. by steam generator tube rupture, ISLOCA)

I = containment impaired

The three damage descriptors of the core and the four of the containment together result in 12 different damage states. For each of these, appropriate strategies or groups of strategies can be defined, as not all strategies are effective in all plant damage states.

The plant damage descriptors are elaborated in Appendix 1: Plant damage states, which also includes damage descriptors for the Spent Fuel Pool.

Note: the challenge of RPV meltthrough is not incorporated in this scheme, but various SAMG programs include mitigation of this challenge, as an RPV failure results in a serious aggravation of the situation (may lead to core-concrete interaction, which sets free a large volume of gases, which can lead to overpressurisation of the containment).

It should always be assumed that strategies may also fail, as NPPs have not been designed against severe accidents: resources (such as water, AC, DC) may not be available, necessary equipment may have failed, or actions would have severe negative consequences which outweigh the benefits. Therefore, always strategies should be available to mitigate releases from various FPB failures, even if the probability of their failure is considered to be low e.g. due to accident management measures.

The potential strategies are often called 'Candidate High Level Actions' (CHLAs), which will be described later. A list of CHLA is presented in Appendix 2: Candidate High Level Actions.

Philosophy of Severe Accident Management

Neither the evolution of a severe accident nor the effects of countermeasures can be predicted with sufficient accuracy. Hence, precise procedures, to be followed verbatim like in EOP domain, cannot be developed. The countermeasures therefore always take the form of guidelines, where deviations are possible and may even be necessary.

As it will appear, certain countermeasures to mitigate one FPB challenge, may create a new challenge for another FPB. For example, spraying a containment to reduce its pressure may deinert the containment atmosphere and provoke a hydrogen combustion.

Hence, the decision making in severe accident management may be quite complex and, hence, requires insights and a knowledge base which are usually not available at the operating staff. This necessitates the presence of a qualified group of people that support or even determine the appropriate series of counter-measures. This is further discussed in Module 3 and Module 4.