CHAPTER 4: Introduction to Accident Management

Basic structure of development of accident management

Accident management (AM) is essential to ensure effective defence in depth at the third and/or fourth level (depending on each Member State's approach in the consideration of DEC without significant fuel degradation, whether at level 3b or at level 4a, cf. TECDOC-1791 for more information on this topic).

Accident management may be preventive or mitigative (or both), where 'preventive'  means directed to prevent or delay fuel damage, and 'mitigative' means directed to mitigate the consequences of fuel damage.

Accident management is based on strategies, which are developed in the preventive domain as well as in the mitigative domain. In the preventive domain, they are directed to achieve the fundamental safety functions. Where this fails, fuel damage must be expected and strategies are developed to protect remaining fission product boundaries and mitigate the consequences of a severe accident.

As discussed, accident management uses a top down approach: first, objectives are defined, then strategies developed, then needed equipment ('measures') identified and made available, and then procedures and guidelines to execute the strategies.


Figure 1-7: The structure of accident management guidance: Top-down approach (SSG-54).

Strategies are high-level actions that serve to achieve one or more of the objectives of the accident management. Examples of such high-level actions are: injection into the RCS (to cool the core), flooding the RPV from outside (to try to keep the corium in-vessel), intentional burning of hydrogen (to prevent dangerous accumulation of hydrogen in the containment), and venting the containment (to mitigate over-pressure). Further detail is in Chapter 2 of Module 2.

Then all measures are defined that are needed to execute the strategies. This includes plant equipment, added hardware features (if any), and portable equipment (if available).

The strategies are then transformed into procedures and guidelines, so that they can be executed by trained plant staff.

Preventive actions are usually proceduralised, the instructions being given in the Emergency Operating Procedures (EOPs) - Other names are also used, e.g. Emergency Procedure Guidelines (EPGs)-. The effectiveness of preventive measures can be quantified using Level 1 Probabilistic Safety Analysis (PSA), which results in a quantification of the fuel damage frequency.

Mitigative actions are usually provided in structured guidelines, rather than rigid procedures, the Severe Accident Management Guidelines (SAMG). Some other name: Operating Strategies for Severe Accidents (OSSAs).The effectiveness of mitigative measures can be quantified using Level 2 PSA, which results in a quantification of fission product release frequency and magnitude.

These two methods will be further elaborated in the next subsection.