SAMG strategies cannot be executed all at the same time and using the same resources (AC, DC, water, staff); hence, priorities should be assigned to them. An appropriate selection mechanism for priorities should be developed.
Assignment of priorities
In principle, priorities should be assigned depending on the chronology and severity of challenges to fission product barriers and the magnitude of on-going or anticipated releases. For example, a high priority should be assigned to an on-going release or to a potentially imminent failure of an important fission product barrier (assuming there is not yet any release).
Potentially, early failures of important fission product barriers are a steam generator tube creep rupture and a high-pressure melt ejection (and associated direct containment heating), as have been described in Module 2. Hence, the associated mitigation strategies are, in this case, placed in high priority, notably if the RCS pressure is still high.
Another system of selecting priorities depends on the potential loss of important safety functions. For example, in severe accident conditions, threats exist that may result in losing the core cooling, the availability of a heat sink or the containment integrity. The larger the threat is, the higher the priority.
These priorities, however, may change during the evolution of the accident. What can be important in the beginning can be less important or irrelevant later. For example, a steam generator tube creep rupture is an early challenge to an important FPB. So, the first severe accident guideline may be directed to prevent this creep rupture. Later in the accident, when the debris is released from the failed RPV, the creep rupture risk has disappeared, and so has the priority of this guideline. Therefore, the decision makers should regularly check the proper sequence of priorities, on the basis of their understanding of progress of the accident evolution.
Logic diagram
A useful tool for the operating staff to integrate the prioritization of the SAMG strategies is a logic diagram, where questions are asked on the basis of observed parameters and the answers are directing the staff to the initiation of one or more strategies. The parameters are selected such that they represent the chronology and severity of important fission product barrier challenges, or similar other well-defined selection criteria. The logic diagram should be repeatedly consulted during the course of the accident, preferably with regular time intervals. The logic diagram should be independent from the underlying scenario that led to the severe accident.
As stated before, the logic diagram should be checked from time to time whether it still represents the proper priorities for the various severe accident guidelines.
Various SAMG approaches have quite different logic diagrams, as can be seen in the SAMG industry products, Chapter 11.
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