Candidate High Level Actions
In this section a summary of strategies is presented, which are the 'Candidate High Level Actions' discussed before.
The strategies discussed consist of high-level actions that serve to achieve one or more of the objectives of the accident management. Potentially successful actions are here called 'Candidate High Level Actions' (CHLA). As EPRI TBR, sec. 3.1 states: ''CHLAs are those measures that could be taken to manage the course of a severe accident. They reflect steps that would typically be taken by the plant (in consultation with support personnel such as those staffing the technical support centre) to protect the available fission product barriers and to mitigate releases to the environment.'' Read more →
The high-level actions selected to fulfil protective or mitigative functions will depend on the extent of damage to the fuel and to the primary and secondary containment structures. In addition, the initial operating state of the plant will determine which types of actions can be performed.
Each of these actions will further rely on support functions or services such as electrical power, instrument air or water. The extent to which such supporting infrastructure is available or must be recovered as part of implementing a specific CHLA depends on the nature of the initiating event and subsequent failures that led to fuel damage.
The feasibility of implementing specific actions can be further compounded by the degree to which conditions can be controlled across the site beyond a single unit.
In conjunction with the plant and its status, the symptoms indicating the extent of core degradation and challenge to the containment function provide a means of choosing and prioritising different mitigation strategies.
The following is a list of CHLAs for LWRs, specified for PWRs and BWRs where applicable.
  (1) Inject into (make up to) RPV/RCS;
  (2) Depressurize the PRV/RCS;
  (3) Spray within the RPV (BWR);
  (4) Restart RCP (PWR);
  (5) Depressurize Steam Generators (PWR);
  (6) Inject into (feed) the Steam Generators (PWR);
  (7) Operate isolation condenser (BWR);
  (8) Spray into containment;
  (9) Inject into containment;
  (10) Operate fan coolers;
  (11) Operate hydrogen recombiners;
  (12) Operate hydrogen igniters;
  (13) Inert containment with stream (PWR) or non-condensable gases (BWR);
  (14) Vent primary containment;
  (15) Spray secondary containment;
  (16) Flood secondary containment;
  (17) Inject into Spent Fuel Pool;
  (18) Spray Spent Fuel Pool;
  (19) Vent/ventilate reactor building/auxiliary building;
  (20) Spray building to scrub releases.
Note that the effect of a CHLA depends on the PDC where it is applied, as not all CHLAs are effective in all PDCs. Further information is available in EPRI TBR, ch. 3 Read more → This chapter provides a detailed description of the effect of the CHLAs for the various PDCs.
Basing the SAMG on the PDCs makes also recognition of the PDC necessary. However, the diagnosis of PDCs may be difficult. For example, recognition of the EX PDC may be difficult or even impossible. Therefore, various SAMG programs abstain (largely) from recognition of the PDC and define actions upon any observed potential threat to a fission product barrier. This is further discussed in Module 3.
The strategies discussed in this chapter cannot yet be applied by the operating staff, they must first be converted to operator guidelines, which tell staff at which parameter thresholds they should initiate, throttle (if applicable) or terminate actions. Needed equipment must be identified, as well as priority of actions. This will be treated in the next Module.
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