CHAPTER 2: SEVERE ACCIDENT PHENOMENA AND MITIGATION STRATEGIES
Long term provisions (exit of SAMG)
Challenges
The SAMG are guidelines for the short term (days) response to a severe accident. They should be followed by long term (weeks, months, years) provisions, for which measures should be determined on the basis of the actual situation. On the longer term, failed systems will have been brought back to service or replaced by portable equipment. Power will be available again, as is cooling water. Sufficient staff will be again available and shift change again will be possible (assuming it was lost for some time). Depending on the evolution of the accident and the re-availability of essential systems, further measures are taken.
SAMG are used until a longer term stable (controlled state) and/or safe (safe state) has been achieved and the transition to long term provisions can be made.
An example of conditions for such a stable and safe configuration is:
  - Site releases under control and are small or decreasing;
  - Core debris covered and subcooled (e.g. core exit temperature < 100 ° C and stable or decreasing);
  - RCS pressure low and stable;
  - Containment pressure low (near or equal ambient pressure) and stable or decreasing;
  - Hydrogen content in the containment clearly and permanently below flammability limits;
  - Containment sump flooded and being kept flooded (to prevent revolatilisation of FPs).
Strategies
In order to determine whether the SAMG can be exited, one should repeatedly question the existence of one or more of the threats discussed. A logic diagram which contains parameters which indicate the existence of a certain threat is a helpful tool to determine if all exit conditions have been met.
Once the exit conditions are reached, the SAMG is exited. However, it can be useful to check the continuing existence of the exit conditions and take appropriate action if one or more of the exit conditions do not exist any longer.
In the beginning of the accident the emergency staff may place emphasis on RCS injection and containment injection; later, the injection flow should be throttled to what is actually needed, notably if not yet a recirculation flow could be established, as the waste water must be processed. It is important to also define a clear path to reject the decay and other process heat to an Ultimate Heat Sink (UHS). It should be noted that most SAMG do not clearly focus on such a heat rejection path to an UHS (included in the French OSSA) – but it is a clear objective in the long term.
|