CHAPTER 2: SEVERE ACCIDENT PHENOMENA AND MITIGATION STRATEGIES

Spent Fuel Pool damages

Challenges

Although probabilistically rare, the spent fuel pool (SFP) can be damaged either directly by the initiating event (e.g. a seismic event) or later in the course of the accident by failure of the cooling systems. Simple loss of heat rejection from the SFP will require generally many days before any threat of spent fuel uncovering is imminent, allowing significant time to restore cooling and any water lost from evaporation. Low likelihood seismic events have the potential to produce larger coolant loss through liner damage and present a significantly more urgent threat of large radiological releases. And since spent fuel pools are often not in a containment structure, the potential for an unmitigated release from perhaps several offloaded cores of spent fuel, the consequences of SFP fuel damage accidents can be larger than for the reactor itself.

A large leakage of the liner most likely will develop into a fast overheating of the fuel, as the cooling water may drain quickly.

If no leakage occurs, then the fuel will only be overheated after considerable time, which depends on the amount (large - small) and type of fuel (freshly unloaded - old).

Criticality seems to be a minor risk but nevertheless often considered in accident management strategies, as it is difficult to imagine that the compact racks have been destroyed where the fuel bundles and the cooling water are still in place.

As in most plants the SFP is outside containment, fission products may easily escape to the environment. As also hydrogen is formed, combustion is possible in volumes (rooms) which have not been designed against pressure loads. Explosions may easily occur, followed by collapse of those structures.

Strategies

Major strategies to mitigate SFP challenges are:

  - Filling the SFP, also with fire water or using temporary equipment (hoses, fed from outside), will cool the fuel and scrub eventual fission products.

  - Spraying the SFP, as no flooding is available or if the leak is so large that water drains immediately from the pool.

  - Ventilate compartments to prevent accumulation from combustible gases and to take fission products to ventilation system filters. Where no ventilation is available, doors may be opened or structures deliberately damaged to create leakage paths.

  - For once-though cooling methods, run-off water should be captured and stored.

  - As accidents may also happen during shutdown and refuelling modes, care should be taken that access roads to the SFP remain free from maintenance equipment (to allow people to carry hoses to the pool).

  - Finally, perhaps most importantly it should be emphasized that once the fuel in the SFP becomes uncovered, the dose rates in the SFP room will become lethal to emergency personnel, so positioning of any spray mitigation equipment must be completed prior to the uncovering of the fuel and spray actions must be accomplished remotely.