CHAPTER 2: SEVERE ACCIDENT PHENOMENA AND MITIGATION STRATEGIES

Large release at the onset of or during the accident

Challenges

Generally speaking, large early releases of radioactivity to the environment are thought to be among the rarest types of postulated accidents and are generally associated with very energetic initiating events such as steam explosions, strong reactivity events or direct containment heating events where the containment is quickly failed, such as occurred in the Chernobyl accident. Fission product inventories accumulated in the fuel after some time of operation contain essentially every element in the periodic table. These fission products are released from the fuel as noble gases and vapours and many of them will combine chemically to form molecular compounds, mostly condensing after release to form aerosols. These aerosols will rapidly agglomerate to form airborne particles in the size range of 0.01 μm to as large as 60 μm. Any aerosol particles that agglomerate to bigger than this have such large gravitational settling velocities that they do not remain airborne long enough to be released to the environment. Those that remain airborne of course can be transported through the reactor coolant system, containment and ultimately to the environment resulting in land contamination and radiological dose to the population.

Some of the principal chemical species of fission products associated with water reactor accidents include noble gases (Xe, Kr), molecular iodine (I2), iodides (CsI, CH3I), various oxides (BaO, SrO, TeO2, others), hydroxides (CsOH), and metals (Sb). Depending on their relative volatility and the local atmosphere temperature, these fission products can be gaseous (I2, CH3I), volatile (Cs, I), medium volatile (Ba, Ce) or non-volatile (Sr, Ru) forms.

Later in an accident development a second intense source of radioactive aerosols can arise from core-concrete thermal attack (see further at MCCI). MCCI aerosols are comprised of non-radioactive concrete decomposition products and ongoing release of radioactive aerosols from the fuel content in the debris. The aerosols appear as smoke or fume owing to the very large quantity of suspended particulate.

While among the more rare postulated events, large early releases can pose the largest consequences both due to lack of time for natural depletion processes to occur (such as gravitational settling) and due to lack of time to coordinate protective measures such as evacuation.

As mentioned earlier, such releases can be caused by explosions in or near the spent fuel pool (if outside the containment) or by other destructions of major fission product boundaries (reactivity accidents, airplane crash, etc.).

Large releases require immediate measures to protect the staff and the public. Releases may hamper work in the control room and in areas where local manual actions must be taken, or where repairs are needed, or portable equipment must be mobilized.

First, the source location should be determined: the containment, the steam generators, the auxiliary building or the safeguards building. Strategies should then be developed to stop or mitigate the releases.

In addition, the Emergency Preparedness and Response plan (EPR) should be initiated, to protect the staff and the public. This is described further in Module 4. As discussed before, EPR serves as the fifth level in the concept of Defence-in-Depth.

Strategies

Mitigation of releases, while difficult, is possible through various strategies.

If it is possible, a first importance is to determine the release location. This could be leakage from the containment to the auxiliary building or safeguards building. The next action is to try to isolate the leak. Isolation valves should be checked and closed, if found open. In shutdown mode and with an open containment hatch, the hatch should be closed as soon as possible (but this may take several hours).

Unfiltered leakages from the containment can also be reduced by sprays, both internal sprays (from the sump or suppression pool) and external sprays (from outside reservoirs), and by controlled venting via a filter. For the external spray, also the fire brigade can be called in to spray the leak location (assuming the fire brigade people are protected against local radiation).

Filters are effective against aerosols, but do not capture noble gases and are quite transparent for organic iodine. Releases from buildings can also be reduced by starting the ventilation systems, if these are operable.

For all leakages, it is essential to immediately stop ongoing work in the affected locations. Protective clothing and breathing apparatus should be available and should be distributed as is appropriate.