CHAPTER 2: SEVERE ACCIDENT PHENOMENA AND MITIGATION STRATEGIES

Release of FP to the environment

Challenges

(See 2.2.1 as well, large release at onset of accident)

Most presently operating NPPs hhave not been designed to withstand severe accidents, so that it must be considered that such accidents, if they occur, may not be fully mitigated, even not under the execution of the plant SAMG, and that - depending on the severity of the accident - fission product barriers may fail and releases may occur. The possible failure mechanisms have been discussed in previous parts of this Module.

Various paths then may exist along which fission products may escape to the environment: containment leakage, bypass or failure, containment venting, spontaneous steam generator tube rupture or induced steam generator tube creep failure, Isolation Condenser IC tube failure (BWR), drywell liner failure (BWR), spent fuel pool building and/or liner failure, basemat failure.

Releases may hamper work in the control room and at places where local actions must be taken. This includes the accessibility of the site and the areas where portable equipment must be hooked on. Shifts must be changed, long term provisions (water, fuel) brought to various locations on the site. Large releases also threaten the public near the site and further away.

In a shutdown state, the evolution of the accident and the associated releases may be slower, but the accident may occur when the containment hatch is open and reclosure may take much time (~ 3 hours).

It should be emphasized that not all fission products that are released from the damaged fuel are ultimately released to the environment. In fact, it is entirely likely that most fission products are retained within the RPV, PWR steam generators, BWR steam driers and suppression pools and within the containment volume itself. This is because there are a great many natural depletion processes that capture or sequester fission products as they transport in the RCS and containment. Aerosol particles may deposit on surfaces by processes such as gravitational settling and diffusiophoresis from condensing action of steam. Physical scrubbing by suppression pools or by containment sprays can also have significant attenuation effects. Generally speaking, the longer the containment integrity can be maintained during a severe accident, the lower the airborne radioactivity in containment will be at the time of failure, and likewise the lower will be releases to the environment. This delay also makes protective measures such as population evacuation more effective.

Strategies

(See 2.2.1 as well, large release at onset of accident)

Strategies are largely like those that have been described in the section 2.2.1. Some additional considerations:

In many cases, FP release may not be expected during the first few hours, say up to 24 hours. After that time, it may be needed to vent the containment which will be one of the major sources of FP releases, if no other FP barriers have failed before. The major event before is the RPV failure, as it may bring large amounts of non-condensable gases in the containment and, hence, give rise to the need for protection against containment overpressure. Without such failure, there is a reasonable possibility to keep the containment pressure below any dangerous value and, thereby, avoid the need for containment venting.

Therefore, in each application of SAMG preparations should be made for large releases after some time. Preparations should include measures if access to the site will become hampered or impossible, such as proper lodging and food for staff which cannot leave the site, long term provisions (diesel fuel, water, power, breathing air), medical provisions. Communications to off-site contacts should be secured.

It can also be recommended to bring and keep the sump /pool at a pH-level of 7 - 9, by adding chemicals (e.g., NaOH) to prevent formation of organic iodine. Such water may become acid (pH < 7) due to the operation of sprays in a CO2-rich environment. Note that spraying chemically aggressive water may damage components by corrosion, which means spurious operation should be absolutely avoided. As the containment spray may have brought a considerable amount of fission products in the sump, sump dry-out may cause revolatilisation of these deposited fission products and, hence, should be prevented.

Long term provisions should include consideration of additional damage later, such as seismic events, bad weather and subsequent flooding from nearby lakes or rivers, etc.

Long term provision should include capture and storage of contaminated run-off water. Note that this may be large amounts (recall the many tanks needed at Fukushima-Daiichi).

Finally, as mentioned earlier, delaying containment venting can reduce the airborne radioactivity in the containment through natural or engineered processes and reduce the source term to the environment.