Fukushima Meltdown -- The Worst Case Scenario

I have been speaking about the ongoing Fukushima nuclear power saga. It may be necessary to consider in detail the worst case scenario for the plant, which could transpire in the next few days.
  1. The operators fail to cool the reactors;
  2. The fuel and fuel cladding melt, creating a radioactive magma;
  3. The fuel magma melts through the bottom of the reactor vessel;
  4. The magma hits the concrete, and causes chemical explosions which lifts some of the magma into the air;
  5. The magma contaminates the immediate vicinity of the reactor; making the site unusable. Lower levels of radiation contaminate the nearby area (land or sea depending on the direction of the wind).
To prevent this, the reactors need to be cooled which means that water needs to be pumped into the reactors. It is essential to get power to the remaining pumping equipment.

This means:
a) A sufficient power source (e.g. a grid connection or sufficient diesel generators - there should be others on the site)
b) Sufficient diesel to power those pumping devices (this could be shipped in by helicopters).
c) Sufficient pumping power is needed (fire engines may not be sufficient - the other reactors on site that are closed down could surely be used in some way).

Furthermore, actions need to be taken to prevent further explosions and build up of pressure in reactor core.

If this eventuality is not prevented, it would be a level 6 accident on the 1-7 nuclear accident scale. It would lead to a one-off release of radiation; but not an ongoing release such as happened at Chernobyl (level 7). The immediate site would be contaminated. Further contamination would depend on the wind direction. It would not however lead to major loss of life or health risk beyond those in the near vicinity.

Difficult decisions need to be made for personnel, such that they are rotated and monitors such that life-threatening levels of radiation are not received; and that individuals past a childbearing age are used, because of the risk of infertility.

The worst case scenario is probably 50% likely at the present time. To stop it (if that is possible at present), heavy pumping equipment needs to be used to pump water into the core (its not clear whether the fire engines used are sufficient). The main consequence of the worst case scenario would be possible injury to the people engaged in trying to prevent it. To prevent this, their radiation needs to be monitored.
 
A clear chain of command needs to exist, and decisions need to be taken promptly, with knowledgeable people in charge, under advice from the designers at GE. It's not clear whether a more serious accident than has happened already is avoidable.

There will be lessons to be learned from this accident. Despite this worst-case scenario (lets not forget this was one of the 5 biggest earthquakes of the last 100 years, and the reactor was built in the early 1970s, without the passive safety systems in modern designs) the main people at risk from death or injury are those who are bravely battling the nuclear problem. In terms of objective measures such as lives lost or loss of life expectancy, nuclear energy remains one of the safest energy sources available.

3 comments:

Stephen Stretton said...

This does the same job as my article http://www.scientificamerican.com/article.cfm?id=fukushima-core&page=2

SteelyGlint said...

A couple of technical points:
1) The problem will surely compound as leaks from one reactor reduce the ability to access the others. Maybe there should be a greater separation distance between reactors to ensure independent access (and reduce contention for available resources, e.g. water supply).
2) I was told when onsite that the collapse of the Chernobyl sarcophagus would release more radioactive material than the original accident. If Chernobyl was 7 on a scale of 1-7 would would such an event be?!

Whether nuclear can be made safe may be moot. It's public perception that will matter. And to be honest, the public may have a point. This accident was entirely predictable. As even isolated Pacific islanders have known for millennia: "after the ground shakes, the sea will invade the land". Similarly in the UK, we have reactors that would be vulnerable to a storm surge (or possibly a tsunami, the jury is not yet unanimous) such as occurred in the Bristol Channel in 1607.

We not only need to address the weaknesses in our societal risk assessment processes (of which Deepwater Horizon is another example), we also need to convince the public that we have done so. Now, that's what I call a big ask!

It seems to me that Fukushima is a huge hit to the prospects of nuclear power. And the wise crowds seem to think so too: I see this morning that my renewable energy fund is finally perking up - in the face of a massive sell-off of the broader market.

Stephen Stretton said...

Steely Glint: all good points, and some related ones that I would like to make. I think that dealing with the risks financially, as you sort of suggest, is a good bet. The financial risks of nuclear are much larger than the risks to life and limb. Assessing the market-expected risk is important to determine whether a particular nuclear technology makes sense.