A friend of my daughter's wrote me an email: Can you write about those Japanese reactors in a way that ordinary people can understand? I decided to try, despite the fact that there are many excellent resources out there. I particularly recommend this post at the Energy Collective.
I am oversimplifying, and I don't have all the facts I would like to have. But waiting for all the facts would take a long time.
Starting with the Reactor
A boiling water reactor is a bunch of fuel rods (uranium oxide pellets with a zirconium coating to hold them together) and water circulating around them. The pellets get hot because a fission reaction liberates heat. The hot pellets heat the water, and the water boils and becomes steam. The steam turns a turbine and makes electric power.
Now, let's assume something goes wrong. Say, an earthquake. The sensors in the reactor feel the earthquake, and they shut down the reactor by putting control rods into it. These rods grab up neutrons and stop the fission reaction.
However, the uranium pellets are still hot, and so you have to keep water circulating to remove the heat. The circulating water is driven by pumps, the pumps are driven by electricity, but where does the electricity come from if the reactor shuts down? In general, the electricity comes from the grid when a nuclear plant stops making power.
However, in an earthquake or other emergency, the grid may well be unavailable. Therefore, nuclear plants have diesel generators, which can drive the pumps for a long time, until the grid goes back on-line, or it is safe to restart the plant again.
And now, we switch to talking about the Japanese reactors.
The Earthquake and the First Consequences
When the Japanese nuclear plants sensed the earthquake, they shut down quickly and simply, as they were designed to do. Within seconds, the diesels at all the plants started, and cooling water began to circulate. (The earthquake was about five times as strong as the design-basis earthquake, and everything worked very well. This was a triumph of engineering.) An hour went by, with the diesels working, the plants fuel rods cooling and everything fine.
Then the tsunami came. It knocked out the diesels at the three plants closest to the water, and they could not be started again. The Japanese also had back-up batteries, but they were not strong enough to keep the correct level of pumping going for very long.
Questions arise. Why couldn't they get more diesels? Why were the diesels placed where they were so vulnerable to flooding? I don't know. On the other hand, it is easy to ask questions on a nice spring day in America. It is much harder to work hard in the middle of a crisis.
The Second Consequences
Once the diesels and therefore the cooling had failed, it became very hard for the engineers to keep up with the situation. There were two problems:
- Rising temperatures led to rising pressures of water vapor, and it is hard to pump more water IN to a situation of high pressure
- When the zirconium fuel rods overheat, they react with water and liberate hydrogen, which is explosive.
- Since the water around the fuel rods was slightly radioactive, the engineers wanted to vent as little of it as possible
- Since the zirconium was overheating, hydrogen was being liberated and could explode.
A set of good graphics at the Capacity Factor blog show the that the explosion blew out panels, above the main containment area. The concrete area begins below the area that blew out.
Third Consequences: Borated Seawater
The next set of announcements were about adding borated seawater to the reactor vessels. Boron stops neutrons, and would probably aid the cooling of the fuel. This choice probably means that the reactor will be ruined. However, it is not a strange or unforeseen measure. Nuclear power plants have defense in depth and protocols for everything. Margaret Harding led nuclear safety assessment groups at GE for many years. I asked her about borated water addition and seawater additions. Her response:
Flooding BWR Reactor Pressure Vessel with borated water is a STANDARD part of the BWR design. We don't use it unless we have to because it does odd things in a reactor that bubbles, but it is certainly not "untested". I've explained that to several reporters.
Also, explain that some plants have seawater injection as a STANDARD emergency back-up option - it isn't used because we don't want that mess in the reactor unless we have to have it. Just like boron.
As far as I can tell, this set of techniques worked for reactors 1 and 3. They are still adding water and bleeding steam, but the temperatures are dropping within the reactor vessels and the chances of overheating are fading.
The hydrogen explosion at unit 3 was terrible, injuring 11 people. But those two reactors appear to be under control. Assuming all continues to go even moderately well, a few weeks from now the reactors will be ruined, but nothing much worse will have happened.
The situation is different at reactor 2.
Fourth Consequence: Big Problems at Unit 2
In the cases of units 1 and 3, the water flooding and bleeding have led to bringing the units close to safe shutdown. In these cases, the containment was not breached and the full rods spent most of their time covered in water. In unit 2, however, the situation is different. The fuel rods have been uncovered for long periods of time because a stuck valve defeated the feeding and bleeding operation. A hydrogen explosion may have damaged the integrity of the containment and/or the torus, which is that large donut-shaped object, filled with water, at the bottom of the drawing at the top of this blog.
Latest Developments (some updates)
There have been significant releases of radioactivity from unit 2. Bulletins about the plant are coming in practically by the minute, and some are contradictory. For example, now I have learned that a fire at unit 4 came BEFORE the explosion at unit 2. Reactor 4 had been shut down before the quake. The fire was at the fuel pool, and is apparently put out. I do not know if the radioactivity release is primarily from unit 2 or from the fire. You can watch English translations of the main Japanese news sessions at the the NHK World site. You will then know as much as I do about the latest developments.
It has been a confusing day, with fires starting and stopping. I sometimes think I should just stop writing about Japan at all for a while--it has been too hard to follow. Cheryl Rolfer has already made that decision: her post Cooling Off or Melting Down is worth a read.
To understand today's events, though, Barry Brook's summary at Brave New Climate seems the best way to update this blog post.
Wednesday Morning Photo Update
This morning, commenter Martin Langeveld sent links to some excellent pictures of the plant. Thank you Martin! I don't want the links to be buried in the comments, a section people may or may not read.
For a recent photo of the plants (you have to scroll to see them all) see this picture. For a set of pictures, go to this site.