Growing up in my family, when one of the kids was worried about something, my Dad would say "What's the worst that could happen?" We'd go over it, and generally, it became clear that even if the worst scenario happened (no hits and many errors in baseball game), it wouldn't really be that bad. But when you look at the design of the ten reactors at two power stations in Fukushima Japan, and look at Dad's question, the answers aren't pretty. And perhaps, the answers aren't acceptable: (and I appologize in advance and ask for corrections in the comments for the errors that must be in this article...after all it comes from only a few hours of research)
Dad: What's the worst that can happen in a Boiling Water Reactor accident?
BWR Designer: Well, the worst case would seem to be an earthquake that damages the plant and then something that knocks out the emergency cooling systems.
Dad: What would happen then?
BWR Designer: The plant would shut down right away. The rods would go in, and the nuclear reaction would stop. (Hopefully, unless the earthquake shook the plant even worse than we could ever imagine.) But, the core still generates about 3% of the full power heat even in shutdown. The cooling systems must run to take away that heat.
Dad: What if the cooling systems got knocked out too?
BWR Designer: That's very unlikely. We have multiple backup diesel generators, and even batteries to run the cooling pumps for 8 hours if those don't work.
Dad: But lets just say something really bad happened. Maybe a tsunami from that same earthquake. It floods all the generators and knocks out the power grid. What happens then?
BWR Designer: The batteries keep the pumps going for eight hours, and by then a new source of backup power is set up and the cooling pumps are driven from that power.
Dad: But let's just say things are really a mess. Remember, we're talking here about "what's the worst that could happen." Let's say after the batteries run out, we can't get power to the pumps.
BWR Designer: This is a scenario that is not supposed to happen. You must keep the reactor core cooled or it will overheat.
Dad: What happens if the reactor core can't be cooled?
BWR Designer: Well, that's bad, but things can still be contolled. The core overheats, steam is produced. The steam flows down to the torus below the reactor, and that has a huge amount of water in it. This water supresses the pressure of the steam, keeps the containment intact in time for additional cooling to be brought to bear.
Dad: But what if that cooling is still not available?
BWR Designer: Now things are getting bad, but we thought of that too. Now the reactor is going to start making lots of steam. The reactor pressure vessel is designed to contain only a certain amount of pressure, so much like a pressure cooker, we have extensive pressure relief valves, and they will allow pressure to be released.
Dad: But what if those valves don't work right, or maybe our control systems are damaged and we can't run them.
BWR Designer: Okay, now you're getting to one of our worst case scenarios. If the pressure got too high in the Reactor Pressure Vessel, it could explode. And we have the same issue with next line of defense - the containment. In fact, early on three engineers at GE resigned because they believed the reactor was too vulnerable to pressure failure. We later agreed, and added pressure relief systems. (Editor's Note: As I understand it, they have been venting steam, and that steam, which also contains hydrogen and oxygen, caused explosions in the reactor buildings at Fukushima)
Dad: What happens if the Reactor Pressure Vessel or the Containment does explode from pressure buildup?
BWR Designer: That gets us to the the truly "worst case". In this case, the nuclear fuel is uncooled. It melts, creates intense heat and pressure, and the pressure vessel fails, probably explodes (but not a nuclear explosion...a pressure explosion). That explosion could probably explode the containment as well, and would carry radioactive fuel into the atmosphere. In the case of Chernobyl, the intense fire carried the radiation high to the jet stream. If we're lucky in this case, there won't be fire and the radiation won't spread as widely.
Dad: What happens if the Reactor Pressure Vessel and the containment doesn't explode, but you just can't get any cooling there?
BWR Designer: There's a lot debate about what would happen. Clearly, the nuclear fuel will heat up and melt. Clearly, lots of steam and pressure will be created. Hopefully, the melted mass will move down from the Reactor Pressure Vessel and meet the containment structure. At that point, we hope that the material begins to spread out and cool down, although there are some people who believe that this molten mass might be hot enough and concentrated enough to melt through the containment. At Three Mile Island, there was a partial meltdown, and the molten mass did not get close to penetrating the containment.
Dad: Well, we have to go all the way. What if the melted fuel did get through the containment?
BWR Designer: (Ed note: this answer is my summary of what I've read. Again please forgive errors and correct in the comments). First of all, we don't believe, but can't be certain that the melted fuel could re-gain "criticality" and begin producing large amounts of fission-based heat. So let's just assume it's a molten mass and gets through the containment. Now we expect it would meet water-bearing earth, large amounts of steam and other byproducts would be produced, and a pressure explosion of some sort could be expected. This explosion could then spread the radioactive fuel into the atmosphere.
Dad: So, have we arrived at the answer to what's the worst that could happen?
BWR Designer: Not really. These plants tend to be built in groups. In fact, Japan has been the most aggressive at using nuclear power. Because of cooling needs, these plants are located right near the water. So I guess if you want to really think about the absolute worst case scenario, maybe you could have a huge earthquake that shuts multiple plants down, but then something else like the tsunami and a widespread blackout that would disable all of the backup systems at an entire power station. For example, Fukushima I in Japan has six reactors located right next to the ocean.
Dad: So, have we arrived at the answer to what's the worst that could happen?
BWR Designer: Um, not really. One more thing. Since we don't really know how to store the spent fuel for the reactor, we keep it in a pool inside the reactor building and next to the Reactor Pressure Vessel. If something goes wrong with the cooling systems, this fuel, which also needs cooling, will begin to boil off the water that protects it and contains radiation. That could cause fuel rod melting, and could force workers away from the area of the building due to high radiation. Also, in the event of an explosion of the Reactor Pressure Vessel, the spent fuel right next to the reactor would also be spread into the atmosphere.
Dad: Okay, so you actually can imagine six reactors having catastrophic problems at once, involving all current and spent fuel from those reactors, and potentially releasing all that radiation into the atmoshpere?
BWR Designer: Imagine it, yes, but it's just absurdly unlikely. So many things would have go wrong at once, so many backups and backups of backups would have to fail at the same time for this to even get started, let alone occur.
Dad: But you could imagine it?
BWR Designer: Yes.
I started with Wikipedia, and found out that there are two power stations with the name Fukushima (I and II.) Fukushima I, the center of the current catastrophe, is the older station, and it lies about 14 miles north of Fukushima II. Amazingly, Fukushima I Unit 1 (the oldest of the 10 reactor units at Fukushima) was scheduled for end of life shutdown on March 26, 2011 (the end of its 40 year life).
Next, Wikipedia provided a list of all Boiling Water Reactors (BWR), where I found the 10 units described in this post. (There are two more planned for Fukushima I, but it seems construction has not started.)