The earthquake and tsunami in Japan was a horror on its own, but the effects on several nuclear reactors has added to the emergency. There has almost certainly been a partial “meltdown” in two reactors at the Fukushima Daiichi and there may be more. This is still unlikely to lead to a catastrophic failure and release of a large plume of radioactive material, but even a small chance multiplied by the potential for thousands more deaths makes the situation very tense.
There will be some “fallout” from this event no matter what – at least in terms of our interest in expanding or even maintaining our nuclear plants. There is likely to be another tense event here in Minnesota in the next few weeks that will face a lot of scrutiny. What has happened in Japan, and can it happen here?
The Daiichi plant first came online in 1974 and contains six moderate sized nuclear reactors. The technology used is actually quite simple in practice and has a lot of built-in backup procedures used as safeguards to prevent anything catastrophic from happening – but several of these have already failed.
Nuclear power is made from atoms that are so large that they are unstable. In nature, they fall apart on their own given enough time. When they decay in this way, smaller pieces of the atom fly out with great energy – “radiation”. A fairly common Uranium atom found in nature radiates mainly neutrons that used to be part of the atom itself.
If you purify this material so that the unstable Uranium is close together, the neutrons from one atom falling apart cause others to split, flying off even more neutrons. A highly concentrated mass of a certain size will explode in an atomic bomb. Control the flow of neutrons, however, and you control the rate of the chain reaction and thus how much energy it releases as heat. This can be done easily enough with two atoms that tend to soak up neutrons – Boron and Carbon.
The reactors in question use a solution of boric acid – a water soluble form of Boron – to keep everything under control. As long as there are pumps to keep the solution moving there is no problem at all. The decay of Uranium is under control.
At the Daiichi plant, the earthquake cut power to the pumps that keep this boric acid solution moving. The backup generators, apparently diesel fired, were then shut down by the tsunami. The plant went without pumping capability, and thus control, for some time. While we do not know exactly what happened in there we do know that there have been two explosions of vented Hydrogen.
Hydrogen could only have come from water that became so hot it split apart at a temperature in excess of 2000C. That is hot enough to melt the Uranium fuel rods – a situation known as “meltdown”. You do not simply dump more water on something that has become that hot without creating a lot of steam, and thus pressure, which has to be vented. Once meltdown is reached a reactor has to be cooled very carefully to pull the energy away gradually – while maintaining some control over the neutron flow.
The final backup system is a steel shell called a “containment vessel” that surrounds the entire reactor. The workers in this plant are certainly doing their best to keep the steam pressure in this vessel as low as possible so that it does not break. The government of Japan assures us that these vessels are holding pressure so far, which is to say they are intact despite the earthquake. They are the last line of defense in this heroic struggle to regain control.
A very similar system from the same era is located at Prairie Island, Minnesota. This facility will never face an earthquake, but it is certain to encounter some serious flooding in the next few weeks. There are 24 “Dry Casks” of Uranium stored in the floodplain of the Mississippi River at this location – barrels fuel that has decomposed to the point where it does not generate enough heat for a power plant but still has a lot of radioactive material.
When the floodwaters rise this plant will likely shut down as a precaution. It is much easier to keep the neutron flow under control if you lose pumping capability if you are starting from a cold reactor. That means that we will lose a lot of generating capacity for the duration of the flood. The status of this plant will likely be a major item in the press, given what has happened in Japan.
Is the stored Uranium safe on the floodplain? It is very likely to be surrounded by water in the next few weeks, but it is so heavy that it is unlikely to go anywhere. But the scene will not look good for an industry that is already reeling and usually prides itself on multiple redundant safety systems. Once again, the possibility of danger is low but a large potential for devastation if something goes wrong makes the situation intolerable.
Prairie Island’s license to operate comes up for renewal in two years. If it is not renewed, Xcel Energy will have to scramble to find other sources of electricity.
We do not know exactly what is happening at the Daiichi plant right now other than it is a very difficult operation that dedicated workers have been slaving for days to bring under control. Their fatigue is the biggest worry that we have right now. What we do know is that they are still fighting. While we all hope that they win, we also know that there is another situation at a similar plant waiting to happen here – one that won’t be anywhere near as dangerous, but will certainly scare the bejeezus out of many people.