Posted by jlwile on March 14, 2011
I have gotten questions via E-MAIL, Facebook, and phone regarding what is going on at the nuclear power plants in Japan, so I thought I would post my thoughts here. I am not privy to any special information regarding the disaster in Japan. I am getting my news from the same sources that anyone else can use. However, as a nuclear chemist, I can easily spot the flaws in many of the news reports and commentaries related to the Japanese nuclear power plants.
Before I discuss my take on what is happening in Japan, let’s get one thing straight:
In order to produce a nuclear explosion, you must have a runaway nuclear chain reaction. To have such a runaway nuclear chain reaction, you must have a very specific amount of nuclear fuel in a very specific geometry. A nuclear power plant (by design) does not have the right amount of fuel in the right geometry. So even if all the safety measures completely stop working, there is no chance that the nuclear reactor will turn into a nuclear bomb. Basic physics says this is not possible.
Now, of course, just because a nuclear reactor can’t become a nuclear bomb, that doesn’t mean bad things can’t happen at a nuclear power plant. Indeed, as the video above demonstrates, explosions can happen at a nuclear power plant. Those explosions will never produce the kind of destruction that is produced by a nuclear bomb, but they can still have some devastating consequences.
A nuclear reactor contains many elements, but the two most important are the fuel rods and the control rods. The fuel rods, of course, contain the fuel that undergoes nuclear fission. In this process, a large nucleus is hit with a neutron, and the result is two smaller nuclei, a few neutrons, and energy. The neutrons produced in that fission reaction travel away from the reaction, and if they hit another large nucleus, they can start another fission reaction. In this way, the reaction is self-sustaining: one reaction produces what is necessary to start more reactions.
The control rods are made of chemicals that absorb neutrons. Since the fission reactions happen when neutrons hit the large nuclei in the fuel rods, if I put control rods in between the fuel rods, they will absorb any neutrons produced, effectively stopping more reactions. A nuclear power plant is designed so that when the control rods are pushed all the way in, they absorb the vast majority of neutrons emitted in the reactor. As a result, the fission reactions effectively stop. They don’t stop completely, because the earth is naturally bombarded by neutrons, but the rate of fission reactions becomes so slow that no appreciable energy is produced.
All nuclear power plants of which I am aware are designed so that the default position of the control rods is all the way in. It takes energy and mechanisms to pull the control rods out. Thus, if power or mechanisms fail, the control rods automatically move to their “all the way in” position, which effectively stops the nuclear reaction. I do not know if the nuclear reactors in Japan are designed like that, but it doesn’t matter. We know that the control rods are currently pushed all the way in at the nuclear power plants that were affected by the earthquake. As a result, there are essentially no fission reactions taking place in those nuclear reactors.
So…if there are essentially no fission reactions taking place in those reactors, what’s the big deal? Why is there a problem? The problem comes from the fact that the products of these nuclear fission reactions are radioactive. As a result, even though the fission reactions (which are the main source of heat in a nuclear reactor) are essentially gone, the products of the reactions that have already occurred are undergoing radioactive decay. Radioactive decay also produces energy, so right now, the nuclear reactors are still being heated by radioactive decay.
So what? Nuclear reactors are supposed to heat up, aren’t they? Yes, they are. That’s how they produce electricity. However, the rate at which they heat up is tightly controlled, because they produce so much energy that they can heat up to the point that they melt themselves and everything holding them. When that happens, all that radioactivity can escape and contaminate a wide area. This is called a “meltdown,” and that’s what everyone is worried about. A nuclear reactor cannot produce a nuclear explosion, but it can experience a meltdown, which will contaminate the area with lots of radiation.
To avoid a meltdown, then, the reactor must be constantly cooled. Unfortunately, the cooling systems on the Japanese reactors, as I understand them, ran on electricity that was being supplied by the nation’s electrical grid. When that went down, the cooling systems had to be run on battery backup, and they just aren’t too efficient when they do that. As a result, they could not keep the reactor at a safe temperature as the radioactive decay going on in the reactor continued to heat it up.
Now the system that cools the reactor is completely contained so that the radioactive chemicals in the reactor can’t escape to the outside world. However, as the reactor temperature increased, the cooling water’s temperature increased, and eventually, the water molecules started breaking down to produce hydrogen gas, and that’s most likely what caused the explosions you see in the video above. So the explosions are from the cooling system, not the reactor itself.
Of course, if the cooling system explodes, that releases some radiation into the surroundings. It doesn’t release as much as a meltdown does, because most of the radioactive products of the fission events are contained in the fuel rods. Only a small amount is dissolved into the water in the cooling system. Nevertheless, radiation levels are currently higher than normal in the areas around the Japanese nuclear power plants. For example, the U.S.S. Ronald Reagan was operating about 100 miles northeast of one of the Japanese nuclear power plants, and it was repositioned after its radiation detectors noticed slightly more radiation in the area than what normally exists there. They say that the amount was very slight – about what you get from a month of natural exposure. Nevertheless, it was enough to make them move.
So we know that the cooling systems on some of the reactors have blown, which has released some radiation into the area. Is there enough radiation released to cause serious harm? I doubt it. As I said, there just isn’t enough radioactivity in the cooling water to cause high levels of exposure over a large area.
As I said, the real potential disaster is a meltdown. If that happens, serious problems will occur in the area around the nuclear power plants. Obviously, then, there is a lot of work going on to try and prevent that from happening. To that end, they have started using seawater and boron (I expect in the form of boric acid) to try to cool the reactor. This will most likely cause them to never produce electricity again, because the salt in the seawater and the caustic nature of boric acid will simply destroy the inner workings of the system. Nevertheless, it is better than a meltdown.
In case you are wondering why boron is being used, it absorbs neutrons. If a reactor is starting to melt down, its control rods will melt, which will allow neutrons to start more fission reactions again. That’s the last thing you want if you are trying to cool the reactor, so boron is being added to the seawater in order to “back up” any control rods that have been damaged.
The key thing to realize is that the technicians at these plants just have to hang on for a few days. If they can keep the reactors cool enough to avoid a complete meltdown, the heat generated by radioactive decay will begin to decline, because the reactor isn’t producing any significant amounts of new radioactive products. Thus, the amount of radioactive products is declining, because they are decaying. As a result, over time, there will be fewer radioactive products and less heat. Eventually, the reactors will still be radioactive, but not so much that they will heat up significantly.
So what’s the worst case scenario? What happens if the reactors cannot be cooled until the radioactive decay declines enough to no longer heat the reactors significantly? The reactors will melt down, which is what happened in the Chernobyl disaster. What were the consequences of that disaster? According to the International Atomic Energy Agency, it put 400 times as much radiation in earth’s atmosphere as the atomic bomb that was dropped on Hiroshima. To put that in perspective, however, nuclear weapons testing done in the 1950s and 1960s put 100-1,000 times as much radiation in the atmosphere as did the Chernobyl disaster! In addition, an area of 4,300 square kilometers around the plant has been evacuated and is considered uninhabitable by people. Some plant life and animal life is flourishing in the region, however.
In terms of human health consequences, the long-term effects of the Chernobyl disaster have been surprisingly hard to detect. The only definite long-term human health consequence of the disaster is an increase in thyroid cancer in the northern part of Ukraine and in Belarus. Other than that, no long-term health consequences have been discovered. In addition, the severe environmental consequences were short term.
So while the danger of meltdown in Japan is quite real, and while that could lead to some short-term and long-term consequences, it must be emphasized that even the worst-case scenario will not be nearly as devastating as some reports might lead you to believe.