Accident at Tokaimura
The plant and the accident
... lower doses
... doses at Toakimura
Fission and chain reactions
... how fission happens
... Picture the nucleus
... What are the odds?
... an odditiy of neutrons
... thermalizing neutrons
... No, Tokaimura wasn't a bomb!
Criticality in the Japanese accident
... Nuclear self-control is
... apparently, that's what happened
(A letter publsihed in The Pueblo Chieftain)
First, the accident occurred at a fuel processing plant, not a nuclear power plant; no so-called meltdown happened or is in the offing. Second, three workers are ill from radiation exposure, and two of them are (at the moment of this writing) in critical condition. The two will likely not survive. Third, the accident involves criticality. Some discussion of these facts is in order.
The fuel for nuclear power plants is pellets of uranium oxide, and the job of the plant is to produce the pellets. The uranium used in reactors is about 1,000,000 times less radioactive than radium. The radiation that has caused these injuries, therefore, did not come directly from uranium. Apparently, some workers accidentally put in too much uranium into a tank that contains water and some soluble compounds of uranium. A chain reaction started, which produced heat and radiation. There is no chance whatsoever of it exploding like a bomb. The only hazard can come from the radiation.
We are all exposed to radiation on a daily basis. In fact, Coloradans receive about twice as much radiation from outer space and from the soils as do people who live in coastal cities. In terms of the rad, the unit commonly used to describe radiation exposure, we annually are exposed to about 1 third to a half of a rad of radiation. There is a so-called LD-50, the dose of radiation that will be lethal to 50% of the people who are exposed to it during a short period of time. The LD-50 for radiation is about 450 rads, about 1000 years’ worth of background radiation condensed into a brief time. A dose of 1000 rads causes near certain death.
Below about 200 rads, the exposed person does not become sick or feel ill. However, there may be long-term effects, primarily the chance of contracting leukemia. If 25 people are each exposed to 200 rads, then over the course of years, there will be one case of leukemia (or other cancer) resulting from the radiation exposure. There will also be approximately four cases of cancer that would occur naturally, without that exposure to the radiation.
From this brief review of the health effects of radiation, we may conclude that the two workers who are in critical condition probably had doses in excess of 200 rads. Since the two were wheeled out of the plant on gurneys, we may suppose that their exposures were probably in excess of the 450-rad LD-50 dose, and therefore, they are very likely not to survive.
It is important to distinguish between a radioactive material and the radiation it produces. It is somewhat similar to the distinction between a light bulb and the light that strikes your skin. Radioactive materials emit radiation. How much exposure you get to that radiation depends upon how much the source produces, how much distance lies between you and the emitter, how much shielding exists between you and the emitter, and how long you are exposed. Time, distance and shielding are the watchwords of radiation protection.
If a certain isotope of uranium atom, namely the U-235 one, absorbs a neutron, it becomes a U-236 and rather abruptly undergoes fission. The U-236 breaks into two separate atoms of much lower mass, say an atom of molybdenum and one of barium. (Many other combinations are possible.) When the uranium atom breaks apart, it releases two or three neutrons. That is, a certain kind of uranium atom absorbs a neutron and then suddenly the uranium atom is gone. In its place are two atoms, both of which are usually radioactive, and two or three neutrons that are emitted with high speed. If, and that’s a big "if," one of the emitted neutrons is absorbed by another U-235 atom, it will break up the same way the first one did. It is the making of the chain reaction.
All of the activity takes place within the nucleus of the atom. Imagine that an atom is enlarged to the size of a room. The nucleus would then be the size of a pinhead and be in the middle of the room.
How likely is it that neutrons emitted by one atom are absorbed by another? Point a laser in a random direction. What are the chances the laser light shines on somebody? If people are packed close together, the odds are very high. If there is only one person per square mile, the odds of a hit are pretty remote. The same arguments hold for the likelihood that a neutron hits and is absorbed by a uranium nucleus.
But, neutrons are not exactly like laser beams. The uranium nucleus almost never absorbs fast neutrons, but readily absorbs slow ones. The high-speed neutrons released by the fission process will therefore usually not be absorbed by other uranium atoms.
However, it is easy to slow down neutrons. As they pass through water, they slow down dramatically within a little over an inch of travel. These so-called "slow" or "thermal" neutrons are the ones that can be absorbed by a U-235 atom. The result is another U-236, which breaks apart, releasing two or three more neutrons. Only when the uranium atoms are close enough together is it possible to induce the well-known chain reaction, in which a few neutrons generate many, which in turn induce more. The term criticality refers to the condition in which exactly enough neutrons are absorbed to keep the reaction going.
One huge difference between bombs and nuclear power plants is that it is physically impossible for the power plant to explode like the bomb, because the fuel is mostly U-238, a less reactive isotope of uranium that always keeps the U-235 atoms too far apart for the chain reaction to get out of control.
How are they going to shut down the chain reaction that is going on in Japan? One idea is to get a lot of boric acid into the bath. The element boron absorbs neutrons much better than U-235 does, so mixing boric acid into the vat will absorb the neutrons that make the sustain the chain reaction and shut it off.
But something else may help, and that is simply letting the liquid heat up. The neutrons that are sustaining the reaction will not be slowed down as much, so the likelihood of being absorbed is diminished. Moreover, the expansion caused by the heating will increase the distance between any uranium atom and its neighbor. These processes are well known ones that keep nuclear reactors in check without any human intervention.
To quote from the New York Times coverage of the accident, "Radiation levels around the plant were 10,000 times higher than normal at one point." The crucial part is in the last three words, "at that point." Evidently, the radiation intensity dropped rapidly. Why? Not because anybody did anything; indeed, they probably didn’t have time. I speculate that the liquid heated up, thereby shutting down the reaction in the tank below criticality. There would still be some radiation coming from the fission by-products, but not terribly much. We may learn more in the days to come.
What about the Three Mile Island accident? The TMI accident was unique: it was the only multi-billion dollar accident in history in which nobody was harmed. One reason was that the "containment building," a huge concrete structure with four-to-six-foot thick walls surrounding the reactor, provided shielding and keep the radioactive materials inside, just as it was designed to do. The Chernobyl plant in the Ukraine had no such containment building, and was also of a design that was rejected in the US decades ago because of the possibility that such an accident could occur.
There is no such thing as "safe energy." You’d be asking for gasoline that doesn’t burn. Just as a reminder, we refer to a recent article in the Pueblo Chieftain [8/15/99], talking about the hazards of coal dust. Seven workers were injured — one with third-degree burns over 90% of his body — in Myrna, Georgia, in an explosion in a coal-fired power plant. A cleaning chemical interacted with coal dust and caused the explosion. The press made no big deal of the coal-dust accident, nor does anybody normally write blaring headlines about hundreds of coal-mine accidents, refinery accidents, oil-well accidents, and natural gas explosions that regularly kill or maim the people who work to deliver energy to us.
Howard C. Hayden
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