![]() In the accident at Three-Mile Island there was not a meltdown. have a containment building - a large concrete structure that covers the whole reactor, and should prevent release of radioactive material to the environment even in the event of an explosion.Īll of the nuclear accidents that have occurred have been at least partially caused by loss of coolant. Iodine is volatile (evaporates into the air very readily), and it can be concentrated in the thyroid gland. Here iodine-131 ( 131 53I), with half-life 8 days, is one of the important isotopes. Also, gaseous elements could be released to the air. It could then contaminate water supplies. If coolant is lost and the emergency system does not work, there can be a meltdown, which means that a large mass of fuel and other radioactive matter melts its way through the floor of the reactor, and, since it is very hot, melts down into the earth. The system has been tested successfully under experimental loss of coolant conditions. Some people question whether it would work reliably under accident conditions. There is an "emergency core cooling system", which is supposed to flood the reactor with water in case of a loss of coolant. However, even though there is no fission, heat continues to be generated by the radioactivity of the materials in the fuel rods. And so, by design, there cannot be a nuclear explosion. If water is lost, the moderator is also gone, and so the chain reaction immediately stops. There is an important point here that follows from the fact that the water in the reactor is both the coolant and the moderator. This system is where various failures and repairs have occurred in reactors over the past few decades of their operation. Thus the reactor includes a sophisticated plumbing system for containing and transporting this pressurized water. For water at high pressure, the boiling point can be increased from 100 o C, its value at normal atmospheric pressure, to several hundred degrees. It is an important property of all matter that the temperature at which it changes phase (the boiling point of a liquid, the melting point of a solid) is not a constant: it varies with the pressure of the environment in which the matter is located. In order to maintain H 2O in its liquid form, the water used in a nuclear reactor is kept at high pressure. The heat produced in the fuel rods is more than enough to raise the temperature of cooling water to its boiling point. If for some reason the flow of water is stopped or slowed - for example if a pipe breaks - the fissioning fuel rods could become so hot that they could melt. Nuclear scientists have always felt that the greatest risk in operating a reactor is the loss-of-coolant accident. ![]()
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