Nuclear scientists have always felt that the greatest risk in operating a reactor is the loss-of-coolant accident. 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.

Pressurized water

The heat produced in the fuel rods is more than enough to raise the temperature of cooling water to its boiling point. In order to maintain H2O in its liquid form, the water used in a nuclear reactor is kept at high pressure. 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. For water at high pressure, the boiling point can be increased from 100o C, its value at normal atmospheric pressure, to several hundred degrees. [For a more complete discussion, see the following link; it is not a required part of this site.] Thus the reactor includes a sophisticated plumbing system for containing and transporting this pressurized water. This system is where various failures and repairs have occurred in reactors over the past few decades of their operation.

Coolant as moderator

There is an important point here that follows from the fact that the water in the reactor is both the coolant and the moderator. If water is lost, the moderator is also gone, and so the chain reaction immediately stops. And so, by design, there cannot be a nuclear explosion. However, even though there is no fission, heat continues to be generated by the radioactivity of the materials in the fuel rods.

Emergency Core Cooling System

There is an "emergency core cooling system", which is supposed to flood the reactor with water in case of a loss of coolant. Some people question whether it would work reliably under accident conditions. The system has been tested successfully under experimental loss of coolant conditions.

Meltdown, Radioactive iodine

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. It could then contaminate water supplies. Also, gaseous elements could be released to the air. Here iodine-131 (13153I), with half-life 8 days, is one of the important isotopes. Iodine is volatile (evaporates into the air very readily), and it can be concentrated in the thyroid gland. [Taking pills with (non-radioactive) iodine is a preventative measure, since this will reduce the amount of radioactive iodine that can be absorbed from the atmosphere.]


All civilian reactors in the U.S. 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.


All of the nuclear accidents that have occurred have been at least partially caused by loss of coolant. In the accident at Three-Mile Island there was not a meltdown. In the Chernobyl accident there most likely was. More important though were the explosions which occurred at Chernobyl, blowing the roof off of the building, and releasing radioactive material to the air. The first explosion was water suddenly flashing to steam. It was followed by a hydrogen explosion. The Chernobyl-type reactors do not have a containment building.

The most important difference between most reactors in this country and the Chernobyl-type reactors is that in the latter the moderator is not water, but graphite. Thus, when coolant is lost fission does not automatically stop. Under some conditions, the Chernobyl-type reactor can behave in the reverse way: loss of water can increase the rate of fission. This happened in the accident. The Russians have modified the operation of these reactors to eliminate this problem, although the reactors still do not have a containment building. Western experts are skeptical about whether these reactors are safe enough to continue operating.

There are a small number of graphite reactors in the U.S., although they don't have the design flaws of the Chernobyl-type reactors.


The Fukushima accident that began in March 2011 exposed a different kind of risk. The reactor design is similar to many in the West and not considered faulty. A combination of unexpected (but not extremely improbable) external events, an earthquake followed by a tsunami, destroyed the electrical grid that powered the reactor, including its water-circulating system. A backup diesel generator was also destroyed.

The accident highlighted another issue not frequently discussed in this country. Since there has been no decision on a long-term repository for nuclear waste, the spent fuel rods that have been taken out of reactors are stored in cooling pools (in the U.S. as well as in Japan) on the reactor site. These rods are also subject to overheating and melting if they are not continuously cooled.


  • Loss of coolant; pressurized water
  • Meltdown
  • Emergency core cooling
  • Containment building
  • Radioactive iodine
  • Chernobyl accident: moderator was not water
  • Fukushima: the natural world intervenes