Answer to Question #1462 Submitted to "Ask the Experts"

The three accidents you mention, Three Mile Island in 1979, Chernobyl in 1986, and Tokaimura in 1999, were very different in cause and effect. Following is a brief summary of the three events:

Three Mile Island—The accident at Three Mile Island Unit 2 (TMI 2) in 1979 was caused by a combination of equipment failure and the inability of plant operators to understand the reactor's condition. A gradual loss of cooling water to the reactor's heat-producing core led to partial melting of the fuel rod cladding and the uranium fuel, and the release into the environment of a small amount of radioactive material. The TMI 2 accident caused no injuries or deaths. In addition, experts concluded that the amount of radiation released into the atmosphere was too small to result in discernible direct health effects to the population in the vicinity of the plant. At least a dozen epidemiological studies conducted since 1981 have borne this out. This information was taken from the Nuclear Energy Institute's Web site, where there is a much longer discussion of the accident.

Chernoby—The Chernobyl facility in the Ukraine was a large nuclear reactor that was undergoing some tests. In order to perform these tests, which were on the nonnuclear part of the reactor, the operators had to disconnect some of the safety circuits and, additionally, had to put the reactor into a very unstable condition. Then, when they tried to put the reactor back into its normal condition (that is, running at full power) an event called a super-criticality occurred. In a nuclear reactor, the fission chain reaction is controlled by the use of elements that absorb neutrons. In reactors, this is generally done using boron (which is dissolved as a salt in the water in the reactor) and control rods, which are solid metal rods (usually cadmium) which are inserted or withdrawn from the core, the area where the nuclear fuel is located.

In the Chernobyl reactor this was also the case, but what happened was that in the very low-power condition where the operators had put the reactor for the test, there were so few neutrons that moving the control rods out of the core region only very slowly caused the power to go up (which was what the operators wanted). So, the operators, who were in a hurry, pulled the control rods out almost all the way very quickly, and this had a huge effect. This is called a super-criticality, which is when a uranium atom absorbs a neutron and fissions, creating energy and releasing three other neutrons on average, and each of these three also causes a uranium atom to fission and release three neutrons each, which then also cause more fissions and more neutrons. This is the normal sequence of a chain reaction, but if most of the neutrons cause new fissions (because there are no control rods, for example) then very quickly (within a microsecond) there are billions of fissions and lots of energy is released. This is not the same amount of energy as released in an atomic bomb, but in the case of Chernobyl it was plenty to very quickly vaporize the water around the fuel into steam, which, like steam in an old piston locomotive, quickly built up pressure and caused a steam explosion. This broke lots of pipes, making the reactor impossible to control, which caused more steam explosions. The Chernobyl reactor is also made up of lots of graphite blocks, and these caught fire, causing more heat and damage. So the steam explosions and all that heat caused lots of the fission products in the reactor to be thrown out of the reactor containment building. At Chernobyl, this reactor containment building was a very simple, thin-metal-walled building, not like the three-to-six-foot thick steel-reinforced concrete containments we have in the United States and all countries other than Russia and the former Soviet Union countries.

The fission products were projected high into the sky by the very intense heat and were carried by the wind literally around the world. When it rained, the fission products were washed out of the sky and deposited on the ground, on crops, on houses, on roads, etc. The highest levels were within approximately 50 miles of the plant, where there was significant deposition of radioactive iodine on grass and crops. The grass was eaten by cows, primarily because no one told the people that there had been an accident, so none of the farmers knew that the grass was contaminated and that their cows should not eat it. Iodine concentrates in cow's milk, which in the case of Chernobyl was consumed by the children in the area. When iodine enters the human body it is almost all concentrated in the thyroid gland. This caused, in the years following the accident, thyroid cancer in about 2,000 children. Fortunately, thyroid cancer is easy to cure if you find it, and almost all of the children exposed have been cured. Tragically, three or four children died of this cancer. In addition, 30 people, mostly workers fighting the graphite fire at the Chernobyl site, also died. They died of radiation poisoning because they were exposed to lots of radiation because to fight the fire they had to be very close to the reactor core, which was at that time open and unprotected.

Other than these discussed above, there have been no deaths due to Chernobyl. Around the world a number of people have been exposed to the radioactive material released from Chernobyl. For example, in northern Sweden, Finland, Scotland, and Ireland there is still radioactivity (mostly ¹³⁷Cs) on the ground such that sheep grazing on this grass cannot be eaten because they intake too much radioactivity and it goes to their muscles. But otherwise the doses to people have been so low that no cancer effects have been observed. This includes the populations in what is now Belarus and the Ukraine, who have been living in contaminated areas for 15 years now. The World Health Association (WHO) has been following the health of these populations since the accident and will continue to do so. To this point, there has been no statistical increase in the number of cancers (other than the thyroid cancers mentioned above) due to the Chernobyl accident.

If you want more information about this, look on the Web site of the Nuclear Energy Agency at "Chernobyl: Assessment of Radiological and Health Impacts," which you can download for free.

Tokaimura—The Tokaimura accident was completely different. Here, three workers were mixing uranium in a small vat which was then to be sent to another part of the factory to be made into uranium fuel for a small, experimental nuclear reactor. They decided, because they were in a hurry, to bypass some safety procedures and to pour the uranium by hand into the vat, rather than to let it be pumped slowly through a small pipe. So here, again, a super-critical chain reaction was started; because they poured in too much too fast, all three were exposed to varying levels of neutron and gamma radiation. The two individuals working in closest contact with the process died in the months following the accident. During this event, there was no fire and there was no steam explosion, so there was no radioactivity spread around. However, because workers could not get into the room for some time because of high radiation levels, the chain reaction continued (not at a super-critical state, but it was a critical mass), and this gave off neutron and gamma radiation that exposed some of the people in the plant and living around the plant. The mayor of the town evacuated about 350 people who lived nearest the plant, but their exposures were very low. There have been no cancers at all in the populations exposed in this accident, and none are expected because the doses were so low. A good, but very detailed, summary of this event can be found on the University of Buffalo Web site.

Linda Sewell, CHP 
Edward N. Lazo, CHP, PE, PhD