Chernobyl
The accident in the Chernobyl nuclear power plant in the Ukraine in 1986 was the most devastating event of its kind that has taken place. Prior to discussing its impacts, it would be helpful to describe this plant and the reasons that the accident occurred and that its impacts were so devastating. As a first observation, it is to be noted that the building in which the reactor was housed was not a thick steel-reinforced concrete containment, such as those in use in other countries of the world including the United States.
Another reason for the release of radioactive materials was that the Chernobyl reactor operated very differently than other power plants. Nearly all plants operate on the principle called a “self-sustaining nuclear fission chain reaction,” where neutrons bombard or hit atoms in the fuel, causing fission. When a neutron gets absorbed in the uranium fuel, two or three new neutrons are released, which allows the fission process to be self-sustaining. The process, however, needs to be controlled—there must be multiple methods to control the rate at which the chain reaction is permitted to occur. The use of methods to control this chain reaction is where the Chernobyl reactor differed greatly from other reactors.
Neutrons produced in the fissioning process are released at high velocity. For these neutrons to be effectively absorbed by other uranium atoms in the fuel and cause subsequent fissioning events, they must first be slowed down. Slowing down the neutrons requires what is called a moderator (i.e., a material in which the neutrons can “bounce around” and be slowed down. All power reactors in the United States use water as both a coolant and a moderator. As such, water completely surrounds the uranium fuel and moderates (slows down) the neutrons at the same time that the water is removing heat from the uranium fuel. The water is subsequently converted into steam, in boiling water reactors, which flows through piping to the turbines, which, in turn, rotate and generate electricity. Although the Chernobyl reactor was also cooled by water, the water was essentially only used for cooling, but not slowing down the neutrons. Instead, enormous blocks of graphite surrounded the fuel and were used to slow down the neutrons.
If something goes wrong in a reactor that is both water-cooled and water-moderated, the heat generated causes the water surrounding the fuel to boil and be converted into steam. While water is an excellent moderator for the neutrons, steam is not. When the slowdown of the neutrons decreases (because some or all of the water has turned to steam), the neutrons will no longer be able to continue the fission chain reaction, and the reactor will shut down. This is why most reactors inherently respond to prevent any disastrous increase in pressure and the potential consequences of that.
In the case of Chernobyl accident, the sudden increase in power did cause the cooling water to boil but, because it was not water-moderated, the graphite blocks continued to moderate the neutrons, allowing power to increase until it reached devastating consequences.
It is also noteworthy that just prior to this incident, operators were conducting tests in which they chose to disconnect certain safety circuits so the tests would not take as long. The lack of one of those safety circuits actually allowed power to increase rapidly.
The graphite blocks caught fire causing more heat and damage. The steam explosions and all the heat forced the reactor core cover off its mountings and caused a lot of the fission products in the reactor to be thrown out of the reactor building. And remember, at Chernobyl, the reactor was housed in a thin-metal-walled building and did not have a three-to-six-foot thick steel-reinforced concrete containment such as those used in the United States and countries other than Russia and in the former Soviet Union.
The number of deaths due to acute radiation syndrome (ARS) during the first year following the Chernobyl accident is well documented. According to the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR 2000), ARS was diagnosed in 134 emergency workers. In many cases the ARS was complicated by extensive beta radiation skin burns and infection in the blood stream. In addition, 30 people, mostly workers fighting the graphite fire at the Chernobyl site, died of ARS caused by their exposure to a large amount of radiation from the open and unprotected reactor core. Two other people died due to other injuries sustained after the accident.
Subsequent to the Chernobyl accident, there were predictions of the large number of cancer deaths that would occur. Of particular concern were potential health effects of the unborn children of pregnant women who potentially had been exposed during the accident. Because of this concern and bad information these women were getting, over 2,500 unnecessary, elective abortions were performed in Greece alone. Similar observations were reported in Italy (an average increase of 30 to 60 abortions per day), West Germany, Denmark, and Norway. While there was the potential for an increase in thyroid cancers in children who were exposed before birth, very few would have died from the disease.
After the accident, about 4,000 cases of childhood thyroid cancer were diagnosed. Most of these are thought due to the 131I (iodine-131) released during the accident. It is also believed that up to 4,000 additional persons who were in the highest radiation exposure group may get cancer (that is in addition to the 100,000 cancers this population would be diagnosed with without additional radiation exposure). However, to date, there is no clear increase in the number of solid cancers or leukemia in the population receiving the highest exposure.