In April of 1986 there was a major nuclear accident at the Chernobyl nuclear power plant in Ukraine about two miles Prypiat, a city of about fifty thousand people on the Dnieper river near border with Belarus Ukraine was part of the Soviet Union at that time and the power plant was under the direct control of authorities in Moscow. As with Kyshtym, there was not enough attention paid to safety systems and procedures.
The Chernobyl power plant contained 4 reactors built between 1970 and 1983 based on a unique Soviet boiling water reactor design. Ordinary water was used as a coolant with graphite acting as a neutron moderator. Control rods were raised or lowered to control the power output of the reactors.
As the reactor heats up, bubbles or voids of steam reduce the density of the water in the core which causes a drop in neutron absorption and an increase in the reactivity of the core. In this particular reactor design, the amount of bubbles or the void coefficient has a very strong influence over reactor temperature and output.
On April 25 the operators were preparing the reactor number 4 for a test of the turbines ability to continue to spin and drive the circulation pumps after main electrical power was shut off. Early on April 26 automatic shut down mechanisms were disabled before the test.
With the reactor in a very unstable condition, the control rods were inserted into the core and caused a severe power surge. The very hot fuel and cold water reacted to produce fuel fragmentation, increase in steam production and an increase in pressure. The pressure opened the one thousand ton reactor cap which ruptured the fuel channels and jammed the control rods which had only been inserted halfway. Water dumped into the core by the ruptured cooling system fed increased steam production throughout the core. A huge steam explosion released fission products into the atmosphere. A few seconds later a second explosion likely caused by hydrogen from zirconium alloy interacting with steam blew out fragments of the fuel and graphite from the core. About three hundred tons of graphite was thrown out of the core resulting in the fuel becoming incandescent and staring fires. These fires were the main cause of the release of around fourteen times ten to the eighteenth power Becquerels of radioactivity into the environment.
For twelve hours, around three hundred tons of water per hour were dumped into the half of the reactor that hadn't been destroyed but was ultimately stopped to prevent flooding of reactors 1 and 2. Over the next eight days, five thousand tones of boron, dolomite, sand, clay and lead were poured into the burning core from helicopters in an attempt to put out the fires and stop the release of radioactivity. Over half a million works were ultimately involved in trying to contain the disaster. The eighteen billion ruble cost of fighting the fires and cleaning up the site dealt a severe blow to the economy of the Soviet Union. Five years later the Soviet Union disintegrated.
The Chernobyl accident was the worst release of radioactivity in history from a civilian nuclear power plant. Serious disruption of the social and economic life of large populations in Ukraine, Belarus, Russia and countries in Europe resulted. The radiation release spread over much of Europe Iodine-131 and cesium-137 were a major threat to public health. Thirty operators were killed in the accident and a few more died later. Over two hundred people received radiation injuries and around thirty later died from the effects of the radiation. Many children developed thyroid cancer as a result of exposure to iodine-131. The accident was rated as a level 7 on the International Nuclear Event Scale.
