- Date:
- April 26, 1986 - May 4, 1986
- Location:
- Ukraine
Chernobyl disaster
When did the Chernobyl disaster occur?
What happened in the Chernobyl disaster?
How many people died as a result of the Chernobyl disaster?
What caused the Chernobyl disaster?
How big was the exclusion zone created after the Chernobyl disaster?
What effects did the Chernobyl disaster have?
News •
Chernobyl disaster, accident in 1986 at the Chernobyl nuclear power station in the Soviet Union, the worst disaster in the history of nuclear power generation. The Chernobyl power station was situated at the settlement of Prypâyat, 10 miles (16 km) northwest of the city of Chernobyl (Ukrainian: Chornobyl) and 65 miles (104 km) north of Kyiv, Ukraine. The station consisted of four reactors, each capable of producing 1,000 megawatts of electric power; it had come online in 1977â83.
The explosion at Unit 4
The accident at the Chernobyl nuclear power plant resulted from the combination of a reactor design with inherent control problems and the reckless and deliberate disregard of established safety procedures by the plant operators. The disaster occurred on April 25â26, 1986, when technicians at reactor Unit 4 attempted a poorly designed experiment. Workers shut down the reactorâs power-regulating system and its emergency safety systems, and they withdrew most of the control rods from its core while allowing the reactor to continue running at 7 percent power. These mistakes were compounded by others, and at 1:23 am on April 26 the chain reaction in the core went out of control. Several explosions triggered a large fireball and blew off the heavy steel and concrete lid of the reactor. This and the ensuing fire in the graphite reactor core released large amounts of radioactive material into the atmosphere, where it was carried great distances by air currents. A partial meltdown of the core also occurred.
Timeline of the Chernobyl disaster
- 1:00 am: Reactor operation was reduced to 50 percent of full power as the first step in setting up test conditions.
- 2:00 pm: Thirteen hours later, the emergency core cooling system was disconnected, at which time further preparations were postponed because power from the reactor was needed for the local energy grid. The reactor operated in this condition for the next nine hours, when test preparations were resumed.
- Just after 11:00 pm: The power level was further reduced, and computer control of some of the automatic control rods was switched off. At low power levels, however, the operators had trouble maintaining a stable output from the reactor (in part because of the previous prolonged operation at low power), and it took almost two hours to get the power output to the desired level.
At this point almost all the safety features of the reactor had been disconnected or turned off.
- Shortly after 1:00 am: The operators turned on additional water circulation pumps to begin the final steps in setting up the test. At the 7 percent power level at which the reactor was operating, this action dropped the steam pressure and the water level in the steam separation drums below emergency levels, but the operators blocked the signals to the emergency reactor protection system.
- 1:19 am: The operators attempted to build up the water level by introducing additional water into the steam system. This led to further control problems.
- 1:23 am: The operators decided conditions were satisfactory for the actual test to begin. Blocking off further emergency protection signals so that the test could be repeated, the operators shut off the flow of steam to the turbine, which began to coast down to a stop. As a result, the circulation pumps also slowed, and the flow of coolant to the reactor declined. This led to a sharp temperature rise in the fuel channels of the reactor. Because of the reactor design this meant that the reactor was out of control.
Realizing an emergency was at hand, the supervisor ordered a âscram,â the term used in the nuclear power industry for the rapid insertion of all control rods to shut the reactor down. The reactor was already so overheated, however, that most of the control rods could not go in all the way.
- 1:24 am: Within three seconds the power output of the reactor rose from 7 to 50 percent of full power. This caused the fuel elements to burst apart, placing the superheated fuel particles in direct contact with the cooling water, which immediately flashed to steam. At least two explosions occurred in rapid succession. There was an almost instantaneous pressure shock that blew off the top of the reactor vessel, blew the roof off the building, and caused the servicing crane to come crashing down on the reactor.
- 1:28 am: The first firefighters arrive on the scene. They are unaware of the lethal levels of radiation at the scene.
The rupture of the fuel elements and cooling tubes also brought steam in direct contact with the zirconium coating on the fuel elements and with the graphite blocks surrounding the cooling tubes. Both zirconium and graphite react with steam at high temperatures to produce hydrogen. With the reactor exposed to the atmosphere, the hydrogen ignited and caused fires that burned out of control for almost four hours.
Evacuation of Prypâyat and initial containment efforts
On April 27 the 30,000 inhabitants of Prypâyat began to be evacuated. A cover-up was attempted, but on April 28 Swedish monitoring stations reported abnormally high levels of wind-transported radioactivity and pressed for an explanation. The Soviet government admitted there had been an accident at Chernobyl, thus setting off an international outcry over the dangers posed by the radioactive emissions. By May 4 both the heat and the radioactivity leaking from the reactor core were being contained, albeit at great risk to workers. Radioactive debris was buried at some 800 temporary sites, and later in the year the highly radioactive reactor core was enclosed in a concrete-and-steel sarcophagus (which was later deemed structurally unsound).
Deaths, radioactivity, and the creation of the Chernobyl Exclusion Zone
Some sources state that two people were killed in the initial explosions, whereas others report that the figure was closer to 50. Dozens more people contracted serious radiation sickness; some of them later died. Between 50 and 185 million curies of radionuclides (radioactive forms of chemical elements) escaped into the atmosphereâseveral times more radioactivity than that created by the atomic bombs dropped on Hiroshima and Nagasaki, Japan. This radioactivity was spread by the wind over Belarus, Russia, and Ukraine and soon reached as far west as France and Italy.
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Millions of acres of forest and farmland were contaminated, and, although many thousands of people were evacuated, hundreds of thousands more remained in contaminated areas. In addition, in subsequent years many livestock were born deformed, and among humans several thousand radiation-induced illnesses and cancer deaths were expected in the long term. The Chernobyl disaster sparked criticism of unsafe procedures and design flaws in Soviet reactors, and it heightened resistance to the building of more such plants. Chernobyl Unit 2 was shut down after a 1991 fire, and Unit 1 remained on-line until 1996. Chernobyl Unit 3 continued to operate until 2000, when the nuclear power station was officially decommissioned.
Following the disaster, the Soviet Union created a circle-shaped exclusion zone with a radius of about 18.6 miles (30 km) centred on the nuclear power plant. The exclusion zone covered an area of about 1,017 square miles (2,634 square km) around the plant. However, it was later expanded to 1,600 square miles (4,143 square km) to include heavily radiated areas outside the initial zone. Although no people actually live in the exclusion zone, scientists, scavengers, and others may file for permits that allow them to enter for limited amounts of time. With the dissolution of the Soviet Union in 1991, control of the site passed to Ukraine. In 2011 the Ukrainian government opened parts of the exclusion zone to organized tour groups, and Chernobyl and the abandoned city of Prypâyat became popular destinations for so-called âdark tourists.â During the Russian invasion of Ukraine in 2022, Russian forces attacking from Belarus captured Chernobyl after a brief but pitched battle. Combat at the site of the worldâs worst nuclear disaster led to concerns about damage to the containment structure and the possibility of widespread radioactive contamination.