The Three Mile Island nuclear power plant was named after the island on which it is located, about three miles downstream from Middletown on the Susquehanna River in Pennsylvania, USA.

The Three Mile Island nuclear power plant is located near Harrisburg, Pennsylvania, USA, and originally consisted of two pressurised water reactor (PWR) units. Unit 1 began commercial operation in 1974 and operated for more than 40 years before shutting down in 2019. Plans are currently under way to return the unit to operation. Unit 2 began commercial operation in 1978, but on 28 March 1979, only a few months later, it suffered a serious accident involving a partial meltdown of the reactor core. The unit never operated again and is being decommissioned.

The accident began at about 4 a.m. on 28 March 1979 with a failure in the secondary system that interrupted the flow of feedwater to the steam generators. The turbine tripped and, as temperature and pressure increased in the primary circuit, the reactor automatically shut down. A power-operated relief valve (PORV) on the pressuriser opened to reduce the rising pressure. When the pressure subsequently fell, the valve should have closed, but it became stuck open. The control-room indication nevertheless showed that the electrical signal to close the valve had been sent, not its actual mechanical position. Coolant therefore continued to escape through the open valve, creating a small loss-of-coolant accident.

As the primary circuit pressure fell, the emergency core cooling system automatically injected additional water. At the same time, steam voids formed in the reactor coolant system, causing the water level indicated in the pressuriser to rise even though the total amount of coolant in the system was decreasing. The operators had no direct indication of the water level in the reactor vessel and, believing that the pressuriser was becoming too full, reduced the emergency injection of water. They were also unaware that the relief valve remained open. As coolant continued to escape, parts of the reactor core became uncovered and overheated, causing severe fuel damage and a partial core meltdown.

Radioactively contaminated water discharged through the stuck relief valve eventually entered the containment sump. Some of this water was automatically pumped to the auxiliary building outside the containment, where radioactive gases released from the water passed through the building ventilation system and into the atmosphere. The vast majority of radioactive material nevertheless remained confined within the plant.

The Chernobyl Nuclear Power Plant. A memorial dedicated to the workers who took part in the immediate response to and cleanup of the accident stands about 100 metres from the destroyed Unit 4.

The Chernobyl Nuclear Power Plant is located in northern Ukraine, about 130 kilometres north of Kyiv and around 20 kilometres from the border with Belarus. The plant had four RBMK-1000 reactors, commissioned between 1977 and 1983. Two additional units were under construction when Unit 4 was destroyed in the 1986 accident and were never completed. The remaining three units continued to operate after safety modifications, with the last reactor shutting down in 2000. The nearest town, Pripyat, had about 49,000 inhabitants at the time of the accident and has remained largely abandoned since its evacuation. The town of Chernobyl, which gave the power plant its name, lies about 15 kilometres from the plant.

The accident occurred during a safety test carried out on the night of 25—26 April 1986, before a planned shutdown of Unit 4 for maintenance. The test was intended to determine whether the inertia of the slowing turbine generator could temporarily provide sufficient electrical power to essential equipment after a loss of external power, until the emergency diesel generators started. Reactor power was gradually reduced in preparation for the test, but the reduction was interrupted for several hours at the request of the grid controller because electricity was still needed. Some automatic safety systems, including the emergency core cooling system, were disabled as part of the test preparations, although this did not directly cause the accident.

After midnight, reactor power unexpectedly fell to about 30 MW thermal, far below the level intended for the test. At such low power, the reactor was strongly affected by xenon-135, a fission product that absorbs neutrons and suppresses the chain reaction. In an attempt to restore power, the operators withdrew an excessive number of control rods from the core, reducing the operating reactivity margin below the level required by operating procedures. The reactor was eventually stabilised at about 200 MW thermal, but it was now operating in a highly unstable condition with only a small number of control rods remaining sufficiently inserted in the core.

Pripyat, the town closest to the Chernobyl Nuclear Power Plant, was evacuated about 36 hours after the accident. Once home to around 49,000 people, it has remained largely abandoned ever since and lies within the Chernobyl Exclusion Zone.

The test began with the reactor operating at about 200 MW thermal. As the turbine generator ran down, the electrical power supplied to several circulation pumps decreased and coolant flow through the reactor gradually fell. More steam began to form in the reactor channels. Because the RBMK-1000 had a large positive void coefficient under these operating conditions, the increasing amount of steam increased reactor reactivity and power instead of reducing it.

At the end of the test, the AZ-5 emergency shutdown button was pressed to insert all control rods. However, the RBMK control rods had graphite displacers at their lower ends. When rods that had been almost completely withdrawn began to enter the core, these graphite sections initially displaced neutron-absorbing water and briefly increased reactivity in the lower part of the core. Under the extremely unstable conditions that existed at the time, reactor power rose catastrophically within seconds. Fuel channels ruptured and the rapid generation of steam produced a powerful explosion that destroyed the reactor core and lifted the massive upper biological shield. A second explosion followed shortly afterwards, and burning graphite and fragments of fuel were ejected into the reactor building and surrounding area.

The massive concrete and steel Shelter, commonly known as the “sarcophagus”, was constructed around the remains of Unit 4 in the months following the accident to limit further releases of radioactive material. In 2016, it was enclosed by the much larger New Safe Confinement, designed to provide long-term protection and enable the eventual dismantling of the destroyed reactor.

Firefighters initially fought fires in and around the reactor building, while thousands of tonnes of boron compounds, sand, clay, dolomite and lead were subsequently dropped onto the destroyed reactor from helicopters. The exposed graphite in the reactor core continued to burn for several days, carrying radioactive material high into the atmosphere. Large quantities of radioactive material, particularly iodine-131 and caesium-137, were dispersed over Ukraine, Belarus, Russia and other parts of Europe, although much of the radioactive material remained within or close to the destroyed reactor.

During the months following the accident, the remains of Unit 4 were enclosed in a massive concrete and steel structure known as the Shelter or “sarcophagus”. Because this hastily constructed structure gradually deteriorated, a much larger arched structure, the New Safe Confinement, was subsequently built and, in 2016, moved into position over the reactor and the original Shelter. It now provides a controlled environment for dismantling unstable structures and eventually retrieving radioactive materials from the destroyed reactor.

The Chernobyl accident was classified as Level 7 on the INES scale, the highest level. It resulted from a combination of serious deficiencies in the RBMK reactor design, operation of the reactor in an unstable condition, violations of operating procedures and fundamental weaknesses in the Soviet nuclear safety and regulatory system. Important characteristics of the reactor, including its large positive void coefficient and the positive reactivity effect that could occur during emergency insertion of the control rods, were not adequately understood or communicated to the operators.

At the time of the Chernobyl accident, two additional RBMK-1000 reactors, Units 5 and 6, were under construction at the site. Following the accident, construction was suspended and the two units were never completed.

Hundreds of thousands of emergency and recovery workers, commonly known as liquidators, participated in the response and cleanup during the years following the accident. Those who responded during the first hours and days received the highest radiation doses. Acute radiation syndrome was diagnosed in 134 workers, mainly firefighters and plant personnel, and 28 of them died within the first few months. Two workers also died from injuries sustained during the accident itself.

The most clearly demonstrated long-term health effect of the accident has been a substantial increase in thyroid cancer among people who were exposed to radioactive iodine as children or adolescents. Thousands of such cases have been diagnosed, although thyroid cancer is generally highly treatable and relatively few have been fatal. Apart from this increase, studies have not demonstrated a clear radiation-related increase in overall cancer incidence among the general population exposed to the Chernobyl fallout.

The earthquake epicentre, the location of the Fukushima Daiichi Nuclear Power Station, nearby towns and the three evacuation zones are shown on a map of Japan.

Many Japanese nuclear power plants are located on the coast, where seawater can be used as the ultimate heat sink. The Fukushima Daiichi Nuclear Power Station is located on the Pacific coast of Honshu, about 90 kilometres south-east of Sendai. It consisted of six boiling water reactors (BWRs), commissioned between 1971 and 1979. Because of its coastal location, the plant was protected against tsunamis, but the design basis did not anticipate an event of the magnitude that occurred on 11 March 2011. The exceptionally powerful earthquake and tsunami devastated a large part of north-eastern Japan, causing around 20,000 deaths and missing persons and overwhelming the flood protection at Fukushima Daiichi.

On 11 March 2011, a magnitude 9.0 earthquake occurred off the north-eastern coast of Japan. Known as the Great East Japan Earthquake or Tohoku Earthquake, it was the strongest earthquake ever recorded in Japan. Fukushima Daiichi Units 1, 2 and 3, which were operating at the time, automatically shut down as designed, while Units 4, 5 and 6 were already shut down for inspection and maintenance. The earthquake caused the loss of external grid power, but emergency diesel generators started automatically and initially supplied the electrical power needed for reactor cooling and other essential systems. Although the fission chain reactions had stopped, decay heat from the fuel still had to be continuously removed.

About 50 minutes after the earthquake, a tsunami approximately 13—15 metres high reached Fukushima Daiichi and overwhelmed the plant’s coastal defences. Seawater flooded low-lying buildings and equipment, disabling most emergency diesel generators, electrical switchgear and seawater pumps. Units 1—4 consequently lost almost all AC electrical power, and much of the DC instrumentation and control capability was also eventually lost as batteries were depleted or flooded. This combination of a station blackout and loss of the ultimate heat sink severely impaired the systems needed to remove decay heat from the reactors.

Some emergency cooling systems initially continued to operate without AC power, using steam-driven pumps or batteries for control, but they could not maintain cooling indefinitely. Attempts were made to restore electrical supplies using mobile generators, but damaged and flooded equipment made connection difficult. As cooling was progressively lost in Units 1, 2 and 3, water levels in the reactor vessels fell, exposing the fuel. Fuel temperatures rose sharply, the fuel assemblies were severely damaged and substantial parts of all three reactor cores eventually melted.

As the fuel overheated, zirconium-alloy fuel cladding reacted with steam, producing large quantities of hydrogen. Operators attempted to reduce reactor pressure, vent steam and gases from the containments, and inject water into the reactor vessels. When conventional supplies were no longer available, seawater was eventually injected as an emergency measure. Venting and leakage released radioactive material to the environment. Hydrogen that escaped from the containments accumulated in the reactor buildings and caused powerful explosions in Units 1 and 3.

Model of the Fukushima Daiichi Nuclear Power Station, which suffered a severe accident in March 2011. The tsunami flooded low-lying buildings and disabled most emergency diesel generators and electrical equipment, resulting in an almost complete loss of power and severely impairing reactor cooling.

Unit 4, which had been shut down and contained no fuel in its reactor vessel, was also damaged by a hydrogen explosion. Initially, there were concerns that the spent fuel pool had lost water and its fuel was overheating. Later investigations showed that the fuel had remained covered by water and that hydrogen had most likely migrated from Unit 3 into the Unit 4 reactor building through a shared ventilation system. The accident therefore caused severe damage to four reactor buildings, while the reactor cores of Units 1, 2 and 3 suffered meltdowns.

Efforts to stabilise the damaged reactors continued for many months, and a cold shutdown condition was declared in December 2011. Radioactive material was released to both the atmosphere and the Pacific Ocean through venting, leakage and contaminated water. The most radiologically significant releases included iodine-131 and cesium-137. Although Fukushima Daiichi was classified at the same INES level as Chernobyl, UNSCEAR estimates that its releases of radioiodine and radiocaesium were only about one tenth of those from the Chernobyl accident, and most of the atmospheric release from Fukushima was dispersed over the Pacific Ocean.

Residents were progressively evacuated as the accident developed. Evacuation initially covered areas close to the plant and was subsequently extended to a 20-kilometre radius, while additional evacuations were later ordered in some more distant contaminated areas. Tens of thousands of people were displaced. No cases of acute radiation sickness occurred among members of the public, and UNSCEAR has found no adverse health effects among Fukushima residents that can be directly attributed to radiation exposure from the accident. Future radiation-related increases in cancer rates are also considered unlikely to be detectable.

More than 20,000 emergency workers participated in mitigation and recovery activities at the plant during the first year. Their average effective radiation dose was about 13 mSv, while 174 workers received more than 100 mSv. No acute radiation syndrome occurred among the workers. Several workers were injured during the earthquake, tsunami and explosions, and two workers at the plant were killed by the tsunami.