The Three Mile Island nuclear power plant was named after the island situated three miles downstream on the Susquehanna River near Middletown, Pennsylvania, USA.

The Three Mile Island nuclear power plant is located near Harrisburg, Pennsylvania, USA. There are two blocks with pressurized-water reactors, PWR. The first block, TMI 1, was commissioned in 1974 and delivers 800 MW of electricity to the grid. It is one of the best reactors in the USA, with a minimum number of shutdowns. The second block, TMI 2, with a power output of 906 MW, was commissioned in 1979 and three months later there was a serious accident, and it was shut down, permanently.

The accident began at 4 a.m. on March 28, 1979. A small failure in the secondary circuit resulted in a temperature increase in the primary circuit. The automatic safety systems dropped the shutoff rods into the core, resulting in reactor shutdown. For effective cooling of the shutdown reactor, the cooling water pressure was automatically reduced using a relief valve that was supposed to immediately close. However, the valve was stuck open and the cooling water started to escape from the reactor core. Due to the resulting pressure drop, the pumps started to deliver more water into the primary circuit. However, the mixture of water and steam continued to escape through the stuck valve into the pressurizer, raising its level. The pressurizer maintains a constant pressure in the primary circuit (it contains a mixture of water and steam). The pressurizer cannot fulfill its function if it is full of water. However, the plant operators did not have any other information about the water level inside the reactor except the pressurized level. Moreover, they thought the relief valve was closed. This led to the wrong conclusion that there was too much water in the primary circuit and the operators tried to release it. This actually helped start the accident. Since the fuel rods were partially exposed from the cooling water, about one-third of the fuel had melted down. The released cooling water contaminated by the fission products was later automatically re-pumped into a tank in the auxiliary building outside of the containment boundary and, through a ventilation stack, radioactive gas was released into the atmosphere. The reactor was stabilized in a few days and there was no additional release of any radioactive material.

The Chernobyl nuclear power plant. The memorial to the individuals who helped, without any special tools, to clean up the accident and thus helped to “save the world”. The memorial is located about 100 meters from the destroyed block 4.

The Ukrainian nuclear power plant Chernobyl is located 130 kilometers from Kiev and 20 kilometers from the Belorussian border. There are four RBMK 1000 type reactors, commissioned in 1970–1983. There were two additional blocks under construction when block four was devastated by disaster. These blocks were never completed. The remaining three blocks were modernized and operated until 2000. The town nearest to the power plant is Pripyat. At the time of the accident, there were 49,000 inhabitants, and it is now deserted. The village Chernobyl, that gave the power plant its name, is 15 kilometers away.

The disaster began at night on April 26, 1986 after a series of wrongful and irresponsible actions performed during the previous day. Prior to a routine reactor shutdown, a test to see how long a rundown turbine can still generate electricity (in the case of loss of external power) to power the main pump of the primary cooling circuit. To be able to carry out this experiment, the plant operators disabled the reactor emergency cooling system. The reactor power output ramp-down was initiated, but the experiment was interrupted for several hours as requested by the grid operator since more electricity was required.

Pripyat, the nearest town to the destroyed Chernobyl block 4, was evacuated with a one-day delay. It has been deserted for more than 25 years and is part of the so-called dead zone.

The actual experiment was initiated late at night. After midnight, the power output dropped down to only 30 MW_t. due to an operator error, and the reactor became unstable. The operators attempted to increase the power output and withdrew most of the control rods from the reactor core. The regulations stipulated at least 15 inserted rods, at the time of the accident there were only 8 rods inserted in the core. A reactor at low power suffers from xenon poisoning — the accumulation of xenon, an effective neutron absorber. However, xenon is promptly burned up during a quick power increase of the fission reaction, and thus contributed to a steep power output increase.

Finally, the plant operators stabilized the reactor power output at 200 MW_t and initiated the experiment. The electric pumps were supplied by the generator driven by a turbine running down, the flow of coolant water decreased somewhat, and warmer water entered the lower portion of the reactor and started to boil. At that moment, for reasons unknown, an instruction was given for an emergency reactor shutdown (scram) and the graphite tops of the inserted shutoff rods also led to the local increase of the fission reaction. The positive void coefficient, an inherent property of all BRMK reactors, in conjunction with the fast loss of xenon, caused a quick increase in reactor power and the coolant water started to boil. The generated steam pressure lifted the 1,000-ton concrete slab placed above the reactor, which came crashing down, destroying the cooling pipes as well as the control rods. Moments later, there was a second explosion — hydrogen was generated and exploded. A portion of the reactor core was torn by the explosion, throwing out hot graphite debris that ignited the asphalt roof of the building.

The huge concrete sarcophagus of block 4 was constructed a few months after the accident in order to prevent any additional release of radioactive substances into the atmosphere. This sarcophagus is already near its service-life end and its replacement is being designed.

After the accident, the reactor was cooled by water and from helicopters was covered by tons of sand, lead, dolomite, and boron. It took 10 days to extinguish the fire and for this entire time radioactive material was being released into the environment from the breached reactor core. The majority of the radioactive debris remained near the reactor; the rest (mostly iodine and cesium) was carried by wind throughout Europe. The total released radioactivity was about 14 × 10¹⁸ Bq, representing about 5% of all the radioactive material contained in the reactor before the accident. Approximately 300 tons of hot graphite and uranium were blown into the environment. In the end, using remotely controlled machines, a concrete sarcophagus was constructed above the reactor. However, it requires permanent maintenance, and construction of a new envelope is anticipated.

At the time of the Chernobyl nuclear power plant accident, there were two more blocks with the same reactor, block 5 and 6, under construction. Due to safety concerns, these blocks were never completed.

The Chernobyl accident was classified as INES level 7: the most serious accident in the history of the nuclear reactor. This accident was caused by a series of serious operator errors, insufficient operator training and know-how, as well as gross breach of safety regulations. An inept reactor design also played its part.

In total, 600,000 people participated in the cleanup of this accident: 600 of them, summoned immediately after the accident, were exposed to a high radiation dosage. 237 individuals were diagnosed as suffering acute radiation sickness, of which 28 individuals died. Additionally, two people died during the explosion and another individual suffered a fatal heart attack as a direct result of these events.

Earthquake epicenter, location of the Fukushima nuclear power plant, the nearest towns and the three evacuation zones are depicted on a map of Japan.

All Japanese power plants are constructed on the seacoast, because they use seawater for cooling. The Fukushima Daiichi nuclear power plant is located on Honshu Island, 90 km from Sendai. There are six boiling water reactors (BWR), commissioned in 1971—1979. Since it is located on the seacoast, there is the danger of a tsunami. A system of dykes was constructed to protect the plant against a wave about 7 meters high. On March 11, 2011, an earthquake followed by a tsunami, with about 30,000 dead and missing persons, was bigger than the plant designers had expected.

On March 11, 2011 there was a very strong earthquake located 130 kilometers from the coast of Japan. This is referred to as the “Great East Japan Earthquake” (Also called the Tohoku Earthquake), classified as 9 on the Richter scale. At the time of the earthquake, the nuclear reactors were shutdown automatically by their shutoff rods. At Fukushima, this concerned reactor 1, 2, and 3, the other reactors were already in cold shutdown for planned maintenance. The quake itself did not cause any damage to the plant. Because the grid was down, the emergency generators came online to power the electronics and the coolant systems, since a shutdown reactor still generates lots of heat.

However, one hour later, the tsunami, at least 14 meters high, had reached the coast. Water went over the dykes and flooded the machine room, damaged the cooling piping leading to the sea, destroyed the fuel tanks for the diesel generators, and also flooded some standby batteries: the power plant lost almost all of its power supply. Without cooling, the reactors, still generating about 30 MW, started to overheat. For a short while, some blocks still had the emergency cooling running since it can function with a DC battery supply, but it also eventually failed. It took 8 hours to bring spare power supplies to partially restore cooling. However, it was not possible to effectively remove all the generated heat, because the cooling circuit was damaged by the tsunami. The cooling water started to boil in the reactors, generating steam, and the exposed fuel rods melted.

At this point, the plant operators decided that only a prompt flooding of the reactors with seawater could have cooled the reactor. To do that, the reactor pressure had to be reduced and part of the steam was vented into the atmosphere. Radioactive material was released and most importantly, a reaction between the nuclear fuel zircalloy cladding and the steam produced hydrogen, which exploded and damaged the roofs of all the reactor blocks. The actual power plant containment remained intact. Block 4 was also damaged; the water level in the pool used to store spent fuel decreased and there was also a hydrogen-air explosion.

Model of the Fukushima nuclear power plant that suffered a great disaster in March 2011. A tsunami wave flooded the machine room, damaged the tanks for diesel generators, leaving the power plant without electricity.

The struggle to stabilize the reactors continued until the end of 2011: radioactivity was released into the atmosphere and coastal waters several times. The total amount of radioactivity released from the power plant was about 770 × 10¹⁵ Bq, primarily iodine and cesium.

The Fukushima power plant’s surrounding area was gradually evacuated, the evacuation radius was extended to 20 km. None of the local inhabitants were injured or exposed to a dangerous dose of radiation. A few of the plant’s employees who were present during the accident were exposed to a higher dose of radiation: two received a dose of about 500 mSv, the international limit for workers participating in rescue operations. No radiation sickness was recorded, but two workers who waded through radioactive water suffered beta-burns of their skin. However, they were cured without any lasting effects. A few people in the facility were injured or killed by the tsunami or by falling debris.