Three Mile Island in 2010. Deactivated Unit 2 visible on the left.

5 Worst Nuclear Accidents in History

The development of nuclear energy has been plagued by many accidents – some of them could be described as minor difficulties, while others fully deserve their place among the worst man-made disasters humanity has ever experienced.

The severity of nuclear accidents is measured according to the INES, or the International Nuclear and Radiological Event Scale, established in 1990 by the International Atomic Energy Agency (IAEA). The INES is far from perfect representation of the seriousness of the situation at the time it’s happening. INES rating is assigned after the accident occurred and, in most cases, had been dealt with. One of the reasons for it is a genuine lack of information – right after a major disaster strikes, no one really knows what’s exactly going on. Full picture of the damage and destruction is unavailable. The other reason is that there is no central body to manage the rating. Because of that, incidents may be assigned a rating by the facility operator, high-level scientific authorities or even single experts. Finally, even in the democratic countries authorities often intentionally do not reveal the true extent of the disaster, to avoid causing panic among the populace and the markets. The Fukushima disaster is a very good example of such an approach.

There are 8 Levels on INES scale. “Level 0”, or “Deviation”, is assigned to minor anomalies without safety significance. The most serious nuclear accidents are ranked “Level 7” – “Major disaster”. Fukushima and Chernobyl disasters are the only two “Level 7” events to date. INES is a logarithmic scale, meaning that every next level is about 10 times more severe than the previous one (e.g. “Level 1” accident is 10 times more severe than “Level 0”).

5) Lucens, Vaud, Switzerland – 1969 (INES Level 5)

The construction of the Lucens reactor began in 1962. It was a small, Swiss-designed, carbon dioxide gas-cooled pilot reactor, built in an underground cavern. Moderated with heavy water and fueled by enriched uranium, it had an output of 30 megawatts of heat (MWt), used to generate over 8 megawatts of electricity (MWe). In comparison, the Kashiwazaki Kariwa Nuclear Power Station in Japan houses 7 nuclear reactors – 5 of them with an output of 1,067 MWe each, and 2 generating 1,315 MW each. It goes to show that even such a small reactor as the one in Lucens certainly has the capability to cause serious damage.

On January 21st, 1969, the reactor suffered a loss-of-coolant accident (LOCA) during a startup.  This led to a partial core meltdown and a massive contamination of the cavern. None of the people working on the reactor were irradiated and the contamination was contained inside the cavern, which was then sealed – encasing the nuclear contaminants in an underground cave (or simply in concrete), is called nuclear entombment.  The cavern was later decontaminated and the reactor dismantled.

The accident was caused by corrosion – the accumulated corrosion products impeded the flow of gas coolant.

Lucens, Switzerland.
Lucens, Switzerland.

4) Three Mile Island, United States – 1979 (INES Level 5)

This is considered to be the most serious civilian nuclear accident in the United States. On March 28th, 1979, one of the elements of the power plant’s system malfunctioned, initiating the chain of events that could lead to much more serious consequences. The element was a relief valve. The valve was opened following the increase in the primary loop pressure – this increase being caused by some other valves being closed for routine maintenance (closing them in such a manner was a violation of the key NRC [Nuclear Regulatory Commission] rule). The relief valve was supposed to close automatically after excess pressure had been released, but it didn’t. As a result, large amounts of the reactor coolant escaped.

Three Mile Island - pre-accident photo.
Three Mile Island – pre-accident photo.

Even though it sounds serious, in normal conditions the loss of coolant does not necessarily have to lead to a rapid deterioration of situation. When managed properly, and if both the emergency systems and personnel operate as they are supposed to, it’s not going to unavoidably result in a meltdown. During a Three Mile Island incident, some serious errors were made. The personnel operating the reactor were not trained properly and some control indicators were showing ambiguous information. There was much confusion as to what was actually going on. The crisis was eventually contained, but not before 2.5 million curies of radioactive gases (mostly Xenon and Krypton) and about 15 curies of iodine-131 had been released into the environment. The accident significantly raised the resistance to the nuclear energy among general populace and strengthened the anti-nuclear movement in the United States. The cleanup effort was officially completed in 1993, at a cost of $1 billion.

Three Mile Island in 2010. Deactivated Unit 2 visible on the left.
Three Mile Island in 2010. Deactivated Unit 2 visible on the left.
President Carter leaving the Three Mile Island.
President Carter leaving the Three Mile Island.

3) Kyshtym, Russia (former Soviet Union) – 1957 (INES Level 6)

During the Cold War many cities in the Soviet Union were “closed cities”. Some of those places were off limits not only for foreigners (it was impossible for foreigners, especially Westerners, to travel freely around the Soviet Union), but for the outsiders in general. Others were only inaccessible for foreigners, while the Soviet citizens normally living outside could enter. Back in the days even the Russians living inside the Russian SFSR (today’s Russia, which was one of the republics of the Soviet Union) needed special permits to move around the country. The situation was worse for rural residents, who were not allowed to leave their place of residence for more than 30 days and only with a permit from the selsoviet – or rural council. The closed cities were not marked on any maps (except for classified maps, of course) and there were no road signs to point you in the right direction. They were also omitted from railroad time tables and bus routes. Such cities were usually named like the nearest large city plus some number, e.g. Beloretsk-16, orPetropavlovsk-Kamchatsky-50.

Fissile Material Storage Facility (FMSF), Mayak, Russia.
Fissile Material Storage Facility (FMSF), Mayak, Russia.

As for life in the closed cities, in most case it was not what you’d expect. While living in a close city does sound somewhat like living in some sort of a prison camp, it was not exactly the case (although it may have looked like it from the outside, because those places were surrounded with the barbed wire fences and guarded by armed personnel). For the most part, citizens were allowed to travel outside – although they still had to have proper authorization and pass through checkpoints. But life inside closed cities had many benefits, which were beyond the reach of the majority of the population. For one, the housing conditions were better than average. Citizens also had access to stores that were better supplied than your average store in the Soviet Union. Also, anyone working with classified information or technologies received salary bonus.

What was the purpose of such cities? Some were in border areas, and were closed simply for general “security purposes”. Some cities housed strategically important facilities. For example, Perm was a center for tank production. Chelyabinsk-65 had plutonium production plant. Other ones could be important scientific centers or be associated with the production of nuclear weapons and other military projects. Finally, they could be large military bases – Vladivostok was the base for the Soviet Pacific Fleet (now Russian Pacific Fleet). There are still closed cities in Russia today – 42 that we know of, and probably more that we don’t. They are known as the “closed administrative-territorial formations”, or under the Russian acronym ZATO.

Entry checkpoint at the closed city of Seversk, Russia, 2006.
Entry checkpoint at the closed city of Seversk, Russia, 2006.
Source: Dmitry Afonin

The Kyshtym disaster occurred in one of such closed towns – Ozyorsk, also known as Chelyabinsk-40 and Chelyabinsk-65. As a closed town, it was not marked on the maps, so the disaster was named after the nearest city that was – Kyshtym. Ozyorsk is a home to Mayak plant – a large facility housing plutonium production plant and reprocessing facility. The plant was built in 1945-48 to make plutonium for nuclear weapons. At that time Soviets lagged behind the United States in the field of nuclear weapons and they were desperately trying to close the gap. Since the Soviet authorities did not care about the environment and about the people living around the facility (or working in it, for that matter) the health concerns were irrelevant. The facility kept dumping irradiated water and radioactive waste into nearby lakes and streams. That waste was eventually reaching Arctic Ocean, in the waters of the river Ob (the world’s 4th longest river).

Ozyorsk, Russia. 2008.
Ozyorsk, Russia. 2008.
Source: Sergey Nemanov

Some years after the original construction of the Mayak plant was completed, the storage tanks for dissolved radioactive waste were built. Since the waste was heating itself through decay heat, the cooling system had to be installed. On September 29th, 1957, the cooling system for one of the tanks, storing many tons of liquid nuclear waste, malfunctioned, leading to the evaporation, and eventual explosion of the dried waste – mostly ammonium nitrate and acetates. The explosion had a force of 70 to 100 tons of TNT. Although no one was killed directly by the explosion, it released a massive radioactive cloud. The cloud contaminated an area of over 7,000 square miles (the total area of New Jersey is 7,787 sq mi, or 20,168 sq km).

At least 200 people died due to direct exposure to radiation and 10,000 were evacuated (although no one told them why they have to leave). In total – 470,000 people were exposed to radiation and the Soviet Health Ministry found that over 8,000 people had died within the preceding 32 years as a result of the disaster.

Of course, the accident was kept secret by the Soviet Authorities. Only in 1976 the real extent of the disaster was made public. The CIA also kept quiet (although the Agency did know what happened all along) – presumably to avoid arousing public fears of the nuclear energy in the U.S.

2) Fukushima, Japan – 2011 (INES Level 7)

On March 11th, 2011, 9.0 magnitude earthquake struck off the coast of Japan. The resulting tsunami (49 feet tall) hit the Fukushima I Nuclear Power Plant 51 later, flooding the entire area and destroying or damaging electrical lines, generators and pumps. As a result of this damage, reactors 1, 2 and 3 experienced full meltdown. The full extent of the damage is not known even today. It’s certainly too soon to even estimate the total impact on people and environment. Even though this is number two of the most serious nuclear disasters, it may well become number one in the future.

Satellite image on 16 March of the four damaged reactor buildings
Satellite image on 16 March of the four damaged reactor buildings

1) Chernobyl, Ukraine (former Soviet Union) – 1986 (INES Level 7)

On April 26th, 1983, a series of events led to the explosion of the reactor number four at the Chernobyl Nuclear Power Plant. The resulting fire of the graphite moderator sent huge plumes of heavily radioactive smoke in the air, and it eventually contaminated about 100,000 sq kilometers (over 38,000 sq miles) of what is now Ukraine, Belarus and Russia. The accident was caused by the faulty design of the reactor and a series of errors by the reactor’s operators. There is no agreement as to how many people have died as a result of this disaster, but most estimates place the number of casualties at 5000-6000. About 100 people were killed in the immediate aftermath of the accident. 600,000 workers took part in the decontamination – they were called “liquidators”.

The nuclear reactor after the disaster. Reactor 4 (center). Turbine building (lower left). Reactor 3 (center right).
The nuclear reactor after the disaster. Reactor 4 (center). Turbine building (lower left). Reactor 3 (center right).
Chernobyl corium lava flows formed by fuel-containing mass in the basement of the plant.
Chernobyl corium lava flows formed by fuel-containing mass in the basement of the plant.
Source: International Nuclear Safety Administration

Further reading

International Atomic Energy Agency (IAEA) – Fukushima Daiichi Status Reports
World Nuclear Association – Chernobyl Accident 1986
United States Nuclear Regulatory Commission – Backgrounder on Chernobyl Nuclear Power Plant Accident
Greenpeace – Fukushima Nuclear Disaster
European Commission – Nuclear Safety