Ozyorsk or Ozersk (Russian: Озёрск) is a closed city in Chelyabinsk Oblast, Russia. It had a population of 82,164 as of the 2010 census.

The town was founded on the shores of Lake Irtyash in 1947. Until 1994, it was known as Chelyabinsk-65, and even earlier, as Chelyabinsk-40 (the digits are the last digits of the postal code, and the name is that of the nearest big city, which was a common practice of giving names to closed towns).

Codenamed City 40, Ozersk was the birthplace of the Soviet nuclear weapons program after the Second World War. In 1994, it was granted town status and renamed Ozyorsk.

Within the framework of administrative divisions, it has, together with six rural localities, official status as the Town of Ozyorsk—an administrative unit with a status equal to that of the districts. As a municipal division, the town of Ozyorsk has the official name Ozyorsky Urban Okrug.

Ozyorsk was and remains a closed town because of its proximity to the Mayak plant, one of the sources of Soviet plutonium during the Cold War, and now a Russian facility for processing nuclear waste and recycling nuclear material from decommissioned nuclear weapons.

The plant itself covers an area of approximately 90 km and employs about 15,000 people.

The Mayak is primarily engaged in reprocessing of spent nuclear fuel from the nuclear submarines and icebreakers and from nuclear power plants. Commercially, it produces cobalt-60, iridium-192, and carbon-14.

The town's coat of arms depicts a flame-colored salamander.

Southern-Urals Construction Department (ЗАО "Южноуральское управление строительства") is another major enterprise. Its activities include construction for atomic industry needs, production of concrete constructions and construction materials.

Main products of Plant of Wiring Products #2 (ЗАО "Завод электромонтажных изделий №2") are low-voltage devices for military-industrial establishments.

Ozyorsk and Richland in the US were the first two cities in the world to produce plutonium for use in Cold War atomic bombs.

The Chelyabinsk region has been reported as being one of the most polluted places on Earth, having previously been a center of production of weapons-grade plutonium.

Ozyorsk and the surrounding countryside have been heavily contaminated by industrial pollution from the Mayak plutonium plant since the late 1940s. The Mayak plant was one of the largest producers of weapons-grade plutonium for the Soviet Union during much of the Cold War, particularly during the Soviet atomic bomb program. Built and operated with great haste and disregard for safety, largely out of gaps in information, between 1945 and 1957 the plant dumped and released large amounts of solid, liquid and gaseous radioactive material into the area immediately around the plant. Over time, the sum of radionuclide contamination is estimated to be 2-3 times the release from the explosions from the Chernobyl accident.

In 1957, the Mayak plant was the site of a major disaster, releasing more radioactive contamination than the meltdown at Chernobyl. An improperly stored underground tank of high-level liquid nuclear waste exploded, contaminating thousands of square kilometres of territory, now known as the Eastern Ural Radioactive Trace (EURT). The matter was quietly and secretly covered up, and few either inside or outside Russia were aware of the full scope of the disaster until 1980.

Before the 1957 accident, much of the waste was dumped into the Techa River, which it severely contaminated as well as residents of dozens of riverside villages such as Muslyumovo, who relied on the river as their sole source of drinking, washing and bathing water. After the 1957 accident, dumping in the Techa River officially ceased, but the waste material was dumped in convenient shallow lakes near the plant instead, of which 7 have been officially identified. Of particular concern is Lake Karachay, the closest lake to the plant (now notorious as the most contaminated place on Earth) where roughly 4.4 exabecquerels of high-level liquid waste (75-90% of the total radioactivity released by Chernobyl) was dumped and concentrated in the shallow 45-hectare (110-acre) lake over several decades.

In addition to the radioactive risks, the airborne lead and particulate soot levels in Ozyorsk (along with much of the Ural industrial region) are also very high—roughly equal to the levels encountered along busy roadsides in the era predating unleaded gasoline and catalytic converters—due to the presence of numerous lead smelters.

On Sunday September 29, 1957 at 4:22 pm, in the production association "Beacon" in Ozersk one of the containers exploded, in which high-level waste was kept. The explosion completely destroyed a stainless steel container located in a concrete canyon 8.2 meters deep. In total, there were 14 containers ("cans") in the canyon. One tenth of the radioactive substances were lifted into the air. After the explosion, a column of smoke and dust rose up to a kilometer high, the dust flickered with an orange-red light and settled on buildings and people. The rest of the waste discarded from the tank remained at the industrial site. Reactor plants got into the contamination zone.

Immediately after the explosion at the facilities of the chemical plant, dosimetrists noted a sharp increase in the background radiation. Many industrial buildings, vehicles, concrete and railways were contaminated. The main spot of radioactive contamination fell on the territory of industrial sites, and 256 cubic meters of radioactive solutions were poured into the tank. The radioactive cloud passed the city of atomic scientists and passed by due to the successful location of the city - when it was laid, the wind rose was taken into account.

As a result of the explosion of the container, a concrete slab weighing 160 tons was torn off. A brick wall was destroyed in a building located 200 meters from the explosion site. They did not immediately notice the polluted streets, canteens, shops, schools, and kindergartens. In the first hours after the explosion, radioactive substances were brought into the city on the wheels of cars and buses, on the clothes and shoes of industrial workers. The most polluted was the central Lenin street, especially when entering the city from the industrial site, and Shkolnaya street, where the management of the plant lived. Subsequently, the flow of radioactive substances was suspended. It was forbidden to enter the city from industrial sites for cars and buses. Site workers at the checkpoint got off the buses and passed the checkpoint. This requirement extended to everyone, regardless of rank and official position. Shoes were washed on flowing trays.

The territory, which was exposed to radioactive contamination as a result of the explosion at the chemical plant, was named "East Ural Radioactive Trace" (EURT). Its total length was about 300 km, with a width of 5–10 km. This area was inhabited by about 270 thousand people. Fields, pastures, reservoirs, forests were polluted on the territory, which turned out to be unsuitable for further use.

In a memo addressed to the Central Committee of the CPSU, Industry Minister E.P. Slavsky wrote: "Investigating the causes of the accident on the spot, the commission believes that the main culprits of this incident are the head of the radiochemical plant and the chief engineer of this plant, who committed a gross violation of the technological regulations for the operation of storage of radioactive solutions". In the order for the Ministry of Medium Machine Building, signed by E.P. Slavsky, it was noted that the reason for the explosion was insufficient cooling of the container, which led to an increase in the temperature in it and to the creation of conditions for the explosion of salts. This was later confirmed in experiments carried out by the Central Factory Laboratory (CPL). The director of the plant M. A. Demyanovich took all the blame for the accident, for which he was relieved of his duties as director.

The radiation accident in the Urals posed a whole series of completely new tasks for science and practice. It was necessary to develop measures for the radiation protection of the population. An Experimental Station was created in the Urals, which played a leading role in studying the consequences of the accident and developing the necessary recommendations.

While the environmental impact of the disaster was immense, the contamination of the local populace was equally devastating. The average person living in Ozyorsk, 8 km from the Mayak Nuclear Facility, had a long term radioactive burden on their body at 17 Bq. Because of the large amounts of radioactive materials that were discharged into the atmosphere, over 22 small towns throughout the region were evacuated. Some cities took two years for a full evacuation to take place.

Reports indicated that humans living in the affected area during the time that the disaster took place and their offspring have developed problems with reproductive functions, mortality, age structure, and sex deformities. These populations were recorded at experiencing a radioactive exposure of 40-500 mSv.

There are seventeen different cultural and public-service institutions.

There are sixteen secondary schools, two schools specializing in the English language, one gymnasium, physics-mathematics lyceum, three professional colleges, Southern-Ural Polytechnical College, Music College, Ozyorsk Engineering Institute (an affiliate of National Research Nuclear University MEPhI), and affiliates of Yekaterinburg's and Chelyabinsk's universities.

City 40 is a documentary film about the town, by Samira Goetschel, released in July 2016.

The Half Life of Valery K is a 2022 historical fiction novel by Natasha Pulley. Most of the novel is set in City 40.

• Законодательное Собрание Челябинской области. Постановление №161 от 25 мая 2006 г. «Об утверждении перечня муниципальных образований (административно-территориальных единиц) Челябинской области и населённых пунктов, входящих в их состав», в ред. Постановления №2255 от 23 октября 2014 г. «О внесении изменений в перечень муниципальных образований (административно-территориальных единиц) Челябинской области и населённых пунктов, входящих в их состав». Вступил в силу со дня официального опубликования. Опубликован: "Южноуральская панорама", №111–112, 14 июня 2006 г. (Legislative Assembly of Chelyabinsk Oblast. Resolution #161 of November 25, 2006 On Adoption of the Registry of the Municipal Formations (Administrative-Territorial Units) of Chelyabinsk Oblast and of the Inhabited Localities They Comprise, as amended by the Resolution #2255 of October 23, 2014 On Amending the Registry of the Municipal Formations (Administrative-Territorial Units) of Chelyabinsk Oblast and of the Inhabited Localities They Comprise. Effective as of the official publication date.).
• Законодательное Собрание Челябинской области. Закон №287-ЗО от 28 октября 2004 г. «О статусе и границах Озёрского городского округа», в ред. Закона №124-ЗО от 28 апреля 2011 г «О внесении изменений в Закон Челябинской области "О статусе и границах Озёрского городского округа"». Вступил в силу со дня официального опубликования. Опубликован: "Южноуральская панорама", спецвыпуск, 30 ноября 2004 г. (Legislative Assembly of Chelyabinsk Oblast. Law #287-ZO of October 28, 2004 On the Status and Borders of Ozyorsky Urban Okrug, as amended by the Law #124-ZO of April 28, 2011 On Amending the Law of Chelyabinsk Oblast "On the Status and Borders of Ozyorsky Urban Okrug". Effective as of the official publication date.).

• Kate Brown, Plutopia: Nuclear Families, Atomic Cities, and the Great Soviet and American Plutonium Disasters. Oxford: Oxford University Press, 2013.

• (in Russian) Official website of Ozyorsky Urban Okrug
• (in Russian) News, views and people (information portal of Ozyorsk)
• (in Russian) Information portal of Ozyorsk
• (in Russian) News portal of Ozyorsk
• (in Russian) Website of Ozyorsk
• Article about Ozyorsk and Mayak at uralpress.ru
• Plutopia: Nuclear Families, Atomic Cities, and the Great Soviet and American Plutonium Disasters