Tokyo (/ˈtoʊkioʊ/ TOH-kee-oh, /-kjoʊ/ -kyoh; Japanese: 東京, Tōkyō [toːkʲoː] (About this soundlisten)), officially Tokyo Metropolis (東京都, Tōkyō-to), is the capital and most populous prefecture of Japan. Located at the head of Tokyo Bay, the prefecture forms part of the Kantō region on the central Pacific coast of Japan’s main island of Honshu. Tokyo is the political and economic center of the country, as well as the seat of the Emperor of Japan and the national government. In 2019, the prefecture had an estimated population of 13,929,280. The Greater Tokyo Area is the most populous metropolitan area in the world, with more than 37.393 million residents as of 2020.
Originally a fishing village named Edo, the city became a prominent political center in 1603, when it became the seat of the Tokugawa shogunate. By the mid-18th century, Edo was one of the most populous cities in the world, with a population numbering more than one million. Following the end of the shogunate in 1868, the imperial capital in Kyoto was moved to the city, which was renamed Tokyo (literally “eastern capital”). Tokyo was devastated by the 1923 Great Kantō earthquake, and again by Allied bombing raids during World War II. Beginning in the 1950s, the city underwent rapid reconstruction and expansion, going on to lead Japan’s post-war economic recovery. Since 1943, the Tokyo Metropolitan Government has administered the prefecture’s 23 special wards (formerly Tokyo City), various bed towns in the western area, and two outlying island chains.
Tokyo is the largest urban economy in the world by gross domestic product, and is categorized as an Alpha+ city by the Globalization and World Cities Research Network. Part of an industrial region that includes the cities of Yokohama, Kawasaki, and Chiba, Tokyo is Japan’s leading center of business and finance. In 2019, it hosted 36 of the Fortune Global 500 companies. In 2020, it ranked fourth on the Global Financial Centres Index, behind New York City, London, and Shanghai.
The city has hosted multiple international events, including the 1964 Summer Olympics and three G7 Summits (1979, 1986, and 1993); it will also host the 2020 Summer Olympics, which were postponed to 2021 due to the COVID-19 pandemic. Tokyo is an international center of research and development and is represented by several major universities, notably the University of Tokyo. Tokyo Station is the central hub for Japan’s Shinkansen bullet train system, and the city is served by an extensive network of rail and subways. Notable districts of Tokyo include Chiyoda (the site of the Imperial Palace), Shinjuku (the city’s administrative center), and Shibuya (a commercial, cultural and business hub).
The Chernobyl disaster was caused by a nuclear accident that occurred on Saturday 26 April 1986, at the No. 4 reactor in the Chernobyl Nuclear Power Plant, near the city of Pripyat in the north of the Ukrainian SSR. It is considered the worst nuclear disaster in history and was caused by one of only two nuclear energy accidents rated at seven—the maximum severity—on the International Nuclear Event Scale, the other being the 2011 Fukushima Daiichi nuclear disaster in Japan.
The accident started during a safety test on an RBMK-type nuclear reactor, which was commonly used throughout the Soviet Union. The test was a simulation of an electrical power outage to aid the development of a safety procedure for maintaining reactor cooling water circulation until the back-up electrical generators could provide power. This gap was about one minute and had been identified as a potential safety problem that could cause the nuclear reactor core to overheat. It was hoped to prove that the residual rotational energy in a turbine generator could provide enough power to cover the gap. Three such tests had been conducted since 1982, but they had failed to provide a solution. On this fourth attempt, an unexpected 10-hour delay meant that an unprepared operating shift was on duty.
During the planned decrease of reactor power in preparation for the electrical test, the power unexpectedly dropped to a near-zero level. The operators were able to only partially restore the specified test power, which put the reactor in a potentially unstable condition. This risk was not made evident in the operating instructions, so the operators proceeded with the electrical test. Upon test completion, the operators triggered a reactor shutdown, but a combination of unstable conditions and reactor design flaws caused an uncontrolled nuclear chain reaction instead.
A large amount of energy was suddenly released, vaporising superheated cooling water and rupturing the reactor core in a highly destructive steam explosion. This was immediately followed by an open-air reactor core fire that released considerable airborne radioactive contamination for about nine days that precipitated onto parts of the USSR and western Europe, especially Belarus, 16km away, where around 70% landed, before being finally contained on 4 May 1986. The fire gradually released about the same amount of contamination as the initial explosion. As a result of rising ambient radiation levels off-site, a 10-kilometre (6.2 mi) radius exclusion zone was created 36 hours after the accident. About 49,000 people were evacuated from the area, primarily from Pripyat. The exclusion zone was later increased to 30 kilometres (19 mi) radius when a further 68,000 people were evacuated from the wider area.
The reactor explosion killed two of the reactor operating staff. In the emergency response that followed, 134 station staff and firemen were hospitalized with acute radiation syndrome due to absorbing high doses of ionizing radiation. Of these 134 people, 28 died in the days to months afterward and approximately 14 suspected radiation-induced cancer deaths followed within the next 10 years.
Among the wider population, an excess of 15 childhood thyroid cancer deaths were documented as of 2011. The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) has, at multiple times, reviewed all the published research on the incident and found that at present, fewer than 100 documented deaths are likely to be attributable to increased exposure to radiation. Determining the total eventual number of exposure related deaths is uncertain based on the linear no-threshold model, a contested statistical model, which has also been used in estimates of low level radon and air pollution exposure. Model predictions with the greatest confidence values of the eventual total death toll in the decades ahead from Chernobyl releases vary, from 4,000 fatalities when solely assessing the three most contaminated former Soviet states, to about 9,000 to 16,000 fatalities when assessing the total continent of Europe.
To reduce the spread of radioactive contamination from the wreckage and protect it from weathering, the protective Chernobyl Nuclear Power Plant sarcophagus was built by December 1986. It also provided radiological protection for the crews of the undamaged reactors at the site, which continued operating. Due to the continued deterioration of the sarcophagus, it was further enclosed in 2017 by the Chernobyl New Safe Confinement, a larger enclosure that allows the removal of both the sarcophagus and the reactor debris, while containing the radioactive hazard. Nuclear clean-up is scheduled for completion in 2065. The Chernobyl disaster is considered the worst nuclear power plant accident in history, both in terms of cost and casualties. The initial emergency response, together with later decontamination of the environment, ultimately involved more than 500,000 personnel and cost an estimated 18 billion Soviet rubles—roughly US$68 billion in 2019, adjusted for inflation. The accident resulted in safety upgrades on all remaining Soviet-designed RBMK reactors, of which 10 continue to be operational as of 2019.