In 1989, during a Symposium organised by the IAEA, it was reported that on 29 September 1957, a major radiation accident occurred at the first Soviet nuclear installation, now called the Mayak Production Association (PA), located near the town of Kyshtym in the Chelyabinsk oblast of the Russian Federation, and this accident is now generally referred to as the 'Kyshtym Accident' [1–4]. A chemical explosion with an energy equivalent to about 100 tons of TNT occurred in a concrete tank containing 670–740 PBq of radioactive waste. About 10% of the radioactive content involved in the explosion (74 PBq) was transported into the atmosphere to a height of 1 km and subsequently deposited in parts of the Chelyabinsk, Sverdlovsk and Tyumensk provinces. The contaminated area was called the East Urals Radioactive Trace (EURT). The radioactive material consisted essentially of 90Sr + 90Y, 95Zr + 95Nb, 106Ru + 106Rh, and 144Ce + 144Pr. In the most severely affected area, close to the boundary of the industrial site, the initial 90Sr deposition density reached 150 MBq m−2. Countermasures to protect the population were applied in areas with a 90Sr level greater than 74 kBq m−2. The territory contaminated with 90Sr levels in the range from 3.7 to 74 kBq m−2 had an area of 15 000 km2 and a population of 260 000. Within 7 to 10 d following the accident, 600 people were evacuated from the settlements in the most severely affected areas, and about 10 000 people were evacuated during the 18 months following the accident. The average effective dose received by the most exposed group before evacuation was 520 mSv, including a contribution of 170 mSv from external irradiation. Among the 10 730 evacuees, the average effective dose was 120 mSv. Among the 260 000 non-evacuees, the effective doses were much smaller, with an average of 5 mSv. An analysis of the malignant neoplasms indicated no significant difference between irradiated and non-irradiated populations with regard to incidence and mortality. In addition to the assessment of the health effects, a substantial number of radioecological and biological investigations also were conducted, in particular to study the bahavior of radionuclides in the environment, to organise the agricultural production in ways minimising the radionuclide intakes by the exposed population, and to study the biological effects of radiation on the fauna and flora in a contaminated environment.
This news, provided 32 years after the accident, was a surprise for most of the world's scientists. The Soviet authorities had until then managed to keep a tight lid on any information related to the accident. It is mainly due to the efforts of an exiled Soviet geneticist, Zhorev Medvedev, that rumors started to circulate in the 1970s that a nuclear disaster, associated to a volcano-type explosion of buried radioactive waste, had occurred in the Ural mountains in 1958 and had resulted in the radioactive contamination of a vast territory [5]. Medvedev's allegations were supported in the U.S. by Trabalka et al [6, 7] on the basis of a thorough review of the Russian literature on radioecological studies, but disputed by other scientists [8, 9], who agreed that 'a contaminated area does exist east of Kyshtym', but thought that it was due to either an accumulation of radioactive material resulting from routine discharges into the environment [8] or fallout from one or several nuclear weapons tests conducted at the faraway site of Novaya Zemlya [9]. Therefore, the Kyshtym accident remained an enigma until the Soviet authorities decided to release a substantial amount of information in 1989.
Within a few years after 1989, the description of the Kyshtym accident and of its impact on the environment and the local population became more widely documented and reported [10–16]. In the early 1990s, information also was provided on the activities that were conducted at the Mayak PA, where the first Soviet nuclear reactor for plutonium production was constructed in 1948, where the produced plutonium was separated from the fuel in a radiochemical plant, and where the 'Kyshtym Accident' took place [12, 17–19]. The most dangerous environmental problem in the Mayak PA facilities was the early system of radioactive wastes produced in large quantities. All radiation situations that occurred in the Urals in the 1950s and 1960s were caused by inadequate technology for managing radioactive wastes. The radioactive contamination of the territory in the Urals and the irradiation of the population residing on it was due to: (1) releases of radioactive wastes into the Techa River, mainly in 1950 and 1951; (2) explosion of a radioactive waste tank in 1957 (the 'Kyshtym Accident'); (3) transfer by wind of radioactive material from dry banks of Lake Karachay, an open depot of liquid radioactive waste, in 1967; and (4) atmospheric releases from the radiochemical plant from 1948 to 1972 [19, 20].
International collaboration was undertaken in the 1990s, notably with U.S. scientists. In 1994, a bilateral U.S./Russian agreement to support research and exchange information on radiation health effects was signed [21]. Within the framework of this agreement, extensive research has been conducted on radiation doses and health effects related to (1) current and past workers from Mayak PA, and (2) current and past residents along the Techa River who were impacted from airborne and waterborne releases from Mayak PA [22]. The releases of radioactive waste into the Techa River resulted in external and internal exposures of the members of a cohort of about 30 000 persons who lived at some time in downstream settlements. In order to study the radiation effects in that cohort, individual doses were estimated in the joint Russian/U.S. studies [20]. In part because 5280 members of the cohort were also exposed at some time due to their residence within the EURT area at one time or another, a re-evaluation of the EURT doses was carried out, based on an analysis of the 90Sr concentrations and total beta activities measured in local foodstuffs in 1958–2011 and on the measurements of 90Sr in human bones and whole-body [20, 23, 24]. A EURT cohort, comprising about 21 400 residents of contaminated settlements, was also formed; a review of the environmental monitoring, of the updated dose estimates, and of the health status of the cohort members was published in 2017 in this Journal [23]. The documentation on the various aspects of the 'Kyshtym Accident' and of its consequences was later supplemented with (1) a description and analysis of the protective measures taken during the emergency and remediation phases of the accident [16] and (2) a comprehensive review of the long-term studies of biological effects in plants and animals inhabiting the area contaminated as a result of the 'Kyshtym Accident' [15].
The estimation of the doses received by the Mayak workers has also received considerable attention. During the early years of operation, from 1948 until the mid-1950s, the Mayak workers were exposed to relatively high exposures due to external irradiation and plutonium intakes. The Mayak Worker Dosimetry System has been developed over many years of collaboration between Russian and American scientists, with input from many other countries, to provide estimates of organ absorbed doses for use by epidemiologists in their studies of radiation effects [25, 26]. The Mayak cohort includes workers first employed between 1948 and 1972 and consists of about 20 000 subjects [25].
However, even though much is known on the radiation exposures of the Mayak workers, very little information has transpired on the doses received by the persons who were on site at the time of the explosion and on the recovery operation workers involved in the remediation of the accident. Prior to the publication of the very interesting article authored by Vasilenko et al in this issue [27], the only publicly available information on the doses to the workers present on the site at the time of the accident and to the recovery operation workers seems to be from Kruglov [28] who reported that more than 5000 workers who were on the site were subject to a dose as high as 1000 mSv between the time of the explosion and the beginning of the evacuation and that the nearly 30 000 workers involved in the remediation of the accident in 1957–1959 received doses above 250 mSv.
Much more detailed information is provided in the article authored by Vasilenko et al in this issue [27]. In summary, the average personal dose equivalent to the 4869 individuals exposed on the site at the time of the accident on 29 September 1957 (and evacuated on 30 September) was estimated to be 820 mSv, corresponding to a collective dose equivalent of 4000 man Sv. The remediation of the accident took place from October 1957 to the end of 1959: it involved more than 30 000 recovery operation workers, (including Mayak personnel, civilian and military construction workers, and military guards) who received, on average, an estimated personal dose equivalent of about 100 mSv. The average personal dose equivalent over all persons exposed on the site from the time of the accident to the end of 1959 was 200 mSv, corresponding to a collective dose equivalent of about 7000 man Sv. In comparison to the other two large radiation accidents that occurred in the nuclear industry, namely Chernobyl and f*ckushima, the 'Kyshtym Accident' is in an intermediate position between Chernobyl and f*ckushima in terms of collective dose to recovery operation workers: the collective dose to the 530 000 Chernobyl recovery operation workers was about 60 000 man Sv [29], while the collective dose to the f*ckushima workers was 300 man Sv [30].
The article of Vasilenko et al in this issue [27] is very important, as it unveils detailed information on what is likely to be the last important knowledge gap on the main events that occurred at Mayak PA in the early years of the nuclear age.
