The accident occurred early in the morning of 26 April 1986 during low-power test operation of the Unit 4 reactor. The safety system has been switched off and unstable operation led the uncontrollable power surge to occur, resulting in successive steam explosions that severely destroyed the reactor. A massive amount of radioactive material 5200 PBq (I-131 equivalent) was released into the environment (UNSCEAR 2000 Report). Several hundred subjects were exposed to high doses of beta- and gamma-irradiation either as working personnel within the reactor immediately after the accident (emergency workers). Emergency workers include stuff of the power plant (Units 1, 2, 3 and 4), construction workers at Units 5 and 6, firemen, guards, and stuff of the local medical facilities. Among 134 emergency workers who showed acute radiation symptoms, 19 patients received bone marrow transplantation, and 28 died within few weeks after (19 more subsequently died between 1987-2004, but the causal relation to radiation is not known). The followings are the summary of the exposed populations (UNSCEAR 2000 Report).

Subjects

Reference

Salassidis K, Schmid E, Peter RU, Braselmann H and Bauchinger M: Dicentric and translocation analysis for retrospective dose estimation in humans exposed to ionizing radiation during the Chernobyl nuclear power plant accident. Mutation Res., 311:39-48, 1994.

Characterization of the study population 

Since the paper by Salassidis et al (1994) is only publication that described exposed conditions and chromosome aberration analysis of Chernobyl victims in detail, the exposure conditions of the patients have been re-described from the paper in order to help the understanding of the biological dosimetry of the heavily exposed persons.
[Patient 1] worked at the reactor during the night of the accident, transferred to Hospital N.6, Moscow, 1 day after the accident.
[Patient 2] worked within the power plant at 40 m distance from the reactor during the night of accident. Carried a severely contaminated colleague on his shoulders out of the endangered area and was irradiated for 10 min; walked through heel-high contaminated water for 20 min; transferred to Hospital No.6 1 day after the accident; received a bone marrow transplantation from his sister.
[Patient 3] worked outside the power plant at about 500 m distance from the reactor; his entire body was covered with dust and ashes; transferred to Hospital No. 6 after 24 h.
[Patient 4] was visiting engineer on an inspection trip during night of the accident. Stayed from 3-4 h at about 50 m distance from the reactor and waited for 1 h outside the reactor for transport to Hospital No.6.
[Patient 5] worked as a consulting engineer with the liquidation team from 1 week to 9 months following the accident. He wore a protective suit, with left the hands and the nape of neck free. Contamination with radioactive water on day 1.
[Patient 6] worked as a technician at the reactor during the night of the accident. Exposed during cleaning procedures for >6 h, contaminated with radioactive water for >1 h; changed clothes and showered after 7 h. Treated for 2 yrs in Hospital No.6. Amputation of left lower leg due to nonhealing ulcers.
[Patient 7] worked at the reactor during the night of the accident at a distance of about 50-70 m from explosion. Spent about 40 min in the contaminated area, washed and changed clothes 1 h later; transferred to Hospital No.6 24 h after accident.
[Patient 8] worked in block IV at the time of the accident at a distance of 30-40 m from the reactor; hands severely contaminated, clothes were wet; remained in the contaminated area for about 1 h; changed clothes and showered after 2 h; treated in Hospital No.6.
[Patient 9] worked in hall 4 at a distance of 100 m from the reactor; severely contaminated through siting on contaminated pipes; changed clothes for the first time 3.5 h after the accident. Transferred to Hospita. No.6 1 day after the accident. Received bone marrow transplantation from unknown donor.
[Patient 10] worked as a watvhman walking at about 150 m distance around the reactor during the night of the accident until 8 a.m.
[Patient 11] worked at the reactor during the night of the accident for 1.5 h; contaminated by radioactive water; exposed for about 10 min at 150 m distance from the reactor.
[Patient 12] arrived at the reactor site 1 week after the accident and worked as a liquidator for several months.
[Patient 13] worked as a watchman during the night of the accident; stayed close to the reactor for 1 day and 2 nights and was covered by graphite dust; lost hair and nails on day 20.
[Patient 14] lived 2 km from the power plant and arrived to work on the morning after the accident; worked for 1 h at 400 m distance from the reactor; received bone marrow transplantation.
[Patient 15] worked as a member of the liquidation teams; protected on the body by read but hands and feet were free; claimed to ave received a whole body dose of 1.6 Gy.