The 25-year-old senior reactor control engineer who pressed the AZ-5 button at Chernobyl's Unit 4
While studying control-rod documentation, Toptunov told his friend Sasha Korol that 'in certain circumstances, the rods could accelerate rather than slow the reaction': three months later, that insight became reality under his own finger on the AZ-5 button.
Leonid Toptunov was the Senior Reactor Control Engineer (SIUR) at Chernobyl's Unit 4, a 25-year-old technician at the control panel during the night of the 26 April 1986 disaster. Only three months into independent duty and facing his first shutdown, he pressed the AZ-5 emergency shutdown button under pressure from deputy chief engineer Anatoly Dyatlov, but a design flaw in the RBMK control rods caused a reactivity surge that triggered the explosion; he returned to the control room afterward to manually open water supply valves, absorbing a lethal 1,300 rem before dying in a Moscow hospital on 14 May.
Activities and affiliations
Career Timeline
- 1960Born in Mykolaivka, Sumy Oblast, Ukrainian SSR
- 1983Graduated from Obninsk Institute for Nuclear Power Engineering
- 1983–1986Chernobyl Nuclear Power Plant: unit control engineer, then Senior Reactor Control Engineer (SIUR), Unit 4
- 1986.04.26Pressed AZ-5 scram button during Unit 4 safety test on night shift; received lethal radiation dose
- 1986.05.14Died of acute radiation syndrome at Moscow Hospital No. 6; buried at Mitinskoe Cemetery
Related historical events
Operator error and indemnified victim: service at Unit 4 and its legal aftermath (1983-1986)
Toptunov began work at the Chernobyl Nuclear Power Plant in March 1983, serving as a unit control engineer and then as a senior reactor control engineer. Russian-language material records that he graduated from the Obninsk branch of the Moscow Engineering Physics Institute, that he did his pre-diploma practice at the Chernobyl plant, and that from 31 March 1983 he worked as a unit control engineer and senior reactor control engineer in reactor shop No. 2. English-language material reports that, while studying the reactor documentation, he remarked to an acquaintance that the control rods could, under certain circumstances, accelerate rather than slow the reaction. How far that remark shows what he understood of the RBMK control-rod defect later confirmed after the accident is described differently across sources, so it cannot be settled here.
The Unit 4 shift was where he stood at the panel in April 1986. One account states that he had roughly two months' experience in operating the reactor and that this was his first shutdown as operator; the figure does not appear in the other and is given differently across documents, so it is not carried over as a fixed number. The same Russian-language account records that thermal power was 720 MW when his shift began at 00:05 on 26 April 1986, that it fell to 30-50 MW at 00:28 during the switchover between automatic power regulators, and that it was raised back to 200 MW and stabilised at about 01:03. At 01:23:39 the AZ-5 button was pressed; about three seconds later signals registered a sharp rise in reactivity and thermal power, and a series of explosions followed at 01:23:46.
The handling after the accident is also documented. The Russian-language article states that criminal proceedings against him were terminated on 28 November 1986 on account of his death, under the criminal procedure code of the Ukrainian SSR, and that by Ukrainian presidential decree No. 1156/2008 of 12 December 2008 he was posthumously awarded the Order for Courage, third degree. An English-language account says his family was told that his death was the only reason he was not prosecuted, and notes that his role was recreated in a 2004 television dramatisation, a 2006 BBC production and the 2019 HBO miniseries. His life thus stands as a record in which a first investigation that assigned blame to operating actions overlapped with a later, posthumous rehabilitation of the men who died.