A new theory by Dr Lars-Erik De Geer challenges the origins of the Chernobyl accident. Khai Trung Le reports.
On 26 April 1986, an uncontrollable power surge in reactor 4 of the Chernobyl nuclear power station resulted in explosions that led to the death of 28 staff and emergency workers and contamination across Belarus, Bryansk and northwestern Ukraine. One explosion sent the heavy lid of the reactor through the roof of the building, followed by a stronger explosion seconds later. While the predominant theory is that both were steam explosions, as hot fuel particles from a ruptured pressure tube reacted with water in the reactor, a consensus has never been reached.
Now, a new paper asserts that the first explosion was not steam but consisted of thermal neutron mediated nuclear explosions. Dr Lars-Erik De Geer, retired nuclear physicist and co-author of the paper, A Nuclear Jet at Chernobyl Around 21:23:41 UTC on April 24, 1986, published in Nuclear Technology, centres his argument on the detection of fresh xenon (Xe) isotopes located in Cherepovets, over 1,000km northeast from Chernobyl, damage to the southeastern part of the reactor and eyewitness accounts of a blue flash above the reactor after the first explosion.
Two short-lived Xe nuclides were detected four days after the accident in Cherepovets. The samples of 133Xe and 133mXe exhibited ratios that indicated they had been partially produced recently in fresh fission, rather than from the inventory of the reactor. The isotopes would have had to be propelled up to 3km high for airflows to carry them across, which is possible after such an explosion. De Geer asserts that the nuclear explosions created a jet of plasma that shot upwards between 2.5–3km through refuelling tubes and lifted the xenon particles up to an altitude where a meteorological route to Cherepovets was available.
De Geer argues that this event is corroborated by known details including damage to the bottom lip of the reactor and a 'well-known eyewitness account'. Damage to the reactor’s bottom lid, a steel-encapsulated serpentinite plate, which De Geer states was vaporised in one quarter, would only be possible in a nuclear explosion. ‘The rest of the lid was pushed four metres down by the second explosion, but less damage to the surface of the remaining bottom lip shows that it happened at a much lower temperature,’ he told Materials World. De Geer also stated that the details of eyewitness Alexander Yuvchenko, a local fisherman, of a blue light ‘suggests a high temperature – around 7,000˚C to produce blue light – which was not the case in the major explosion that caused the fallout in Europe.’
32 years in the making
De Geer has been exploring his theory for several years, and presents results in his recetly published paper. In 2008, he was reminded of a paper he co-authored in April 1986 with the V. G. Khlopin Radium Institute, Russia, that performed meteorological studies regarding the dissemination of Chernobyl debris reaching Sweden. Although the paper focused on emissions trajectories around altitudes of 0.5–1km, De Geer said, ‘We included potential emissions as high as 3km that actually went west before turning around at the Gulfs of Riga and Finland and continued past Cherepovets. That was where my idea was born – there must have been a shot of the residues of a nuclear explosion up at that altitude.’
While design flaws with the four Chernobyl Soviet reaktor bolshoy moshchnosty kanalny (RBMK) reactors, which were unstable at low power owing to control rod design and positive void coefficient – the change in core reactivity from steam in the reactor coolant – are attributed with causing the accident, De Geer believes it was caused by a combination of human error and design flaws. Chernobyl was, at the time, aiming to perform experiments on the use of turbines if they lost electricity. But the test was delayed as a conventional power station in Kiev went down as Chernobyl was reducing output from 3,200MW to 700MW. After remaining at 1,600MW and a staff changeover, Chernobyl received the signal to continue its experiment, which resulted in a power reduction to 30MW. The operators tried to suddenly shut down the reactor by inserting the control rods, which included graphite displacers, increasing reactivity.
De Geer, who retired in 2012, is not well known within the international nuclear community. ‘The Professor of Reactor Physics at KTH and the former head of the Swedish Nuclear Power Inspectorate (SKI) have large international networks,’ said De Geer. ‘When this paper came out, they told me they had been contacted by many global experts asking who I am – having heard my name before and were of course suspicious.’ Lars Högberg, former director of the SKI, supported De Geer’s observations, stating, ‘[He] certainly knows how to use the tools of the trade, some of which he has developed himself. So in all probability he is right in his hypothesis. It provides new insights into the first few seconds of the processes destroying the reactor.’
The Chernobyl accident was predicated on a combination of human error and flawed reactor design long acknowledged and rectified in modern reactors. Similarly, De Geer’s work on the origins of the Chernobyl event is unlikely to have any impact on nuclear safety.
‘The problems with positive void co-efficient and graphite displacers have been recognised and corrected, and RBMK reactors are in the process of dying out,’ said De Geer. ‘But, as a scientist, all work that helps explain something is a good thing’
To read A Nuclear Jet at Chernobyl Around 21:23:41 UTC on April 24, 1986, visit bit.ly/2z8PFs9