New theory could re-write Chernobyl history

Piotr Andryszczak

RESEARCHERS in Sweden have published new analysis of the Chernobyl disaster which suggests that the first explosion was a nuclear explosion, and not a steam explosion as is generally thought.

Two explosions were reported by eyewitnesses within seconds of the start of the Chernobyl disaster at 01:23 local time on 25 April 1986. The generally-accepted course of events is that the first steam explosion was followed by a second explosion between hydrogen produced from the reaction of steam with the zirconium fuel cladding and oxygen in the air. The researchers, from the Swedish Defence Research Agency, Swedish Meteorological and Hydrological Institute, and Stockholm University, base their contradictory theory on new isotope analysis.

Lead author and retired nuclear physicist from the Swedish Defence Research Agency, Lars-Erik De Geer and the team say that seismic data and a blue flash, characteristic of a criticality accident, reported by eyewitnesses, corroborate the hypothesis of a nuclear explosion followed by a steam explosion within three seconds. The first explosion would have sent debris into the high atmosphere, while the second would have sent debris into lower altitudes. They say that there is “solid support” for the new theory.

The first bit of evidence comes from xenon isotopes in samples from Cherepovets, a city north of Moscow, detected by scientists from the VG Khlopin Radium Institute in St Petersburg (then Leningrad), four days after the accident. Cheropovets is far from the main course of the nuclear debris, but the isotopes detected were recent fission products which could only have come from a recent nuclear explosion. Xenon isotopes detected in Scandinavia, the path of the main debris cloud, were equilibrium isotopes of the type found in the core. The researchers believe that the isotopes found at Cheropovets came from debris from high altitudes from the first explosion, while the isotopes found in Scandinavia were from the reactor rupture in the lower altitudes.

De Geer and the team also point to observations of the reactor tank, which suggest that the first explosion resulted in temperatures hot enough to melt part of a 2 m thick bottom plate in the core, which is again consistent with a nuclear explosion.

The researchers say that the new insights could be useful in preventing future disasters.

Nuclear Technology doi.org/cgfw