Fission reactions have been reported again in uranium fuel masses buried beneath the reactor hall at unit 4 of the Chernobyl NPP in Ukraine which was destroyed in the 1986 explosion, Science magazine reported on 5 May. Sensors are tracking a rising number of neutrons, a signal of fission, emanating from one sub-reactor room, Anatolii Doroshenko, from the Institute for Nuclear Power Plant Safety (IPBAE) of the Ukraine Academy of Sciences, reported during discussions about dismantling the reactor. “There are many uncertainties,” said IPBAE’s Maxim Saveliev. “But we can’t rule out the possibility of [an] accident.” The neutron counts are rising slowly, Saveliev noted, and there is still time to find a solution to the threat.
When part of Chernobyl 4’s core melted down on 26 April 1986, uranium fuel rods, their zirconium cladding, graphite control rods, and sand dumped on the core to try to extinguish the fire melted together to form a lava, which flowed into the reactor hall’s basement rooms. There it hardened into fuel-containing materials (FCMs), estimated to contain 170 tons of irradiated uranium.
The concrete-and-steel sarcophagus called the Shelter, erected 1 year after the accident to cover the damaged reactor, allowed rainwater to seep in. Because water slows, or moderates, neutrons, heavy rains would sometimes send neutron counts soaring. The NPP later installed gadolinium nitrate sprinklers in the Shelter’s roof to dampen such reactions but these do not reach the basement rooms.
Chernobyl officials assumed the criticality risk would diminish when the €1.5 billion New Safe Confinement (NSC) was installed over the sarcophagus in 2016 so it could be stabilised and eventually dismantled. The NSC also keeps out the rain, and initially neutron counts in most areas in the Shelter remained stable or decreased.
However, they are now increasing in some locations, nearly doubling over four years in room 305/2, which contains tons of FCMs buried under debris. A runaway fission reaction in an FCM would likely stop out once heat from fission boiled off the remaining water. Nevertheless, Saveliev noted that, although any explosive reaction would be contained, it could bring down unstable parts of the aged sarcophagus, filling the NSC with radioactive dust.
Other problems caused by the intense radiation and high humidity are disintegration of the FCMs producing more radioactive dust that complicates plans to dismantle the Shelter. Early on, an FCM formation known as the Elephant’s Foot was so hard scientists had to use a Kalashnikov rifle to shear off a chunk for analysis. “Now it more or less has the consistency of sand,” Saveliev said.
Ukraine has long planned to remove the FCMs and store them in a geological repository. By September, with help from European Bank for Reconstruction and Development, it aims to have a comprehensive plan for doing so. But the task remains complex.
Following publication of the Science article and its reproduction in other media, IPBAE issued a statement confirming that an increase in the neutron flux density at the periphery of certain FCM accumulations had been observed. “However, to date, this increase does not exceed the established safety limits controlled by the standard nuclear safety monitoring system (NSC IASK),” it said.
Over the past decade, the IPBAE has accumulated knowledge and developed models of the behaviour of the main accumulations of nuclear hazardous fissile materials. Institute scientists in 2015 had developed a scientific hypothesis predicting an increase in the neutron flux density as a result of excluding atmospheric precipitation from entering the sarcophagus after the NSC was installed. “The current experimental data only confirmed the scientific hypothesis developed by IPBAE specialists even before the construction of the NSC.” The institute said.
This hypothesis is based on the fact that, in sub-reactor room 305/2 there are two waterlogged accumulations of FCM with an increased concentration of fissile materials. Analysis of the results of experimental data and modelling, suggests the "southern" accumulation is deemed potentially dangerous, as it was the epicentre of the formation and spreading of the lava-like FCM. “According to our data, the accumulations of FCM in this room may have a two-layer porous structure: the upper layer is black lava-like FCM, and the lower layer is a potentially critical composition.”
Before installation of the NSC, the FCMs were completely waterlogged and the dynamics of the neutron flux density remained within the seasonal trends due to the regular influx of moisture from atmospheric precipitation and condensate, that is, there were no potentially dangerous changes in the level of subcriticality of these accumulations. “After the NSC was installed, as a result of the exclusion of atmospheric precipitation into the Shelter, the process of evaporation and loss of moisture from the porous structure of lava-like FCMs began, which led to a recorded increase in the neutron flux density,” IPBAE said.
“Based on predictive estimates, it is expected that in the future there will be an increase in the neutron flux density, which will be determined by the process of moisture loss in the layer of the waterlogged medium of lava-like FCM. That is why the scientists of the IPBAE are focusing on the need for an in-depth study of the condition of the FCM and a study of the factors affecting the nuclear safety state of the sarcophagus and, accordingly, we have developed the FCM monitoring programme.”
IPBAE pointed out that this scientific hypothesis and predictive estimates had been repeatedly published in domestic and foreign scientific publications and reported at scientific conferences. In 2021, these materials were presented in a report at the international conference INUDECO, where the risks of nuclear safety of the Shelter were discussed.
IPBAE is the only scientific institution that has been consistently studying the conditions inside the Shelter since the accident and forecasting changes in their state over time. “These scientific studies are very important, since the data obtained provide the basis for substantiating the radiation and nuclear safety of the Shelter,” said IPBAE, adding that it co-operated closely with Chernobyl NPP as a scientific advisor organisation, providing “relevant scientifically substantiated recommendations”. In view of the complexity of the problem facing the Chernobyl NPP during the transformation of the Shelter into an environmentally safe system providing for the handling of nuclear hazardous materials, “in 2016 the Institute developed a special Programme for monitoring the FCM of the Shelter. However, at the current time, Chernobyl NPP is not provided with the appropriate resources for implementation of this programme”.
Photo: The New Safe Confinement has been built over the damaged reactors (Credit: EBRD)