Chernobyl’s nuclear fuel is ‘smoldering’ again and could explode

    22 May 2021

    Nuclear reactions are smoldering again in an inaccessible basement of the wrecked Chernobyl nuclear power plant in Ukraine, Live Science reports.

    Researchers monitoring the plant — which infamously exploded in a deadly 1986 meltdown — have detected a steady spike in the number of neutrons in an underground room called 305/2. The room is full of heavy rubble, concealing a radioactive mush of uranium, zirconium, graphite and sand that oozed into the plant’s basement like lava, before hardening into formations called fuel-containing materials (FCMs).

    Rising neutron levels indicate that these FCMs are undergoing new fission reactions, as neutrons strike and split the nuclei of uranium atoms, creating energy.

    For now, this radioactive waste is smoldering “like the embers in a barbecue pit,” Neil Hyatt, a nuclear materials chemist at the University of Sheffield in the U.K., told Science magazine.

    However, it’s possible that those embers could fully ignite if left undisturbed for too long, resulting in another explosion.

    This potential explosion wouldn’t be anywhere near as devastating as the one that s

    hattered the plant in 1986, which resulted in thousands of deaths and spewed a radioactive cloud over Europe, Maxim Saveliev, a senior researcher with the Institute for Safety Problems of Nuclear Power Plants (ISPNPP) in Kyiv, Ukraine, told Science. If the nuclear material ignites again, the blast will be largely contained within the steel and concrete cage known as the Shelter, which officials built around the plant’s ruined Unit Four reactor one year after the accident.

    Still, even a contained explosion would make the long-term mission of removing the plant’s FCMs much harder, Saveliev said. The Shelter is old and could easily crumble from the force of an explosion, filling the area with heavy debris and radioactive dust. (The Shelter itself is contained in a larger steel structure called the New Safe Confinement, which was completed in 2018.)

    Neutron levels have been steadily rising in room 305/2 for four years, Saveliev said, and could continue rising for several more years without incident. It’s possible these nuclear nuggets will fizzle out on their own in that time. But if neutron levels keep rising, scientists will have to intervene.

    The threat can’t be ignored. As water continues to recede, the fear is that “the fission reaction accelerates exponentially,” Hyatt says, leading to “an uncontrolled release of nuclear energy.”

    There’s no chance of a repeat of 1986, when the explosion and fire sent a radioactive cloud over Europe. A runaway fission reaction in an FCM could sputter out after heat from fission boils off the remaining water. Still, Saveliev notes, although any explosive reaction would be contained, it could threaten to bring down unstable parts of the rickety Shelter, filling the NSC with radioactive dust.

    Addressing the newly unmasked threat is a daunting challenge. Radiation levels in 305/2 preclude getting close enough to install sensors. And spraying gadolinium nitrate on the nuclear debris there is not an option, as it’s entombed under concrete. One idea is to develop a robot that can withstand the intense radiation for long enough to drill holes in the FCMs and insert boron cylinders, which would function like control rods and sop up neutrons. In the meantime, ISPNPP intends to step up monitoring of two other areas where FCMs have the potential to go critical.

    The resurgent fission reactions are not the only challenge facing Chernobyl’s keepers. Besieged by intense radiation and high humidity, the FCMs are disintegrating—spawning even more radioactive dust that complicates plans to dismantle the Shelter. Early on, an FCM formation called 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 says.

    Ukraine has long intended 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 with life still flickering within the Shelter, it may be harder than ever to bury the reactor’s restless remains.

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