From the journal Science, May 5, 2021:
Thirty-five years after the Chernobyl Nuclear Power Plant in Ukraine exploded in the world's worst nuclear accident, fission reactions are smoldering again in uranium fuel masses buried deep inside a mangled reactor hall. "It's like the embers in a barbecue pit," says Neil Hyatt, a nuclear materials chemist at the University of Sheffield. Now, Ukrainian scientists are scrambling to determine whether the reactions will wink out on their own—or require extraordinary interventions to avert another accident.
Sensors are tracking a rising number of neutrons, a signal of fission, streaming from one inaccessible room, Anatolii Doroshenko of the Institute for Safety Problems of Nuclear Power Plants (ISPNPP) in Kyiv, Ukraine, reported last week during discussions about dismantling the reactor. "There are many uncertainties," says ISPNPP's Maxim Saveliev. "But we can't rule out the possibility of [an] accident." The neutron counts are rising slowly, Saveliev says, suggesting managers still have a few years to figure out how to stifle the threat. Any remedy he and his colleagues come up with will be of keen interest to Japan, which is coping with the aftermath of its own nuclear disaster 10 years ago at Fukushima, Hyatt notes. "It's a similar magnitude of hazard."
The specter of self-sustaining fission, or criticality, in the nuclear ruins has long haunted Chernobyl. When part of the Unit Four reactor'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 into a lava. It flowed into the reactor hall's basement rooms and hardened into formations called fuel-containing materials (FCMs), which are laden with about 170 tons of irradiated uranium—95% of the original fuel.
The concrete-and-steel sarcophagus called the Shelter, erected 1 year after the accident to house Unit Four's remains, allowed rainwater to seep in. Because water slows, or moderates, neutrons and thus enhances their odds of striking and splitting uranium nuclei, heavy rains would sometimes send neutron counts soaring. After a downpour in June 1990, a "stalker"—a scientist at Chernobyl who risks radiation exposure to venture into the damaged reactor hall—dashed in and sprayed gadolinium nitrate solution, which absorbs neutrons, on an FCM that he and his colleagues feared might go critical. Several years later, the plant installed gadolinium nitrate sprinklers in the Shelter's roof. But the spray can't effectively penetrate some basement rooms.
Chernobyl officials presumed any criticality risk would fade when the massive New Safe Confinement (NSC) was slid over the Shelter in November 2016. The €1.5 billion structure was meant to seal off the Shelter so it could be stabilized and eventually dismantled. The NSC also keeps out the rain, and ever since its emplacement, neutron counts in most areas in the Shelter have been stable or are declining.
But they began to edge up in a few spots, nearly doubling over 4 years in room 305/2, which contains tons of FCMs buried under debris. ISPNPP modeling suggests the drying of the fuel is somehow making neutrons ricocheting through it more, rather than less, effective at splitting uranium nuclei. "It's believable and plausible data," Hyatt says. "It's just not clear what the mechanism might be."....
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