Nuclear reactions are restarting at Chernobyl

nivek

As Above So Below
Nuclear reactions are restarting at Chernobyl and scientists don’t rule out the possibility of an new critical radioactive accident

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.

New signal of fission from a sealed room

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.”

Self-sustaining fission

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 just not clear what the mechanism might be.

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.

Daunting challenge

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.

FCMs are disintegrating

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.

Worst of all, scientists still aren’t quite sure why the nuclear reaction is restarting… We are doomed! [Science mag]

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nivek

As Above So Below
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.

They cannot say this with complete certainty, given enough heat and pressure I think it is possible that the top could blow off of Chernobyl and here we are again with a repeat of '86 but worse...

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coubob

Celestial
Ukraine seized vodka made from Chernobyl apples. The scientists who made it want it back

A team of British and Ukrainian scientists are battling Ukraine's government in court for the return of 1,500 bottles of liquor made from "radioactive" apples grown near the Chernobyl nuclear power plant.

The liquor, branded as Atomik, is part of a four-year experiment by the scientists to see if they could produce a product safe to consume from crops grown in an area that was contaminated during the 1986 nuclear disaster at the plant.

The team sourced the apples from a farmer inside the outer ring of the 18-mile "Exclusion Zone." Around 10,000 people still live in that outer zone and the scientists hoped that farming, which is still officially banned there, can now be safe to resume.

The first bottle of the liquor was produced in 2019 and scientists used rye grain and water from the Exclusion Zone. This year, the scientists had hoped to ship their first batch to the United Kingdom, with profits intended to go to the local community near Chernobyl.

But in March they hit a roadblock: Ukraine’s security service, the SBU, seized the bottles at a distillery plant in western Ukraine. The reason, according to Ukrainian prosecutors, wasn’t radiation, but problems with the bottles’ customs documents.

"I have no idea why. The reason they gave was they thought the bottles had forged duty stamps on. But they clearly had the U.K. stamps on. We hope it was just a mistake,” said Jim Smith, a professor of environmental science at the University of Portsmouth and one of the scientists behind the liquor.

The city prosecutor’s office in Kyiv told ABC News that the British stamps for the bottles didn’t correspond to examples submitted by the plant for registration.

Smith said his team won a first ruling in court to return the impounded liquor but are now awaiting a new hearing after the prosecutors appealed.

The liquor was intended mainly as a way of drawing attention to the scientists' real work in the Exclusion Zone, where they have spent years studying how the landscape around Chernobyl has recovered following the disaster.

Smith and Gennadiy Laptev, a scientist from Ukraine’s Hydrometeorological Institute who also took part in the cleanup of the disaster, believe their studies show contamination in the outer ring is so weak that restrictions on farming no longer make sense.

Although there are still hot spots in the Exclusion Zone where radiation levels are potentially dangerously high, in most areas -- even much closer to the power plant itself -- levels are normal and nature in recent years has thrived there.

The apples used in the liquor showed slightly elevated radiation levels but were below the limit considered safe for consumption by Ukrainian law. That radiation was then filtered out in the distillation process, Smith said.

To prove the liquor’s safety, Smith sent it to be analyzed by scientists at the University of Southampton in 2019. The scientists found no sign of unusual radiation, he said. The only trace of radiation in the liquor, Laptev said, was Carbon-14 -- a radioactive isotope that naturally occurs in spirits.

The same Southampton University scientists were due to analyze the batch for sale before they were impounded.

The spirit was expected to sell for around $50 in Britain. The scientists said they had already received a lot of interest.

"We’re getting emails from people all over the world -- Australia, U.S., Canada, France -- from people saying, 'Where we can buy some?'" Smith said.
 
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