Sunday, May 9, 2021

The craziest nuclear story you will read today

Via Science magazine, this remarkable story: 
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...

...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....

Keeping this site secure and stable has got to be a huge task - the engineering challenges must be enormous, and the difficulty of getting good information is daunting. 

2 comments:

  1. 1. I think they are asking for more funding to take care of this long-going problem so maybe things are not as dramatic
    2. Water seeping in is bad - apart from acting as a moderator, it can wash away Gd and B that were used to prevent the melted fuel from going critical again. So you can see unpredictable changes in neutron radiation, depending on the weather. Water seeping in means that the sarcophagus is not sealed and stuff can leak out, and the pooled water will flood and corrode stuff, this could make it much harder to access the area in the future.
    3. If some the old melted fuel goes critical, it would produce cyclic spikes in neutron radiation (which are bad to people within about 50 meters radius) and new source of radioisotope fission product pollution for the area, and some heat. But a criticality excursion, dangerous it is, would likely not mean explosion or fire

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  2. "scientists had to use a Kalashnikov rifle to shear off a chunk for analysis"

    The group meeting before this happened must have been epic.

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