They warn that Chernobyl started generating new nuclear reactions 35 years after the nuclear accident

35 years have passed since the explosion of the Chernobyl nuclear power plant in Ukraine in the world’s worst nuclear accident. But as memory, the heating still not turned off.

Is this the fission reactions continue to burn in masses of uranium fuel buried deep in a reactor room that exploded. “It’s like embers in a barbecue,” said scientist Neil Hyatt, a nuclear materials chemist at the University of Sheffield. Now Ukrainian researchers are working to determine whether the reactions will go away on their own or whether extraordinary interventions will be needed to prevent another crash.

Is that sensors are tracking an increasing number of neutrons, a clear sign that the fission process is underway, sinking from an inaccessible room, Anatolii Doroshenko of the Institute for Nuclear Power Plant Safety Problems (ISPNPP) in Kyiv, Ukraine, reported last week during discussions about dismantling the reactor. “There are many uncertainties. But we can’t rule out the possibility of an accidentISPNPP’s Maxim Saveliev said.

According to Saveliev, the number of neutrons is slowly increasing, suggesting that managers still have a few years to figure out how to quell the threat. Any remedy that he and his colleagues can think of will be of great interest to Japan, which is face the consequences of their own nuclear disaster 10 years ago in Fukushima, Hyatt notes. “It’s a similar magnitude of danger.

The specter of self-sustaining fission, or criticality, in nuclear ruins has long haunted Chernobyl. When part of the Unit Four reactor core melted on April 26, 1986, uranium fuel rods, their zirconium coating, graphite control rods, and sand were thrown into the core in an attempt to to put out the fire. It flowed into the basement rooms of the reactor room and hardened into formations called fuel-containing materials (FCM), which are loaded with about 170 tonnes of irradiated uranium, 95% of the original fuel.

The concrete and steel sarcophagus called Refugio, erected a year after the accident to house the remains of Unit Four, allowed rainwater to seep in. Because water slows or moderates neutrons and thus increases their chances of hitting and dividing uranium nuclei, the rains sometimes increased the number of neutrons. After a downpour in June 1990, a “stalker”, a Chernobyl scientist who risked exposure to radiation to venture into the damaged reactor room, rushed over and sprayed a gadolinium nitrate solution, which absorbs neutrons, in an FCM that he and his colleagues feared would become critical. Several years later, the factory installed gadolinium nitrate sprinklers on the roof of the Refuge. But the spray cannot effectively penetrate some rooms in the basement.

Chernobyl officials speculated that any risk of criticality would disappear when the enormous A new secure containment (NSC) slipped over the refuge in November 2016. The 1.5 billion euro structure was intended to seal the shelter so that it could be stabilized and eventually dismantled. The NSC also avoids rain, and from its location the neutron count in most areas of the Refuge is either stable or declining. But they started to go up in some places almost doubling in 4 years in room 305/2, which contains tons of FCM buried under the rubble. The ISPNPP model suggests that drying the fuel somehow makes the neutrons bouncing through it more, rather than less, effective at dividing uranium nuclei. “This is credible and plausible data,” says Hyatt. “What the mechanism might be is just not clear.”

The threat cannot be ignored. As the water continues to recede, the fear is that “the the fission reaction accelerates exponentiallyHyatt states, leading to “Uncontrolled release of nuclear energy”. There is no possibility of repeating what happened in 1986, when the explosion and fire sent a radioactive cloud over Europe. An uncontrolled fission reaction in an FCM could sizzle after the heat of fission boils the remaining water. Yet, Saveliev notes, while any explosive reaction would be contained, it could threaten to topple unstable parts of the dilapidated shelter, filling the NSC with radioactive dust.

Tackling the newly unmasked threat is a tall order. Radiation levels in 305/2 prevent getting close enough to install sensors. And spraying gadolinium nitrate on nuclear debris is not an option, because it is buried under concrete. One idea is to develop a robot capable of withstanding intense radiation long enough to punch holes in FCMs and insert boron cylinders, it would work like control rods and absorb neutrons. At the same time, the ISPNPP intends to intensify oversight of two other areas where FCMs have the potential to become critical.

The resurgence of fission reactions is not the only challenge facing the custodians of Chernobyl. Besieged by intense radiation and high humidity, the FCMs disintegrate, generating even more radioactive dust that complicates Vault dismantling plans. At first, an FCM training called Elephant’s Foot was so difficult that scientists had to use a Kalashnikov rifle to cut a piece for analysis. “Now it has more or less the consistency of sand,” says Saveliev.

Ukraine has been trying for a long time to get rid of FCMs and store them in a geological repository. By September, with the help of the European Bank for Reconstruction and Development, it intends to have a comprehensive plan to do so. But with the life still sparkling inside the safe, it can be harder than ever to bury the stirred remains of the reactor.

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