Nuclear fission reactions resumed in the masses of uranium buried in the reactor number four of the Chernobyl nuclear power plant that exploded in 1986. Scientists from the Ukrainian government are trying to understand if these reactions will fade by themselves or if an intervention will be necessary to avoid an accident, even if obviously there is no mention of consequences comparable to what happened 35 years ago but of a much smaller event. . “It’s like there are embers in a barbecue,” says Neil Hyatt, a nuclear materials chemist at the University of Sheffield, in an interview with Science magazine. The sensors recorded an increasing number of neutrons, the signal of a nuclear fission reaction, arriving from one of the inaccessible rooms of the reactor, explained Anatolii Doroshenko, of the Institute for the safety problems of nuclear plants in Kiev. “There are many uncertainties, but we cannot rule out the possibility of an accident,” added his colleague at the Institute, Maxim Saveliev, stating that “the neutron count slowly increases.” When the reactor core melted, the rods of uranium used as fuel, their zirconium coating, graphite rods, and sand poured out, like lava. They fell into the cellar of the reactor entrance where they petrified in what is called Fcm (Fuel containing material) in which 170 tons of irradiated uranium are found.The ‘sarcophagus’ built around the reactor one year after the accident had left pass rainwater, water that slows down the neutrons and therefore increases the probability of a collision with uranium, and then generates others in a chain through the fission of the uranium nucleus. Coinciding with heavy rains, the neuron counter spiked, but then returned to normal levels. The new mantle was supposed to protect the reactor from water as well, and so it did. Until in some places, the neutrons started to grow again, almost doubling in four years, at room 305/2. The hypothesis is that with the drying up of the FCM, collisions between neutrons and uranium atoms are easier. If so, the fission reaction could also accelerate exponentially, releasing nuclear energy in an uncontrolled way. Not like what happened in 1986, Ukrainian scientists point out. It would be contained, but it could still collapse some unstable sections of the building, releasing radioactive dust into the new protective structure.