The number of neutrons released in an inaccessible room at the Chernobyl nuclear power plant ruins is increasing, suggesting that the rate of nuclear fission is increasing.

The rise in neutrons was predicted, and scientists don't believe it would result in an explosion, but they can't be sure and are keeping a close eye on things.

When a section of the plant's reactor core melted down in 1986, uranium fuel rods, their zirconium cladding, graphite control rods, and sand poured on the core to try to put out the fire melted together to form lava, according to Science.

This poured into the basement rooms of the reactor hall and solidified into fuel-containing materials (FCMs).

Ironically, the surge in neutrons from room 305/2 is thought to be linked to the late-2016 construction of the New Safe Confinement (NSC) system over the plant's decommissioned Reactor 4.

The NSC fully encased a previous makeshift shelter that had been erected in haste following the accident and had failed to keep all rainwater out of the reactor's ruins.

Water seeping through the previous shelter was thought to be restricting neutron activity in room 305/2.

One theory, according to Neil Hyatt, a professor of radioactive waste management at the University of Sheffield, is that before the NSC was installed, the amount of water entering room 305/2 was higher than the optimal amount for maintaining a fission chain reaction.

“The neutrons may be absorbed by other elements inside the fuel containing mass, or by water, under certain conditions,” Hyatt told the BBC.

“Now that the new secure confinement is in place and the fuel-containing mass is drying out, you're getting closer to the optimal amount of water to keep a fission reaction going.”

There was assumed to be insufficient fissile material in the correct configuration inside the space for an explosion to occur.

“Basically, there isn't enough uranium or plutonium, and it's not structured in a way that will result in, say, an explosive release of energy.” However, we can't be certain.” According to Hyatt.

“What we expect to happen is that when we get closer to that optimum ratio, the reactivity will increase, then decrease as the material dries out further as we get further away from it.”

The rise in neutron release was beginning at a very low level, and for the time being, the best thing to do was to wait and see what happened.

“We don't want to put people in danger because it's structurally degraded... there's a risk to staff going in to interfere, as well as a radiation risk,” she says.

If the rate of neutron output began to increase exponentially, there were options to explore. It would be feasible, for example, to drill a hole into the wall into the room and insert something that would absorb the excess neutrons and prevent further fission.

“For the time being, it's a case of being worried but not alarmed,” Hyatt said.