Spikes in Radioactivity at Chernobyl Prompt Fears of Future Accident

Scientists have picked up signs that nuclear activity at the Chernobyl nuclear plant is stepping up, but it is unclear exactly how.

The nuclear plant, located in northern Ukraine, was the site of the world's worst-ever nuclear accident in 1986 when a runaway nuclear reaction partially destroyed a reactor unit.

The subsequent fire and exposed reactor released massive amounts of radiation into the environment, so authorities covered the entire unit with a large dome sometimes called the "sarcophagus" to prevent radiation from escaping.

The sarcophagus was eventually replaced by a new containment system called the New Safe Confinement structure, which was only completed in 2019.

Today, radioactive uranium fuel still lies buried within the old Unit 4, and Neil Hyatt, a nuclear materials chemist at the University of Sheffield in the U.K., told Science Magazine the remaining fuel is like "embers in a barbeque pit."

The publication reports scientists have detected an increasing number of neutrons from one particular room within the old unit, which is a signal of nuclear fission—the process that nuclear reactors use to produce energy.

Currently the neutron counts are not rising fast, but Hyatt added there were concerns they could accelerate exponentially.

An uncontrolled reaction within the tomb of Unit 4 would not cause a disaster on the scale of the original Chernobyl accident, Science Magazine reports. This is because scientists think that leaking rainwater has been speeding up the remaining reactivity, and a runaway reaction would eventually cause that water to evaporate, cutting the incident short.

However, such a reaction could damage the delicate structure of the original sarcophagus and cause it to collapse.

The mechanism of the increasing radioactivity is still unclear. As mentioned, rainwater is believed to have increased neutron counts in the past, and this is because it slows them down—which makes it more likely that they will collide with uranium nuclei and split them, releasing heat and radiation.

The New Safe Confinement has been cutting rainwater off from Unit 4, but new data suggests that the drying of the fuel is somehow causing radioactivity to increase in certain locations, rather than decrease. "It's just not clear what the mechanism might be," Hyatt told Science Magazine.

The 1986 Chernobyl disaster occurred after a low-power test went wrong. Power fell too low, and operators were instructed to remove safety measures that usually kept the reactor operating within safe power levels.

At one point, power quickly surged, and attempts to re-introduce the safety measures failed. This caused a further runaway power increase and explosions that blew the reactor open.

The giant protective dome that currently covers Unit 4 of the Chernobyl plant, taken April 13, 2021. The dome was built to contain the old radioactive core. Sergei Supinsky/AFP/Getty