Chernobyl is back in the news due to an unexpected turn of events

Neutron counts under the Chernobyl sarcophagus rose, but the best evidence says the spike came from shifting moisture inside the wrecked reactor, not an approaching chain reaction. 

The modeling shows that a self-sustaining reaction would require far more uranium in one spot than the debris actually holds.

In plain terms, sensors recorded a jump in neutron activity in 2019, then a glide toward a new steady level deep inside Unit 4.

A new study links the pattern to how water moved in and out of the debris after the giant enclosure over the site changed the building’s humidity and drainage.

Neutron spike at Chernobyl

The work was led at the Institute for Safety Problems of Nuclear Power Plants, National Academy of Sciences of Ukraine (ISPNPP). The team studies how fuel containing materials behave inside the sealed structure at the site.

Chernobyl’s on site network of detectors, called the Nuclear Safety Monitoring System, watches clusters of fuel debris for slow shifts that could hint at risk. 

The system’s design and performance have been described in detail by the team that built it, including how Chernobyl neutron and gamma signals ebb and flow across dozens of points in the ruins.

One probe sits in a particularly dense debris flow inside the fourth steam dump valve area near the base of the ruined core.

That location had the biggest rise in counts, which drew attention because it sits close to large volumes of melted material.

Here the key quantity is neutron flux density, the number of neutrons passing through a square centimeter each second.

Small changes in that number can signal how the debris is moderating and reflecting neutrons in the hidden spaces between fragments.

Water and Chernobyl’s neutron spike

The paper’s explanation is straightforward. As the structure dried out after the enclosure went into place, water drained from cracks and cavities, which changed how neutrons slow down and bounce around.

Engineers also saw water condense and later evaporate inside the borehole that holds the sensor.

That local layer of water temporarily muted the signal, then lifted as the water level fell, which explains part of the rise without invoking new fissions in the debris.

Here the controlling idea is criticality, a self sustaining nuclear chain reaction.

The authors show the cluster stayed on the safe side of that line the entire time, with the observed changes driven mainly by water movement through porous debris.

The debris itself consists of lava like fuel containing materials, melted fuel mixed with concrete, sand, and steel fragments.

That mix is chemically complex and riddled with voids that can hold and release water as the building’s climate shifts.

Modeling the cluster

To test worst cases, the team modeled how neutrons behave in the exact geometry of that debris flow. Their simulations used standard Monte Carlo tools to track particle histories and evaluate different water and uranium loadings in the cluster.

They focus on the effective multiplication factor, Keff, the ratio of neutrons produced to neutrons lost. When Keff is less than one, the material is subcritical and any fission fizzles out.

“The critical mass in the volume of the 4th SDV is reached with a mass content of 45 percent of uranium in brown LFCM,” wrote Kostiantyn Sushchenko, a researcher at the Institute for Safety Problems of Nuclear Power Plants (ISPNPP).

The model shows the debris would need to be far richer in uranium to cross the critical line. 

That threshold is well above the measured averages in similar material from nearby corridors. The analysis therefore points to a comfortable margin under real conditions, even after accounting for uncertainties in burnup and composition.

What has changed

The New Safe Confinement, the arch that now covers the site, altered the building’s humidity and kept rain out.

Ukraine’s plant operator records that the structure was handed over for operation on July 10, 2019, with pilot operation beginning in April that year.

Before that handover, water repeatedly found its way into lower rooms and boreholes, which created a complicated microclimate around the debris.

After the enclosure, the interior air and surfaces began to dry, and trapped water started to drain or evaporate.

That shift tracks neatly with the timing of the neutron signal. The counts crept up as water thinned between the sensor and the debris, then settled as the local environment stabilized.

Outside observers had flagged rising neutrons years earlier, which raised fair questions about whether fission might be picking up in one hard to reach spot. A detailed article from 2021 captured those concerns as the monitoring data first came to light.

By 2025, with more data and a tighter model tied to the exact debris flow, the picture looks calmer. The analysis indicates that moisture dynamics, not a shift toward self sustaining reaction, drove the change.

Risk from Chernobyl neutrons

The team calculated how much the signal could grow from water movement alone. They found the maximum growth in neutron counts from moisture leaving an over saturated matrix stayed modest, even in conservative cases.

In their words, “The calculated growth factor of the NFD due to the increase in Keff does not exceed 1.27,” wrote Sushchenko. That ceiling fits the gradual rise measured by the sensor over the same period.

When they tied the signal to water levels in nearby rooms, the match improved further. The borehole itself briefly held a thin water layer, which attenuated neutrons until it evaporated, and then the count rate rose.

The authors also state the bottom line plainly. “Based on modelling and analysis of the observed neutron flux density, it was concluded that the possibility of criticality in the volume of the steam dump valve is unlikely to occur,” wrote Sushchenko.

Neutrons, water, and Chernobyl lessons

The safest path is to keep measuring and keep testing the models as the building’s climate evolves. The enclosure will continue to change humidity and temperature inside the old structure, which can keep nudging the water balance in debris.

A priority is maintaining the detector network and its cabling in a tough industrial environment. Reliable trends depend on stable hardware and well documented recalibrations after any maintenance.

It also helps to watch for slow seasonal swings and long term drying that could make counts drift over years. Those changes tell engineers how the microclimate and the debris continue to settle toward a new normal.

The central message is steady and clear. “The cluster has been and will continue to stay subcritical,” wrote Sushchenko.

The study is published in Nuclear and Radiation Safety.

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