Tests on bore holes around the giant sarcophagus that encloses the remains of Reactor 4, which exploded in 1986 causing the worst nuclear disaster in history, have detected signs of thritium, a radioactive isotope of hydrogen, in subsoil water.
This has raised fears that it could eventually find its way into the nearby Dneiper River, or into the water table, and - after many years - into the drinking water of Ukraine's 50 million population. The issue underscores the deteriorating condition of the shelter itself, rapidly erected by remote control in the aftermath of the disaster, which sent a radioactive cloud across the northern hemisphere, caused more than 100,000 people to be evacuated, and led to a sharp rise in illnesses.
Despite efforts to patch up the shelter, it is dotted with cracks and holes. Taken together, about 100 square metres is exposed to the atmosphere. Inside the 74m structure, there is a lethal cocktail of fresh nuclear fuel, plutonium, piles of highly radioactive waste and - critically - an estimated 34 tons of lethal dust.
Russian scientists monitoring the shelter know water is inside, and acknowledge that there is a risk that it will react with the contents causing "neutron bursts" - releasing even more radioactivity into the atmosphere. Worse, they believe it could be seeping out from within the reactor core, through holes in its base.
A leak has not finally been confirmed as the culprit for the thritium traces, which are currently below "permissible levels", but it is the "main suspect". Dr Sergei Bogatov, head of the department of radiation at Chernobyl, told The Independent: "If we don't take prevention measures now, there is a real risk that the consequences will one day be catastrophic. This could take dozens, or even hundreds of years, but it could get into the water supply."
He is pressing for a scheme to collect the polluted water in a tank, and pump it off for safe storage. The issue now worrying scientists is what would happen if the shelter's rickety roof, where most of the dust is concentrated, collapses, releasing its deadly payload.
According to Dr Bogatov, estimates of a collapse put the annual odds at one in 10.
n A full report on the continuing hazards of Chernobyl will be featured this week in the Independent on Sunday.Reuse content