The soldier was part of a unit that captured the Chernobyl plant in the first days of the war. The troops then occupied the 20-mile exclusion zone around the plant, where people are banned from living due to dangerous levels of radiation left over by the nuclear disaster in 1986.
Ukraine’s state nuclear company Energoatom said Russian troops had dug trenches in the radioactive mud of the most contaminated part of the exclusion zone and had received “significant doses” of radiation.
They had reportedly camped in the dangerous zone known as the Red Forest, where radiation levels are at their highest and become ill “very quickly”.
Troops also drove their trucks along dirt roads without wearing radiation gear, disturbing poisonous radioactive dust, Reuters reported.
One employee at the plant said the move was “suicidal” because any dust inhaled by soldiers would likely cause internal radiation in their bodies.
He said: “The convoy kicked up a big column of dust. Many radiation safety sensors showed exceeded levels.”
But Kremlin forces have now reportedly retreated to Belarus, which is a short distance from Chernobyl, where they are being treated for radiation poisoning.
One soldier is thought to have died from being exposed to the high levels of radiation, the Telegraph reported.
Ukraine has said that members of the unit that occupied Chernobyl retreated due to sickness caused by the conditions.
The Ukraine defence ministry said: “The Russian occupiers have left the Chernobyl nuclear power plant.
“Two key reasons: losses caused by the Ukrainian army and radiation exposure.”
Seven busloads of Russian soldiers believed to be suffering from the effects of radiation poisoning later arrived at the Belarusian Radiation Medicine Centre in Gomel, according to the Ukrainian news agency UNIAN.
Energoatom said that troops had withdrawn from the defunct plant after “panicking at the first sign of illness”.
The company said: “The information is confirmed that the occupiers, who seized the Chernobyl nuclear power plant and other facilities in the exclusion zone, have set off in two columns towards the Ukrainian border with the Republic of Belarus.”
Russian troops seized Chernobyl at the beginning of Vladimir Putin’s invasion of Ukraine on 24 February.
Staff who were maintaining the facilities were held hostage by Kremlin troops and not allowed to leave.
Senior US defence officials said on Wednesday that Russian troops had now withdrawn from Chernobyl over a month later.
Energoatom confirmed on Friday that there were no longer “outsiders” at the plant and said that some Russian troops had been seen leaving for the Belarusian border earlier in the week.
In a statement on Thursday, Energoatom said: “This morning, the invaders announced their intentions to leave the Chernobyl nuclear power plant.”
There have been reports that some soldiers did not realise they were camped in a radiation zone at the site of the world’s worst nuclear accident.
The exclusion zone is off limits to anyone who does not work at Chernobyl or has special permission to be there.
The Red Forest is so highly contaminated that even nuclear plant workers are not allowed to go there for risk of being exposed to dangerous levels of radiation.
The site got its name when dozens of square miles of pine trees in the forest turned red after absorbing radiation from the explosion of a nuclear reactor in 1986.
The Russian military has claimed that troops stayed within a normal range of the plant.
The International Atomic Energy Agency (IAE) said it would seek an independent assessment of the site in the coming days and would send an “assistance and support mission” to Chernobyl.
Rafael Grossi, head of the UN’s atomic watchdog, said he would lead a visit to the area “very, very soon” in the first of such visits to Ukrainian nuclear power plants.