A Russian soldier at Chernobyl picked up radioactive material with his bare hands, its staff are reported to have said.
Employees say troops, who seized the nuclear plant early in the war, spread radiation across the toxic site.
One soldier is reported to have died from radiation poisoning with pictures also showing Kremlin forces actually dug trenches in the Red Forest near the plant, a known location where radiation has settled since the 1986 disaster.
Shocked journalists discovered evidence of food and cooking in the Red Forest which suggests Russian troops spent an extended period of time in the trenches.
One Russian military ration box discovered exhibited radiation levels 50 times above naturally occurring values, CNN reported.
Signs of a fire in the area, with the forest providing the fuel, also means radioactive smoke might have spread.
Staff at the Chernobyl said the Russian soldiers contaminated the power plant with radioactive material they carried back from the forest on their shoes.
The radiation levels increased at the power plant as a result, Ukrainians at the plant said.
"It's crazy, really," Ukrainian Energy Minister German Galushchenko told CNN. "I really have no idea why they did it.
"But we can see they went in there, the soldiers who went there, came back here and the level of radiation increased."
Plant officials say radiation levels increased during the Russian occupation to slightly above what the World Nuclear Association describes as naturally occurring radiation.
In one incident a Russian soldier handled a source of cobalt-60 at one waste storage site with his bare hands, according to Valeriy Simyonov, the site's chief safety engineer.
The radiation was so intense it went off the scales of a Geiger counter.
Chernobyl is not an active power station but staff maintain the site of the 1986 nuclear disaster to avoid further radiation leaks.
Russians held the plant for a month and access to the site opened this week, providing evidence of how little regard the Russian soldiers had for nuclear safety.
