Its purpose is not to shield radioactive emissions but to prevent the release of radioactive dust and other materials, and to keep out rainwater, which could carry contaminants into the water table.
And air movement must be minimized to avoid stirring up and resurrecting the bane of shelter workers, radioactive dust.
The Chernobyl nuclear power plant accident in the Soviet Union sent radioactive dust particles hundreds and even thousands of kilometers away, with health effects occurring close in.
Both British and US forces in the conflict used armour-piercing shells tipped with depleted uranium, and veterans' groups have long argued that radioactive dust from the shells could have caused illnesses.
After reactor no. 4 exploded at Chernobyl in 1986 due to errors in both design and operation it sent plumes of radioactive dust as far away as Japan and the U.S..
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