The drums containing sand were taken from beaches near the Sellafield nuclear installation in Cumbria. They have been stored for several months in the yard outside Greenpeace's Islington office.
The drums, showing low level contamination, were said by the group to have been taken to show the dangers to public areas from radioactive emissions from nuclear power stations.
However, the organisation defended its actions, stating that the drums were properly sealed and posed "absolutely no health risk". A spokesman said: "It is far better to have this sand properly and safely stored in our own yard than to simply dump it on a rubbish tip."
Clothing worn by activists who entered the Capenhurst nuclear plant in Cheshire is also being stored in the fenced yard, which lies beside a public footpath. The Environment Agency has confirmed it will seek an explanation and carry out tests if required.
t At least 17 cases of leukaemia have been diagnosed around the Greenham Common nuclear air base, according to ITV's World In Action, to be screened tonight.
The number of victims is 2.5 times higher than the national average and were all from two miles around the Berkshire base, according to the programme.Reuse content