Insects may be to blame for trace contamination at 13 spots on the 560-square-mile Hanford Nuclear Reservation in Washington State.

Fluor Daniel Hanford Co, the US Department of Energy’s prime contractor at the Reservation, has not yet determined how radiation spread. But insects, ranging from fruit flies to gnats and ants, are believed to be part of the problem.

The greatest contamination was found at a landfill used by the nearby city of Richland, where 10 items, mostly wet garbage such as apple cores that contaminated flies might have feasted on, measured 10 to 12 millirads per hour.

About 35 tons of Hanford trash was covered over with tarpaulin to keep birds away. The trash will be returned to Hanford.

About 10 acres with office buildings and trailers have been closed to workers because of the discovery of trace amounts of radiation. Construction of a storage building for nuclear waste at the site was halted temporarily after contamination was found in a trash can.

A fixative with a sugar base that Hanford officials use as a spray to prevent radioactive materials from becoming airborne may be contributing. The fixative, which consisting mostly of glucose, is sprayed onto equipment destined for maintenance work. It then dries to a hard finish, supposedly immobilising any radioactive materials. But Hanford officials suspect it may be attracting insects.

Hanford officials have stopped using the sugar-based fixative and are fumigating garbage trucks, garbage bins and other areas at the site.

No workers are believed to have received any radiation from the contamination.