Dounreay Site Restoration Ltd

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The silo

As the shaft filled up with intermediate-level waste, those in charge of the fast reactor experiment in the mid-1960s realised a second facility would be needed. In 1971, a new facility came into service – the ILW silo.

The silo is a concrete box set into the ground, with its roof just above ground level. It has a capacity of 720 cubic metres.

For the next 20 years, a variety of radioactive waste from every aspect of site operations was deposited in the silo through ports in the roof on more than 16,000 occasions.

Much of the waste was metallic and included failed and redundant equipment, fuel element transit cans and debris from fuel element breakdown such as cladding, end fittings, wrappers and swarf. Other waste includes plastic, glass, paper and filters.

The waste containers were moved into place above the posting ports using an overhead crane relocated from the shaft. Once secured, the base of the container was opened and the waste dropped into the silo.

In 1977, a cover building was constructed around the crane to give protection from the weather.

A second silo was constructed in the 1980s but never used. Standards had changed, and the site started to store this type of waste in drums stacked inside ventilated above-ground stores. The last disposal to the silo was in 1998.

The silo has always been partly filled with water to absorb some of the radiation. On occasions, more water was added to level the “pyramids” of waste that built up beneath the roof ports.

The overhead crane and cover building were dismantled in 2007.

Retrieval of waste from the silo will begin within the next decade. Robotic equipment will grab the waste and transfer it to a series of shielded handling cells where it will be checked, segregated and mixed with cement in steel drums that can be stored on-site as conditioned intermediate-level waste. Once empty, the silo can be decontaminated and demolished.