Low Level Waste
Solid low level waste (LLW) is generated wherever nuclear materials are handled and may comprise paper, rags, tools, glass, concrete, clothing and scrap metal components.
It is not dangerous to handle, the radioactivity being below the level at which special precautions such as thick shielding are needed. Its radioactivity is however greater than the level allowed in ordinary landfill sites.
Historically, this waste was disposed in a series of shallow pits at the site. These pits are full and will be emptied as part of the site clean-up programme.
By volume, LLW accounts for more than 80 per cent of all radioactive waste that needs to be managed during site closure. By radioactivity, it is less than 0.1 per cent.
The LLW collected in drums from site decommissioning is managed at the Waste Receipt Assay Characterisation and Supercompaction facility, or WRACS. Here, the drums are scanned to confirm their compliance with the LLW acceptance criteria before being supercompacted to a fifth of its size, to minimise the volume for disposal. The compacted drums, referred to as “pucks”, are stored in half-height ISO containers.
Other LLW items too large to fit into drums can be packed directly into the half-height ISO container.
Once a half-height ISO container is full, it is taken to the encapsulation plant, filled with grout and allowed to cure before being disposed of at the Low Level Waste Disposal Facility, adjoining the licenced site.
The site is authorised by the Scottish Environment Protection Agency (SEPA) to discharge low-activity effluent to the sea. Conditions attached to this authorisation and other licences regulate how and when this happens. The site applies constant downward pressure to keep discharges to the minimum necessary.
The operation, clean-out or decommissioning of facilities can produce radioactive effluent. Areas of contaminated land also produce radioactive effluent as a result of contact with groundwater and rainwater.
Effluent from facilities can be filtered or put through ion exchange columns to clean it up, before it goes to a low-active drain that carries potentially contaminated liquid from all the facilities to a collection point known as the Low Level Liquid Effluent Treatment Plant (LLLETP).
This plant also receives potentially contaminated groundwater diverted from areas of contaminated land, the low level waste disposal pits, boreholes and the shaft., as well as from the neighbouring Vulcan site. It is designed to remove any solvents and suspended solids and neutralise pH.
Once treated, the effluent is transferred to sea discharge tanks where samples are taken to measure its radioactivity and suitability for discharge. The effluent is sampled again during discharge to sea and these measurements are reported routinely to SEPA. The effluent is carried in a pipeline in a subsea tunnel to a diffuser approximately 600 metres offshore, where it is dispersed in the sea.
It is a condition of the authorisation that the site measures the impact of these discharges. This is done by taking samples of water and marine life. The results are reported routinely to SEPA and appear in the Radioactivity in Food and the Environment reports published annually by regulators.
Airflows that come into contact with nuclear materials have the potential to pick up airborne contamination, so plants are designed to divert this airflow away from areas where people are working.
There are approximately 50 facilities at Dounreay where airflows are partly or wholly ventilated to prevent unnecessary exposure of the workforce.
Some facilities have their own ventilation stacks while others, such as the older plants in the Fuel Cycle Area, share a common ventilation system that discharges from a 55-metre high chimney.
Airflows are filtered and monitored at each facility. Samplers attached to stacks measure the levels of activity being released and the results are reported routinely to the Scottish Environment Protection Agency (SEPA).
SEPA sets limits on the amount of radioactivity that can be released to the atmosphere and these are specified in the site’s authorisation.
SEPA requires the site operator to monitor the environment around the site to measure the potential impact. These results are reported to SEPA and appear in the Radioactivity in Food and the Environment series of reports published annually by regulators.
Higher Activity Waste
This type of waste contains radioactive materials in sufficient quantities that it can be a significant health hazard. Those which emit beta and gamma radiation can be contained safely through the use of thick shielding to block the radiation or, for others with alpha emissions only, by keeping them within sealed containers to prevent inhalation or ingestion.
This type of waste was previously called intermediate level under a previous classification.
At Dounreay, higher activity waste has arisen from historical activities such as reactor operations, fuel reprocessing, fuel manufacture, and a variety of other experimental tasks. They typically consists of components from the reactors, and equipment that has been used to handle fuel during processing operations and so on.
The waste exists at Dounreay in the form of solids, liquids and sludges. Historically, there were three primary means of storing this waste – by placing high radiation solid waste in the shaft and silo, storing it as liquid in tanks or keeping the unconditioned waste inside special drums in above-ground stores.
During the course of the site closure programme, the historical waste will be retrieved, and together with waste from the clean-out and dismantling of redundant plants, it will be processed to allow it to be mixed with cement inside steel drums or boxes, converting it into a passively safe form which is suitable for longer-term storage or disposal.
The Scottish Government policy for the long-term management of this waste type is “near site, near surface”. However a significant proportion of Dounreay’s waste is unsuitable for near surface disposal and will be stored on site for an extended period of time.
By 2030, when the site has been cleared, the volume of higher activity waste that needs to be managed is expected to be in the region of 15,000 cubic metres.
This waste will remain at Dounreay after the redundant facilities have been cleared away.
Raffinate is a liquid waste, produced during the reprocessing of spent nuclear fuel. The higher activity raffinates at Dounreay are stored in specially constructed tanks housed in concrete vaults. By 1996, when reprocessing ceased, 21 tanks were in use.
Dounreay’s approach is to progressively empty the tanks, neutralise the acidity of the liquor and immobilise it with cement inside 500-litre drums. This is carried out remotely in a heavily shielded series of cells. This solid form of higher activity waste then becomes passively safe for long-term storage or disposal.
Once the tanks have been emptied of their contents, the storage facility can be cleaned out and dismantled.