Dounreay Site Restoration Ltd -Raffinate

Raffinate

Reactors do not extract all the energy contained in nuclear fuel elements. Most it remains unused. This unused portion of the fuel element can be recycled through a process of chemical separation known as reprocessing. This separates the unused fuel from that which is “spent” and no longer has any use.

Reprocessing was an important part of the “fuel cycle” at Dounreay. Plants were built and operated to reprocess fuel from the site’s three reactors as well as other reactors in the UK and abroad until 1996 when this work ceased.

The spent fuel was dissolved in acid and the re-usable fuel separated from the waste, or fission products, using solvent extraction.

The acidic liquor containing the waste, known as raffinate, was transferred to underground storage tanks.

Eight tanks were constructed below ground at Dounreay in 1956/57 and used to store the raffinate from early reprocessing of fast reactor and research reactor fuel. Additional tanks were installed as more fuel was reprocessed. By 1996, when reprocessed ceased, a total of 21 tanks had been installed and were in use.

The waste falls into the category known as intermediate-level. Originally, the waste from reprocessing of Prototype Fast Reactor fuel was categorised as high-level because it was assumed that some of the spent fuel would be so hot that artificial cooling would be needed. However, none of the spent fuel that was reprocessed needs to be cooled and in 2004, following public consultation, the waste was reclassified as intermediate-level.

The raffinate is highly radioactive, accounting for approximately 80 per cent of the waste inventory at Dounreay in terms of radioactivity. The total volume of raffinate generated was1200 cubic metres.

Inspection of the tanks carried out using remotely-operated equipment has confirmed they are safe for continued storage. However, indefinite storage of hazardous liquor in underground tanks does not provide a long-term solution.

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 solid form of intermediate-level waste then becomes safe for long-term storage or disposal.

Liquor from the reprocessing of research reactor fuel is now being transferred from the tanks and immobilised in this way in a cementation plant. The rest of the waste liquor will be conditioned in a second treatment plant being proposed as part of the site clean-up.

Once the tanks have been emptied of their contents, the storage facility can be cleaned out and dismantled.