Loss of liquor from the Magnox Swarf Storage Silo original building primary containment

The nuclear site licence holder for the Sellafield site has determined that radioactive liquid is being lost from the original building of the Magnox Swarf Storage Silo.

The Magnox Swarf Storage Silo facility is used to store the waste cladding material that was removed from used Magnox nuclear fuel. The original silos were built in the 1960s and the facility was extended three times during the 1970s and 1980s. The waste inside is classified as intermediate level waste. Most of the radioactivity inside the silo (99.5%) is solid waste, the remaining 0.5% is held within the water (described as liquor). There have previously been historic below ground leak from the original building and the potential leak paths remain. As part of routine arrangements, water levels within the silos are monitored and clean water is routinely added to the silo to maintain levels to make up for natural losses (e.g. evaporation).

In November 2019 the site licence holder notified the regulator that liquor loss from the original silo structure had exceeded expected levels since July 2019. Analysis has shown that approximately 1 meter cubed of liquor is being lost per day. This equates to the loss of approximately 8 terabecquerel of Caesium-137 (the dominant radioactive isotope) over the period July to November 2019.

The radioactivity is being discharged into the ground beneath the silos. Monitoring of groundwater boreholes and in-ground gamma activity around the facility has not indicated a release to the environment. The migration of contamination through the ground is predicted to be very slow therefore any risk to the public and environment is low. The nature of the facility and structural design of the original building (singled skin silos with the lower level below ground level) prevents practicable mitigation options.

The site licence holder is investigating to determine the cause and further characterise the losses. Structural data for the facility continues to be monitored and reviewed. Radiological surveys are being undertaken to identify signs of changes in the immediate area. There has been no detectable change to general radiological conditions in and around the facility and there are no immediate safety concerns to the public or the environment. The barriers and systems which are designed to protect against accident progression within the facility continue to operate normally. The existence of leak paths through the silo structure is not considered to affect the structural integrity of the facility, which is a reinforced concrete structure. A long term strategy is progressing to achieve retrievals of the waste inventory into modern standard storage facilities. This will facilitate the removal of solid and liquid waste and allow the remediation of the facility.

The UK Office for Nuclear Regulation and Environment Agency are continuing to maintain close regulatory oversight in relation to this matter.

Location: Sellafield Site - Magnox Swarf Storage Silo
Event date: Tue, 12-11-2019
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