Quarterly statement of civil incidents reported to ONR - 1 October 2019 to 31 December 2019

During the reporting period from 1 October 2019 to 31 December 2019 there were two civil incidents at nuclear licensed sites within Great Britain that met the Ministerial Reporting Criteria (MRC) as defined within the Nuclear Installations (Dangerous Occurrences) Regulations 1965 and ONR guidance in relation to notifying and reporting incidents and events.

Sellafield Ltd - Redundant Settling Tank (RST) Sludge Sump - 19 October 2019

On 19 October 2019, Sellafield Ltd (SL) reported it had evidence of a loss of radioactively contaminated water ('liquor') from the Redundant Settling Tank (RST) facility. The evidence of a leak was based on the abnormal frequency of water top-ups required to maintain water level within the sump of the RST.

The RST is an open-topped cast concrete tank comprising a series of tanks and chambers located within the separation area of the site. It stopped receiving significant routine liquid waste in 1986 and Post Operational Clean Out (POCO) was completed in 1995. Although POCO removed the bulk of the sludge, a quantity of Intermediate Level Waste (ILW) sludge and an assorted of debris still remain within the sub chambers of the tank. The RST has been in a state of control and surveillance since POCO.

The licensee has carried out an investigation to determine the basis of the loss of liquor. It has concluded that there is a leak to ground however it has not migrated beyond the immediate vicinity of the RST facility. The most probable source of the leak is believed to be from historic leak paths to ground from small cracks in the structure of the sump. As part of the preparations to permanently seal connections from the RST to other areas of plant, remediation activities were being undertaken within the sump. These activities are considered to be the most likely cause of this incident.

The loss of liquor is occurring at a steady rate but investigation to date has determined the movement of liquor is not via a known engineered route. There remains no confirmed release to the environment. Monitoring is being conducted on all potential receiving systems across the site including lagoons, active drain trenches and groundwater boreholes. No abnormal arisings have been identified.

There have been no radiation dose consequences from the event to the workforce or the public and ONR is satisfied there is no risk of public water supply boreholes being affected by the leak or drawing any contaminated groundwater towards them.

SL had commenced a programme to remediate the sump and seal the source of the leak. This will involve removing the remaining inventory in the sump and entombing the sump in concrete. However, the leak will result in additional contamination of the ground at Sellafield which will ultimately require clean up.

SL has subsequently reported the incident as a release or spillage of radioactive substance, which is likely to exceed the quantity specified in Ionising Radiations Regulations 2017 (IRR17) schedule 7.

The incident has been classified as Level 1 (Anomaly) on the International Nuclear and Radiological Event Scale (INES).

Sellafield Ltd - Magnox Swarf Storage Silo Original Building - 12 November 2019

On 12 November 2019, Sellafield Ltd (SL) reported an increased leakage rate from the Magnox Swarf Storage Silo Original Building (MSSS OB).

The MSSS OB stores waste cladding material that was removed from used Magnox nuclear fuel rods. It was built in the 1960s, comprises six compartments and the lower level of each compartment is below the ground. The MSSS facility was extended three times during the 1970s and 1980s. The compartments are filled with water to shield, cool and prevent fire in the waste. The MSSS OB only has primary containment. The MSSS extension buildings have increasing levels of secondary containment as well as leak detection and management equipment.

SL's existing leak detection arrangements for MSSS OB have provided evidence of a loss of radioactively contaminated water ('liquor'). SL reported the leak based on the conclusions of its mass balance tool (the Liquor Balance Model) which has detected losses from MSSS OB. The liquor is believed to be leaking into the ground from cracks in MSSS OB structure below ground level. The potential leak mechanism is the reopening of an existing crack associated with a previous leak detected in the 1970s. Building movement monitoring data indicates no unusual movement of the facility that could have influenced this latest incident. SL reported that the rate of leakage increased compared to previous months, comparable to estimated leak rates in the 1970s.
There have been no radiation dose consequences from the event to the workforce or the public. There has been no detectable change to general radiological conditions at the plant. However, the leak will result in additional contamination of the ground at Sellafield which will ultimately require clean up.

SL's groundwater modelling and underpinning research concludes that any migration of the more significant contamination would be very slow. As such, any risk to the environment and public would be very low and over an extended timescale. A more rapid migration of contamination in features such as surface water drains is unlikely but monitoring is being undertaken by SL to confirm this. Based on knowledge of regional and local groundwater movement, ONR is satisfied that there is no risk of public water supply boreholes being affected by the leak or drawing any contaminated groundwater towards them.

Due to the nature of the facility and the large quantities of stored radioactive waste, ONR judges that the most effective way to remediate the leak is for SL to continue with its long-term, programme to remove the waste as its highest priority. ONR continues to maintain close regulatory oversight of SL's actions in relation to this matter and we are conducting a joint independent investigation with the EA into the event to consider what enforcement action may be appropriate.

ONR and EA have issued a joint response to SL which expects them to review the MSSS environment/safety case for leak to ground to ensure risk from any leakage remain As Low As Reasonably Practicable (ALARP) and subject to the application of Best Available Techniques (BAT).

Sellafield Limited has reported on this event on their website page as part of their incident and notice reporting arrangements, which has resulted in a number of news items appearing in the local media.

The leak has been classed as a release or spillage of radioactive substance, which is likely to exceed the quantity specified in Ionising Radiations Regulations 2017 (IRR17) schedule 7.

This incident has been classified as Level 2 (Incident) on the International Nuclear Radiological Event Scale.

Revisions to ONR Ministerial Reporting Criteria

Table 1 – Ministerial Reporting Criteria for Transport Events (MRCT)

Under the United Nations classification scheme for transport of dangerous goods, radioactive material is classified as Class 7 material. Hence, in these reporting criteria, radioactive material is referred to as ‘Class 7 goods’ which is consistent with the terminology used in UK regulations.

Table 2 – Ministerial Reporting Criteria for Incidents Occurring on Nuclear Licensed Sites and Associated Operations (MRC)