Office for Nuclear Regulation

This website uses non-intrusive anonymous cookies to improve your user experience. You can visit our cookie privacy page for more information, including details on how to opt-out.

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.

MRCT Publication Criteria Description of Criteria
MRCT1 A radiation emergency as defined in the Carriage of Dangerous Goods (Amendment) Regulations 2019 (CDG (Amendment) Regulations 2019): means a non-routine situation or event arising during the carriage of Class 7 dangerous goods that necessitates prompt action to mitigate the serious consequences
  1. Of a hazard from that situation or event;
  2. Of a perceived risk arising from such a hazard; or
  3. To one or more of the following: human life; health and safety; quality of life; property; the environment.
MRCT2 Theft or loss of High Consequence Radioactive Material in carriage.
MRCT3 An occurrence during loading, carriage or unloading of Class 7 dangerous goods involving (a) any release of activity greater than A2 of radioactive material from the packages or from the conveyance if being transported unpackaged; or (b) exposure leading to a breach of the limits set out in IRR17 to workers or members of the public.
MRCT4 An occurrence during loading, carriage or unloading of Class 7 dangerous goods where there is reason to believe that there has been a significant degradation in any Type B(U); Type B(M); Type C or Fissile package safety function (containment; shielding; thermal protection; or criticality) that may have rendered the package unsuitable for continued carriage without additional safety measures.
MRCT5 An occurrence where Class 7 dangerous goods have been transported with any non-compliance regarding radiation or contamination levels where those levels are greater than ten times the prescribed regulatory limits.
MRCT6 Events involving Class 7 goods in carriage likely to attract, or that have attracted, significant national media or public attention.
MRCT7 Events involving Class 7 goods in carriage categorised as INES level 2 and above.

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

MRC Publication Criteria Description of Criteria
a Dangerous occurrences reportable under the Nuclear Installations (Dangerous Occurrences) Regulations 1965.  Summary of occurrences from  Regulation No 3 are as follows:-
3(a) any occurrence on a licensed site involving the emission of ionising radiations or the release of radioactive or toxic substances, causing or likely to cause death, or serious injury, on or off the site.
3(b) any occurrences during transport causing or likely to cause death or serious injury or the breach of containment of a transport package.
3(c) any explosion or fire on a licensed site affecting or likely to affect the safe working or safe condition of the nuclear installation.
3(d) any uncontrolled criticality excursion.
b Confirmed exposure to radiation of individuals which exceed or are expected to exceed, the dose limits specified in Schedule 3 to the Ionising Radiation Regulations (IRRs) 2017.
c Examination, inspection, maintenance, test or operation of any part of the plant revealing that the safe operation or condition of the plant may be significantly affected.
d Abnormal occurrences leading to a confirmed release to atmosphere or spillage of a radioactive substance which exceeds or is expected to exceed, the limits set out in Column 5 of Part 1 of Schedule 7 to the IRRs 2017 except where the release is in a manner specified in an Authorisation under the Environmental Permitting Regulation 2010 or Environmental Authorisations (Scotland) Regulations 2018 (EASR18).
e Abnormal occurrences leading to a release or suspected release or spread of radioactivity, on or off site, which requires special action or special investigation by the Operator.
f Events likely to attract, or that have attracted, significant national media or public attention.
g Events that are categorised as INES level 2 and above.