During the reporting period from1 April 2022 – 30 June 2022 there were two 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.
Dounreay
Incident description
Dounreay Site Restoration Ltd is currently conducting operations within the Prototype Fast Reactor Sodium Tank Farm. This is part of the overarching programme to remove the residual alkali metal used as coolant within the Prototype Fast Reactor and Dounreay Fast Reactor vessels and associated plant. The process involves the controlled introduction of warm moist air into the tank. This removes the sodium residues by producing hydrogen and a caustic solution which are then discharged via gaseous and liquid waste routes approved by the Scottish Environment Protection Agency (SEPA.)
At approximately 20:00hrs on 20 April, a higher than expected pressure excursion was detected, activating the call-out of the site’s fire and rescue service. The pressure excursion damaged some internal components of the tank, causing the release of a small amount of caustic liquor (approximately 1 litre) and associated fumes. There were no injuries to personnel, and only minor damage to pipework and internal components. The process involved very low levels of radioactivity and consequently the radiological risk to workers and the public were negligible. There was no nuclear safety consequence.
Dutyholder's response
Dounreay Site Restoration Ltd (DSRL) fire and rescue team attended the scene in accordance with the site’s emergency arrangements and secured the area. DSRL has subsequently completed a site investigation and identified the contributory factors that led to the loss of process control and subsequent build-up of a combustible atmosphere that led to a small, contained fire within Tank 2 and the associated pressure excursion. DSRL has submitted the 60-Day Follow-Up Report (FUR) required by ONR’s arrangements, which provided additional details regarding the incident, informed by its investigation.
DSRL has reviewed the actions resulting from its investigation and identified improvements to those actions to address corporate level concerns. These have been captured within their action management system and ONR will require these to be adequately addressed prior to re-commencement of operations.
ONR's action
ONR has conducted enquiries into the incident including a site visit to the incident facility.
From the information gathered, ONR identified that DSRL had contravened one or more relevant statutory provisions.
ONR applied its Enforcement Management Model to consider the event and associated factors to determine the appropriate level of enforcement, which for these circumstances has resulted in ONR issuing an Enforcement Letter to DSRL seeking the necessary improvements in its arrangements to prevent a reoccurrence and return DSRL to compliance.
ONR has instructed DSRL to implement improved arrangements prior to the commencement of the following activities:
- Active commissioning of ‘enhanced weathering’ activities in support of residual sodium removal from Tank 2.
- Active commissioning of water vapour nitrogen activities in support of the removal of residual alkali metal from the Prototype Fast Reactor pressure vessels and associated plant.
The UK National INES Officer has confirmed with DSRL that the incident meets the criteria for a Level 1 event [‘Anomaly’] on the International Nuclear Event Scale (INES) [“Any fire or explosion within a nuclear facility involving low level radioactive waste”].
Sellafield
Incident description
A routine internal dosimetry monitoring programme (urine sample) identified an individual with a positive result for internal radiation contamination. The approved dosimetry service undertook an assessment. When the individual’s internal and external dose components were combined, the dosimetry service concluded the individual had received a radiation dose that just exceeded the annual statutory dose limit of 20 milli-Sieverts set in the Ionising Radiation Regulations 2017 for the calendar year of 2021.
Dutyholder's response
Upon receiving the positive result, Sellafield Ltd undertook further urine samples from the individual and the individual’s work colleagues to confirm the result and to assess the extent of condition. Sellafield Ltd requested that the approved dosimetry services undertake a radiation dose assessment for the individual and placed the individual on restricted activities on the plant (to minimise further radiation dose).
The individual’s colleagues have also been tested and returned negative results.
Sellafield Ltd has continued to consult with the individual throughout this process. The individual is showing no signs of ill health and their welfare is being managed by the plant and safety team.
Sellafield Ltd is carrying out detailed investigations to identify how and during what activity the internal dose uptake occurred.
ONR's action
ONR considers Sellafield Ltd’s immediate response to the incident to be appropriate.
ONR is investigating the incident to gain an understanding of the root cause of the internal radiation dose intake, the outcome of the dose assessment, the extent of the condition, and any corrective actions required to prevent a recurrence.
This incident meets the Ministerial Reporting Criteria under criteria 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 2017.
The incident has been provisionally rated as an International Nuclear Event Scale (INES) 1 rating by Sellafield Ltd as the cumulative exposure to worker was in excess of the statutory annual limits.
If it is concluded that the statutory annual limits have been exceeded as a result of a one-off exposure event, this is likely to meet the criteria for INES level 2. This will be reviewed by the UK National INES Officer on completion of further enquiries.