During the reporting period 1 April 2024 - 30 June 2024 there were two events 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.

Sellafield

Incident description

On 25 January 2024, Sellafield Ltd staff entered a nuclear material storage area to undertake routine maintenance activities. This area is subject to the application of safeguards measures by the International Atomic Energy Agency (IAEA).  

On this occasion, an IAEA seal was removed from the access door without prior notification to the IAEA, which is a requirement under the IAEA safeguards measures. Later, it was confirmed that the seal had not been properly connected by staff upon exiting the store.

As there are backup systems such as IAEA surveillance cameras in place, there were no actual consequences in terms of IAEA's ability to maintain continuous monitoring of the area.

No radioactive materials were affected by this incident and there were no safety consequences. 

Dutyholder’s response

Sellafield Ltd confirmed to ONR that the IAEA safeguards measures applied to the store were in place and performed a check of future maintenance plans, which may necessitate access to the storage area to ensure the IAEA are notified in advance of any store entry.

Sellafield Ltd are conducting an investigation to establish the facts and ensure adequate measures are put in place to avoid this situation in future.

ONR’s action

ONR considers Sellafield Ltd’s immediate response to the incident to be appropriate.

Following the outcome of Sellafield Ltd’s investigation into the event, ONR will conduct an independent review of the findings and Sellafield Ltd’s arrangements to seek the necessary assurance that similar incidents are prevented in the future.

Dounreay

Incident description

NRS Dounreay routinely monitors the water level within a redundant carbon bed filter located in an external area near fuel cycle facilities. This carbon bed filter is no longer in use and is awaiting decommissioning. When in operation, the system was a dry system and water was introduced to the system after operation ceased. 

Monitoring has indicated a small quantity of water is lost from the bed each day. The decrease in the water level has been estimated at 200mm over 12 months (a rate of water loss of around 1 litre per day). Previous sampling of the water reported activity levels of up to 61,000 Bq per litre, with Caesium (Cs-137) the dominant isotope. 

The rate of decrease has been steady for several months with no evidence of change. Dounreay’s investigation has yet to establish the pathway for loss of the water. A release to the environment has not been detected by the current monitoring regime. A small leak is the most likely reason for the decrease in water level, but this has not been confirmed.

Dutyholder’s response

NRS Dounreay staff have initiated a site level investigation and both ONR and the Scottish Environmental Protection Agency (SEPA) were notified of the situation. The carbon bed filters are subterranean and not accessible, therefore establishing the cause and mechanism of this event is likely to take some time.

Ground water sampling and monitoring in the area has been enhanced from monthly to weekly. The results of this enhanced monitoring has not identified a change in activity level, in either the ground or drain networks, in the vicinity of the carbon bed filter.

ONR’s action

ONR inspectors have discussed this matter with NRS Dounreay’s Environment, Health, Safety and Quality (EHS&Q) lead for the affected area. We have confirmed that there were no immediate consequences from the water losses and consider that NRS Dounreay’s follow up actions are reasonable. 

It was considered appropriate to allow Dounreay time to further investigate and establish its forward action. This will be assessed by ONR, including NRS Dounreay’s understanding of the source of the loss and the proposed actions to prevent further losses. 

No site workers have been harmed or exposed to radiation as a result of this issue.