During the reporting period 1 January 2024 - 31 March 2024 there were four 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.

Torness

Incident description

A planned shutdown of Reactor 2 at Torness for off load refuelling and graphite inspection programme commenced at 11pm on 15 November 2023. Post-shutdown, three gas circulator variable frequency converter units (VFCs) and one Variable Speed Drive (VSD) failed, preventing the associated gas circulators from continuing to run at reduced speed as designed. 

Adequate post trip cooling of the reactor was maintained by the remaining four gas circulators (GC) running.
Failure of the three VFCs and the VSD unit does not challenge the applicable safety case or operational limits but represents an unexpected reduction in defence in depth at the site.

Dutyholder’s response

EDF’s decision making process was established to review the findings, consider available options and recommend a way forward. The outcome was to proceed with the planned reactor gas pressure blowdown to 0.5 bar, once a further GC was repaired and became available. 

EDF established an event recovery team at Torness to focus on understanding the cause of the failures and to initiate a repair strategy.

Torness had already planned a programme of examination, inspection, maintenance and testing for the Reactor 2 outage, following similar VFC failures, which occurred in January 2023. This programme will now be extended to include the findings and associated rectification work from these failures.

Before Reactor 2 returned to service, a full review was undertaken by EDF’s Operational Safety Review Committee. 

OPEX has been shared with Heysham 2 who have the same equipment.

ONR’s action

The ONR site inspector followed up this incident to ensure Reactor 2 was in a safe state and that EDF’s processes had been implemented appropriately to establish the cause of the failures.

ONR engaged with Torness prior to the restart of Reactor 2, to ensure similar VFCs/VSD failures are prevented. 

Capenhurst

Incident description

On 5 February 2024, at the Tails Management Facility (TMF) on the Capenhurst Site in Cheshire, a metal box containing uranium powder  was dropped from a forklift truck.

When the box was dropped, it struck other equipment within the facility and was damaged, resulting in 3.3 Kg of uranium powder being released into the facility. This is a relatively small amount of material by mass. However, the amount of radioactivity released is above the statutory reporting criteria.  

Following the powder release, monitors detected the presence of radioactivity in air. The incident occurred inside the facility, adjacent to a metal door which opens to the outside of the facility. During the incident the door became obstructed by the metal box/forklift truck, and was unable to be closed.  

Two workers were involved in the incident in the facility. They were not injured and were monitored and found to be “clear” of any external radioactive contamination. Subsequent investigations revealed neither worker had accrued any internal radiation dose. 

There was no significant risk to members of the public as a result of this event.

Dutyholder’s response

Following the incident, the dutyholder deployed their on-site emergency response teams and immediately stopped operations in the TMF. All staff on shift successfully mustered. The incident was reported to ONR on the same day.

Recovery operations were undertaken, including cleaning up the facility to remove the spilled uranium powder and closing the roller shutter door. The dropped box was sealed and moved to a secure location on the site. The dutyholder is undertaking enhanced monitoring of the box to detect any signs of further leakage of the uranium powder. They are currently considering the safest way to transfer the contents to a new box.

The dutyholder has implemented changes to the way they handle boxes of uranium powder at the facility, including establishing new operating instructions. The facility remained off-line for approximately one week while these changes were implemented, but is now operating again. The dutyholder also carried out their own investigation into the incident.

ONR’s action

ONR’s site inspector visited the facility the day after the incident and spoke with members of the Urenco Chemical Plant’s Senior Leadership Team to gain a fuller understanding of the event.

ONR has continued to monitor the dutyholder’s response to the incident to gain assurances on their recovery operations and the actions being taken to prevent a reoccurrence. 

At this stage, we are satisfied that the dutyholder has taken appropriate measures to prevent a reoccurrence. 
However, we believe this incident could have been significantly worse, and should have been preventable. Therefore, ONR is currently undertaking its own investigation.

Hartlepool

Incident description

In January 2024, EDF took the decision to shut down all four reactors at the Heysham 1 and Hartlepool power stations. This was due to a steam valve failure at Heysham 1 during operation that challenged the continued safe operation of these valves, which are used at both stations. The decision was taken to shut down all the reactors to investigate the fault.

This failure of the Heysham 1 steam valve was the subject of a separate ONR investigation that resulted in an improvement notice being issued to Heysham 1 (which has now been complied with) but is referenced here for context as the reason for the Hartlepool reactors being shut down.

When the Hartlepool reactors were shut down, the flow rate of (demineralised) water used for post-trip cooling of the reactors was observed to be far higher rate than was assumed by the safety case. This was later confirmed as being due to leakages within the system and other genuine system usages that hadn’t been considered within the safety case.

The situation presented a potential safety challenge as there is an expectation that the site will be able to maintain adequate water supplies to cool down both reactors when they shut down. This is achieved by ensuring defined water stocks are maintained in the five reserve feed tanks, which are continuously monitored. In justifying the required water stocks, the safety case assumed that there would be minimal losses from the water system to ensure that the water in the reserve feed tanks is sufficient to cool down both reactors on shutdown. This assumption was demonstrated to be incorrect when the reactors were shut down in January 2024 with water usage being significantly higher than that assumed in the safety case.

As all water supplies, and all safety systems, were available at the time of shutdown, there was no actual safety consequence due to this event. The potential, however, was that if water supplies to the station had been interrupted while shutting down the reactors, this would have challenged the ability to provide adequate post-trip cooling to the reactor fuel.

Dutyholder’s response

EDF entered their internal investigation process to ascertain why the level of water usage was so high and what actions were required to ensure future usage was sustainable for post-trip cooling of both reactors. In their investigation, EDF identified several process shortfalls, and that daily water usage had steadily risen over several years without any intervention being taken to ascertain why.

A systematic review of the system has now been carried out, with  several defects rectified, which has brought water usage down significantly. In addition, system isolations have been identified that can be made when the reactors are shut down to minimise non-essential water usage, thus maximising water available for post trip cooling. The investigation noted that claims made in the safety case regarding water usage had not been adequately validated and were subsequently shown to be incorrect. EDF has also implemented process changes to enhance daily water usage monitoring, with clear criteria on how to respond to rising usage.
EDF has made several commitments to update the safety case to ensure it accurately reflects validated water usage during all phases of system operation and to ensure system losses are appropriately rectified. While these commitments are being progressed, EDF have installed additional temporary water production capabilities on site which provides confidence in continued operation.

ONR’s action

ONR inspectors have continued to engage with and monitor EDF’s response since the identification of the issue, through to the return to service of the reactors.

Prior to return to service of each reactor, we reviewed EDF’s interim justification for continued operation and scrutinised the commitments that were made and the actions identified in their internal investigation. ONR will ensure that EDF delivers against the commitments and post investigation actions, ensuring they are completed in a timely manner.

ONR conducted preliminary enquiries and concluded that EDF had failed to adhere to the requirements of site licence conditions, and we issued an enforcement letter. This enforcement letter requires EDF to provide ONR with a detailed plan of how they will address all commitments and investigation actions. EDF was required to provide this response in writing by 30 June 2024, although ONR has monitored EDFs progress in implementing improvements since the return to service of the reactors.  We have raised a level 3 regulatory issue to maintain formal oversight of the actions required in the enforcement letter.

Adherence to licence conditions and safety case assumptions is routinely confirmed by ONR through regular compliance inspections at all licenced sites. The information from the event at Hartlepool was promptly shared with all EDF stations and has not been identified as an issue at any other operating station.  

Dounreay

Incident description

When plant operators lifted a personnel access hatch to the Prototype Fast Reactor (PFR) Surge Tank Pit to inspect for water ingress and the condition of the surge tanks, a significant accumulation of water in the pit was observed. Samples of water taken from within the pit determined elevated levels of tritium. 
No other specific radionuclides have been detected.

Dutyholder’s response

A site level investigation was as convened, and additional borehole sampling undertaken to confirm the extent of the elevated levels.

The water in the pit was scheduled to be pumped out through the normal authorised route for radioactive aqueous discharges. A substantial amount of the water from the pit has since been discharged via the normal authorised route for radioactive aqueous discharges.

ONR’s action

ONR’s site inspectors followed up the event during their visit to site in March 2024. During the inspector’s visit, site personnel confirmed that monitoring of the surrounding boreholes had confirmed that there were no elevated levels of tritium in the surrounding ground water. 

ONR is satisfied that the offsite discharge of the water from the pits was within the authorised limits set by the Scottish Environment Protection Agency (SEPA).

Dounreay’s site investigation team has not been able to definitively identify the source of the water or tritium in the pits, however, they consider the most likely source being historical operations which contaminated the pits with tritium which was mobilised by the presence of water from either rain or ground water. ONR considers the dutyholders analysis of these matters to be reasonable.

Damage to some of the tank supports have also been identified and Dounreay has developed a recovery plan and is progressing this. ONR inspectors are monitoring the dutyholder’s progress via routine site interactions. 
ONR does not consider the levels of tritium detected present a substantive challenge to the site in managing potential exposure to site personnel, and that it does not present a concern to persons off the site.