Office for Nuclear Regulation

Statement of civil incidents meeting the Ministerial Reportable Criteria (MRC) reported to ONR - Q4 2017

1 October 2017 to 31 December 2017

A statement on incidents at nuclear installations in Great Britain and during the transport of radioactive material, which meet Ministerial Reporting Criteria (MRC), is provided to the Secretary of State for Business, Energy and Industrial Strategy and the Secretary of State for Scotland and Wales, and is published each quarter by the Office for Nuclear Regulation.

For the period, 01 October 2017 to 31 December 2017, there were two incidents which met the MRC.

Sellafield Ltd - Sellafield Storage of Chemicals - 20 October 2017

Ministerial Reporting Criterion - Examination, inspection, maintenance, test or operation of any part of the plant revealing that the safe operation or condition of the plant maybe significantly affected.


As a result of an audit of chemicals in a laboratory on the Sellafield site, Sellafield Ltd sought specialist advice regarding a small number of bottles containing non-radiological chemicals.

Specialist advice stated that Tetrahydrofuran (THF) posed a theoretical (albeit very low) risk of self-detonation. As a result, SL’s Analytical Services facility, which also contains radioactive inventory, implemented its emergency arrangements, evacuating the building, mobilising the Sellafield Emergency Control Centre (SECC) and declaring an Operational Alert. 

As part of normal UK protocol, the SECC informed the Ministry of Defence Explosive Ordnance Disposal (EOD).  On arrival at Sellafield site, the EOD conducted a visual inspection of several chemicals, concluding a number of the chemicals should be disposed of in a controlled manner, to established procedures.

The EOD removed the identified chemicals from Analytical Services, transferred them to a safe, pre-prepared location on the Sellafield site, and disposed of them in a controlled manner, using established procedures. Sellafield then instigated a site wide search for similar chemicals, and further chemicals were identified, and similar disposal processes were followed.

ONR’s follow-up enquiries found that Sellafield Ltd took prompt and appropriate action, and adopted a precautionary approach once the nature of the chemical hazard and its risk had been identified. ONR also considers that Sellafield Ltd took timely and appropriate action to establish the extent of the issue across the site. Fundamentally Sellafield Ltd responded well to the event once it occurred. However, they should have made and implemented arrangements to avoid having the event in the first place.

ONR considers that Sellafield Ltd has been in breach of legal requirements, namely: the Control of Substances Hazardous to Health Regulations 2002; and the Dangerous Substances and Explosive Atmospheres Regulations 2002.

ONR concluded that the proportionate regulatory action was to issue a formal enforcement letter that required Sellafield Ltd to undertake a review of its arrangements for managing waste chemicals within the Analytical Services Group, and across all other site-wide operating units. ONR will review Sellafield Ltd’s response and engage further as appropriate. 

EDF Energy Nuclear Generation Ltd - Sizewell B - Identification of weld defects in steam generator - 3 November 2017

Ministerial Reporting Criterion - 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.


The Sizewell B reactor was shut down for its 15th Refuelling Outage on Friday 3rd November 2017.  In the early stages of the outage, an examination of the primary coolant circuit is undertaken to identify any indications of leakage.  These are identified by the accumulation of boric acid crystals, which appear as white powder deposits close to any leak sites.

On Saturday 4th November 2017, examination of the Sizewell B Steam Generator D revealed deposits of boric acid crystals. The boric acid crystals were analysed by the licensee and confirmed to have originated from the primary coolant during the recent operating cycle. EDF Energy NGL identified evidence of a pinhole defect in a weld associated with a drain line in the Steam Generator channel head bowl.  The defect, which was caused by primary water stress corrosion cracking, was sufficiently small such that leakage was not identifiable during power operation.

Non-destructive testing (NDT) performed by NGL identified defects in similar locations in the other three Steam Generators at Sizewell B.  NGL took the decision to repair all of the station’s Steam Generators by plugging the drain lines with material more resistant to primary water stress corrosion cracking.

The company promptly notified ONR in accordance with arrangements under condition 7 of the site licence.  ONR is satisfied that the leak was extremely small, well within the capacity of the station’s normal provision for making up cooling water; hence, we consider that the leak did not pose any threat to core cooling or fuel integrity.  No increases in radioactive discharges to the environment, or radiation doses to the public occurred.  The incident was categorised as level one (an anomaly) on the International Nuclear Event Scale. (INES).

Under the conditions attached to the nuclear site licence, NGL is required to obtain ONR’s consent to start-up the reactor at Sizewell B following each refuelling outage.  As part of the consent process, ONR’s specialist inspectors undertook a detailed assessment of NGL’s drain line repair.  Our inspectors were satisfied with the safety of the chosen design and measures implemented to ensure the repair was performed to the required high standards and quality.  NGL has reviewed the vulnerability of similar welds in the primary coolant circuit and has provided ONR with appropriate assurance of their integrity.  We are satisfied that the licensee’s safety case adequately demonstrated the safety of reactor operations.  Accordingly, ONR granted consent to start up the reactor at Sizewell B on 26 January 2018.

The licensee has undertaken an internal investigation which has identified lessons learned.  ONR’s site inspector will monitor the licensee’s response to this learning, ensuring effective corrective action is implemented.

Sizewell B is the UK’s only civil nuclear pressurised water reactor.  The other operating Nuclear Power Plants in the UK are Advanced Gas Reactors, which are not affected by this fault.