Office for Nuclear Regulation

Statement of Nuclear Incidents at Nuclear Installations

A statement on incidents at nuclear installations in Britain, which meet Ministerial Reporting Criteria (MRC), is provided to the Secretary of State for the Department of Energy and Climate Change and the Secretary of State for Scotland, and is published each quarter by the Office for Nuclear Regulation, an Agency of HSE.

For the period 1 October 2012 to 31 December 2012 there were two incidents at licensed nuclear installations, which met the MRC and reports are published below.

Dungeness B - December

In December 2012, EDF Energy Nuclear Generation Ltd (NGL) notified ONR that the primary sea defence at the Dungeness B nuclear power station (a shingle bank) was not as robust as previously thought.

NGL has initiated an investigation into the causes for this discrepancy, which will be reviewed by ONR. However, the immediate focus of both NGL and ONR in response to this incident has been to ensure that safety improvements have been made and will continue to be made in a prompt manner.

Recommendation 10 in the interim Weightman report into implications for the UK from Fukushima required the UK nuclear industry to review external flooding studies. In response, NGL commissioned sea flooding studies using modern data and a modern and consistent method for all its sites.

For Dungeness B, this analysis indicated higher over topping rates than previously thought this is the rate at which sea water comes over the top of the primary sea defence. On 14 December 2012, NGL advised ONR that they no longer had confidence in this aspect of their safety case and agreed to provide ONR with a justification for continued reactor operation or to place the site into the safest state by 21 December 2012.

On 21 December 2012, NGL provided ONR with a draft justification for continued operation at Dungeness B valid up to 31 January 2013, which ONR has reviewed and considered adequate. This was based primarily on:

  1. Reactor shut down on receipt of a Severe Flood Warning.
  2. Implementation of temporary local flood protection measures. These were completed by 23 December 2012. The justification for continued operation was also finalised by NGL on that date.
  3. Commitments to:
    1. Provide more permanent local flood protection by 31 January 2013 and
    2. Implement a permanent engineered solution against external flooding, restoring protection against the 1 in 10,000 year event and taking due cognisance of the need for a margin against more severe events, on a timescale of months.

ONR will continue to monitor NGLs programme of safety improvements at Dungeness B until permanent engineered protection is in place, taking enforcement action if necessary. ONR will also monitor NGLs investigation of this incident to ensure that appropriate lessons are learnt.

This incident has been rated at Level 1 on INES, which is an anomaly on the 7 level scale.

Dungeness B - 5 November

On 5 November 2012, EDF Energy Nuclear Generation Ltd (NGL) found that an incorrect control rod had been loaded into Reactor 22 at Dungeness B in March 2003. The misload was identified during routine control rod maintenance after their removal from the reactor.

There are a total of 57 control rods of two different types in each reactor at Dungeness B: 36 more absorbing black rods and 21 less absorbing grey rods. The issue related to a grey rod being loaded where a black rod should have been.

At the time this misload was identified, Reactor 21 was in a shutdown state on a planned refuelling outage whilst Reactor 22 was operating. NGL produced a justification for continued operation of Reactor 22 until its next planned refuelling outage in January 2013. NGL also placed embargoes on refuelling and subsequent re-start of Reactor 21 due to the possibility of such a loading fault on that reactor too. These embargoes were lifted progressively as additional information became available, culminating in a justification for the re-start of Reactor 21. ONR was satisfied by NGLs decision-making process throughout.

To support the safety justification for the re-start of Reactor 21, NGL produced evidence of control rod inventory and maintenance and movement documentation. ONR inspectors are content with the conclusions of these review reports.

ONR considers NGLs investigation and corrective action plan to be adequate, subject to extension of a planned human factors assessment to cover the process for generation and control of records for both reactors. NGL is taking this ONR recommendation through its due process and has indicated its intent to progress it.

NGL has established that this incident was caused by a procedural failure prior to the misloading incident in 2003.

Due to the age of the incident, the improvements made by NGL and the corrective actions identified, ONR is not planning further investigation but will continue to discuss with NGL potential improvements to safety cases dealing with control rod misload.

This incident has been rated at Level 1 on INES, which is an anomaly on the 7 level scale.