Office for Nuclear Regulation

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Statement of Nuclear Incidents at Nuclear Installations - Q2 2013

A statement on incidents at nuclear installations in Britain, which meet Health and Safety Executive Publication Criteria, 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 April 2013 to 30 June 2013 there were three incidents at licensed nuclear installations, which met the Publication Criteria and reports are published below.

Dungeness B

In its statement of nuclear incidents at nuclear installations for the fourth quarter of 2012, ONR reported that EDF Energy Nuclear Generation Ltd (NGL) had notified the regulator that the primary sea defence at the Dungeness B power station (a shingle bank) was not as robust as previously thought. This incident arose from work commissioned by NGL in response to recommendation 10 in the interim Weightman report into the implications for the UK from Fukushima, which required the UK nuclear industry to review external flooding studies. For Dungeness B, this review indicated higher over topping rates for the primary sea defence than previously thought - this is the rate at which sea water comes over the top of the primary sea defence.

Following this incident in December 2012, NGL installed local flood protection around key safety related equipment. Improvements were also made to operating procedures and further extensive modelling and physical testing was commissioned to obtain more certainty in the predicted sea water levels on site during 1 in 1,000 and 1 in 10,000 year events.

This programme of modelling and physical testing was completed in May 2013. The results confirmed that the over topping rates in use since the December 2012 incident were conservative. However, the results also indicated that outflanking of the primary sea defence could be a significant contributor to sea water levels on the site - this is where sea flooding to the sides of the primary sea defence in front of the site leads to water on the site. The new work indicated that the local flood protection work already completed needed to be extended and improved to further enhance the protection against sea flooding hazards.

Having reviewed all aspects of the new hazard predictions, NGL decided to shut down Reactor 22 whilst a programme of improvements to plant, procedures and training was

implemented. Reactor 22 was shut down on 20 May 2013. At that time the other reactor on the site, Reactor 21, was already shut down on a planned refuelling outage. NGL decided to prepare a sea flooding safety case at Category 1 (the highest safety significance category and hence employing the most robust NGL approval process) to support the return to service.

A multi-disciplinary team of ONR specialist inspectors, together with the ONR site inspector and an ONR project inspector monitored the evolution of NGLs return to service safety case so that ONR was satisfied that:

  1. An adequate safety case was prepared in terms of plant, procedures and training at return to service.
  2. An adequate forward programme of post return to service improvements is in place to ensure that safety continues to improve until a final long term position is reached.

Reactor 22 returned to service under the sea flooding return to service safety case on 14 July 2013 and Reactor 21 will return to service several days after this.

ONR will continue to monitor post return to service sea flooding improvements until the sea flooding design basis for Dungeness B has been fully restored (protection against the 1 in 10,000 year sea flooding event with a margin against more severe but less likely sea flooding). This will happen when a protective wall has been built around the site, the safety benefit from which is claimed in a final long term sea flooding safety case.

NGL has committed to have the protective wall in place by the end of 2013.

The incident originally reported in December 2012 was rated by NGL at Level 1 on INES, which is an anomaly on the 7 level scale

Sellafield & Calderworks

An asset inspection of Pipe bridge 39 in March 2013 identified contamination around its base and steel structure. Further isotopic analysis of material samples identified that the total radioactivity exceeded the quantity specified in Schedule 8 of the Ionising Radiation Regulations 1999, which requires notification to HSE in accordance with Regulation 30.

Sellafield Ltds analysis of samples showed the contamination to be historic based upon isotope decay profiles. Sellafield Ltds investigation concluded that the source of contamination was a degraded, redundant ventilation duct previously connected to an active facility that had been demolished in the early 1990s.

ONR's Site Inspector followed-up this event and agrees with Sellafield Ltds conclusion regarding the source of contamination. ONR has confirmed that Sellafield Ltd has followed its arrangements for reporting the event and that it has responded appropriately by sealing the contamination to prevent it spreading and recording it on the Site Contaminated Land Register. ONR is also satisfied that Sellafield Ltd has work in hand to remove the redundant ventilation duct and that there was no serious risk of harm to the public or workforce from this event.

The ONR Inspector has evaluated this event against HSE's Enforcement Management Model and concluded that further enforcement action is not appropriate.

This incident has been provisionally rated at Level 0 on INES.

Sellafield & Calderworks

During routine radiological survey work on the Sellafield site in March 2012, surface contamination was discovered around part of a building floor slab, whose structure had been demolished in the 1980s. At the time of the discovery, contamination levels were considered to be below the notification levels specified in Schedule 8 of the Ionising Radiation Regulations 1999. Following further intrusive ground sampling by Sellafield Ltd, it was able to confirm in June 2013 that the level of contamination was significantly higher than that originally reported and immediately notified the increased levels of contamination to ONR. 

Sellafield Ltds investigation into the event concluded that the cause of the contamination was historic in nature based upon isotope decay profiles of soil samples, and knowledge of the operations previously undertaken in the since demolished facility.

ONR's Site Inspector followed-up this event and agrees with Sellafield Ltds conclusion regarding the source of contamination. ONR has confirmed that Sellafield Ltd has followed its arrangements for reporting the event and that it has responded appropriately by sealing the contamination to prevent it spreading and recording it on the Site Contaminated Land Register. ONR is also satisfied and that there was no serious risk of harm to the public or workforce from this event.

The ONR Inspector has evaluated this event against HSE's Enforcement Management Model and concluded that further enforcement action is not appropriate.

This incident has been provisionally rated at Level 0 on INES.