Office for Nuclear Regulation

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Conditions on the graphite cores in nuclear reactors

Information requested

Conditions on the graphite cores in nuclear reactors.

Information released

Requested Item ONR Reference number Document type or another reference number and comments
The latest three yearly statutory stoppage reports dealing with the condition of the graphite cores in the nuclear reactors. We are sending six ONR documents. Four of these are what we term assessment reports, one is what we term an intervention report and one is an email.    
Hunterston B reactor 3 2012/382553 Assessment report ONR-CNRP-AR-12-116
Hunterston B reactor 4 2011/445013 Assessment report AR 41-2011
Hinkley B reactor 3 2012/454322 Assessment report ONR-CNRP-AR-12-140
Hinkley B reactor 4 2011/629484 Assessment report AR 63-2011
Torness reactor 1 2010/529538 This is an email sent 21 October 2010. We are not sending the attachments.
Torness reactor 2 2012/274685 Intervention report ONR-TOR-IR-12-021
The results of the most recent Periodic Safety Reviews for: - The pairs of AGR power stations such as Hinkley Point B and Hunterston B are normally considered together, as they have very similar designs. Thus there is only one set of documents for two power stations. The most recent periodic safety review (PSR) from Hinkley Point B and Hunterston B dates from 2005 and things have moved on since then. We have therefore supplied a document from 2009 instead, which is an assessment of an interim review carried out more recently. The Licensee agreed to carry out an interim review in 2009 at the time of the 2005 PSR. However, as even this document is not recent, we are also supplying an assessment of the graphite related aspects of a document relating to what the Licensee termed a 'lifetime technical review', written in 2012. Strictly, this is not part of the PSR process, but was a voluntary submission made by the Licensee to obtain ONR's views at the time the Licensee was developing plans for plant lifetime extension (PLEX). For Torness, the PSR was carried out for that power station and the sister power station Heysham 2 at the same time and we are supplying our assessment, which was carried out in 2009. We are also supplying a 2014 email sent internally to clarify some of the matters arising from that PSR.    
Hinkley  B and Hunterston B (note one PSR was done for this pair of power stations) 2009/201442 2012/302031 Assessment report of PSR2 Interim review in 2009. ND DIV1 AR No. 53/2009
We are sending another assessment report as well ONR-CNRP-AR-12-033
Torness 2009/498298 2014/72047 Assessment report ND Div. 1 PAR No. 94/09 issue 2
We are sending an ONR email sent 7 February 2014 as well
The safety case for re-starting each of these reactors after their last inspection - we have identified a number of documents produced by the Licensee that we hold. However, it should be noted that the term 'safety case' does not necessarily refer to one particular document. In practice, the complete safety case, even just for the graphite integrity aspects of a particular reactor may consist of many documents. The ones that we are supplying are associated with the request from the Licensee for ONR to grant consent that the reactor can be returned to service. They therefore summarise the position of the findings of the Licensee's examination and inspection processes that have been performed during a statutory outage in order to confirm that the results from the inspection are in line with the expectations of the safety case. The documents we are supplying as response to your first request are therefore ONR assessments of the documents supplied under the third request. When ONR assesses the documents associated with the return to service request for consent, we ensure that we are satisfied that there are no significant outstanding safety issues. We then grant the Licensee permission to restart the reactor. However, that consideration is not based purely on the assessment of the documents prepared for the return to service request by the Licensee. Aspects of the entire safety case are taken into account in reaching our decision and we ensure that there are no significant deviations from limits set in the safety case. Note the following ONR reference numbers are not marked on the documents below, as they are the Licensee's  
Hunterston B reactor 3 2012/384790 EC 347336
Hunterston B reactor 4 2011/444773 EC343780
Hinkley  B reactor 3 2012/451402 EC 347677
Hinkley  B reactor 4 2011/611103 EC 344384
Torness reactor 1 2010/557094 EC 340646 We do not appear to hold the actual document, but have the attached 'independent nuclear safety assessment' performed by the licensee's internal regulator.
Torness reactor 2 2012/307161 This is an email sent 1 August 2012 with EC341885 as an attachment, only section 3.1.2.1 is relevant to the graphite and only that is supplied.

Exemptions applied

Some information has been redacted because it is personal data, so has been withheld using the exception in Regulation 13 EIR.

This is an absolute exception so does not require a public interest test.

PIT (Public Interest Test) if applicable

Not applicable.

Annex 1 - EDF NGL (the Licensee) - Additional Briefing

To assist, it is accepted by the regulator, the Office for Nuclear Regulation (ONR) and the industry in general, that cracks will occur in some of the graphite bricks as part of the normal ageing process within the graphite reactor core. This is a phenomenon known about and anticipated for within the safety case that underpins continued safe operation.

It has been demonstrated that the cores are tolerant to a wide range of cracking within these bricks before there is a challenge to safe operation. The current level of cracking is low and well within the recognised safety margins. This is after over 38 years of electricity generation at our lead stations, Hinkley Point B and Hunterston B.

The cores are tolerant to such cracking and our monitoring has clearly demonstrated that there has been no material change in the fuel and control rod channel profiles as a result of these cracks, as a change in the profiles could prevent insertion of either the fuel stringers or control rods.

As our understanding has developed over the years, so too have our monitoring techniques and we have also increased the level of inspections during appropriate statutory outages (effectively a three-yearly MOT for the reactor and its associated conventional plant).

Description of graphite core, current cracking and safety case margin

Core ageing and degradation

  1. AGR's are composed of just over 300 fuel channels and 10 layers of Graphite bricks to make up the core. Each brick is loosely connected to its 8 neighbouring bricks by graphite keys and there are also 4 keyways at the top and bottom of each brick.
  2. This structure then provides pathways for the fuel to be loaded and control rods to enter the core. This is the major safety requirement of the core.
  3. This structure is highly redundant and therefore a number of keys could fail or bricks could crack before there is any affect to the overall distortion of the core.
  4. Due to operation of the reactor radiation, this degradation leads to stresses being generated in fuel bricks which are predicted to lead to cracking, not all at the same time. This is progressive and the number of cracked bricks will increase with time.
  5. At the start of life there is a large safety factor between maximum core distortion and that required to stop control rods entering.

Inspection

  1. At each AGR there is a level of inspection defined to meet the safety case requirements. This inspection determines the number of cracked bricks in the core and the current level of core distortion.
  2. The most inspection is carried out at the lead stations of Hinkley Point B and Hunterston B.
  3. At the current time in the lead stations, there is less than 10% cracked bricks in the reactor, but the majority of cracks are horizontal and will have no affect to core distortion.
  4. Currently there are only 1% of bricks axially cracked in the core.
  5. It is predicted later in life this number could increase beyond current level.
  6. At the rest of the AGRs the level of cracking is less than that at the leading stations.
  7. Samples are also cut (trepanned) from the core. These samples are analysed to give material properties to support the assessment of the graphite core.

Consequence of ageing

  1. At the current time the levels of cracking is low, and the distortion is limited
  2. Assessments have been made with regards to the distortion that could be generated in a severe earthquake and this analysis has shown that even up to 30% cracked bricks the worst distortion only occurs in a few channels and only for a brief time.
  3. At this level of cracking there is still a large margin to the distortion to stop control rods.
  4. The reactors were built with a second way to be able to stop the nuclear reaction in AGR's. This is to introduce nitrogen gas into the circuit. This system has recently gone through an upgrade to ensure there is complete confidence in the ability to shut down the reactor under all circumstances.

Lifetime

  1. Brick cracking could be one of the lifetime limiting features of the AGR
  2. An increase in cracking is not expected for a number of years although there is uncertainty in this analysis
  3. There is a programme of work underway to demonstrate increased tolerance to brick cracking

Weight loss

The Graphite also losses mass during operation; this is called radiolytic oxidation. This reduction in mass slightly affects the efficiency and moderation of the reactor and there is therefore an upper bound on this value.

GCPT Group Head
EDF Energy Nuclear Generation Limited
5 March 2014