Conditions on the graphite cores in nuclear reactors.
|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 188.8.131.52 is relevant to the graphite and only that is supplied.|
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.
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).
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