Office for Nuclear Regulation

Transcription

1 Office for Nuclear Regulation Civil Nuclear Reactors Programme Doel 3 and Tihange 2 Reactor Pressure Vessel Inspection Findings and their implications for Sizewell B and Hinkley Point C Assessment Report: ONR-CNRP-AR April 2013

2 Office for Nuclear Regulation ASSESSMENT REPORT Site: Project: Title: Sizewell B and Hinkley Point C Investigations of defects found in Doel 3 & implications for UK programmes Doel 3 and Tihange 2 Reactor Pressure Vessel Inspection Findings and their implications for Sizewell B and Hinkley Point C Licence Numbers: 63 and 97 Licence Condition(s): 17, 23, 28 IIS Rating: EDF NGL 3 (Adequate) NNB GenCo 3 (Adequate) Document Identifier Identifier Revision TRIM Reference(s) ONR-CNRP-AR / Step-based Document Review Step Description Role Name Date TRIM Revision * 1 Initial draft, including identification and mark-up of SNI/CCI Author 13/03/ Main editorial review Author 3 Peer Review in accordance with AST/005 Issue 1 Peer Reviewer 4 Assessor update / sentencing of comments and return to Peer Reviewer Author 25/03/ Final editorial / clean draft review Author 25/03/ Acceptance review in accordance with AST/003 Issue 4 AUH 27/03/ Report Sign-off Author / Peer Reviewer / AUH 27/03/ Review of Rev, 1 including factual accuracy edits following replies from NNB GenCo, EDF NGL and FANC Author/AUH 22/04/ /04/ Document Acceptance * TRIM revision to be identified upon completion of activity and incorporation of any changes to document.

3 Office for Nuclear Regulation ASSESSMENT REPORT Role Name Position Signature Date Author HM Inspector 22/04/2013 Peer Review HM Inspector 24/04/2013 Acceptance HM Superintending Inspector 29/04/2013 Revision History Revision Date Author(s) Reviewed By Accepted By Description Of Change 0 28/03/2013 First formal issue. 1 29/04/2013 Inclusion of factual accuracy & commercially confidential information edits Circulation (latest issue) Organisation Name ONR Where required in accordance with ONR How2 Document AST/005 Issue 1. Hard-copy of document signed-off, TRIM version updated with authors / approver / acceptor names and dates and record finalised.

4 Office for Nuclear Regulation ASSESSMENT REPORT

5 Crown copyright 2013 COPYRIGHT You may reuse this information (excluding logos) free of charge in any format or medium, under the terms of the Open Government Licence. To view the licence visit write to the Information Policy Team, The National Archives, Kew, London TW9 4DU, or Some images and illustrations may not be owned by the Crown so cannot be reproduced without permission of the copyright owner. Enquiries should be sent to Unless otherwise stated, all corporate names, logos, and Registered and Trademark products mentioned in this Web site belong to one or more of the respective Companies or their respective licensors. They may not be used or reproduced in any manner without the prior written agreement of the owner(s). For published documents, the electronic copy on the ONR website remains the most current publically available version and copying or printing renders this document uncontrolled. Page (i)

6 Page (ii)

7 EXECUTIVE SUMMARY Background Doel 3 and Tihange 2 are Pressurised Water Reactors (PWRs) in Belgium commissioned in 1982 and 1983 respectively. During summer 2012, examination of the Reactor Pressure Vessels (RPVs) of these plants revealed a large number (several thousand) of defect indications in the parent forgings used in the construction of the RPVs. The Belgian Licensee (Electrabel) and the Belgian Nuclear Regulator (FANC) have undertaken an extensive programme of work to understand the root-cause of the defects. ONR participated in international regulatory expert working groups, set up by FANC, to investigate this issue. There is consensus that the indications relate to defects formed during the steelmaking and forging processes by a hydrogen-induced flaking/cracking mechanism. The UK has one PWR reactor at Sizewell B operated by EDF Nuclear Generation Limited (EDF NGL) and NNB Generation Company Limited (NNB GenCo) has a nuclear site licence for construction of a twin PWR of the UK EPR TM design at Hinkley Point C. The manufacture of long lead items for Hinkley Point C, including the RPV, has begun prior to on-site construction. ONR asked these two licensees to provide responses to the findings from Doel 3 and Tihange 2; this report is ONR s assessment of those responses. Defect Indications at Doel 3 and Tihange 2 We concur with the view expressed at the FANC expert working groups that the Doel 3 and Tihange 2 defects are likely to be hydrogen-induced defects and appear to be broadly comparable with the hydrogen induced defects which recently occurred in forgings destined for another utility. The Doel 3 and Tihange 2 defects are believed to have been detectable and reportable by the original manufacturing inspections. The reasons for the defects not being reported have not been explained. Consequently we judge that the concerns about the manufacturing inspections for these two RPVs are not so much about the adequacy of the inspection procedures and their sensitivity, but rather the apparent lack of compliance with the reporting criteria. We also judge that whilst the supplementary reporting criteria specified during manufacture were adequate, the defect acceptance criteria specified in the inspection procedure were not stringent when compared with relevant good practice. Implications for Sizewell B The RPVs for Doel 3 and Tihange 2 were manufactured by a different fabricator to those used at Sizewell B, using forgings from a different forgemaster and a different steel casting manufacturer. EDF NGL supplied evidence of the additional measures taken to control the level of hydrogen taken by suppliers of Sizewell B plant. We consider that all reasonably practicable measures were taken at the time of manufacture to ensure that the probability of hydrogen flaking was minimised. We judge that the Sizewell B RPV was produced in such a way that the hydrogen levels in the forgings were below the levels expected to lead to hydrogen-induced defects. Moreover, the level of non-metallic impurity inclusions for Sizewell B were below those for the Doel 3 RPV. This gives confidence that the likelihood of hydrogen flaking in Sizewell B is lower than for Doel 3 and Tihange 2. Multiple and independent inspections were performed on the Sizewell B forgings during manufacture and we are satisfied with the recording and acceptance levels specified for these inspections. Consequently, we judge that defects of the type discovered in the Belgian plants should have been detected and reported using the inspection techniques employed for Sizewell B during manufacture. From the records kept at Sizewell B, there is evidence that defects were recorded properly and sentenced in accordance with the specifications. The inspections at Sizewell B were validated rigorously which is an additional source of confidence in their reliability. Page (iii)

8 This is because the validation process demonstrates not only the intrinsic capability of the procedures, but also that the operators had the training and experience to apply the procedures effectively. We have confidence that the precautions taken during the manufacture of the RPV for Sizewell B has minimised the likelihood of hydrogen-induced defects occurring in the Sizewell B forgings. We also have confidence that, should there have been any defects of the type seen at Doel 3 and Tihange 2, these would have been detected and recorded. We judge that to bring forward inspections ahead of the ten-yearly inspection due in 2016 is not consistent with the ALARP (as low as reasonably practicable) principle. We make this judgement based upon: The confidence in build quality at Sizewell B, The depth and breadth of the manufacturing records, The improved inspections used at Sizewell B, compared with Doel 3, The resulting small residual risk of significant hydrogen defects actually being present, The increased radiation dose to workers arising from the additional inspections, The increase in the number of complex lifts to enable RPV inspections to take place. We judge that the validity of the extant RPV safety case for Sizewell B is not affected by the recent observation of hydrogen-induced defects in the Doel 3 and Tihange 2 RPVs. We are content for Sizewell B to continue to operate subject to satisfactory periodic safety reviews being carried out, and satisfactory results from routine maintenance, inspection and testing that continue to support the plant safety case. EDF NGL has undertaken to review the RPV weld inspection programme to assess the benefits and disbenefits of extending the extent of future inspections beyond the immediate area of the identified welds. We recommend that this review be expedited to ensure that, if any enhanced inspections are agreed, they may be planned into the ten-yearly outage in Implications for Hinkley Point C We judge that NNB GenCo has undertaken a well reasoned comparison between the manufacturing routes for Hinkley Point C forgings with those used at Doel 3 and Tihange 2. We judge that this comparison has identified a number of factors likely to influence the formation of hydrogen-induced defects and that controls are in place to minimise the likelihood of formation of hydrogen-induced defects in Hinkley Point C forgings. The justification for no defects of significance entering service also depends on the adequacy of manufacturing inspection. NNB GenCo has expanded the range of the manufacturing inspections and provided detailed assessments of inspection capability. We judge that the NNB GenCo inspection techniques and procedures are adequate to detect and report defects of the type reported at Doel 3 and Tihange 2. Page (iv)

9 LIST OF ABBREVIATIONS ALARP ASME BWE CNRP EDF NGL FANC FBH HIC HOW2 HPC HSE IPI ISI JSW LC ONR OPEX NDT NNB GenCo NSSS PSI RCC-M RDM PWR RPV SDH SAP SZB TAG TJ TSC UK EPR As low as is reasonably practicable American Society of Mechanical Engineers Back Wall Echo Civil Nuclear Reactor Programme EDF Energy Nuclear Generation Limited Federal Agency for Nuclear Control (Belgian Nuclear Regulator) Flat Bottom Hole High Integrity Components ONR Business Management System Hinkley Point C Health and Safety Executive In-Process Inspection In-Service Inspection Japan Steel Works Licence Condition Office for Nuclear Regulation (an agency of HSE) Operating Experience Non-Destructive Testing NNB Generation Company Limited Nuclear Steam Supply System Pre-Service Inspection Règles de Conception et de Construction des Matériels Mécaniques des Ilots Nucléaires (Design and Construction Rules for the Mechanical Components of PWR Nuclear Islands) Rotterdam Dockyard Pressurised Water Reactor Reactor Pressure Vessel Side drilled hole Safety Assessment Principle(s) (HSE) Sizewell B Technical Assessment Guide(s) (ONR) Technical Justification Technical Support Contractor EDF and AREVA UK specific pressurised water reactor design Page (v)

10 TABLE OF CONTENTS COPYRIGHT... I EXECUTIVE SUMMARY... III Background... iii ONR Judgement on Implications of the Defect Observations at Doel 3 and Tihange 2... iii Implications for Sizewell B... iii Implications for Hinkley Point C...iv 1 INTRODUCTION Background UK Response PURPOSE OF REPORT SCOPE METHODOLOGY ASSESSMENT STRATEGY Standards and Criteria Safety Assessment Principles Technical Assessment Guides National and International Standards and Guidance Use of Technical Support Contractors Integration with other Assessment Topics Out-of-scope Items LICENSEE S SAFETY CASE EDF NGL Sizewell B Metallurgy Non-Destructive Testing ALARP Justification NNB GenCo Hinkley Point C ONR ASSESSMENT Scope of Assessment Undertaken Interactions with the Belgian Federal Agency for Nuclear Control EDF NGL Sizewell B NNB GenCo Hinkley Point C Assessment Participation in FANC Expert Working Groups and key insights EDF NGL Sizewell B NNB GenCo Hinkley Point C Comparison with Standards, Guidance and Relevant Good Practice Comparison of ultrasonic inspection procedures with EN : SZB manufacturing procedures for ultrasonic inspection HPC manufacturing procedures for ultrasonic inspection Page (vi)

11 4.3.4 Doel 3 and Tihange 2 manufacturing and in-service inspections CONCLUSIONS AND RECOMENDATIONS Conclusions Doel 3 and Tihange EDF NGL Sizewell B NNB GenCo Hinkley Point C Recommendations EDF NGL Sizewell B NNB GenCo Hinkley Point C REFERENCES Tables Table 1: Relevant Safety Assessment Principles Considered During the Assessment Annexes Annex 1: procedures Annex 2: SG forgings Comparison of recording and acceptance levels for ultrasonic forging inspection Comparison of ultrasonic procedures and capability statements for HPC RPV and Page (vii)

12 1 INTRODUCTION 1.1 Background 1 Doel 3 and Tihange 2 are pressurised water reactors (PWRs) operating in Belgium since 1982 and 1983 respectively. Non-destructive examinations of the Doel 3 reactor pressure vessel (RPV) conducted in June and July 2012 identified several thousand defect indications occurring in clusters in parent forging materials. Following the observations at Doel 3, similar inspections were undertaken at Tihange 2, whose RPV is of identical design and construction. These inspections also revealed a high number of defect indications, albeit fewer than found at Doel 3. 2 The RPV is a key component of the reactor unit safety case for Doel 3/Tihange 2. As a result, the Licensee, Electrabel, decided to keep both units shut down, at least until indepth analyses had been completed and submitted to the Belgian Federal Agency for Nuclear Control (FANC) in preparation of a possible restart of operation. 3 In addition to work undertaken by Electrabel, FANC commissioned several national and international expert groups to provide scientific and technical advice. 1.2 UK Response 4 The UK has one PWR reactor at Sizewell B (SZB), operated by EDF Energy Nuclear Generation Limited (EDF NGL), and NNB Generation Company Limited (NNB GenCo) has a nuclear site licence for construction of a twin unit PWR of the UK EPR TM design at Hinkley Point C (HPC). The manufacture of long lead items for HPC, including the RPV, has begun prior to the start of on-site construction. 5 ONR engaged with FANC through participation in technical working groups comprising representatives from regulatory authorities worldwide. EDF NGL and NNB GenCo have engaged with Electrabel and industry forums to consider the safety implications. 1.3 Purpose of Report 6 This report presents a summary of ONR s interactions with the FANC expert working groups and the findings of ONR s assessment of the responses of EDF NGL and NNB GenCo to defect indications in the RPV s at Doel 3 and Tihange 2. The EDF NGL response is presented in Engineering Change Proposal EC and supporting documentation (Ref. 1). The NNB GenCo response is presented in letters to ONR (Refs 2 and 3). 1.4 Scope 7 The scope of this report covers the insights gained from participation in FANC expert working groups obtained during the period August 2012 to February 2013 and interactions on this topic with EDF NGL and NNB GenCo over the same period as documented in CNRP Intervention Project Record NGL-067 (Ref. 7). 1.5 Methodology 8 The methodology for the assessment follows ONR HOW2 document PI/FWD, Purpose and Scope of Permissioning (Ref. 4) 9 This assessment has concentrated on reviewing the defect indications found at Doel 3 and Tihange 2 and assessing the potential safety implications for Sizewell B and Hinkley Point C. Attention has focussed on the underlying causes of defect formation during the manufacturing of RPV steel forgings, the likelihood of detection of defects during non- Page (1)

13 destructive examination and the criteria for reporting and sentencing any defect indications. 10 Information on Doel 3 and Tihange 2 was obtained largely through attendance at an information meeting organised by FANC for international regulators and participation in FANC regulatory expert working groups. ONR structural integrity assessors attended each of the three working groups: WG 1 - Non-Destructive Examination Techniques WG 2 Metallurgical Origin/Root Causes of the flaw indications WG 3 Structural Mechanics & Fracture Mechanics Approach for Justification File 11 Following the initial FANC information meeting in August 2012, ONR requested EDF NGL and NNB GenCo to consider the implications of the Doel 3 and Tihange 2 findings in the context of Sizewell B and Hinkley Point C respectively. EDF NGL and NNB GenCo presented their preliminary findings at a level 4 meeting with ONR in October 2012 (Ref 30). EDF NGL subsequently submitted the findings of their review of the safety justification for Sizewell B in a category 2 Engineering Change proposal EC (Ref.1). NNB GenCo has presented the results of their findings in two letters to ONR. (Refs 2 & 3). These documents form the basis of the Licensees safety justifications for ONR assessment. 12 Additional information on the manufacturing processes and manufacturing inspection of long lead item forgings for HPC were obtained through the ONR Hinkley Point C nuclear site licensing intervention (Ref. 8) and reported in ONR Structural Integrity Topic Report (Ref. 9). This workstream is continuing during the proposed HPC construction and commissioning phase (Ref 10). These interventions have involved regular level 4 meetings with NNB GenCo on the demonstration of structural integrity and the procurement of Nuclear Steam Supply System (NSSS) forgings. As part of these interventions ONR assessors have sampled NNB GenCo s proposed procedures for manufacturing inspections. Also as part of these interventions, inspection visits have been made by ONR to manufacturers of NSSS forgings for HPC to review the quality systems and adherence to procedural controls. Particular attention was given to the procedural controls to limit the occurrence of hydrogen induced defects during manufacture of NSSS forgings. This follows the detection of hydrogen cracking during the preliminary stages of manufacture in two forgings being produced at the Creusot Forge facility in France during 2012 and intended for another utility. The mechanism of defect formation in these two recent French forgings is thought to be the same as found in Doel 3 and Tihange 2. The assessment of NNB GenCo s response to Doel 3 and Tihange 2 OPEX has therefore been substantively supported and informed by the ONR HPC NSSS interventions both before and after granting of the HPC nuclear site licence. Page (2)

14 2 ASSESSMENT STRATEGY 13 The intended assessment strategy is set out in this section. This identifies the limits to the scope of the assessment and the standards and criteria that were applied. 14 For SZB, we asked the Licensee to review the manufacturing and inspection records for the RPV and justify continued operation. 15 For HPC, we asked the Licensee to assess the implications of the indications found at Doel 3 and Tihange 2 on the adequacy of the manufacturing controls for High Integrity Components (HICs). 2.1 Standards and Criteria 16 The relevant standards and criteria adopted within this assessment are principally the Safety Assessment Principles (SAPs), Ref. 5, internal ONR Technical Assessment Guides (TAGs), Ref. 6, relevant national and international standards and relevant good practice informed from existing practices adopted on UK nuclear licensed sites. The key SAPs and relevant TAGs are detailed within this section. Where appropriate, national and international standards and guidance are referenced and relevant good practice cited within the assessment report. 2.2 Safety Assessment Principles 17 The key SAPs applied within the assessment are included within Table 1 of this report. High integrity components such as the RPV and steam generators of a PWR require the highest level of assurance of structural integrity and therefore EMC 1-3 are relevant SAPs as well as EMC 4, 5, 8, 12, 17, 18, 19 and Technical Assessment Guides 18 The following Technical Assessment Guides have been used as part of this assessment (Ref. 6): Integrity of metal components and structures. T/AST/016 Issue 3. August Procurement of nuclear safety related items or services. T/AST/077 Issue 1. HSE August National and International Standards and Guidance 19 Sizewell B was designed and constructed and is operated using the ASME Boiler and Pressure Vessel Code augmented and adapted for use in the UK. Hinkley Point C is being designed and manufactured in accordance with the French RCC-M Code augmented and adapted for use in the UK. Both these codes are examples of internationally recognised codes applicable to the design and manufacture of PWRs which form the basis for good practice. 20 However the inspection procedures and quality arrangements implemented at SZB and being applied during manufacture of forgings for HPC impose additional controls and criteria which are specific to each site. Some of these additional requirements are derived from supporting standards (e.g. Ref 28) which have been used during our assessment where appropriate. 2.3 Use of Technical Support Contractors 21 No technical support contractors were used in this assessment. Page (3)

15 2.4 Integration with other Assessment Topics 22 As previously noted in Section 1.3 this assessment is closely interfaced with the HPC NSSS interventions both before and after granting of the HPC nuclear site licence. Any recommendations arising from this assessment relating to HPC will be pursued via the HPC NSSS post licensing intervention. 2.5 Out-of-scope Items 23 This assessment has focussed on the likelihood of occurrence of the formation of defect indications with characteristics similar to those of the type seen at Doel 3 and Tihange 2 in parent forging materials, and the ability of non-destructive examination techniques to detect such defects. Wider aspects, such as the procedures for the demonstration of the structural significance of these defects, have not been assessed in detail. Should the presence of defects similar to those seen at Doel 3 and Tihange 2 be considered to be conceivable for high integrity components at Sizewell B or Hinkley Point C, then further assessment of the appropriate defect assessment methodology would be required. Only those parts of the SZB and HPC RPV safety cases relevant to the OPEX have been considered. Page (4)

16 3 LICENSEE S SAFETY CASE 3.1 EDF NGL Sizewell B 24 As stated above, the only operating civil nuclear power reactor in the UK with a steel pressure vessel is Sizewell B (SZB). Implications for the civil UK fleet are, therefore, limited to SZB only. EDF NGL has provided a safety case update or Engineering Change (EC) (Ref. 1) to justify the operation of SZB, without additional inspection, up until the next statutory ten-year outage, in Spring The EC has been produced in a Claims- Arguments-Evidence format. The principal reference for the EC is Engineering Advice Note E/EAN/BBIB/0105/SZB/12 (Ref.19). This provides supporting evidence to the EC, for example tables of chemical composition data, and provides some of the evidence for claims made in the EC Metallurgy 25 The case is based upon a comparison between the SZB forgings and those produced at Rotterdam Dry dock (RDM) for Doel 3 and Tihange 2. Evidence is presented in Reference 1 of the production route for the SZB and this is contrasted with the production routes for the Belgian forgings. 26 Claim 1 of the EC states that The Sizewell B RPV forgings have been manufactured to enhanced material specifications and standards and the incidence of hydrogen flaking is unlikely. Arguments and evidence provided against this claim are outlined below. 27 Argument 1.1 states that Enhanced Quality Standards (Chemical Composition, inspection and testing) were applied during Sizewell B ingot manufacture to ensure that the material specification remained within acceptance limits. Evidence notes that the forgings were produced to an appropriate materials standard: SA 508 Class 3. Evidence 1.1.2, states that there is a distinction between the carbon levels in the Doel 3 and SZB forgings. SZB has, through specific amendments to the specification, significantly lower carbon levels than Doel 3. This leaves it less hardenable and, thereby, less susceptible to most hydrogen cracking mechanisms. Evidence gives information on the pre- and post-weld heat treatments. This gives confidence in the manufacturing programme more generally. 28 Argument 1.2 states that the appropriate steelmaking processes were used in the casting of the ingots to improve desulphurisation and to minimise the hydrogen levels in the Sizewell B forgings. Evidence gives detail on the difference in dehydrogenisation treatments during the steelmaking process between the producers of the SZB and Doel 3 RPVs. 29 Evidence relates to the measured hydrogen values of the SZB forgings and how these compare with the Doel 3 forgings. The levels of hydrogen reported for SZB forgings during product analysis are, generally, below the 0.8ppm level often quoted in the literature as being a level above which hydrogen flaking might occur. For forgings exhibiting defects at Doel 3 and Tihange 2, hydrogen levels were significantly above the 0.8ppm threshold Non-Destructive Testing 30 EDF NGL state that the case for the absence of significant defects in the RPV at SZB is supported by evidence that the rigour and quality of the inspections, including the arrangements for reporting of defects, were more robust for SZB than for the Belgian plants RPVs and that low levels of defects were found. Details of the reporting and rejection levels for both SZB and the Belgian plants are compared in Reference 19 and Page (5)

17 summarised in Annex 1. Details of the levels of qualification for the SZB inspections, both in-service and pre-service are discussed and details of indications recorded are also given. 31 Claim 2 states that The inspection programs for the Sizewell B RPV have the capability of detecting indications similar to those found on the Doel 3 RPV and have found no significant or reportable indications. This claim is supported by five arguments listed below. Evidence supporting these arguments is summarised below, but given in detail in Reference Argument 2.1 states that An independent validation process has been deployed at each stage in the Sizewell B RPV, from IPI (in-process inspection) through to ISI to qualify the procedure, equipment and operators. Three items of evidence are given in Reference 1 outlining what techniques and organisations were involved in demonstrating that the inspections were suitably validated. 33 Argument 2.2 states that Diverse and redundant inspection techniques have been applied during the manufacture of the Sizewell B forgings and provided the capability of detecting flaws similar to those found on Doel 3. Four items of evidence are provided against this claim. These give an outline of the different types of inspection techniques used and the capability of these inspections to detect flaws of the type seen at Doel 3 and Tihange Argument 2.3 states that The Sizewell B PSI examinations provided the capability of detecting flaws in the RPV forgings, similar to those found at Doel 3. One item of evidence here presents the Pre-Service Inspections (PSI) performed on the forgings. This discusses the capability of these inspections to detect flaws of the type seen at Doel 3 and Tihange Argument 2.4 states that The extent of Sizewell B in-service volumetric examination coverage provides the capability of detecting both weld defects and defects in the adjacent vessel shell. Three items of evidence are presented against this argument. These give an outline of the inspections performed at outage and how they might find defects of the type seen at Doel 3 and Tihange Argument 2.5 states that The Sizewell B IPI, PSI and ISI programs have identified no significant or reportable defects. Three items of evidence are presented in Reference 1 against this claim. These provide an overview of the records from In-Process Inspection (IPI), Pre-Service Inspection (PSI) and In-Service Inspection (ISI) respectively ALARP Justification 37 Ref. 1 also argues that it is not ALARP to perform additional inspections on the Sizewell B RPV in the very near future. Claim 3 states that It is ALARP to continue operation of the Sizewell B RPV and maintain the planned schedule of RPV inspections. This claim is supported by the three arguments listed below. 38 Argument 3.1 states that It is not ALARP to inspect the Sizewell B RPV immediately. Evidence given by NGL supporting this argument is based around there being no evidence of defects and no reason to expect they might be there, the existence of a station safety report containing a fracture analysis for the RPV and the disbenefits of instituting a forced outage. 39 Argument 3.2 states that it is not ALARP to inspect the Sizewell B RPV during the next two refuelling outages (RO12 or RO13). The first of these outages is due to take place in May Evidence given by NGL supporting this argument is based on the Page (6)

18 understanding that defect extension, should any defects exist, is unlikely, and the disbenefits of instituting a forced outage. NGL make particular mention of the increased radiation dose and increased number of complex lifts that would be incurred by bringing forward these inspections. 40 Argument 3.3 states that the plan for future in-service inspections of the Sizewell B RPV incorporates inspections of the main welds in the RPV. EDF NGL has also undertaken to review the RPV weld inspection programme for the major refuelling outage (RO14) in Spring 2016 to assess the benefits and disbenefits of extending the inspection extent beyond the immediate area of the identified welds. 3.2 NNB GenCo Hinkley Point C 41 NNB GenCo provided an initial response in Ref. 2 based on early information noting that studies relating to Doel 3 and Tihange 2 were ongoing. This response is summarised in the subsequent paragraphs. 42 NNB GenCo intends to ensure that the RPV for HPC is manufactured and inspected to standards in excess of the basic design code to provide the highest confidence that it will be free from defects of structural concern. 43 NNB GenCo notes that HPC RPV forgings are being procured from two highly experienced steelmakers/forgemasters: Japan Steel Works (JSW) and Arcelor-Mittal /Creusot Forge in France. 44 NNB GenCo reports that the ladle degassing and stream degassing casting production process at JSW consistently produces ingot hydrogen contents <0.5 ppm, which is below the threshold value for hydrogen cracking of 0.8 ppm determined by AREVA in the 1980s. As far as NNB GenCo are aware there is no known reported incidence of hydrogen cracking in a JSW forging for nuclear or turbine applications in the last 25 years. 45 Arcelor-Mittal employ ladle degassing for solid ingots and allow air-pouring for hollowingots. For the HPC RPV all solid ingot forgings produced by Arcelor-Mittal will be vacuum degassed. It is probable that some solid ingots and all hollow ingots will have hydrogen contents in excess of the 0.8 ppm threshold immediately following casting. Ingots for Hinkley Point C are required to have hydrogen contents of <1.5 ppm. Following hot forging, components are subject to an extended heat treatment to allow the hydrogen to diffuse out of the steel. This process is calculated to reduce the hydrogen content to less than the 0.8 ppm threshold value for hydrogen cracking. NNB GenCo note that there appears to be some uncertainty surrounding the level of hydrogen in the Doel 3/Tihange 2 ingots on casting and whether or not there was any degassing heat treatment employed. 46 NNB GenCo states that the HPC forgings are subject to full ultrasonic inspection to the highest standard and that this is carried out at the rough machined stage and followed by a further inspection at the final machined stage. NNB GenCo notes that these inspections will be reviewed and reinforced by the production of inspection capability statements. NNB GenCo is confident that if they were to form then any hydrogen induced defects similar to those found at Doel 3 and Tihange 2 would be detected at an early stage in the production process. 47 In addition to inspection of the forgings, NNB GenCo states that the construction welds will be inspected using qualified ultrasonic inspection techniques, and a further inspection of selected areas will be carried out following cladding of the inside surfaces of the RPV. NNB GenCo states they have full confidence that any defects of significance due to hydrogen induced cracking or any other mechanism would be detected. Page (7)

19 48 On the basis of this accumulated evidence NNB GenCo believes that hydrogen induced cracking will not occur in the UK EPR and in the extremely unlikely event of a system malfunction during manufacture then any defects would be detected and appropriate actions would be taken. 49 Following the opportunity to secure more details of the ongoing studies relating to Doel 3 and Tihange 2, NNB GenCo wrote to ONR on 5 November 2012 (Ref. 3). In this letter NNB GenCo outlines a number of factors where comparisons between HPC and Doel 3/Tihange 2 indicate that HPC forgings will be less susceptible to hydrogen induced cracking. These include: Hydrogen contents in the original steel ingot (being less in HPC ingots) Partial working of Doel 3/Tihange 2 ingots prior to delivery to the forgemasters Removal of more ingot segregation in the central regions of solid ingots for HPC forgings The possible absence of an intermediate degassing heat treatment for Doel 3/Tihange 2 forgings The higher carbon content for Doel 3/Tihange 2 forgings Allowing Doel 3/Tihange 2 forgings to reach a low temperature prior to forging and the quality heat treatment The suggestion that Doel 3/Tihange 2 forgings were subject to inadequate deoxidation of the steel during steelmaking 50 In addition, NNB GenCo notes that there were inconsistencies in the application of NDT for Doel 3/Tihange 2 which suggests that the defects should have been detected and reported at the time of manufacture. However it is probable that under the inspection regime applied at the time, the defects would have been judged to be code compliant. 51 NNB GenCo concludes that from their comparison of Doel 3/Tihange 2 with HPC forgings there is a reduced risk of hydrogen induced cracking for HPC and combined with a more reliable non-destructive technique, then this should eliminate the possibility of forgings with hydrogen induced cracking entering service. 52 NNB GenCo note however that recent experiences at Creusot Forge with forgings destined for another utility have shown that even with modern practice, hydrogen induced cracking is possible when prevention strategies are eroded, for example by poor control of hydrogen or inadequate heat treatment. However their experience also showed that with well controlled non-destructive testing examinations then hydrogen induced cracking defects are readily detected, recognised and appropriate actions taken. 53 NNB GenCo considers that the existing ultrasonic inspections at forging suppliers are very thorough, but nevertheless NNB GenCo has required these procedures to be augmented with additional scans and has subjected the inspection techniques to detailed critical review. (ONR note: These enhancements were undertaken by NNB GenCo during their normal review of documents in advance of the operating experience from Doel 3 and Tihange 2 or the occurrence of hydrogen defects in forgings at Creusot Forge.) 54 NNB GenCo notes that forgings are well specified to minimise risk and that second and third party inspections should detect any shortfall in the control of production process. 55 NNB GenCo concludes Consequently, we are confident that the measures we have in place which will minimise the risk of hydrogen induced cracking in our forgings and in the Page (8)

20 very unlikely event of a system malfunction which is not detected, the NDT inspections give us further confidence that any defects will be reported and assessed appropriately. Page (9)

21 4 ONR ASSESSMENT 4.1 Scope of Assessment Undertaken Interactions with the Belgian Federal Agency for Nuclear Control 56 Interactions with the Belgian regulator, FANC, were achieved through attendance at an initial information exchange meeting and then through participation in three regulatory technical working groups. Background and progress papers were made available for reference by members of the working groups on a confidential basis. The preliminary findings of the FANC investigations have been reported in summary and in more detail through announcements and publications on the FANC website (Ref 13, 14) EDF NGL Sizewell B 57 SZB is the only civil nuclear reactor currently in operation in the UK with a steel reactor pressure vessel, so ONR s assessment is limited to this plant only and not the operating AGR or Magnox fleet. EDF NGL, the licensee for SZB, has produced a safety case presenting their justification for continued operation of the plant up to the statutory 10- yearly outage planned for Spring The safety case provided a review of the manufacturing records at SZB, how these compared with Doel 3 and Tihange 2, and how these are linked to their plans for future operation. 58 The scope of this assessment was limited to review of the safety case provided by EDF NGL (Ref. 1) and its supporting documentation. The objective of this assessment is to review the claims made in the safety case and judge whether the evidence available is sufficient to support these claims NNB GenCo Hinkley Point C 59 ONR has been assessing the NNB GenCo arrangements for control of the procurement and manufacture of long lead item forgings for HPC both before and after the granting of a nuclear site licence for HPC (Refs 8 and 9). These interventions have included a review of the controls applied to prevent hydrogen cracking and to detect it at an early stage should it occur. These assessments have been extended to take account of the Doel 3 and Tihange 2 experience. 60 As part of these interventions, inspection visits have been made by ONR to manufacturers of NSSS forgings for HPC to review the quality systems and adherence to procedural controls. Particular attention was given to the procedural controls to limit the occurrence of hydrogen induced defects during the manufacture of NSSS forgings. This follows the detection of hydrogen cracking in two forgings being produced at the Creusot Forge/Arcelor Mittal facility in France during 2012 and intended for another utility. No hydrogen-induced defects have been found in HPC forgings. 61 NNB GenCo provided copies of the ultrasonic inspection procedures applicable to certain forgings being procured from the two suppliers, and ONR compared the capabilities of these procedures with those applied at Doel 3 and Tihange 2, at Sizewell B and with relevant good practice. This comparison has concentrated on detection of hydrogen flake defects but also considered the wider range of defects addressed in the capability assessments provided by NNB GenCo. Page (10)

22 4.2 Assessment Participation in FANC Expert Working Groups and key insights 62 One of the authors attended a technical meeting in August 2012 organised by FANC to report findings from non-destructive inspections on the Doel 3 reactor pressure vessel in June and July The aim of this meeting was to appraise international regulators of the non-destructive inspections at Doel 3. Attendance for this meeting was primarily intended for those regulators with PWRs with reactor pressure vessels manufactured by Rotterdam Dockyard (RDM), the fabricator used for the Doel 3 reactor pressure vessel. Although the only UK PWR was not manufactured by RDM, ONR attended the meeting to keep abreast of relevant international OPEX and to ensure that ONR was well informed of any generic nuclear safety implications. 63 At this meeting FANC, together with Belgian technical support organisations, provided an overview of inspections performed at Doel 3, a preliminary review of the initial manufacturing records for Doel 3 and some initial interpretation. This information is summarised below: 64 In June 2012 Doel 3 was shut down with the fuel removed and undergoing its third ten yearly inspection of RPV welds in accordance with the requirements of Section XI of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code. In addition to the routine inspections a newly qualified ultrasonic inspection technique was adopted to look for potential defects underneath the stainless steel cladding applied to the internal surface of RPV forgings. This inspection technique is optimised for the innermost region of the RPV forging up to 30 mm depth from the cladding interface. This was the first occasion that extensive regions of the RPV shell forging had been examined since the original manufacture. No under-clad defects were detected but the inspection identified numerous other indications. 65 As a result of the initial inspection in June, further ultrasonic inspections were performed in July These inspections covered a high proportion of the RPV surface area including two shell forgings, the transition ring and nozzle ring forging. The inspection sampled material through the full wall thickness of the pressure vessel. These inspections identified several thousand defect indications, concentrated in the two shell forgings, in a region 30 mm to 100 mm from the inner surface and remote from circumferential welds. The size of the defect indications was estimated as typically 4 mm to 14 mm in diameter with a maximum size of 30 mm and lamellar in orientation (i.e. essentially parallel to the forging inner surface). Although not specifically qualified for defects of this type, the lamellar orientation was confirmed for a limited number of indications and all indications were considered likely to be associated with lamellar type defects. 66 Such a large number of defect indications together with the lamellar orientation prompted attention to the initial manufacture as being the potential origin. A preliminary review of manufacturing records and in-service inspection records was reported to attendees together with a preliminary interpretation. 67 The forgings for the Doel 3 RPV were supplied from Krupp steelmaker and then transported to RDM for forging. Initial welding and cladding was completed in Belgium by Cockerill before transportation and completion of RPV fabrication in France by Framatome. Based on a number of factors, and wider experience with hydrogen cracking during manufacture of PWR reactor components, AREVA provided a preliminary interpretation that the defect indications observed in the 2012 inspections were consistent with the root-cause being hydrogen-induced cracking (also known as hydrogen flaking) Page (11)

23 during the initial manufacture. Factors for the Doel 3 RPV known to promote susceptibility to hydrogen flaking include: hydrogen levels of ppm recorded in steel ingots, no confirmation of a dehydrogenation heat treatment to reduce the hydrogen content in forged components to less than 0.8 ppm, removal of reduced amount of the central ingot core material known to contain material richer in solidification segregation of alloying elements and impurities carbon levels in the steel (0.23 wt%) towards the upper end of the specification (0.24 wt%). 68 A review of the manufacturing inspection records also revealed that a large region of defect indications had initially been reported for one of the RPV ring forgings (the upper shell forging) but subsequent inspections did not report these defects. In any event, the forging was accepted as being compliant with the contract acceptance criteria which were based on the ASME III Code. The reporting and acceptance criteria are discussed more fully in Section 4.3 and Annex 1 below, but whereas we consider the reporting criteria to have been adequate, we judge that the acceptance criteria were not stringent when compared with relevant good practice. 69 The reasons why the defects were not apparently reported reliably during manufacturing inspections remain unresolved but the discrepancies between repeat inspections of the upper shell forging provide circumstantial evidence of non-compliance with the reporting requirements. 70 A review of the history of in-service examinations for the Doel 3 RPV welds confirmed that there had been a total of seven in-service inspections. These inspections are focussed on the structural welds and the immediately adjacent forging material and had not reported any lamellar type defect indications. However it was noted that the regions of material examined by the routine ASME XI inspections do not cover the forging material exhibiting a high density of lamellar defects in the 2012 inspections. 71 The number, size, orientation, location of the defect indications, and the manufacturing history were concluded by AREVA to be indicative and consistent with hydrogen flaking during manufacture being the root cause mechanism for defect formation. 72 Other regulators present at the FANC August meeting provided preliminary feedback on experience in their countries. There were no reported similar instances of large numbers of lamellar defects in parent forgings although it was recognised that there had not been similar inspections of forgings at other plants other than those performed during the initial manufacture. 73 FANC presented an outline of the future work programme and regulatory response inviting technical experts from international regulators to join one or more of three regulatory working groups addressing: Non-destructive inspection Metallurgy/Root cause investigations Fracture mechanics assessments/ routes for re-justification of operation. 74 It is noteworthy that lamellar oriented defects are not generally the focus of assessment methods for defects found during service. Through-wall defects are generally considered most damaging as these present the risk of a breach to the pressure boundary. The high Page (12)

24 number and lamellar orientation of defect indications found at Doel 3 present some novel features to be addressed when applying conventional defect assessment methodologies. 75 From the information gained at this initial FANC meeting in August the ONR Inspector present judged there to be insufficient evidence to limit the potential for occurrence of defects to RPVs manufactured at RDM. The proposed mechanism of formation (i.e. hydrogen flaking) is not limited to a single manufacturing route. It is generally accepted that without sufficient controls in place hydrogen flaking may occur in pressure vessel steels. 76 The ONR Inspector therefore judged it prudent to convey the results of this initial information exchange meeting to EDF NGL and NNB GenCo and to request that they assess the implications of the Doel 3 findings with respect to the UK. (Ref. 12). 77 Following the initial FANC information exchange meeting an intervention project record was written outlining ONR s intervention activities and participation in each of the FANC regulatory expert working groups (Ref 7). Working Group meetings, attended by ONR Structural Integrity inspectors took place on 16 October (Ref. 16) and 8/9 January 2013 (Ref 17). In addition FANC set up an International Expert Review Board and a National Scientific Expert Group to provide independent advice. 78 Between June 2012 and December 2012 the Licensee for Doel 3 and Tihange 2, Electrabel, completed a number of studies to investigate the root-cause of the defect indications, to assess the interpretation of the inspection results, to provide data on material mechanical and fracture properties and to assess the structural significance of the defect indications on the integrity of the RPVs. 79 Confirming the earlier conclusion, there was general consensus expressed at the working group meetings that the reported defect indications most likely arose from hydrogen induced flaking during the original manufacture. 80 Discussion within the technical groups focussed on the suitability and uncertainties inherent in the non-destructive testing procedures, the material properties and the structural integrity assessment methodology adopted by the Licensee in presenting their justification for the proposed restart of Doel 3 and Tihange There was substantive, but not complete, agreement reached within individual working groups. In the time available it was agreed that the Licensee had completed a large amount of work to high standard. The key topics of outstanding debate include the necessity for qualification of an inspection technique prior to return to service, the appropriate allowance for an increase in the fracture toughness transition temperature for material local to a hydrogen flake defect, and desirability of a hydrotest with the aim of providing additional confidence in RPV integrity. 82 No final recommendation was made by the working groups on the adequacy of the safety justification for continued operation of Doel 3 or Tihange 2. This assessment will be made by FANC based on the totality of information received by FANC. Subsequently FANC have summarised their findings (Ref. 14): The Federal Agency for Nuclear Control received on 5 December 2012 two Safety Case reports on the flaw indications found in the Doel 3 and Tihange 2 reactor pressure vessels. Based on the data provided these Safety Case Reports and the conclusions released by Bel V, AIB-Vinçotte and the national and international experts groups, the Federal Agency for Nuclear Control has drawn its conclusions which can be Page (13)