ONR Review of NGL DNB PLEX Technical Overview Report · Generation Ltd (NGL) Plant Life Extension (PLEX) Technical Overview Report and its supporting documentation for Dungeness B

Title of document

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PROJECT ASSESSMENT REPORT

Unique Document ID and Revision No:

ONR-CNRP-PAR-14-008 Revision 0

TRIM Ref: 2014/321878

Project: EDF Energy Nuclear Generation Ltd (NGL) Dungeness B (DNB) Plant Life Extension (PLEX)

Site: DNB

Title: ONR Review of NGL DNB PLEX Technical Overview Report

Licence Instrument No: (if applicable) N/A

Nuclear Site Licence No: 61

Licence Condition: LC15

Document Acceptance and Approval for Issue / Publication

Role Name Position Signature Date

Author Principal Inspector

14 August 2014

Reviewer Principal Inspector

14 August 2014

Accepted by1 Superintending Inspector

14 August 2014

Approval for publication2

Superintending Inspector

14 August 2014

Revision History

Revision Date Author(s) Reviewed By Accepted By Description of

Change

A 3 August 2014 n/a 1st draft for review

B 13 August 2014 n/a 2nd draft incorporating comments

0 14 August 2014 First accepted issue

1 - Acceptance of the PAR 2 - Although this PAR does not relate to a permissioning decision, it will be published on the ONR web site due to public interest in NGL’s PLEX decision relating to DNB.

Template Ref: ONR-DOC-TEMP-005 Revision 8 Page 1 of 25

Report ONR-CNRP-PAR-14-008 TRIM Ref: 2014/276957

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Circulation (latest issue)

Organisation Name Date

Office for Nuclear Regulation

Files: Programme Report File 4.4.1.2346 Relevant Subject Files 4.6.1380

Environment Agency Nuclear Regulator

Licensee Dungeness B, NGL

Office for Nuclear Regulation Page 2 of 25


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EDF Energy Nuclear Generation Ltd (NGL) Dungeness B (DNB) Plant Life Extension (PLEX)

ONR Review of NGL DNB PLEX Technical Overview Report

Project Assessment Report ONR-CNRP-PAR-14-008 Revision 0

14 August 2014



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© Office for Nuclear Regulation, 2014 If you wish to reuse this information visit www.onr.org.uk/copyright for details. For published documents, the electronic copy on the ONR website remains the most current publicly available version and copying or printing renders this document uncontrolled.


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EXECUTIVE SUMMARY This Project Assessment Report (PAR) details ONR’s assessment of the EDF Energy Nuclear Generation Ltd (NGL) Plant Life Extension (PLEX) Technical Overview Report and its supporting documentation for Dungeness B power station (DNB). This is an early engagement exercise which serves three purposes:

1. to provide an early pre-Periodic Safety Review (PSR) review of potential issues with two key irreplaceable reactor components (graphite core and boiler tubes).

2. to provide a brief review of the arrangements intended for through life management of components considered to be largely replaceable (fuel route).

3. to allow the process for PSR3 [the third suite of PSR submissions in the Advanced Gas-cooled Reactor (AGR) lifecycle] to give early indication on any potential issues prior to NGL’s formal PSR3 submission.

No statement in this document should be interpreted either as a commitment for ONR to accept a safety case that has yet to be submitted, or as an implication that NGL’s Lifetime Technical Review (LTR) restricts the necessary scope of any future safety case or PSR3 submission. Permission Requested This early engagement activity falls outside the requirements for formal permissioning arrangements however; it is directly associated with NGL’s next submission of the DNB PSR3 in January 2017 and; therefore, with nuclear site licence condition 15 Periodic Review. ONR was approached as part of NGL’s early PSR3 engagement to provide views from a regulatory perspective. This PAR and the associated letter to NGL records ONR’s position. Background NGL is managing its UK fleet of AGR reactors through their final years of operation to their end of electrical generation and eventual entry into decommissioning. As part of this, NGL is undertaking studies aimed at optimising the remaining lifetime and generating capacity as part of its lifetime management project. The submission date for DNB PSR3 falls after the intended formal announcement by NGL of the lifetime management study results. This carries the risk for NGL that the future PSR3 submission might reveal a significant issue that could then negate its lifetime management project aspirations. To address this, NGL has undertaken a series of reviews which have provided input into its PLEX Technical Overview Report. ONR was asked, as part of NGL’s early PSR3 engagement, to provide its regulatory perspective. Owing to the importance of the future DNB PSR3 submission, ONR agreed to undertake a limited review of the PLEX Technical Overview Report and its supporting documentation. ONR was able to draw upon the assessment work it had carried out in connection with the DNB Reactor 21 periodic shutdown to support ONR’s review of NGL’s intention for potential plant extension at DNB Assessment and inspection work carried out by ONR in consideration of this request ONR’s assessment was primarily focused on NGL’s PLEX documentation covering the fuel route, as part this assessment an inspection of the fuel route was also undertaken at site.



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Furthermore, ONR assessment work from the DNB Reactor 21 periodic shutdown 2014 covering boilers, graphite core, and the reactivity effects of boiler tube failure faults was also used to support ONR’s review of NGL’s PLEX proposal. Matters arising from ONR's work ONR’s review has established that there are no immediate nuclear safety issues associated with fuel route, the boilers or the graphite core that would preclude adopting NGL’s lifetime management project proposals for DNB. However, some uncertainty regarding the ageing of the graphite and the boilers, which are all irreplaceable, is likely to require NGL to implement boiler modifications to support future claims that the associated risks remain As Low As Reasonably Practicable (ALARP) for operation beyond the 2017/2018 periodic shutdowns for the two reactors at DNB. ONR note that NGL regard degradation of other irreplaceable components (such as the reactor internals) as a lower threat to achieving a lifetime of 2028. Notwithstanding this, ONR will continue to expect appropriate examination, inspection, maintenance and testing to be conducted throughout life capable of identifying any degradation likely to compromise safe operation of the plant. ONR accept that currently NGL has adequate arrangements in place to monitor such degradation, however, these arrangements will need to be reviewed. For example, any future inherent uncertainties in the predicted condition may reveal issues which require the need for increasing levels of inspection later in life. Conclusions I conclude that:

a) ONR’s review of the submitted documentation, which was well presented, in conjunction with recent periodic shutdown assessment work has not revealed any significant flaws or omissions with respect to nuclear safety that would preclude adopting NGL’s PLEX proposals for DNB.

b) NGL’s review of all the potential degradation mechanisms as part of its PLEX review

has been judged to be well defined; also, mitigation arrangements are either in place or there are plans to develop such arrangements.

c) It is important that the limited nature of NGL’s submission is taken into account and

ONR’s comments are taken in context of the potential for other issues emerging from its formal assessment of the full scope LC15 PSR3 submission scheduled for January 2017.

d) It is important that those issues that have been raised from ONR’s recent assessment

of Reactor 21 return to service from its 2014 periodic shutdown relating to the boiler tube failure faults safety case are taken into account.

e) The nuclear site licence requires adequate safety cases to be maintained over any

extended operating life of the stations. It is not clear that the full extent of the uncertainties involved in the ageing predictions have been considered to assess accurately the likely costs involved with the maintenance of these safety cases and hence the continuing viability of operations.

f) NGL’s PLEX portfolio will require a substantial investment by NGL in both major safety

improvements such as boiler improvements; and a large number of smaller but still significant modifications to improve reliability and/or safety. It is important that safety



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improvements are made in a timely manner so that risks are maintained ALARP. ONR will need to ensure that those commitments that improve nuclear safety over the proposed PLEX are fully funded and resourced in order for them to be delivered at the appropriate time.

g) Regulation of nuclear power station operation is based on the submission of adequate

LC15 PSR submissions, allied to continuing satisfactory performance as judged against inspections of the site and assessment of the safety justification of the plant. This includes the ONR requirement to permission the start up of each of the two reactors at DNB after their three yearly periodic shutdowns. Taking this regulatory background into account, any decision regarding the accounting lifetime of the plant and the cessation of power generation remains a commercial decision solely for the licensee to determine.

Recommendations I recommend that the Superintending Inspector: a. Signs this Project Assessment Report to confirm acceptance of ONR technical and

regulatory arguments supporting issuing letter DNB71259N to NGL b. Signs this Project Assessment Report approving its release for publication, after

redaction where appropriate.



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LIST OF ABBREVIATIONS

AGR Advanced Gas-cooled Reactor

ALARP As Low As Reasonably Practicable

BMS (ONR) Business Management System

CNRP Civil Nuclear Reactor Programme

DNB Dungeness B Power Station

GWL Graphite Weight Loss

HOW2 (Office for Nuclear Regulation) Business Management System

HPB Hinkley Point B

HNB Hunterston B

IAEA The International Atomic Energy Agency

IFD Irradiated Fuel Dismantling Cell

KRC Keyway Root Cracking

LC Licence Condition

LI Licence Instrument

LMP Lifetime Management Project

LSR Lifetime Safety Review

LTR Lifetime Technical Review

MDI Maintain Design Integrity

NDA Nuclear Decommissioning Authority

NGL EDF Energy Nuclear Generation Limited

NSC Nuclear Safety Committee

ONR Office for Nuclear Regulation

PCPV Pre-Stressed Concrete Pressure Vessel

PAR Project Assessment Report

PLEX Plant life extension

PLEX TOR Plant Life Extension Technical Overview Report

PSR Periodic Safety Review

PSR3 Third round of PSR submissions in the AGR lifetime

R21 (Dungeness B) Reactor 21

RTS Return To Service

SHAP System Health Assessment Process

SHIP System Health Indicator Process

SME Subject Matter Expert

TLMS Through Life Management Strategy



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TABLE OF CONTENTS 1 PERMISSION REQUESTED............................................................................................. 10 2 INTRODUCTION ............................................................................................................... 10 3 ASSESSMENT STRATEGY.............................................................................................. 11 4 NGL ASSESSMENT OF THE DNB PLEX......................................................................... 12 5 ONR ASSESSMENT ......................................................................................................... 16 6 CONCLUSIONS ................................................................................................................ 22 7 RECOMMENDATIONS ..................................................................................................... 23 8 REFERENCES .................................................................................................................. 24



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1 PERMISSION REQUESTED

1. This early engagement activity falls outside the scope of ONR’s formal permissioning, however it is associated with the submission of the Dungeness B power station (DNB) Periodic Safety Review 3 (PSR 3) which is due in early 2017 and therefore it is associated with Licence Condition 15 (LC15) “periodic review”.

2. The Office for Nuclear Regulation (ONR) was approached as part of EDF Energy Nuclear

Generation Ltd’s (NGL’s) early PSR3 engagement to provide a regulatory perspective. This PAR and the associated letter to NGL records ONR’s position in response to responds to this request.

2 INTRODUCTION

3. As part of the Lifetime Management Project (LMP) the licensee, NGL, is considering a life extension of 10 years for DNB (currently declared as 31st March 2018). This Project Assessment Report (PAR) details ONR’s sampled review of the NGL Plant Life Extension (PLEX) Technical Overview Report [1] and its supporting documentation.

4. In order for the NGL Board to make a decision to extend the Current Accounting Life of

stations, NGL prepares a suite of life extension assessments to support its decision. These assessments aim to identify the potential risks to safety, technical feasibility, financial viability and political/stakeholder acceptance in order to fully inform the decision. These assessments include consideration of the significance of the technical issues that could prevent safe, commercial operation throughout the PLEX period, as well as considering the efficiency of the processes that manage nuclear safety.

5. For DNB, as the PSR3 submission (January 2017), is due after the intended date for the

life extension decision, NGL considered it prudent to determine if there were any obvious significant nuclear safety issues that would have affected the life extension decision had that information been available beforehand.

6. NGL has therefore decided to undertake a Lifetime Technical Review (LTR) [2] and

Lifetime Safety Review (LSR) [1]. The LSR follows the same structure as PSR3, which is based on International Atomic Energy Agency (IAEA) guidance [3], and is similarly based on a consideration of the efficacy of NGL’s processes that manage nuclear safety. PSR3 will, later, provide a detailed and systematic assessment of the adequacy of the safety cases for the stations against modern standards and a review of the continuing suitability of the plant against likely ageing effects that may render the plant unsafe to operate.

7. NGL has requested that ONR reviews its PLEX Technical Overview Report in conjunction with supporting documentation to help it ensure the appropriateness of its assessments. An early response from ONR would give the best opportunity to influence the quality of NGL’s technical submission to NGL group in mid September 2014.

8. A key point to note is that NGL is not asking ONR to agree the conclusions of the PLEX

Technical Overview Report, but is asking ONR for its review to focus on the appropriateness of the approach taken by NGL.

2.1 Background

9. NGL is managing its UK fleet of AGR reactors through their final years of operation to through to their end of electrical generation and eventual entry into decommissioning. NGL is undertaking studies aimed at optimising the remaining lifetime and generating capacity as part of its lifetime management project.



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10. A timing mismatch exists between the PLEX period under consideration and the commencement of DNB’s PSR3 which requires the ONR to form a decision on the adequacy of NGL’s submitted PSR3 by January 2018. Furthermore an agreement with the Nuclear Decommissioning Authority (NDA) requires NGL to confirm its decommissioning preparations to commence three years prior to any declared closure date.

11. In recognition of the regulatory significance of the PSR3 cycle, ONR agreed to review the

DNB PLEX submission to determine if, on the basis of current knowledge and experience, the review of any potential issues regarding the boilers and graphite core [and separately the fuel route] provided reasonable evidence in support of NGL’s decision to proceed with its intended PLEX campaign.

12. ONR has previously undertaken a review of PLEX submissions [4] for Hinkley Point B

(HPB) /Hunterston B (HNB) in 2012. The submissions were based on IAEA guidance [5] which is NGL’s intended structure for all future PSR3 submissions. The HPB and HNB PLEX case had separate LTR and LSR reports presented to different stakeholders. Taking into account feedback and lessons learnt from that case, the reporting approach has been streamlined into a consolidated PLEX Technical Overview Report which references out to supporting documentation.

2.2 Methodology

13. Although, there is no requirement for ONR to permission formally this DNB PLEX submission, the methodology that has been adopted for this review broadly follows ONR BMS document PI/FWD, Purpose and Scope of Permissioning [6] in relation to assessment within the ONR.

14. ONR’s review has focussed primarily on the fuel route specifically in respect to the fuelling machine; buffer store and fuel plug units. In addition, ONR has been able to draw upon the recent specialist assessment work from the DNB Reactor 21 periodic shutdown 2014 with respect to the graphite core, boilers and the reactivity effects of boiler tube failure faults, as these are identified as life-limiting structures and faults within the PLEX Technical Overview Report.

15. ONR Nuclear Safety Permissioning Instruction [7] has been used in the preparation of this PAR, although there is no requirement for any Licence Instrument (LI) to be issued under this PAR. This PAR is instead used to support the content of an ONR letter [DNB71259N] to be issued to NGL, summarising the outcome of ONR’s review.

16. The samples reviewed by ONR were identified within NGL’s LTR [2]; I considered these areas to represent a proportionate and targeted area of assessment consistent with ONR’s enforcement policy statement.

3 ASSESSMENT STRATEGY

17. This is an early engagement exercise which serves the following purposes.

to provide an early pre-Periodic Safety Review (PSR) review of potential issues with two key irreplaceable reactor components (graphite core and boiler tubes).

to provide a brief review of the arrangements intended for through life management of components considered to be largely replaceable (fuel route).

to allow the process for PSR3 [the third suite of PSR submissions in the Advanced Gas-cooled Reactor (AGR) lifecycle] to give early indication on any potential issues prior to NGL’s formal PSR3 submission.


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12].

red the

ns that have been made previously since these are fully reported separately [9].

1. For clarity, the areas considered within this PAR are essentially made up of two parts:-

DNB Reactor 21 Periodic Shutdown 2014

[13, 14].

Fault studies assessment of reactivity effects of boiler tube failure faults [17].

X TOR Fuel Route .

store 23).

Irradiated Fuel Dismantling Cell (IFD).

a nuclear safety perspective, to sample this essential area of the licensee’s safety case.

detailed within the following references [19, 20] and are discussed further in section 5.2.

4 NGL ASSESSMENT OF THE DNB PLEX

of has

ifically this report summarises two key reviews supporting the DNB PLEX case :-

18. NGL has provided ONR with the following documentation:-

The DNB PLEX Technical Overview Report. The LTR. Lifetime Safety Review (LSR) documentation [8].

19. ONR has undertaken recent assessment work to permission the start up of DNB Reactor 21 (R21) following completion of the 2014 periodic shutdown [9] that I consider to be directly relevant to PLEX. I have therefore used this work. The individual specialist inspectors concerned have considered their assessment work and have agreed that their findings and conclusions are directly relevant to and consistent for DNB PLEX [10, 11,

20. This PAR has focussed on the major irreplaceable reactor components that are subject to

significant ageing effects and has considered the findings of ONR’s previous assessmentwork [9]. In the case of the previous assessment work this PAR has only considerelevant findings and conclusions of this previous work. It has not repeated the recommendatio

2

Structural integrity which has assessed boiler tubes Graphite integrity for graphite weight loss [15, 16].

ONR review of the PLE Fuelling machine Fuel plug units. Fuel decay store (buffer Neutron scatter plugs.

22. Although the fuel route, including the radioactive waste vault, is considered to be largely

replaceable by NGL, in its opinion it does not pose a significant threat to DNB being able to achieve its PLEX ambitions. Nevertheless, I have considered it prudent, from

23. Therefore specialist mechanical and fault studies inspectors undertook a site visit at DNB [18] to focus on key areas of the fuel route to gain a better understanding to support their views which are

24. The PLEX project considers three pertinent criteria that need to be met to extend the lifea power station: Feasibility, Financial Viability and Stakeholder Acceptability. ONRfocused on the feasibility element of the PLEX project and; spec



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4.1 Lifetime Technical Review (LTR)

uclear

he ies (TLMSs) and from

discussions with the relevant Subject Matter Experts (SMEs).

safety

eactor the fuel route and the radioactive waste vault

capacities have also been included.

ts

ns AMEC Nuclear agrees that confidence levels are good as proposed by the licensee.

rview Report has been cosidered by the NGL Nuclear Safety Committee (NSC) [21, 22].

4.2 Lifetime Safety Review (LSR)

a more detailed and systematic assessment of the adequacy of the same Safety Factors.

been ements. In total NGL has considered 16

individual Safety Factors (Figure 1 below).

25. AGR lifetimes are determined by the technical considerations of the irreplaceable nisland plant such as the graphite core, boilers, reactor internals, and Pre-Stressed Concrete Pressure Vessel (PCPV). The information for the LTR has been taken from trecently produced / updated Through Life Management Strateg

26. The LTR assesses the major technical issues that are considered life limiting and

case issues that necessitate being addressed for continued operation. The LTR concentrates on the four main areas of nuclear island plant (graphite core, boilers, rinternals, and the PCPV), in addition

27. Independent Peer Review of reference [2] by AMEC Nuclear has been carried out to

support the formal decision on PLEX. An update of the LTR [2] incorporates the commenand responses by NGL to this Independent Peer Review. It is noted that AMEC Nuclear does not disagree with the proposals within the LTR, however; further work is required byNGL to align with its life extension plan for DNB. Notwithstanding this, in relation to core life assumptio

28. In addition, the PLEX Technical Ove

29. The LSR determines whether there are any obvious process shortfalls, which might leave significant safety issues undetected or unaddressed, should a review be carried out after the life extension decision. The PSR3 would follow later and build upon this by providing

30. The LSR investigated the effectiveness of NGL’s management systems using its PSR3

review structure. This PSR3 structure is based on the fourteen Safety Factor approach as specified in the IAEA guidance [5], and is similarly based on a consideration of the efficacy of NGL’s processes that manage nuclear safety. Two additional Safety Factors haveadded to the IAEA list to meet NGL’s requir



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Figure 1 31. NGL’s LSR documents [8] cover each of the 16 Safety Factors. A review of each of these

Safety Factors has been conducted by NGL and documented according to a prescribed format in order to identify the appropriate management systems that support the management of nuclear safety risks which could arise or could significantly increase as a consequence of PLEX if they were not managed. Conclusions from the LSR provide confidence to NGL and it’s stakeholders that DNB has adequate processes to manage safety risks throughout the proposed PLEX period.

32. Safety Factors 1-7 are intended to demonstrate that NGL has arrangements in place to:-

Define the plant, its design requirements, and the adequacy of the plant design to meet the safety case.

Identify the expected and actual current condition of plant, including any qualification features, and assess its adequacy to meet design/safety case requirements.

Manage the ageing of plant (including qualification features), particularly to understand how the plant configuration and condition will change over time so that NGL proactively meets safety case requirements.

Adequately monitor potential changes in nuclear safety standards so that timely action can be taken.

The Technical Governance and Through Life Management Strategies assessed under LSR Safety Factor 4 provide monitoring and subsequent enactment of changes in technical standards.

The Hazards Governance arrangements assessed under LSR Safety Factor 7 provide adequate monitoring of standards.

The reviews of all other Safety Factors have considered that the associated standards are unlikely to change significantly with time i.e. are not significantly affected by a decision to extend plant lifetime.

33. Safety Factors 8-16 are intended to demonstrate that the company has arrangements in place to:



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Monitor the nuclear safety performance of the plant against the requirements of the safety case.

Learn from operating experience feedback (internal and external), and research about how the plant meets the safety case requirements.

Support high levels of nuclear safety through its management system, nuclear safety culture and decision-making processes.

Support high levels of nuclear safety through its procedures and procedural use and adherence.

Support high levels of nuclear safety through its approach to Human Factors (including Human Performance).

Provide emergency plans as the last level of defence in depth. These are provided and controlled under the Emergency Preparedness process.

Control radioactive emissions under its Environmental Management process. Protect staff from radiological exposure under its Radiation Protection process. Ensure that plant is safe to decommission at end of life, including any plant

required to operate post-shutdown. Confidence in this is through the specific technical assessment provided to support the life extension business case. It is managed on an ongoing basis by NGL’s arrangements for managing ageing degradation of its plant (see also Safety Factor 4).

Enact its responsibilities to the Nuclear Decommissioning Authority via the Nuclear Liabilities Funding Agreement, part of its decommissioning process.

34. Overall, NGL concludes that the above processes are considered to be effective.

However, the LSR has identified a single key issue (implementation of the Technical Governance process at DNB) and several minor issues. NGL considers that there is no impediment to prevent DNB achieving its life extension ambitions. Those minor issues that have been identified will be addressed as part of normal business. The implementation of Technical Governance is expected to improve as the process matures; this is expected to occur through normal business and supplemented by further work being commissioned as part of PSR3.

4.3 Overall Conclusions of NGL’s PLEX Submission

35. NGL has concluded that the technical and safety feasibility investigations support the proposed DNB PLEX to extend the operational life of DNB as achievable but that graphite core and boiler issues would be key elements of any PLEX. The safety, technical and process risks have been identified and NGL has identified adequate mitigation to address these risks; or, suitable mitigation arrangements will be developed.

36. For DNB the most significant threats to achieving a lifetime of 2028 are associated with the

graphite core and the boilers. The other irreplaceable components are regarded as a lower threat, but the inherent uncertainties in the predicted condition of all of the irreplaceable plant over such a timescale cannot totally rule out any of these components for consideration as part of the overall PLEX submission.

37. DNB’s latest safety case demonstrates adequacy of the reactor shutdown and hold-down

systems following boiler tube failure faults for a Graphite Weight Loss (GWL) of up to 8%. An extension from the extant safety case limit of 8% and a series of boiler modifications are likely to be needed to support a claim that risks remain As Low As Reasonably Practicable (ALARP) for operation beyond the 2017/2018 periodic shutdowns for the two reactors at DNB.

38. Like all AGRs the DNB boilers are, to all practicable considerations, irreplaceable. These

have generally been running well with no plugged tubes but issues have been identified concerning degradation of the 9% Cr tubing in the boiler evaporator section and the



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potential risks associated with failure of the re-heater tubes. Boiler tube failures lead to water ingress to the core which can cause effects that include an increase in reactivity and loss of forced or natural circulation.

39. Even if the risk of failures were to increase significantly due to 9% Cr oxidation, there are

further mitigations that could be deployed by NGL to allow operations to continue, such as changes in operating parameters to reduce the temperature of the evaporator section tubing. A reduction in evaporator temperature would reduce the oxidation rates and; therefore, the necessity for external tube plugging to manage failed tubes combined with vessel entry to repair and recover tubes if viable.

40. The current condition of re-heaters is not fully established, but they may be vulnerable to

Inter Granular Attack. This mechanism is of particular concern during cold shutdown as nitrogen blanketing cannot be applied due to the unique re-heaters design. Cracking at detuning straps could pose a risk to re-heaters as currently there is no safety case to plug re-heater tubes externally.

41. The main ageing and degradation issues on the fuel route systems are associated with

obsolescence, costs and potential increased dose burden on maintenance staff, rather than the technical feasibility of repair or replacement. There have been fuel route legacy issues, but these have been or will be (in the case of old Neutron Scatter Plugs) resolved. It is considered unlikely that fuel route issues will result in end of station life, as the majority of the plant can be replaced if necessary.

42. NGL further concludes that there is an effective management system, with processes to

identify and manage safety and plant risks that could arise as a result of PLEX. Confidence in the management of safety case quality is derived from continuously improving the Maintain Design Integrity (MDI) processes. Output to support the business case is assured by safety, plant and process enhancements and learning from previous events.

43. An investment plan has been developed to support safe and reliable continued operations

to 2028. Based on findings from the technical safety process, and business case reviews reported in this document and its supporting references, the proposal to extend the operational life of DNB to 2028 is judged by NGL to be achievable.

5 ONR ASSESSMENT

5.1 Structural Integrity

5.1.1 Boilers [13, 14]

44. The structural integrity specialist inspector stated that there is currently no basis for concern regarding the structural integrity of the DNB boilers operating until the deferred start of the Reactor 21 periodic shutdown in 2014 [13,].

45. In addition the specialist inspector’s assessment report covering the DNB boiler tubes [14],

concluded that there was currently no basis for concern regarding their structural integrity. The number of tube leaks is relatively low compared with other stations and there is judged to be nothing apparent that challenges the initiating fault frequencies presented in the consequences case.

46. If ageing or degradation mechanisms are active, they are likely to be revealed through an

increasing incidence of boiler tube leaks. Such a situation would have to be dealt with by the licensee and ONR as part of normal regulatory business. The specialist inspector was



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aware that as part of the PLEX submission the licensee is continuing to develop its safety case and plans to introduce a number of plant safety improvements to support their plans to extend the life of the plant.

5.1.2 Graphite [15, 16]

47. The specialist inspector noted that references [1] and [2] consider the challenges posed by graphite ageing, identifying both graphite weight loss due to oxidation and graphite brick cracking as potentially life limiting features. Reference [23] is a specific review of the lifetime prospects for the graphite core, referenced by Reference [1]. The DNB core is of a somewhat different design to that of the other AGRs, having both a different configuration of control rod channels and different brick dimensions. The resulting under-moderated condition of the core is said by NGL to present a vulnerability that is greater than that of later designs and results in a lower graphite weight-loss limit. The DNB limit is currently 8%, whereas other AGRs have limits near to 15%, although it is plausible that all the current limits could be raised at some point providing that this was substantiated by an adequate safety case.

48. References [1], [2] and [24] also identify the factors that may eventually lead to the onset

of keyway root cracking (KRC). This is not predicted to occur until 2029 i.e. after the end of life in 2028. Reference [23] identifies some aspects that appear to render DNB more vulnerable to cracking e.g. the higher flux experienced by the fuel channels. However, this is balanced by other factors, such as the current fluence, which is significantly lower than that of the other AGRs and a realistic judgement as to the future rate of core usage.

5.1.2.1 Graphite Weight Loss and Weight Loss Limits

49. A number of Level 4 meetings have been held in 2014 in relation to the DNB weight loss and the weight loss limit. As a result, the position has changed since References [1], [2] and [23] were written. The most recent meeting was held on 15 July 2014, with ONR’s contact report summarising the outcome of this meeting [24]. One difference is that although References [1], [2] and [23] state the anticipation that the weight loss limit will be raised to 15%, NGL has now stated that it may only be necessary to raise the limit to 10% by the end of life.

50. The important difference between these two values is that a limit of 10% can possibly be

achieved by more refined calculation of reactivity effects, whereas a 15% limit would almost certainly necessitate plant improvements to limit water ingress. It is understood that NGL is currently identifying various plant improvements to limit water ingress from boiler tube failures and may wish to implement these anyway i.e. even if it can be shown that the weight loss will remain below 10%; however, such a decision has yet to be made. This is because there are a number of potential benefits available from these plant improvements, in that they also have the potential to provide mitigation against other accident scenarios for example, vessel over-pressurisation. [NGL considers the main requirement for these plant improvements as being primarily due to uncertainty in future predicted boiler tube failure frequencies. However, some of the potential plant improvements would directly raise the allowable GWL limit by reducing the amount of steam and water that could enter the core in either shutdown conditions or during operation].

51. ONR’s current view is based on the specialist inspectors own judgement, which is that it

cannot be established with sufficient confidence that the future weight loss will remain below 10% as late as 2028. In ONR’s view, NGL may not be able to substantiate a safety case claim that adequate margin between the graphite weight loss and the weight loss limit exist without undertaking additional plant improvements which would provide further support to any analytical substantiation. NGL has recently written to ONR [25] explaining its proposals which include the implementation of an automated boiler depressurisation



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system (ABDS) and possibly improved moisture detection systems. ONR supports these proposals, however, ONR notes that it may not be until 2020/21 before they are implemented fully.

5.1.2.2 Graphite Brick Cracking

52. NGL has previously indicated to ONR that it considers the ultimate lifetime of the AGRs will be determined by the onset and progression of KRC. It is expected that KRC onset would occur first at either Hinkley Point B (HPB) or Hunterston B (HNB), as these reactors have the highest fluence. In view of this NGL has judged that only limited work was necessary to determine the point of KRC onset at DNB, since this would be adequately ‘enveloped’ by the work in support of the HPB and HNB cores. NGL has yet to submit a category 1 safety case for the second ‘post-stress reversal’ phase of DNB’s operational life. ONR would expect an anticipated date for KRC onset to be defined in this post-stress reversal safety case.

53. However, the available analysis techniques to predict KRC are applicable to all the

reactors and we note the current prediction that KRC onset will not be until after 2028. In our view, only limited confidence can be ascribed to the predicted date of KRC . ONR continues to have extensive engagement with NGL on this important topic.

54. Recent analysis of HPB/HNB data suggests that KRC onset may occur at an earlier date

than previously predicted, which if confirmed would have an equivalent implication for DNB [26]. However, this new analysis is incomplete. It is mentioned here as evidence of the uncertainty inevitably associated with prediction of a phenomenon that has yet to be observed anywhere.

55. ONR understands NGL’s wish to determine the expected operational lifetime of the

reactors and we recognised that this will be based on full consideration of economic and safety matters . However, there is considerable uncertainty in being able to predict with confidence the future behaviour of the graphite core , 14 years ahead. ONR’s assessment and judgment on nuclear safety is based on the NGL’s ability to demonstrate that operations will remain safe until the next periodic shutdown of the reactor, i.e. typically three years ahead. NGL has assured ONR that predictions over a three-year period can be made with some confidence. However, these predictions are strongly based on results from inspections at periodic shutdowns and worst-case predictions of further cracking, against a safety case limit that is currently only 10% of bricks cracked.

56. It is noted that NGL consider that the limit may possibly be increased to 30% of bricks

cracked. Part of the remaining lifetime is obviously taken up by the time it would take for cracking to increase from 10% to 30%. However, a claim that the limit is 30% will require further substantiation that we are aware is currently being pursued for the whole AGR fleet. Thus work to substantiate the seismic analysis and increase diversity of the defect tolerance modelling is underway. Until that work is complete, it is premature to express confidence that it will provide a positive result.

57. Only limited cracking has been found at DNB so far. However, all early life cracking has

been ascribed by NGL to be cracking that originated at the bore, due to the early life stresses, when the bore is in tension. This is plausible and specialist inspector notes that NGL considers the current inspection regime adequate to identify and characterise bore cracking. However as the reactor ages, it may be necessary for the number of channels inspected to be increased. DNB is currently performing the least inspection of all AGRs. ONR appreciates that current fuel-route difficulties make it comparatively difficult to increase inspection at DNB, but notes proposed technological improvements that may



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facilitate such improvement. NGL has already stated that the degree of inspection may need to increase.

5.1.2.3 Improvements to Shutdown Systems

58. Reference 1 discusses the potential need for improvements to the reactor shut down systems, that is, the possible introduction of ‘super-articulated control rods’ (SACR) and the seismic qualification of the nitrogen injection system. Similar modifications are in the process of being completed at HPB/HNB. When they are complete, DNB will be the only AGR without either an entirely diverse shutdown system or a significantly upgraded shut down and hold down system.

59. It is said that SACRs would have limited benefit at DNB because its design is such that

there are greater gaps between the control rods and the graphite channel walls than there are at HPB/HNB. ONR notes this claim. It is also said that seismic qualification of the nitrogen system would not be necessary before 2028 i.e. before KRC is expected by NGL to commence. Reference [23] also notes though that displaced channel shape magnitudes may be up to about 25% greater at DNB compared to the other AGRs, following a seismic event. It is therefore not clear to ONR that the post-stress reversal safety case will be able to establish a claim that there are no challenges to the primary shutdown system until KRC has started and progressed. Should more detailed analysis not confirm the expected damage tolerance of the core, it may be that ONR would not be able to agree to operation of DNB with a degree of cracking significantly greater than is currently believed to be present.

5.1.2.4 Overview of Graphite Core

60. Notwithstanding the above, it is realistic that for the graphite core, a safety case can eventually be made that would justify operation beyond 31st March 2018. However, current judgements are only possible on the comparatively short three-year timescale between periodic shutdowns. ONR’s judgements in issuing a Consent after a periodic shutdown are based on the whole safety case, but are particularly influenced by the inspection results from that periodic shutdown and the conservative judgement that graphite brick cracking is unlikely to increase beyond the current 10% safety case limit in a further three years.

61. The DNB cores have experienced a lower fluence than those of any of the other AGRs.

Phenomena that may end the life of other AGRs may not occur at DNB before 2028. However, the differences in core design between DNB and the later AGRs make it uncertain that the other AGRs will provide sufficient ‘leading data’ in terms of core performance. The lower weight loss limit is likely to require specific measures, such as plant improvements to allow generation to continue until 2028. Whether brick cracking ever becomes a more significant problem at DNB is currently unknown. Therefore, ONR is unable to state with certainty that improvements to the shutdown systems will not become to be seen as essential.

5.1.3 Reactivity Effects of Boiler Tube Failure Faults [17 ]

62. The specialist fault studies inspector’s assessment focussed on those aspects of the proposal that are a change from the extant safety case and that have enabled an increase in the GWL limit from 6.2% to 8%. They have considered the arguments and evidence used to justify the use of the new reactivity analysis methodology and a claim made on feed control in relation to infrequent boiler tube faults whilst on-load.

63. In the context of operation in the longer term, the fault studies inspector notes that NGL

should consider whether there may be further ALARP improvements. In this respect, he has noted that a number of plant modifications are already under consideration to reduce



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the quantity of water which enters the core following a boiler tube failure and that a strategy paper for operation beyond 2017/18 is being developed that will address longer term ALARP considerations.

5.2 Fuel Handling Route

5.3 Lifetime Safety Review – Safety Factor 4

64. No detailed assessment has been undertaken of the LSR documentation for this project assessment report as ONR has a separate intervention for Equipment Reliability [27] across the NGL Fleet. However, the ONR assessment team did sample SF4 (Aging and Lifetime Management) [8] as this was directly applicable to ONR’s review of the fuel route. From the mechanical engineering inspector’s limited review of NGL’s LSR they were satisfied that it provided sufficient oversight of the licensee’s arrangements in supporting its LTR.

65. I have noted that there are a number of time limited safety cases identified within [2]

Section 4. I have discussed with NGL its intended management of these time limited safety cases, NGL’s response appears adequate [28] in that NGL has the appropriate management systems in place to monitor and manage appropriately.

66. In addition as part of a separate ONR intervention I have held discussions with NGL’s fleet

lead for Equipment Reliability [29] project (ER) who provided me with an overview of the project. From the discussions, it appeared that the ER project is well established which is supported by discrete sub-processes that gather data from the plant and process (e.g. System Health Assessment Process and System Health Indication Programme) to support NGL’s overall lifetime management strategy.

5.4 Mechanical [19]

67. The mechanical specialist inspector is broadly satisfied with the claims, arguments, and evidence presented within the licensee’s case for the mechanical engineering elements of fuel route examined and his assessment has not identified any matters of nuclear safety significance that may potentially threaten plant life extension.

68. It is the mechanical engineering inspector’s judgement that overall from a mechanical

engineering perspective the fuelling machine has no significant ageing or degradation mechanism which limit the lifetime of the fuelling machine to 45 years.

69. It is the inspector’s judgement that the irradiated fuel ‘buffer’ store is unlikely to be suitable

in meeting the plant life extension without replacement and upgrading of the air conditioning system. He was however satisfied on the basis of Action 16 captured within LTR Table 1 [2] that NGL intends to do so. Based on the inspector’s review of the additional technical support documents he judged that the buffer store is likely to remain safe in operation for the plant life extension.

70. In general, NGL claim it is unlikely that fuel route issues will result in end of station life, as

the majority of the plant can be replaced if necessary. The inspector has judged from his assessment findings that this statement is correct for the mechanical engineering matters examined on the fuel route principally on the basis he has not determined any matters of nuclear safety significance to indicate otherwise.

71. NGL do however recognise there is the potential for a large economic threat associated

with the fuel route which the inspector judges appropriate given the challenges from



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obsolescence and the cost and programme implications from unpredicted component failure on the plant and equipment.

72. NGL claim on a purely technical basis a high confidence is appropriate and for the

mechanical engineering matters the specialist inspector has examined on the fuel route, he judges this statement to be correct on the basis of their findings.

73. However, the fuel route faced significant challenges in 2011 which resulted in a critical

recovery program being set up. NGL claim it would be inappropriate until this is closed out to state high confidence, despite the fuel route plant being largely repairable with sufficient investment. The inspector has judged this statement to be correct on the basis of their findings.

74. NGL conclude therefore its confidences of MEDIUM/HIGH is deemed appropriate for each

of the LTR dates under consideration, 2018, 2023, 2028 and 2033. The specialist inspector judges from his assessment findings this statement is correct for the mechanical engineering matters examined on the fuel route principally on the basis they have not determined any matters of nuclear safety significance to indicate otherwise.

5.5 Fault Studies [19]

75. The fault studies inspector stated that based on his initial overview of NGL Technical Review documentation, there are not (at this moment) any obvious fuel route fault issues (other than with the buffer store) that would prevent the fuel route lifetime from being extended to meet NGL’s PLEX aims. However, he noted that this is a very preliminary view, based on a limited set of technical documents, which do not at the moment constitute a safety case.

76. The internal state of the buffer store is subject to some uncertainty; this would be more

than adequately resolved by an internal inspection by NGL. From discussions with NGL there appears to be an optioneering review to determine the most expedient means of accessing the buffer store vault, which is currently sealed.

77. The results of the buffer store inspection will be key to determining the safe future

operability of the fuel route. There are also a number of fuel route time-limited safety cases. ONR should be mindful of these cases when giving a more detailed consideration of the arguments for life extension.

78. The Fault Studies specialist inspector has identified two recommendations:-

Recommendation 1 – The proposed replacement of the obsolescent fuelling machine C&I systems such as the VMC and the Fairstat relay interlock systems would benefit from some overview assessment by an ONR specialist C&I inspector when the PSR3 documentation is submitted to ONR circa ~2017, to determine the adequacy of the NGL proposed approach to address the obsolescence issues of these control systems. This issue has been given number 2645 on the issues database and needs to be considered for PSR3.

Recommendation 2 – NGL to provide ONR its findings from the inspection work

at DNB and the wider issues from buffer store inspections at other stations and the optioneering process for the buffer store inspection. ONR should be mindful of the fuel route time-limited safety cases when performing a more detailed assessment for PSR3.



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6 CONCLUSIONS

79. In judging whether NGL’s conclusions are soundly based, I have attached significant weight to the fact that NGL’s approach is based on the IAEA guidance [3] and has been reviewed and endorsed by the NGL NSC [21, 22] and independently peer reviewed by AMEC Nuclear. I have also noted that the independent peer review by AMEC Nuclear was undertaken on the pervious version of reference [2]. Notwithstanding this, in relation to core life assumptions AMEC Nuclear agrees that confidence levels are good as proposed by the licensee. However some further work is required to support its PLEX in a number of areas. NGL agrees with these conclusions but as yet does not specify what requirements are necessary and when.

80. For DNB the most significant threats in achieving the proposed PLEX are associated with

the graphite core weight loss and boiler tube failure. The other irreplaceable components are regarded by NGL as a lower threat, but the inherent uncertainties in the predicted condition of all of the irreplaceable plant over such a timescale cannot totally rule out any of these components.

81. The differences in core design between DNB and the later AGRs make it uncertain that

the other AGRs will provide ‘leading data’ in terms of core performance. There is therefore perhaps an additional uncertainty in making predictions about DNB. The lower weight loss limit is likely to require specific measures, such as plant improvements to allow PLEX to be reached. Whether brick cracking ever becomes a more significant problem at DNB is currently unknown. ONR would not wish to provide any assurance that improvements to the shutdown systems will not become to be seen as essential.

82. A large element of the decisions facing NGL regarding the continued operation of the

reactors will be financial. ONR is focused on issues of nuclear safety; this review has been undertaken without consideration of financial concerns. However, ONR specialist inspectors have noted that some of the assumptions made in their reviews/and earlier assessments require that safety cases will need to be maintained over any extended operating life of the stations. At this stage it is not clear that the full extent of the uncertainties involved in the ageing predictions have been considered to assess accurately the likely costs involved with the maintenance of these safety cases and hence the continuing viability of operations.

83. NGL’s PLEX portfolio will require a substantial investment by NGL in both major safety

improvements such as boiler improvements and a large number of smaller but still significant modifications to improve reliability and/or safety. It is important that safety improvements are made in a timely manner so that risks are maintained ALARP. ONR will need to ensure that those commitments that improve nuclear safety over the proposed PLEX are fully funded and resourced in order for them to be delivered at the appropriate time.

84. In general terms, I consider that the submitted PLEX documentation to be well presented,

ONR’s review of the submitted documentation in conjunction with earlier assessment work has not revealed any significant flaws or omission with respect to nuclear safety that would preclude adopting NGL’s PLEX proposals for DNB. NGL’s review of all the potential degradation mechanisms as part of its review of PLEX appears to be well defined and degradation has been mitigated or mitigation arrangements are in place or will be developed.

85. It is important that the limited nature of NGL’s submission is taken into account and ONR’s

comments are taken in context of the potential for other issues emerging from its formal assessment of the full scope LC15 PSR3 submission scheduled for January 2017 and



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those issues that have been raised from ONR’s resent assessment of Reactor 21 return to service from its 2014 periodic shutdown.

86. It should remain clear that the regulation of nuclear power station operation is based on

the submission of adequate LC15 PSR submissions, allied to continuing satisfactory performance as judged against inspections of the site and assessment of the safety justification of the plant (this includes the ONR requirement to permission the start up of each of the two reactors at DNB after their three yearly periodic shutdowns). Provided that these are maintained, any decision regarding the accounting lifetime of the plant and the cessation of power generation remains a commercial decision solely for the licensee to determine.

87. NGL should note that no statement in this document should be interpreted either as a

commitment to accept a safety case that has yet to be submitted to ONR, or as an implication that NGL’s Lifetime Technical Review (LTR) restricts the necessary scope of any subsequent safety case modification or future PSR3 submission.

7 RECOMMENDATIONS

88. I recommend that the Superintending Inspector: a. Signs this Project Assessment Report to confirm acceptance of ONR technical and

regulatory arguments supporting issuing letter DNB71259N to NGL b. Signs this Project Assessment Report approving its release for publication, after

redaction where appropriate



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8 REFERENCES

[1] TRIM Ref 2014/215738, DNB PLEX Technical Assessment Overview Report, Revision 001, DAO/REP/JIEF/048/DNB/14 [2] TRIM Ref 2014/215738, Lifetime Technical Review, Revision 001, DAO/REP/JIEF/052/DNB/13 [3] IAEA Guidance, Series No. SSG-25, April 10, 2013 No. NS-G 2.10. Periodic Safety Review of Nuclear Power Plants [4] TRIM 2012/418269, ONR Assessment of EDF-NGL HPB/HNB Lifetime Safety Review, ONR-PAR-12-046 [5] IAEA Safety Standards Series, Periodic Safety Review of Nuclear Power Plants, NS-G-2.10 [6] ONR How 2 Business Management System. Purpose and Scope of Permissioning. PI/FWD Issue 3. HSE August 2011 www.hse.gov.uk/nuclear/operational/assessment/index.htm. [7] ONR How2 Business Management System , ONR Nuclear Safety Permissioning Instruction, NS-PER-IN-001 Revision 5, April 2014 [8] TRIM Ref 2014/215738, 16 LSR Safety Factor documents [9] TRIM 2014/154078, EDF Energy Nuclear Generation Limited (NGL) – Dungeness B – Consent under Licence Condition 30(3) to Start-up Reactor 21 Following the 2014 Periodic Shutdown, ONR-DNB-PAR-13-039 [10] TRIM 2014/269348, response from the Structural Integrity specialist inspector (Boilers and Boiler tubes) [11] TRIM 2014/299745, response from the Fault Studies specialist inspector, 11/08/2014 [12] TRIM 2014/299920, DNB PLEX- response and comments from the Graphite specialist inspector [13] TRIM 2013/209223, CNRP NGL Structural Integrity Assessment of the Dungeness B Request for Extending the Operating Interval of Reactor 21 June 2013, ONR-CNRP-AR-13-042 [14] TRIM 2013/240045, Assessment note on the Status of the structural integrity case for Dungeness B Boilers as of 26 June 2013, Structural Integrity specialist inspector [15] TRIM 2013/242264, Return to service of Dungeness B after R21 2014 outage - graphite aspects, including EC 349955 Rev001 justification for continued operation with revised predictions of graphite weight loss, ONR-CNRP-AR-13-039 [16] TRIM 2014/206408, Return to service of Dungeness B after R21 2014 outage - graphite aspects, including EC 349955 V04 justification for continued operation with revised predictions of graphite weight loss, ONR-CNRP-AR-14-020

http://www.hse.gov.uk/nuclear/operational/assessment/index.htm


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[17] TRIM 2014/193487, Fault Studies Assessment of Dungeness B Power Station – Extension to the Safety Case for the Reactivity Effects of Boiler Tube Failure Faults – NP/SC 7467 Addendum 2, ONR-CNRP-AR-14-011 Revision 0 [18] TRIM 2014/251375, Familiarisation Visit to Dungeness B with key focus on the Fuel Route for PLEX submission, ONR-NSS- CR-14-104 Revision 0 [19] TRIM 2014/275451, Assessment of EDF - Dungeness B Plant Life Extension (PLEX) – Initial Assessment of Fuel Route Lifetime Technical Overview ONR-CNRP-AR-034-14 Revision 0. [20] TRIM 2014/259927, Assessment of EDF-NGL Lifetime Technical Review & the Mechanical Engineering aspects associated with the Fuel Route in support of Plant Life Extension, ONR-CNRP-AR-14-033 Revision 0 [21] TRIM 2014/281328, 10-NSC comments consolidated 4005 minus BF [22] TRIM 2014/292763, NSC further comments and responses DNB PLEX NSC 01/08/2014 [23] TRIM 2014/294101, DAO/EAN/JIEC/157/DNB/13 - Dungeness B: Review of Graphite Core Structure Lifetime Prospects. January 2014 [24] TRIM 2014/277092, Level 4 meeting on securing a long term boiler tube failure faults safety case for Dungeness B 15 July 2014, ONR-DNB-CR-14-126 [25] TRIM 2014/283133, letter from EDF, NSLDNB50881N, 28th July 2014, Strategy for Boiler tube failure protection system modifications in support of PLEX [26] TRIM 2014/288813, ONR-CNRP-CR-14-137 L4 on possible early onset of KRC in graphite bricks, as evidenced by brick shape changes and the implications on requirements for SC development 22 July 2014 [27] TRIM Ref 2014/235226, TS 223, [28] TRIM 2014/292767, 01/08/2014, Email from NGL Time Limited Safety cases and their management [29] TRIM 2014/166398, EDF NGL Level 4 Meetings to Discuss the Equipment Reliability Project, PSR3 Introduction, SF2 and SF4, ONR-CNRP- NSS - CR-14-015 Revision 0

Documents

ONR Review of NGL DNB PLEX Technical Overview Report · Generation Ltd (NGL) Plant Life Extension (PLEX) Technical Overview Report and its supporting documentation for Dungeness B