ML14342A812

Presentation Slides for December 10, 2014 Meeting on Alloy 600 PWSCC Mitigation for Rvh Nozzles

ML14342A812
Person / Time
Site:	Byron, Braidwood
Issue date:	12/10/2014
From:	Feimster J, Gullott D, Krejcie J, Joshua Wilson, Youman B Exelon Generation Co
To:	Office of Nuclear Reactor Regulation
Eva Brown, NRR 415-2315
References
Download: ML14342A812 (23)

Text

Exelon Generation Alloy 600 PWSCC Mitigation Plan for Reactor Vessel Head Penetration Nozzles Utilizing Cavitation Peening at Byron and Braidwood NRC/Exelon Meeting, Rockville, MD B. Youman J. Feimster J. Wilson D. Gullott J. Krejcie December 10, 2014

Agenda

- Introductions and Opening Remarks

- Objective

- Braidwood / Byron PWSCC History

- AREVA RPV Head Penetration Peening Technology

- RPV Head Penetration Peening Benefits

- Peening Technical Basis

- Exelon Peening Mitigation Implementation Plan

- Implementation of Peening via 50.59 Process

- Post Peening Inspection Relief Requests

- NRC Interaction

- Summary 1

Objective

- Present Exelons initiative for Reactor Pressure Vessel (RPV) head PWSCC mitigation

• Introduce AREVA Peening process

• Provide benefits of Peening process

• Discuss 10 CFR 50.59 approach

• Discuss Exelons integrated implementation plan including potential future inspection relief requests

- Obtain NRC feedback on Exelons initiative and identify technical and regulatory challenges

• Achieve an understanding of questions or concerns

• Discuss approach for resolving questions or concerns 2

Braidwood / Byron PWSCC History

- Braidwood and Byron RPV heads are B&W T-Cold Heads with the RPV head penetration nozzles fabricated using B&W tubular product

- Inspections to date have identified the following flaws:

• Byron Unit 2 - two nozzles (Spring 2007, Fall 2014)

• Byron Unit 1 - four nozzles (Spring 2011)

• Braidwood Unit 1 - one nozzle (Spring 2012)

- Each flaw has been repaired using an NRC approved embedded flaw overlay

- Under the current regulatory requirements, the three repaired RPV heads require ultrasonic examinations and bare metal visual inspections every refueling outage

- To date, the inspection program has not detected cracking symptoms on the Braidwood Unit 2 RPV head 3

Braidwood / Byron PWSCC History

- History of crack locations are shown below Existing Flaw / Repair Locations Byron Unit 2 (Spring 2007) #68 Byron Unit 1 (Spring 2011) #31, 43, 64, 76 Braidwood Unit 1 (Spring 2012 ) #69 Byron Unit 2 (Fall 2014) #6 4

Braidwood / Byron PWSCC History

- All cracks were outside diameter (OD) axial (toe of weld and below and propagated upwards, behind weld), with no leakage identified CRDM nozzle is nominally 4 inches OD and 2 3/4 ID with 5/8 wall thickness 5

Braidwood / Byron PWSCC History

- Exelons current method for addressing RPV head PWSCC induced flaws is to implement an embedded flaw weld overlay

• Reactive due to unknown scope prior to outage

• Significant dose, outage extension, and cost

• Fails to provide a complete solution for long term asset (i.e., RPV) management or improve reliability of non-repaired penetrations

- Potential discovery of new PWSCC induced flaws in the RPV head penetrations over the next 33 years (assuming License Renewal) is estimated to be between 11 (best case) and 84 (worst case)

- Therefore, Exelon is pursuing proactive RPV head penetration Peening technology with AREVA to improve the RPV head penetration integrity and mitigate PWSCC to reduce vulnerabilities 6

AREVA RPV Head Penetration Peening Technology

- Ultra high velocity jet results in pressure below vapor pressure in water resulting in vapor bubbles forming in the water (cavitation)

- Cavitation bubbles collapse at surface generating high pressures on the material

- The shockwaves caused by collapsing voids imparts compressive stresses in the surface layer of material

• The water-jet force does NOT perform the Peening

• The Peening is performed by the collapse of the vapor bubbles 7

AREVA RPV Head Penetration Peening Technology

- Cavitation Peening Process Diagram bubble 8

AREVA RPV Head Penetration Peening Technology

- Demonstration Video 9

AREVA RPV Head Penetration Peening Technology

- Extensive testing has shown cavitation Peening is an effective process that provides broad coverage in its application and extreme flexibility with applied geometries and process flow rates Given it is NOT the water-jet that is doing the Peening but the cavitation bubbles, the process ensures coverage overlap with wide tolerances on geometry length and flow-rates Documented test results confirm:

o Testing over a variety of flow-rates and distances to target provide the same beneficial results o Post peening benefits show final as left stresses regardless of initial residual stress state o Testing has shown higher stresses relax and respond faster to less initial residual tension state o Peening is effective over a wide applied angle between the water-jet and material being peened showing appropriate coverage on complicated geometries o Documented test results are available for review 10

AREVA RPV Head Penetration Peening Technology

- Cavitation Peening Surface Finish Visual inspection can be used to confirm Peening coverage The as left surface finish does not impact NDE inspection results Aggressive high temperature corrosion testing has validated resistance to PWSCC initiation Peened Non-Peened 11

RPV Head Penetration Peening Benefits

- Primary Safety Benefit

• Reduction in nozzle stresses significantly diminishes the potential for future PWSCC induced cracks and associated leakage paths

• RPV head is in a safer condition and more effectively protects the health and safety of the public

• Reduction in cumulative dose/exposure by not performing reactive repairs - ranges from 731 to 2,181 Rem

- Primary Commercial Benefit

• Extend life and improve reliability of RPV head

• Eliminate need for reactive repairs - savings range from $688M to

$1,200M

• Eliminate outage extensions for reactive repairs - ranges from 161 to 607 days

- Secondary benefits may include additional dose and outage savings related to reduced future inspections 12

Peening Technical Basis

- Initiation of PWSCC flaws requires tensile stress at the surface

- Peening techniques mitigate PWSCC by reversing the tensile stress at the surface, exposed to reactor coolant, to compressive residual stress

- Peening vendors and EPRI performed separate verification experiments, including corrosion cracking and stress relaxation tests, to confirm the effectiveness and sustainability of peening treatments

- Evaluations were conducted related to peening performance methods and effects

• Coverage, magnitude and depth of compressive stresses

• Retention of compressive stresses throughout plant life

• Verification of no unacceptable side effects

• Precludes the initiation of PWSCC 13

Peening Technical Basis

- Key supporting EPRI-MRP reference documents

• MRP-267, Revision 1, 2012, Technical Basis for PWSCC Mitigation by Surface Stress Improvement

• MRP-335, Revision 1, 2013, Topical Report for PWSCC Mitigation by Surface Stress Improvement

• MRP-2014-027, 2014, Supplemental Technical Basis for Peening of Alloy 600 RPVHPNs 14

Exelon Peening Mitigation Implementation Plan

- Exelon will mitigate RPV head penetrations using cavitation water jet Peening

• Contract has been awarded to AREVA with field development in progress

- Exelon will implement Peening during the 2016 and 2017 refueling outages

• Byron Unit 2 - Spring 2016

• Braidwood Unit 1 - Fall 2016

• Byron Unit 1 - Spring 2017

• Braidwood Unit 2 - Spring 2017

- Peening will be implemented as a special process in accordance with 10 CFR 50, Appendix B, Criterion IX

- Peening of the RPV head penetrations will be assessed in accordance with 10 CFR 50.59 15

Exelon Peening Mitigation Implementation Plan

- Scope of work includes the outer and inner surfaces of the susceptible Alloy 600 and 182/82 material in all the 79 nozzles in each of the Byron and Braidwood RPV heads:

53 CRDM nozzles 5 thermocouple nozzles 2 reactor vessel level indicating system (RVLIS) nozzles 18 spare nozzles 1 RPV head vent nozzle 16

Exelon Peening Mitigation Implementation Plan

- The minimum peened coverage areas:

The Peening shall extend at least .25 beyond all PWSCC susceptible material PWSCC susceptible material is defined as Alloy 182/82 and the heat affected zone of the Alloy 600 nozzles that are in contact with the primary water C

17

Exelon Peening Mitigation Implementation Plan

- Applying 10 CFR 50, Appendix B, Criterion IX Process

• Essential variables will be bounded (e.g., high pressure, nozzle stand off distance, back pressure, nozzle position, nozzle speed)

• Training and qualification will be conducted by process demonstration

- The qualification will include testing on mockups at the boundary extremes to demonstrate that the required results can be obtained even at the limits of the essential variables

- The qualification will be performed on full scale mockups with material and geometry constraints

• During implementation essential variables will be controlled/monitored to ensure acceptable results are achieved 18

Implementation of Peening via 10 CFR 50.59 Process

- Exelon intends to implement Cavitation Peening in accordance with 10 CFR 50.59 Cavitation Peening does not adversely affect SSCs nor their associated performance, FSAR design basis, or Technical Specifications Extensive testing has shown Cavitation Peening is less invasive than original grinding activities performed during fabrication and the Peening application process energy levels remain low Exelon will make 10 CFR 50.59 evaluation available for NRC review in advance of Peening implementation 19

Post Peening Inspection Relief Requests

- Exelon continues to evaluate the secondary benefit of RPV head penetration Peening related to reduced inspection frequency

- Changes in the inspection frequency specified in 10 CFR 50.55(a) require NRC approval of relief requests

- Exelon is evaluating an incremental relief request approach 20

NRC Interaction

- Exelon plans to move forward with the Cavitation Peening technology due to the safety and long term asset preservation benefits

- Since this will be the first application of RPV head penetration Peening to mitigate PWSCC in United States, Exelon wants to identify regulatory uncertainties

- Exelon supports open dialogue with NRC to understand/resolve regulatory issues Peening evaluated in accordance with 10 CFR 50.59, which will be made available to NRC for review prior to implementation Requirements during the Peening activity to support potential future relief requests related to post Peening inspection frequency Impact of NRC Testing Program scope on post Peening inspection relief o Available industry test results may reduce NRC Program scope and facilitate review of MRP-335 and CC-729-5 AREVA Half Nozzle repair methodology and subsequent relief request approval in lieu of embedded flaw weld overlay 21

Summary

- Application of the Peening technology on the Braidwood and Byron RPV head penetrations is a proactive approach to mitigate PWSCC

• Improves safety by reducing the potential for nozzle cracking and associated leakage

• Results in significant reduction in dose related to reactive repairs

- Exelon has confidence in the Peening technology based on industry testing, EPRI/MRP published documents and the process controls being implemented

- Exelon will proactively support resolution of NRC questions and ensure NRC remains informed of implementation plans 22