ML23249A220

Transcript of Advisory Committee on Reactor Safeguards - Fuels, Materials & Structures - PWROG Hydrogen-Based Strain Limit Subcommittee Meeting, August 24, 2023, Pages 1-56 (Open)

ML23249A220
Person / Time
Issue date:	08/24/2023
From:	Advisory Committee on Reactor Safeguards
To:
References
NRC-2512
Download: ML23249A220 (1)

Text

Official Transcript of Proceedings NUCLEAR REGULATORY COMMISSION

Title:

Advisory Committee on Reactor Safeguards Fuels, Materials and Structures Subcommittee Open Session Docket Number: (n/a)

Location: teleconference Date: Thursday, August 24, 2023 Work Order No.: NRC-2512 Pages 1-35 NEAL R. GROSS AND CO., INC.

Court Reporters and Transcribers 1716 14th Street, N.W.

Washington, D.C. 20009 (202) 234-4433

1 1

2 3

4 DISCLAIMER 5

6 7 UNITED STATES NUCLEAR REGULATORY COMMISSIONS 8 ADVISORY COMMITTEE ON REACTOR SAFEGUARDS 9

10 11 The contents of this transcript of the 12 proceeding of the United States Nuclear Regulatory 13 Commission Advisory Committee on Reactor Safeguards, 14 as reported herein, is a record of the discussions 15 recorded at the meeting.

16 17 This transcript has not been reviewed, 18 corrected, and edited, and it may contain 19 inaccuracies.

20 21 22 23 NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1323 RHODE ISLAND AVE., N.W.

(202) 234-4433 WASHINGTON, D.C. 20005-3701 www.nealrgross.com

1 1 UNITED STATES OF AMERICA 2 NUCLEAR REGULATORY COMMISSION 3 + + + + +

4 ADVISORY COMMITTEE ON REACTOR SAFEGUARDS 5 (ACRS) 6 + + + + +

7 FUELS, MATERIALS, AND STRUCTURES SUBCOMMITTEE 8 + + + + +

9 OPEN SESSION 10 + + + + +

11 THURSDAY 12 AUGUST 24, 2023 13 + + + + +

14 The Subcommittee met via hybrid in-person 15 and Video Teleconference, at 8:30 a.m. EDT, Ronald 16 Ballinger, Chairman, presiding.

17 18 COMMITTEE MEMBERS:

19 RONALD G. BALLINGER, Chair 20 CHARLES H. BROWN, JR., Member 21 VICKI BIER, Member 22 VESNA DIMITRIJEVIC, Member 23 GREGORY HALNON, Member 24 WALT KIRCHNER, Member 25 JOSE MARCH-LEUBA, Member NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1716 14th STREET, N.W., SUITE 200 (202) 234-4433 WASHINGTON, D.C. 20009-4309 www.nealrgross.com

2 1 DAVID PETTI, Member 2 JOY L. REMPE, Member 3 THOMAS ROBERTS, Member 4 MATTHEW SUNSERI, Member 5

6 ACRS CONSULTANT:

7 DENNIS BLEY 8 STEVE SCHULTZ 9

10 DESIGNATED FEDERAL OFFICIAL:

11 ZENA ABDULLAHI 12 WEIDONG WANG 13 14 ALSO PRESENT:

15 TIMOTHY CREDE, Westinghouse 16 LESLIE FIELDS, NRR 17 GEROND GEORGE, NRR 18 JOSEPH MESSINA, NRR 19 BRIAN MOUNT, Dominion Energy 20 PATRICK RAYNAUD, RES 21 22 23 24 25 NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1716 14th STREET, N.W., SUITE 200 (202) 234-4433 WASHINGTON, D.C. 20009-4309 www.nealrgross.com

3 1 P-R-O-C-E-E-D-I-N-G-S 2 8:31 a.m.

3 CHAIR BALLINGER: The meeting will now 4 come to order, everybody. This is the meeting of the 5 Fuels, Materials and Structures Subcommittee of the 6 Advisory Committee on Reactor Safeguards. I'm Ron 7 Ballinger, chairman of today's subcommittee meeting.

8 ACRS members present and in attendance are 9 Tom Roberts, Jose March-Leuba, Matt Sunseri, Dave 10 Petti, Joy Rempe. I don't know if Robert, Bob Martin 11 is going to be here or not. Vicki Bier, Greg Halnon, 12 and Charles Brown. And on line will be I'm sure Vesna 13 Dimitrijevic, yes, and Walt -- yes, Walt Kirchner.

14 Our consultants, Stephen Schultz and 15 Dennis Bley are also -- Steve is here in person and 16 Dennis is online.

17 Weidong Wang is the Designated Federal 18 Official present in person and Zena Abdullahi is 19 present virtually, as opposed to virtually present, as 20 DFO as well.

21 During today's meeting, the subcommittee 22 will review the NRC's staff safety evaluation for 23 approving PWROG's hydrogen-based transient cladding 24 strain limit topic report. The subcommittee will hear 25 presentations by and hold discussions with the NRC NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1716 14th STREET, N.W., SUITE 200 (202) 234-4433 WASHINGTON, D.C. 20009-4309 www.nealrgross.com

4 1 staff, PWROG, and their consultants.

2 Part of the presentations by the applicant 3 and the NRC staff may be closed to discuss information 4 as proprietary to the licensee and its contractors 5 pursuant to 5 U.S.C. 552(b)(c)(4).

6 Attendance at the meeting that deals with 7 such information will be limited to the NRC staff and 8 its consultants, PWROG, and those individuals and 9 organizations who have entered into an appropriate 10 confidentiality agreement with them. Consequently, we 11 will need to confirm that we have only eligible 12 observers and participants in the closed part of the 13 in-person meeting, as well as those attending 14 virtually.

15 The ACRS Section of the U.S. NRC public 16 website provides our charter, bylaws, agendas, letter 17 reports, and full transcripts of all full and 18 subcommittee meetings including slides presented here.

19 The meeting notice and agenda for this meeting are 20 posted and we have not received any requests for any 21 written statements or requests to make oral statements 22 from the public prior to this meeting.

23 The subcommittee will gather information, 24 analyze relevant issues and facts, and formulate 25 proposed positions and actions, as appropriate, for NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1716 14th STREET, N.W., SUITE 200 (202) 234-4433 WASHINGTON, D.C. 20009-4309 www.nealrgross.com

5 1 deliberation by the full committee.

2 A transcript of the meeting is being kept 3 and the open portion will be made available. Today's 4 meeting is being held in person and over Microsoft 5 Teams for ACRS staff and members, NRC staff, and the 6 applicant. There is also a telephone bridge line and 7 a Microsoft Teams link allowing participation of the 8 public to join the open session.

9 When addressing the subcommittee, the 10 participants should first identify themselves and 11 speak with sufficient clarity and volume so that they 12 may be readily heard. When not speaking, we request 13 that participants mute their computer microphones or 14 by phone press *6.

15 We'll now proceed with the meeting and I'd 16 like to start by calling on Gerond George. You're 17 there, I saw you, to make the NRC opening remarks.

18 Before you go, we will have to decide at the end of 19 this whether we recommend a letter, but we have 20 information indicating that they would like a letter, 21 so keep that in your mind when you ask questions and 22 that's part of the presentation, because we're going 23 to have to come up with a letter.

24 MR. GEORGE: Thank you, Chairman 25 Ballinger. Before I get to my opening remarks, I NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1716 14th STREET, N.W., SUITE 200 (202) 234-4433 WASHINGTON, D.C. 20009-4309 www.nealrgross.com

6 1 would say there was a request not to have a letter.

2 We did not need to have a letter.

3 CHAIR BALLINGER: You're apparently not 4 close enough to the microphone.

5 MR. GEORGE: Okay, can you hear me now?

6 Hello.

7 CHAIR BALLINGER: Yes, that's an AT&T 8 advertisement.

9 MR. GEORGE: Yes, Gerond George, Branch 10 Chief, Licensing Projects Branch. Before I get to my 11 opening remarks, I think there's a little confusion.

12 I guess we had stated we did not need a letter for 13 this review.

14 CHAIR BALLINGER: Oh, okay. I've been 15 under the impression all along that you wanted a 16 letter when you read the 635 page NRC NUREG on 17 statistics.

18 MR. GEORGE: I'm not sure what you're 19 talking about, but I'll get to my opening remarks and 20 I guess we can handle the letter later.

21 CHAIR BALLINGER: Okay, sure. By the way, 22 whether we have a letter or not is not up to me. It's 23 actually not up to you. It's up to the subcommittee.

24 MR. GEORGE: Subcommittee. That's right, 25 that's right. Understand. Thank you.

NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1716 14th STREET, N.W., SUITE 200 (202) 234-4433 WASHINGTON, D.C. 20009-4309 www.nealrgross.com

7 1 Thank you, Chairman Ballinger, thanks for 2 having us today. The NRC staff will present the 3 safety evaluation for the Pressurized Water Reactor 4 Owners Group 21001, the hydrogen-based transient 5 cladding strain limit.

6 The topical report proposes an alternative 7 to the hydrogen-based cladding strain limit currently 8 used in our regs, regulation. We would like to thank 9 the PWR Owners Group for providing the information and 10 committing the resources for this review. Because of 11 these efforts, our tech reviewers were able to 12 complete a comprehensive and efficient review in a 13 very short amount of time. So thank you, guys, very 14 much for that.

15 And that would be it for my opening 16 remarks. Thank you for your time today.

17 CHAIR BALLINGER: Okay, now I'm in a bit 18 of a quandary. Who's doing the presentation first?

19 I mean it's the PWROG folks, but I don't see anybody.

20 MR. GEORGE: I believe they are behind 21 you. They need to walk up to the front. We won't 22 make you guys turn around.

23 MR. MOUNT: Is this close enough to the 24 mic?

25 CHAIR BALLINGER: That sounds pretty good.

NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1716 14th STREET, N.W., SUITE 200 (202) 234-4433 WASHINGTON, D.C. 20009-4309 www.nealrgross.com

8 1 MR. MOUNT: Good morning. My name is 2 Brian Mount. I'm the chairman for the Analysis 3 Committee of the PWROG and I work for Dominion Energy.

4 I'd like to thank the ACRS for taking an interest in 5 our topical report and the NRC' SC on it.

6 Just to give you a brief little history of 7 how we got here, as we've been looking at trying to 8 improve our fuel economics, we keep pushing up against 9 various limits and most recently that was the cladding 10 strain limit that we've pushed into. Whenever we 11 approach a limit, we always try to find ways to 12 recover margin either through improvements and 13 analytical inputs or if there's ways to improve the 14 limit itself.

15 As part of this, we went off and started 16 looking at what other activities were going on in the 17 industry, within the NRC, and we saw that there are 18 already hydrogen-based limits out there. We also came 19 across the National Lab, the PNNL report, where they 20 were looking at hydrogen-based credit -- or clad 21 strain limit. With these ideas and these limits, we 22 were thinking this was an excellent opportunity for us 23 to improve our design limits and support even further 24 fuel economic improvements or now what we are hearing 25 is longer operating cycles, potential power uprates.

NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1716 14th STREET, N.W., SUITE 200 (202) 234-4433 WASHINGTON, D.C. 20009-4309 www.nealrgross.com

9 1 Ironically, every time we find a way to improve our 2 margins, somebody is coming to look for it.

3 So as the NRC mentioned earlier, we are 4 very appreciative of the NRC's review. This was a 5 very well-run review. We are very appreciative of the 6 preparedness that the staff came into. We had an 7 audit with them. They were well prepared. They were 8 very pointed in what they wanted and the information 9 they needed to support filling the holes in the draft 10 that Zena had prepared.

11 And with that, I'd like to turn it over to 12 Mr. Tim Crede, who was our technical lead at 13 Westinghouse for us. And although Tim is here, he was 14 supported by a large number of other experts within 15 Westinghouse to get to this report in such a good 16 position for the NRC review. Thank you.

17 MR. CREDE: Thank you, Brian. Good 18 morning, everybody. Can you hear me all right? Thank 19 you.

20 Just a logistics questions first. Do you 21 want me to share my screen around the slides or who's 22 changing the slides?

23 CHAIR BALLINGER: I guess that's up to 24 you.

25 MEMBER REMPE: I thought you were sharing NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1716 14th STREET, N.W., SUITE 200 (202) 234-4433 WASHINGTON, D.C. 20009-4309 www.nealrgross.com

10 1 --

2 MR. CREDE: Okay. Do you want to try it?

3 Okay.

4 MR. MOUNT: Leslie Fields.

5 MR. CREDE: All right, so my name is Tim 6 Crede. I work for Westinghouse in the Fuel Rod and 7 Thermal-Hydraulic Design Group. I was the technical 8 lead on the project, although as Brian mentioned, I 9 certainly had a very large and excellent support 10 staff.

11 We can go to the next slide.

12 CHAIR BALLINGER: See if you've got a 13 speaker on because we're getting feedback.

14 MR. CREDE: All right, so just the agenda 15 for the open session, I wanted to go through a little 16 bit of an introduction, talk about some background, 17 give a high-level overview of the project, discuss the 18 alternate strain limit that we proposed, a quick 19 summary, and conclusions and then take any questions.

20 Next slide, please. So back in 2019, 21 Westinghouse partnered with the PWR Owners Group to 22 develop a new project. We were looking at an 23 alternative way to evaluate transient cladding strain.

24 Specifically, we were looking at a new limit. So the 25 current limit that Westinghouse uses is taken straight NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1716 14th STREET, N.W., SUITE 200 (202) 234-4433 WASHINGTON, D.C. 20009-4309 www.nealrgross.com

11 1 out of NUREG-0800, the NRC Standard Review Plan. But 2 we wanted a more data-driven and performance-based 3 limit, something that was derived from test data 4 specific to Westinghouse cladding outward. And the 5 limit from the SRP is appropriate. It's conservative, 6 but it is not based on our Westinghouse measured data.

7 So what we were looking for is to develop 8 further, more accurate strain limit that would have 9 the benefit of increasing margin, what is typically 10 the limiting time and life for transient cladding 11 strain analyses which would allow utilities to pursue 12 some of the programs Brian mentioned, uprates, loaded 13 pattern optimization, cycle end extensions, things 14 like that because for many of these plants, their most 15 limiting fuel performance criterion does tend to be 16 transient cladding strain. So we were looking for 17 ways to both make the limit more accurate and a little 18 side bonus is to recover some of that margin.

19 Next slide, please. So just a little bit 20 of a background. If you do look at NUREG-0800 and 21 look at Section 4.2, you will see that this is where 22 we take our strain limit from, the current limit. I 23 won't read it verbatim, but effectively what it says 24 is that during a Condition II overpower, a transient 25 event, we have thermal swelling of the fuel pellet.

NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1716 14th STREET, N.W., SUITE 200 (202) 234-4433 WASHINGTON, D.C. 20009-4309 www.nealrgross.com

12 1 It stands out at a rate faster than cladding expands.

2 And it applies a stress strain on the cladding.

3 There's some pellet cladding, mechanical interaction 4 there.

5 And the purpose of the limit is to prevent 6 an over training of the cladding during these 7 Condition II transient events. So specifically, the 8 outer diameter cladding pre-transient can't increase 9 by more than one percent during the transient events.

10 Next slide, please. So the current one 11 percent limits remains valid and conservative design 12 limits. We are certainly not asking that the SRP be 13 updated or changed. What we wanted to look at, 14 however, is an alternative to that one percent limit 15 that are using a generic one percent limit. We wanted 16 one specifically for Westinghouse fuel. And when I 17 say Westinghouse fuel, that is restricted to ZIRLO and 18 Optimized ZIRLO which were our licensed cladding 19 alloys at the time.

20 And when we looked at the data, what we 21 noticed is that there's a very strong correlation 22 between the material strengths or yield strengths, 23 alternate tensile strength of our alloys and the 24 hydrogen content. And the hydrogen content of 25 cladding changes over time. It's a function of the NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1716 14th STREET, N.W., SUITE 200 (202) 234-4433 WASHINGTON, D.C. 20009-4309 www.nealrgross.com

13 1 oxidation process. As the fuel rod oxidizes, it's 2 creating a zirconium oxide, as well as some free 3 hydrogen, some amount of which is then absorbed back 4 into the cladding.

5 Next slide, please. And so what we saw is 6 not unique to Westinghouse fuel. There is a very well-7 established correlation between hydrogen content and 8 material strength. It's something that we also 9 noticed in a lot of open source material. As Brian 10 mentioned, we saw it in a similar strain limit that 11 was developed by PNNL. A lot of other scientific 12 papers that we were able to find also indicated that 13 there's a strong correlation between the hydrogen 14 absorbed in the cladding and its material strength.

15 And the reason -- the reason is as that 16 hydrogen gets absorbed, it is soluble up to a point.

17 Once it hits the terminal solid and solubility limit 18 instead of being soluble what we would call the excess 19 hydrogen, the hydrogen above the solubility limit 20 starts to form zirconium hydride platelets in the 21 cladding and these hydrides can act as cracked 22 propagation pathways such that it makes it easier to 23 fail when you have high PCMI such as you would expect 24 to see during an Condition II transient.

25 Next slide, please. And so the alternate NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1716 14th STREET, N.W., SUITE 200 (202) 234-4433 WASHINGTON, D.C. 20009-4309 www.nealrgross.com

14 1 limit that we're proposing instead of a flat one 2 percent is a function of uniform elongation. Uniform 3 elongation is associated with the materials' ultimate 4 tensile strength. It is effectively the amount of 5 strain at which the deformation ceases to become 6 stable and uniform. It is bend necking and things 7 like that start to occur and you actually start to see 8 a breakdown of the material. So the limit we're 9 proposing is rather than a straight one percent flat 10 limit, it's actually a function of the uniform 11 elongation.

12 Next slide, please.

13 MEMBER MARCH-LEUBA: Pardon me, just for 14 my education. When we look at all the top materials, 15 you know (unintelligible) like Dr. Ballinger. When 16 you said percent, I see that the units are -- it's 17 percent units per meter? What is the one percent?

18 MR. CREDE: It's percent increase of the 19 cladding diameter. So we take the ratio of the pre-20 transient and the post-transient cladding diameter.

21 MEMBER MARCH-LEUBA: And you go back to 22 the previous slide. So in -- doesn't this .01 in the 23 excess scale, there is one percent?

24 MR. CREDE: Yes.

25 MEMBER MARCH-LEUBA: Okay.

NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1716 14th STREET, N.W., SUITE 200 (202) 234-4433 WASHINGTON, D.C. 20009-4309 www.nealrgross.com

15 1 MR. CREDE: And this is just a generic 2 figure. The numbers are just what I put into Excel to 3 make it plot, just to show what it looks like. These 4 are not specific to any actuals here. But yes, this 5 would be the one percent strain.

6 MEMBER HALNON: Hey, Tim, the process of 7 hydrogen getting into -- is that a consistent process 8 without any variables that could affect it differently 9 causing the calculation to be --

10 MR. CREDE: It is how we model. We 11 assume a fractional hydrogen pickup, so it you have a 12 certain amount of oxidation, we assume a certain 13 fraction of the hydrogen that's been generated is 14 absorbed into the cladding. And that is not something 15 that we created or developed as part of this. We have 16 a preexisting approved corrosion model where we have 17 -- one we did a few years ago for -- ZIRLO and 18 Optimized ZIRLO. And it established the hydrogen 19 pickup fraction that we assumed. But yes, we assume 20 a steady hydrogen accumulation throughout life.

21 MEMBER HALNON: Okay.

22 CHAIR BALLINGER: He's being a little bit 23 more theoretical than he needs to be. These are 24 empirical correlations.

25 MR. CREDE: Yes. Thank you.

NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1716 14th STREET, N.W., SUITE 200 (202) 234-4433 WASHINGTON, D.C. 20009-4309 www.nealrgross.com

16 1 CHAIR BALLINGER: For an oxidizing 2 environment, it's usually seven percent of the 3 hydrogen produced ends up in the cladding. For 4 hydrogen over pressure environment which is PWR, it's 5 like 25 percent usually, right?

6 MEMBER HALNON: Okay, so we're ballparking 7 this.

8 CHAIR BALLINGER: To say the least. But 9 it's been ballparked for 40 years.

10 MEMBER HALNON: Tim, you mentioned it's 11 for ZIRLO and for Optimized ZIRLO Westinghouse 12 cladding.

13 MR. CREDE: Correct.

14 MEMBER HALNON: Is it different for each 15 or have you confined the data so that you're looking 16 for one that would represent both of those cladding 17 materials?

18 MR. CREDE: So we weren't sure how we were 19 going to do it when we first started. When we looked 20 at the data, we saw that the uniform elongations were 21 both alloys, was very, very similar. And so the limit 22 that we developed does apply to single limit for both.

23 If you look at other alloys, they would 24 need slightly different ones. If you look at the PNNL 25 model, their database is mostly zirc 2 and zirc 4. It NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1716 14th STREET, N.W., SUITE 200 (202) 234-4433 WASHINGTON, D.C. 20009-4309 www.nealrgross.com

17 1 behaviors similarly, but it is different. In our 2 cases, they were close enough, we have one model for 3 both. If we would do other alloys, it could be 4 different.

5 MEMBER KIRCHNER: Tim, this is Walt 6 Kirchner. I'm attending virtually. Could you just 7 explain -- I know we're not in a closed session, but 8 just for the public and the record, why ZIRLO and 9 Optimized ZIRLO is different from the older circuloid 10 clads and why this might be appropriate to apply to 11 these newer alloys.

12 MR. CREDE: Yes. So I can give a more 13 detailed answer at the closed session, but for the 14 open session what I will say is that Westinghouse 15 ZIRLO and Optimized ZIRLO alloys have components in 16 them and additives that are added in addition to the 17 zirconium that make the material strength a little bit 18 higher than what you would typically see with zirc 4 19 or zirc 2. It is a material reason that we 20 manufactured to have a little bit more material 21 strength.

22 MEMBER KIRCHNER: And this is manifested 23 in primarily by less hydrogen uptake?

24 MR. CREDE: No, no. The material strength 25 itself is tied to the hydrogen uptake over time, but NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1716 14th STREET, N.W., SUITE 200 (202) 234-4433 WASHINGTON, D.C. 20009-4309 www.nealrgross.com

18 1 even when you would have very low hydrogen and it 2 would be soluble, you would still expect a higher 3 material strength for ZIRLO and Optimized ZIRLO 4 because of the additives and the manufacturing process 5 that it undergoes.

6 MEMBER KIRCHNER: Okay. Thank you.

7 MEMBER ROBERTS: This is Tom Roberts. Can 8 you talk briefly about the limitation that is 9 applicable to Condition II over power events?

10 MR. CREDE: Yes, yes, absolutely. So the 11 reason we sort of limited to Condition II events is 12 because those are the only times really where or how 13 we evaluate it that you would see that high stress.

14 Obviously, you would see high stress at high strains 15 for Condition III and IV, major accidents, but we 16 don't use this limit for those accidents. They have 17 their own methods and procedures and everything to 18 look at LOCA and drop rod, rod ejection, things like 19 that.

20 When we do our fuel performance 21 evaluations, we are focused on ensuring that the 22 cladding will not fail during Condition I and 23 Condition II and we don't look at it for Condition I 24 operation because the strain never gets high enough to 25 be a concern. With older methodologies at 4, we used NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1716 14th STREET, N.W., SUITE 200 (202) 234-4433 WASHINGTON, D.C. 20009-4309 www.nealrgross.com

19 1 to have a steady-state strain limit, but it was 2 recognized when we submit it at 5 that it was overly 3 conservative and they got rid of it. We focus on the 4 Condition II strain limit because that is the most 5 restrictive.

6 MEMBER ROBERTS: Okay, thank you, sir.

7 The Condition III and IV, the more severe action 8 scenarios, the existing limits are not being reached 9 yet, is that the issue?

10 MR. CREDE: Right. We would not apply 11 this to those accidents. So they have different 12 criteria that they use to evaluate. Effectively, the 13 cladding can't fail under those conditions. They use 14 different sets of analyses to not ensure that it 15 doesn't fail, but -- or that we don't under predict 16 how many rods are actually failed. But this is not --

17 this does not change or affect the way we look at 18 Condition III and IV accidents.

19 MEMBER ROBERTS: Okay, thank you, sir.

20 Condition II are essentially AOOs where you're 21 required to show that the plant can still be operated 22 after the event?

23 MR. CREDE: Correct, yes.

24 MEMBER ROBERTS: Okay. Thank you.

25 MR. CREDE: Unless there are other NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1716 14th STREET, N.W., SUITE 200 (202) 234-4433 WASHINGTON, D.C. 20009-4309 www.nealrgross.com

20 1 questions, I think, Leslie, you can go ahead. No, 2 sorry, I'm too far.

3 So the alternate limit that we propose as 4 part of this project that we used are the measure data 5 that we had available to develop a uniform elongation 6 strain limit. We made it a function of the total 7 hydrogen content of both the soluble and the insoluble 8 or excess hydrogen. To determine the solubility 9 limit, we used the Kerns correlation. The Kerns 10 correlation is a very common model for solubility. It 11 was developed back in the late '60s. It's been used 12 ever since. It's part of the more recent NRC Reg.

13 Guide 1.236, the PNNL model that we looked as part of 14 this, and also used the Kerns model. It's a very, 15 very common and widely used method for calculating 16 solubility. That's what we used here as well.

17 And so what the limit looks like is your 18 uniform elongation unit is flat when the claddings has 19 very little hydrogen because when the hydrogen is 20 soluble, it doesn't affect the material strength so 21 the limit itself is a flat line. Once you exceed the 22 solubility, you begin to form those zirconium hydrides 23 in the cladding. That's where you see the strain 24 limit start to degrade and it continues to degrade as 25 we absorb more and more hydrogen into the cladding.

NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1716 14th STREET, N.W., SUITE 200 (202) 234-4433 WASHINGTON, D.C. 20009-4309 www.nealrgross.com

21 1 When you see the form and the form will be 2 in the closed session, I apologize, but it looks very, 3 very similar to the same strain limit that was 4 developed by PNNL. Obviously, the numbers are a 5 little bit different because the materials are a 6 little bit different, but the formula model is exactly 7 the same and what you would expect. It's a flat, 8 uniform elongation limit until you start to degrade 9 the material strength and then the limit drops off the 10 board.

11 Last slide, please.

12 So just to summarize, again, Westinghouse 13 and the PWR Owners Group partnered to develop a new 14 and alternative strain limit, a function of the 15 hydrogen content. We are not trying to replace the 16 current one for said limit. This is an alternative.

17 The one percent limit continues to be a valid method 18 for design. But this limit does represent more of a 19 data-driven, material-based limit that is specific to 20 Westinghouse cladding alloys. It meets the NRC's 21 requirement so that we maintain clarity and integrity 22 during the conditions of operation and that does 23 include Condition I operation as well.

24 It has the benefit of recovering margin.

25 The most limiting time in life is strain which in most NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1716 14th STREET, N.W., SUITE 200 (202) 234-4433 WASHINGTON, D.C. 20009-4309 www.nealrgross.com

22 1 cases tends to be right around the point of cladding 2 gap closure because at that early burn up we don't 3 have a whole lot of hydrogen accumulated in the 4 cladding and it does explicitly count for the strength 5 and that utility loss that we do see with accelerated 6 hydrogen pick up.

7 MEMBER MARCH-LEUBA: So just for context, 8 so I understand. You're saying you want to keep both 9 metals available. Is this because the hydrogen base 10 method is more expensive to put in? You have to do 11 additional calculations? And if you don't need it, 12 you don't have to do that expense because basically 13 you're using either one percent or one and a half 14 percent.

15 MR. CREDE: So yes -- please.

16 MEMBER MARCH-LEUBA: I mean unless you 17 want to go to closed session.

18 MR. CREDE: No, no, no.

19 MR. MOUNT: So the one percent limit 20 that's in the SRP as applicable for BWRs, PWRs, so 21 it's a much more generic, widespread limit. The Owners 22 Group project has been only focused on ZIRLO and 23 Optimized ZIRLO cladding materials and only the select 24 number of participants who have joined the project are 25 able to take advantage of it, so we can't --

NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1716 14th STREET, N.W., SUITE 200 (202) 234-4433 WASHINGTON, D.C. 20009-4309 www.nealrgross.com

23 1 MEMBER MARCH-LEUBA: You can't force other 2 people to use your method, but you plan to use your 3 method?

4 MR. MOUNT: Correct.

5 MEMBER MARCH-LEUBA: It's not an issue of 6 it's very difficult to implement and it will cost a 7 lot of money. It's just other people didn't join your 8 group and therefore, they may not want to --

9 MR. MOUNT: And mainly with -- like I 10 said, the other cladding materials in the boiler, we 11 don't want to leave them without a limit.

12 MEMBER MARCH-LEUBA: The most important 13 part from ACRS's point of view is not a technical 14 reason. It's an implementation; I'm just stating.

15 MEMBER KIRCHNER: Tim, just a general 16 question and Brian, you mentioned earlier that this 17 was an issue -- part of the initiation here was that 18 others were doing this and you mentioned PNNL's work 19 there. What was PNNL's objective? Were they trying 20 to demonstrate that even with this new information 21 that the one percent limit was conservative or to 22 evaluate what level of conservatism was within that 23 limit?

24 MR. CREDE: I'm not -- I believe they were 25 commissioned by the NRC to develop that limit. I NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1716 14th STREET, N.W., SUITE 200 (202) 234-4433 WASHINGTON, D.C. 20009-4309 www.nealrgross.com

24 1 think they were looking more at long term storage 2 concerns and things like that. They were interested 3 because at cold conditions, at storage conditions, 4 when the temperature drops, your solubility drops, so 5 you have a lot more hydrides in the cladding and I 6 think they were looking at the material strength more 7 so for storage concerns.

8 CHAIR BALLINGER: What we're talking about 9 here is at temperature. The solubility for hydrogen 10 at room temperature is basically zero, so anything 11 that's in solution at high temperature is going to 12 come out of solution as soon as they shut the plant, 13 shut the plant down.

14 MEMBER KIRCHNER: Good. Thank you.

15 Appreciate that.

16 MR. CREDE: Absolutely. Are we all set?

17 Unless there are other questions that was all I had.

18 CHAIR BALLINGER: Okay, so we need to 19 transition to the staff and I'm not sure who the 20 presenter is. Okay, thanks.

21 Thank you very much and we'll see you in 22 a little bit again.

23 Who's controlling the slides for you?

24 There you go. Thank you.

25 MR. MESSINA: Good morning. My name is NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1716 14th STREET, N.W., SUITE 200 (202) 234-4433 WASHINGTON, D.C. 20009-4309 www.nealrgross.com

25 1 Joseph Messina. I am a reactor systems engineer in 2 the Nuclear Methods and Fuel Analysis Branch of NRR.

3 Also here today with me on the side of the room is a 4 co-reviewer of the topical report, Patrick Raynaud, 5 who is a senior materials engineer in the Office of 6 Research, who is on rotation to NRR during the 7 duration of this review. So he assisted with it.

8 Next slide, please. So in this 9 presentation, I'll be discussing the applicable 10 guidance and regulations for the review, followed by 11 some background information on hydrogen criteria, 12 hydrogen-based criteria before we get into the review 13 focal points. I'll end the presentation with 14 limitations and conditions, as well as conclusion.

15 Next slide, please. So first, I wanted to 16 provide an overview of what's proposed in the topical 17 report. The topical report, as PWR Owners Group said, 18 proposes a change in cladding strain limit as a 19 function of cladding hygiene content for Condition II 20 transients which are AOOs. I provided a non-21 exhaustive list of some examples of Condition II 22 transients on the slide here and I'm not going to read 23 them out, but just for your reference and this limit 24 is intended to be an alternative to the one percent 25 transient clad strain recommended in SRP 42.

NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1716 14th STREET, N.W., SUITE 200 (202) 234-4433 WASHINGTON, D.C. 20009-4309 www.nealrgross.com

26 1 Next slide, please. So here I'll provide 2 some of the applicable regulation and guidance. So 3 for this review, GDC-10 applies. GDC-10 establishes 4 our SAFDLs, Specified Acceptable Fuel Design Limits, 5 to ensure that the cladding does not fail during 6 steady state or the focus of this review the effects 7 of AOOs.

8 Also, SRP-42, as mentioned, talks about a 9 one percent transient clad strain limit. They say 10 that there's no criterion for failure by PCMI or PCI, 11 but two related criteria should be applied. And 12 that's the one percent transient clad strain limit and 13 avoiding fuel melting.

14 And the PWR Owners Group is obviously 15 requesting an alternative to this recommended one --

16 this one percent.

17 Next slide, please. Before I get into the 18 details of the review, I wanted to provide some 19 background and context on hydrogen-based limits. This 20 is not something brand new. The NRC has accepted 21 cladding hydrogen as a surrogate for burn up effects 22 in comparable regulatory applications. For example, in 23 Reg. Guide 1.236 on control rod drop and control rod 24 ejection accidents, it establishes PCMI curves, as 25 shown in the bottom left, which is peak entropy rise NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1716 14th STREET, N.W., SUITE 200 (202) 234-4433 WASHINGTON, D.C. 20009-4309 www.nealrgross.com

27 1 as a function of excess cladding hydrogen and there 2 are different curves for different temperatures, so 3 like hot zero power, cold zero power, as well as for 4 the different cladding types, SRA, so stress relief 5 annealed cladding, as well as RXA, recrystallizing 6 annealed cladding.

7 Additionally, draft rule though it's still 8 with the Commission, 50.46 Charlie establishes alloy-9 specific hydrogen-based oxidation limits, as shown in 10 the curve on the right, which is your ECR, so 11 equivalent cladding reacted, as a function of hydrogen 12 content. And as you see, hydrogen has a negative 13 effect on the ductility of your cladding in reactor.

14 So realistically, it's better that they provide a 15 transient clad strain limit based on actual data 16 rather than just a steady one percent for the entire 17 lifetime of the rod.

18 Next slide, please. Since there are no 19 specific guidance or review for an alternative to the 20 one percent transient strain clad limit in the SRP, 21 it's important to look at some of the primary focal 22 areas of our review. And these can be broken down 23 into four categories. I'll go into detail in each of 24 these categories in the closed session.

25 In the open session, I'll just briefly NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1716 14th STREET, N.W., SUITE 200 (202) 234-4433 WASHINGTON, D.C. 20009-4309 www.nealrgross.com

28 1 touch on each of them. So the first bullet, 2 validation of the limit, this was the bulk of the 3 review. There were a lot of questions on this, such 4 as is there enough data? Is this data appropriate to 5 model PCMI? Are the figures of merit appropriate?

6 How bounding is the limit? And are there any trends 7 and experimental data that might not be captured? So 8 by that I mean should there be an additional variable 9 that's not considered or should there be a separate 10 limit, for example, Optimized ZIRLO versus ZIRLO 11 cladding. So we looked into all of those as part of 12 our quote unquote validation of the model.

13 Next, this second sub-bullet, the impact 14 of AOO on post-transient fuel performance. So if the 15 cladding undergoes an AOO, it will likely at least the 16 limiting rods will likely exceed the yield strength.

17 And the uniform elongation which is the strain 18 corresponding to the ultimate tensile strain would be 19 above -- if you get close to that, you're above the 20 yield strength. And once you would exceed the yield 21 strength, you put some plasticity on the cladding and 22 there's some strain hardening. And also, if you have 23 enough strain of the cladding, you might get reopening 24 of the pellet cladding gap which is obviously 25 detrimental to the transfer, but also would likely due NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1716 14th STREET, N.W., SUITE 200 (202) 234-4433 WASHINGTON, D.C. 20009-4309 www.nealrgross.com

29 1 to a rise in a fission gas release.

2 CHAIR BALLINGER: I have a question about 3 that. There's the no lift off criteria during 4 operation.

5 MR. MESSINA: Yes.

6 CHAIR BALLINGER: But after an AOO, if 7 you've opened the gap, do they have to redo a 8 calculation or is it basically all over, you have to 9 take the fuel out?

10 MR. MESSINA: So as long as the transient 11 cladding strain limit is not exceed, they can put that 12 fuel back into the reactor and I do discuss some of 13 how the Owners Group would discuss that and the 14 impacts of that in the closed session.

15 The second to last bullet, integration 16 with other methods, this was primarily just a check to 17 make sure that they're using approved models for 18 inputs and how they calculate, how they use the model 19 exactly. And lastly, since the SOP does state that 20 the one percent is for both PCMI and PCI or PCISCC, 21 pellet cladding interaction stress corrosion cracking 22 to be specific, we have to look at well, is there 23 going to be any impact on PCI? Are we going to see 24 more PCI failures? So we looked into that as well.

25 Next slide, please. So this slide just NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1716 14th STREET, N.W., SUITE 200 (202) 234-4433 WASHINGTON, D.C. 20009-4309 www.nealrgross.com

30 1 contains our five limitations and conditions of the 2 topical. They're not very interesting and they're 3 relatively straight forward, nothing controversial in 4 them. So first of all, they should be used for PWRs 5 with Optimized ZIRLO or ZIRLO cladding. It's to note 6 that Westinghouse's most recent cladding axiom which 7 is the evolution of the ZIRLO cladding types, it's not 8 applicable to that cladding. So if they wanted to 9 either -- if they wanted to do something other than 10 one percent transient clad strain before that, they 11 would have to come in to the NRC for approval.

12 CHAIR BALLINGER: Is this a limited issue?

13 MR. MESSINA: I believe so and I believe 14 it has to do with the axiom cladding -- the timing.

15 So the axiom cladding topical report was -- it went to 16 ACRS last October and this topical came in last 17 summer. So I envision they may want to come in in the 18 future with data to support an alternate strain limit 19 for that.

20 The third bullet, it's not applicable to 21 cladding with radial hydrides, oxides following, or 22 hydride boostering. The cladding primarily had 23 circumstantial hydrides in the testing, so those --

24 the limit goes out the door once you introduce some of 25 these things.

NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1716 14th STREET, N.W., SUITE 200 (202) 234-4433 WASHINGTON, D.C. 20009-4309 www.nealrgross.com

31 1 CHAIR BALLINGER: Basically, with some 2 kind of constraint on the quality control for the 3 cladding itself, so that you don't end up with the 4 texture that results in radial hydrides, so there's an 5 indirect effect, right?

6 MR. MESSINA: Yes, and primarily we wanted 7 them to be aware that if they notice anything like 8 that, they should not be continuing to use this limit 9 because it would not be applicable.

10 MEMBER KIRCHNER: Ron, this is Walt. Yes, 11 the radial hydrides, that goes back to fabrication 12 technique, doesn't it in QA?

13 MR. MESSINA: Yes.

14 MEMBER KIRCHNER: So by the trademark 15 ZIRLO and Optimized ZIRLO, you would not expect to see 16 radial hydrides. I mean, isn't this, Joseph, just a 17 different way of putting a limit on the applicability 18 of the actual cladding that this UE can be -- a limit 19 can be applied to?

20 CHAIR BALLINGER: That's what I was 21 inferring.

22 MEMBER KIRCHNER: That's what I'm 23 thinking, Ron.

24 MR. MESSINA: So lastly, the two last 25 ones, it should be used with PAD5, a few performance NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1716 14th STREET, N.W., SUITE 200 (202) 234-4433 WASHINGTON, D.C. 20009-4309 www.nealrgross.com

32 1 codes, so we looked at the models in PAD5 that were 2 used to generate inputs for the cladding strain limit.

3 And then lastly, the Condition II transients as I had 4 discussed previously.

5 CHAIR BALLINGER: Not to put you on the 6 spot, but I will, is there any reason for staff to 7 believe that other types of cladding, modern ones, 8 would not be able to use the same, if they were able 9 to develop the data, use the same limits?

10 MR. MESSINA: I envision M5 --

11 CHAIR BALLINGER: Like M5 for example.

12 MR. MESSINA: I envision M5 would be able 13 to do something similar, but as for the exact number 14 it probably would differ because fabrication has such 15 a great impact on the behavior.

16 CHAIR BALLINGER: But the hydrogen 17 absorption is so much lower.

18 MR. MESSINA: Yes.

19 CHAIR BALLINGER: On these modern 20 claddings, so --

21 MR. MESSINA: Okay, next slide.

22 MEMBER KIRCHNER: Before you go on, 23 Joseph, this is Walt again, I find it a little strange 24 that you limit it to the PAD5 code. Now that's the --

25 if I'm correct, that's the Westinghouse fuel NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1716 14th STREET, N.W., SUITE 200 (202) 234-4433 WASHINGTON, D.C. 20009-4309 www.nealrgross.com

33 1 performance code.

2 MR. MESSINA: Correct.

3 MEMBER KIRCHNER: For example, I want to 4 use the FAST code that NRC developed with PNNL. Why 5 would this model not work in the FAST code?

6 MR. MESSINA: Well, primarily what we're 7 getting at with that condition and limitation is they 8 need to use an NRC-approved code.

9 MEMBER KIRCHNER: Okay, yes.

10 MR. MESSINA: Your example of FAST, they 11 could not use FAST because FAST has technically not 12 been NRC reviewed, despite being an NRC code.

13 MEMBER KIRCHNER: Right, okay, all right.

14 Fair enough. Thank you.

15 PARTICIPANT: And Joseph, on that point, 16 as you said, what you've done -- a detailed review of 17 the PADS code and how this has been input to it. So 18 you basically have done the code review associated 19 with this element.

20 MR. MESSINA: Part of it, yes. Next 21 slide, please.

22 MEMBER MARCH-LEUBA: Basically, you use 23 PAD5 to list how much hydrogen is in the cladding for 24 the experiments. Just kind of embedded into the 25 correlation. If you used a different code, you might NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1716 14th STREET, N.W., SUITE 200 (202) 234-4433 WASHINGTON, D.C. 20009-4309 www.nealrgross.com

34 1 have come up with a different hydrogen content?

2 MR. MESSINA: Possibly, yes. I believe 3 so.

4 MEMBER MARCH-LEUBA: Because you used PAD5 5 to develop your correlation data. It's more accurate 6 to continue to use PAD5, so if you want to use 7 something different, you have to use the 5 or 8 reanalyze the raw data.

9 MR. MESSINA: Thank you.

10 CHAIR BALLINGER: Again, it's not the 11 issue, it's the way PAD5 does the calculation. That's 12 independent of the amount of hydrogen that's actually 13 absorbed.

14 MR. MESSINA: True.

15 CHAIR BALLINGER: During certain 16 operations.

17 MR. MESSINA: And actually the test -- the 18 data for hydrogen was mostly measured data, not a 19 calculated, I believe.

20 So the conclusions, we find -- the NRC 21 staff finds the proposed limit to be acceptable 22 because the data is adequately representative of PCMI.

23 The limit is reasonably bounding of the data to 24 sufficient confidence that arrives that means the 25 limit should not fail during a Condition II event by NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1716 14th STREET, N.W., SUITE 200 (202) 234-4433 WASHINGTON, D.C. 20009-4309 www.nealrgross.com

35 1 PCMI. The limit integrates acceptably with existing 2 fuel performance methodologies to capture the relevant 3 phenomena associated with transient cladding strain 4 and fuel behavior before and after AOOs as well as 5 during AOOs because PAD5 has their method for how they 6 approach these Condition II events.

7 And lastly, it's not expected to impact 8 the number of PCI SEC failures.

9 That's it for me. Any questions?

10 CHAIR BALLINGER: Questions from the 11 members?

12 Okay, this is going to be the end of the 13 open session, so we now need to go out and ask for 14 public comment. So if there are members of the public 15 that wish to make a comment, please state your name 16 and your organization and then make your comment.

17 Hearing none so far, so thank you again.

18 So now we need to make a transition to the closed 19 session and so I would suggest that we make a break 20 until 9:30. Again, everything is squared away on 21 who's going to be here or not. So we'll take what now 22 amounts to a ten-minute break. Thank you.

23 (Whereupon, the above-entitled matter went 24 off the record at 9:20 p.m.)

25 NEAL R. GROSS COURT REPORTERS AND TRANSCRIBERS 1716 14th STREET, N.W., SUITE 200 (202) 234-4433 WASHINGTON, D.C. 20009-4309 www.nealrgross.com

Global Expertise

• One Voice ACRS Fuels, Materials, and Structures Subcommittee - Open Session PWROG-21001-P/NP, Hydrogen-Based Transient Cladding Strain Limit Brian L. Mount (Dominion), Tim M. Crede (Westinghouse) August 24, 2023 Westinghouse Non-Proprietary Class 3

Agenda

• Introduction

• Background and Overview

• Proposed Alternate Cladding Strain Limit

• Summary and Conclusions Westinghouse Non-Proprietary Class 3

Introduction

• In 2019, the PWR Owners Group (PWROG) approved a project to develop a new design limit for the transient cladding strain fuel performance criterion

• The current strain limit for Westinghouse fuel is contained in Section 4.2, Fuel System Design of NUREG-800, the NRC Standard Review Plan (SRP)

• The project objective was to develop an alternative, data-driven and performance-based design limit derived from test data and specific to current cladding materials o The new strain limit is a more accurate reflection of measured strain data o It also increases margin at the limiting time in life, to support licensees who are limited by the cladding strain design criterion when they pursue increased fuel duty, optimized loading pattern development, extended cycle lengths, etc.

Westinghouse Non-Proprietary Class 3

Background and Overview

• NUREG-0800, Section 4.2 Revision 3, SRP Acceptance Criteria B. Fuel Rod Failure, vi. states:

The first criterion limits uniform strain of the cladding to no more than 1 percent. In this context, uniform strain (elastic and inelastic) is defined as transient-induced deformation with gauge lengths corresponding to cladding dimensions.

• The objective of the strain limit is to prevent fuel failures from over-straining the cladding due to thermal swelling of the fuel pellet during Condition II overpower transient events Westinghouse Non-Proprietary Class 3

Background and Overview

• The current 1% cladding strain limit is a valid and conservative design limit for zirconium-based cladding

• However, a review of measured strain data for Westinghouse fuel determined that a 1% cladding strain limit does not reflect the true behavior of current cladding alloys, ZIRLO and Optimized ZIRLO' High Performance Fuel Cladding Material

• Additionally, the material yield strength (YS) and ultimate tensile stress (UTS) are impacted by the hydrogen content of the cladding and change over time o Hydrogen is absorbed into the cladding material as a result of the oxidation process A fraction of the free hydrogen is Zr + 2H2O ZrO2 + 2H2 absorbed into the cladding material ZIRLO and Optimized ZIRLO are trademarks or registered trademarks of Westinghouse Electric Company LLC, its affiliates and/or its subsidiaries in the United States of America and may be registered in other countries throughout the world. All rights reserved. Unauthorized use is strictly prohibited.

Westinghouse Non-Proprietary Class 3 Other names may be trademarks of their respective owners.

Proposed Alternate Cladding Strain Limit

• There is a well-established relationship between the hydrogen content and material strength o This is based on Westinghouse measured data, as well as other scientific organizations

• The material strength of the cladding is reduced as excess hydrogen begins to form zirconium hydride (ZrH) platelets o Hydrogen in the cladding is soluble until it reaches the terminal solid solubility (TSS) limit, which is a function of the cladding temperatures o Hydrogen above the TSS is excess hydrogen and leads to the formation of hydrides

• Hydrides act as crack propagation pathways through the cladding which make it more likely to result in cladding failure due to pellet-cladding mechanical interaction (PCMI)

Westinghouse Non-Proprietary Class 3

Proposed Alternate Cladding Strain Limit

• Instead of a constant 1% strain limit, the new design limit is based on uniform elongation (UE) o UE corresponds to the UTS of the cladding o UE represents the strain at which deformation ceases to be uniform and stable Westinghouse Non-Proprietary Class 3

Proposed Alternate Cladding Strain Limit

• Westinghouse used available measured data to develop an alternative UE strain limit as a function of the total (soluble and excess) hydrogen content o The TSS is determined using the Kearns solubility correlation, which is a common model utilized by the NRC for PCMI cladding failure thresholds (see NRC Regulation Guide 1.236)

• The UE limit is a constant value at low hydrogen values (i.e., when the hydrogen is soluble) and degrades slowly once excess hydrogen begins to form ZrH platelets in the cladding o The UE limit is comparable to a strain limit developed by Pacific Northwest National Laboratory (PNNL) for zirconium-based cladding Westinghouse Non-Proprietary Class 3

Summary and Conclusions

• The PWROG approved a project to develop a hydrogen-based design limit for transient cladding strain o This is an alternative to the 1% strain limit contained in Section 4.2 of the SRP

• It represents a data-drive, performance-based design limit based on measured strain test data for Westinghouse cladding alloys

• The hydrogen-based strain limit:

o Meets the NRC requirement for maintaining cladding integrity during Condition II overpower transient events, o Recovers margin at the historically limiting time in life (near the point of gap closure), and o Explicitly accounts for the cladding strength and ductility loss with hydrogen pickup Westinghouse Non-Proprietary Class 3

Questions?

Westinghouse Non-Proprietary Class 3

NRC Review of PWROG-21001, Hydrogen-Based Transient Cladding Strain Limit ACRS Fuels, Materials, and Structures SC Open Session August 24, 2023 Joseph Messina, Patrick Raynaud Nuclear Methods and Fuel Analysis Office of Nuclear Reactor Regulation 1

Outline

• Applicable guidance and regulations

• Historical background

• Review focal points

• Limitations and conditions

• Conclusion 2

Overview

• In PWROG-21001, the PWROG proposed a transient cladding strain (TCS) limit that is a function of cladding hydrogen content for condition II transients Condition II transients include:

o Feedwater (FW) malfunctions causing a decrease in FW temperature o FW malfunction causing an increase in FW flow o Excessive increase in secondary FW flow o Loss of normal FW o Inadvertent opening of a steam generator safety or relief valve o Uncontrolled rod cluster control assembly bank withdrawal at power o Uncontrolled boron dilution o Inadvertent ECCS actuation at power

• H-based TCS limit intended to be an alternative to the 1% TCS limit currently utilized 3

Applicable Regulations and Guidance

• GDC-10: Establishes specified acceptable fuel design limits to ensure that the fuel is not damaged during normal operation, including the effects of anticipated operational occurrences (AOOs)

Not damaged means that fuel rods do not fail, fuel system dimensions remain within operational tolerances, and functional capabilities are not reduced below those assumed in the safety analyses during normal operation and AOOs

• SRP 4.2 paragraph II.1.B.vi: States that no criterion exists for fuel failure resulting from pellet-cladding interaction (PCI) or pellet-cladding mechanical interaction (PCMI), but that two related criteria should be applied:

1) the strain of the cladding during a transient should not exceed 1%; and

2) fuel melting should be avoided 4

Hydrogen-Based Limits - Historical Perspective

• The NRC has accepted cladding hydrogen content as a reasonable surrogate to BU effects in other comparable regulatory applications

• Historically, there is precedent for the use of hydrogen-based limits:

RG 1.236 establishes hydrogen-based enthalpy failure criteria for PCMI during PWR control rod ejection and BWR control rod drop accidents Draft final rule 50.46c establishes alloy-specific hydrogen-based oxidation limits Fig. 5 of RG 1.236 Fig. 2 of Draft RG 1.224 (to support 50.46c) 5

Review Focal Points

• The NRC review focused on several areas:

Validation of limit o Quantity and quality of experimental data?

- Is the experimental data representative of PCMI?

o Are the figures of merit appropriate to accurately model PCMI?

o Does the limit sufficiently bound the data?

- Are there any potentially non-conservative regions?

o Are there any trends in the experimental data that may not be captured by the limit?

Impact of AOO on post-transient fuel performance Integration with other methods Impact on PCI-SCC 6

Limitations and Conditions The PWROG hydrogen-based TCS is limited to:

1. Use in Westinghouse or Combustion Engineering PWRs

2. Use with ZIRLO or Optimized ZIRLO cladding

3. Not applicable to cladding with radial hydrides, oxide spalling, or hydride blistering

4. Use with PAD5 fuel performance code

5. Condition II transients 7

Conclusion The NRC staff find the hydrogen-based TCS limit in PWROG-21001 to be acceptable because:

1. the data presented is adequately representative of the PCMI phenomenon;

2. the proposed hydrogen-based TCS limit is reasonably bounding of the data so that there is a high degree of confidence that fuel rods that meet the limit will not fail due to PCMI during a Condition II event;

3. the proposed hydrogen-based TCS limit integrates acceptably with existing fuel performance methodologies to capture the relevant phenomena associated with TCS and fuel behavior before and after AOOs; and

4. the proposed hydrogen-based TCS limit is not expected to impact the number of PCI-SCC failures 8

Questions?

9