ML23216A218
| ML23216A218 | |
| Person / Time | |
|---|---|
| Site: | 07201032 |
| Issue date: | 08/03/2023 |
| From: | Yen-Ju Chen Storage and Transportation Licensing Branch |
| To: | Holtec |
| Shared Package | |
| ML23216A217 | List: |
| References | |
| Download: ML23216A218 (1) | |
Text
HI-STORM FLOOD/WIND Amendment No. 7 Clarification Call on RAI 3-10 August 3, 2023 The original request for additional information (RAI) was issued on March 23, 2023 (Agencywide Documents Access and Management System [ADAMS] Accession No.ML23074A101). Holtec International (Holtec) provided responses on May 8, 2023 (ML23128A302). A clarification call was held on June 15, 2023 (ML23177A278). Supplemental information was provided following the clarification call on June 30, 2023 (ML23181A192) and July 11, 2023 (ML23192A031). The following summarizes questions and clarifications shared between the U.S. Nuclear Regulatory Commission (NRC) staff and Holtec prior to the official clarification call on August 3, 2023.
RAI 3-10 Clarification Question. Justify the use of material properties at 275°C in the technical justification for the maximum permanent deflection criterion in Holtecs proprietary position paper, DS-331, revision 2, Structural Acceptance Criteria for the Metamic-HT Fuel Basket.
The technical basis for the maximum permanent deflection criterion of the fuel baskets is based on an ANSYS analysis of two Metamic-HT fuel baskets, F-37 and MPC-89. In the paper, Holtec compares determines the stress results in the basket panels at the 0.5% permanent deflection and compares that to the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel (B&PV) Code section III, division 1, subsection NG level D allowable stress limits. Holtec concludes that these two baskets, at 275°C, deflected to the permanent deflection criterion, experience stresses slightly below the ASME stress limit, and therefore, the deflection criterion is an acceptable design criterion for demonstrating the structural integrity of the fuel basket.
However, the bounding temperature distributions for the fuel baskets used in the tipover analysis in HI-2200503 for the MPC-44 CBS and MPC-37 CBS include temperatures as high as 380°C. See figure C.1-4 of HI-2200503. These temperatures values are considerably higher than the 275°C temperature used in the analyses supporting the technical basis for the deflection criterion. The staff notes that the yield and ultimate stress of the material will decrease with higher temperatures. The staff understands that there is some relationship between permanent deflection and the yield stress as it changes with temperature. However, the ultimate stress of Metamic-HT decreases at a different higher rate with increasing temperatures than the yield stress decreases.
The staffs concern with DS-331 analyses is that performing the analyses at a higher temperature, closer to that experienced by the inner core of the fuel basket, will result in the stress experienced at the deflection limit being closer to the ultimate stress, since the ultimate stress will be reduced at the higher temperature. More broadly, the staff is concerned that using a deflection criterion does not maintain parity with temperature dependent material properties in the way a more traditional structural design criteria does (e.g., ASME stress limits).
Additionally, the staff is reviewing the acceptability of using 90% of the ultimate stress listed in the Metamic-HT sourcebook. The staffs ongoing review is focused on the use of the ultimate stress values provided by the Metamic-HT sourcebook material property data as acceptance criteria for level D service conditions compared to previously accepted material property data Attachment
2 documented in ASME B&PV Code section II. Consider providing any information to support the staffs review.
This information is needed to determine compliance with the regulatory requirements in title 10 of the Code of Federal Regulations section 72.236(c).
Additional Follow-up Clarification: Previously, DS-331 states that 0.5% total deflection was roughly equivalent to the yield strength. There are several reasons why the analyzed temperature was less concerning when comparing deflection to the yield strength. As noted in the question,
- 1) Both yield strength and deflection will change less with temperature than the ultimate strength.
- 2) As DS-331 mentions multiple times, tying the deflection criteria loosely to the yield strength is quite conservative. So, the previous total deflection criterion is a bit nebulous, but the conservatism makes up for the variability.
DS-331 revision 2 states that 0.5% permanent deflection is roughly equivalent to 90% of the ultimate strength. This question is related to Flood/Wind (FW) Amendment 7 and the changes in DS-331 revision 2 that are related to this amendment. Specifically,
- 1) As temperature changes, the ultimate strength is changing at a larger rate than the yield strength and permanent deflection, meaning that changing from yield to ultimate makes temperatures a greater concern.
- 2) Tying the deflection criterion to 90% of the ultimate strength is significantly less conservative than tying it to the yield strength as the previous revisions of DS-331 had done. Therefore, inaccuracies or variability are more concerning as they are more likely to result in failure.
In summary, the higher temperature concern is new to FW Amendment No. 7, the change from total deflection (elastic) to permanent deflection (plastic) is a concern, and the 90% of ultimate strength is also new as the basis for the deflection criterion.
Holtec Clarification:
This information will help us respond more precisely to the staffs clarification questions and hopefully minimize further iterations as we work to resolve the outstanding technical questions.
To that end, there are two statements that we would like to address and/or clarify immediately, which are:
(1) the change from total deflection (elastic) to permanent deflection (plastic) is a concern (2) the 90% of ultimate strength is also new as the basis for the deflection criterion Regarding (1), as we have stated before, Holtec maintains that the structural acceptance criterion for the Metamic-HT fuel baskets in the HI-STORM FW final safety analysis report (FSAR) has always been, and still remains, that the permanent deflection of the fuel basket panels shall not exceed 0.5% of the unsupported panel width. We also acknowledge that, until recently, the text matter in HI-STORM FW FSAR Chapters 2, 3 and 6 was conflicting in certain places (which we regret), but the history of Holtec paper DS-331 from its origin in 2009 (beginning with the HI-STAR 180 SAR) until now shows a consistent pattern of use, wherein the
3 acceptability of the Metamic-HT basket panels was evaluated based on their permanent deflection. This same concern was previously raised by the NRC Staff in RAI 3-10 (ML23074A101) and formally responded to by Holtec in our RAI response submittal (ML23128A303), and further clarified in our follow-up letter dated June 30, 2023 (ML23181A193).
Regarding (2), the reference to the ASME Level D stress limit (i.e., 90% of ultimate strength) for plastic system analysis is not new to Holtec paper DS-331. Revision 1 of DS-331 (which dates back to 2009) clearly states on page 3:
For plastic system analysis, the maximum primary stress intensity is limited to 0.90 u for Level D conditions per paragraph F-1341.2 of the ASME Code, where u is the true ultimate strength and equals 20,780 psi for Metamic-HT at 275°C (see Figure 3).
DS-331 refers to paragraph F-1341.2 and 0.9 u because a key objective of the paper is to show that the basket deflection limit is conservative relative to the analogous ASME Level D stress limit for plastic analysis. The reason that the plastic analysis limit is considered is because the cask tipover analysis utilizes an elastic-plastic material model for the Metamic-HT fuel basket. In revision 1 of DS-331, it was demonstrated that a total deflection limit equal to 0.5% of the unsupported panel width is extremely conservative relative to the ASME plastic analysis limit, as the calculated stresses barely exceeded the yield strength of Metamic-HT at 275°C. This finding, namely the large disparity in the safety margin to failure between the ASME Code stress limit and the postulated deflection limit, is what prompted Holtec to evaluate the Metamic-HT fuel baskets based on their maximum permanent deflection as opposed to their maximum total deflection. In summary, the explicit calculations showing the stresses in a Metamic-HT beam strip, when the maximum permanent deflection reaches 0.5% of the unsupported panel width, are new to DS-331, but the comparison with 90% of the ultimate strength is not.