ML24100A030
| ML24100A030 | |
| Person / Time | |
|---|---|
| Site: | 07201032 |
| Issue date: | 04/08/2024 |
| From: | Holtec |
| To: | Office of Nuclear Material Safety and Safeguards |
| Shared Package | |
| ML24100A027 | List: |
| References | |
| 5018117, CAC 001028, EPID L-2021-LLA-0053 | |
| Download: ML24100A030 (1) | |
Text
Holtec Letter 5018117 Attachment 2 Page 1 of 6 Holtec Responses to NRCs Clarification Questions documented in Conversation Record
[1] on HI-STORM FW Amendment 7 Clarification Call dated February 28, 2024 Docket No. 72-1032 Holtec International HI-STORM FW Multipurpose Canister Storage System Certificate of Compliance No. 1032 Amendment 7 Clarification Question 1:
Provide additional justification that the MPC-32ML and MPC-37-CBS fuel baskets, both analyzed for the tipover accident condition using an enhanced LS-DYNA finite element analysis (FEA) model in a Version E overpack, bound the stress analysis results of the remaining fuel basket/overpack pairings currently presented in the SAR.
In order to demonstrate that the presented basket/overpack selection criteria consistently selects the basket that produces the maximum stresses, the applicant is requested to 1) perform additional studies, by employing the enhanced FEA model on a targeted sample of basket/overpack pairings, to validate that the Beta parameter consistently determines/correlates to the magnitude of basket stresses, and 2) create a new method, to replace the basket permanent deflection value, of indicating the dynamic response of the basket used to choose the overpack pairing.
Alternatively, the applicant may analyze all basket/overpack pairings using the enhanced FEA model to determine the resulting stresses without attempting to develop a methodology for selecting the bounding baskets for stress analysis.
In SAR section 2.2.8, the applicant introduced a Beta parameter for all baskets in the FW system, as tabulated in Table 2.2.15, which is intended to predict the basket that will produce the most limiting stress results. Although it is not specifically stated in the SAR, the LS-DYNA model employed to produce the limiting basket stress results is actually an enhanced model that incorporates three major changes to the chosen basket models, as described in section G.2 of report HI-2200503, revision 8. The Beta parameter is based on basket-specific geometry and static fuel assembly weight. Therefore, the effects of the dynamic response of the basket in specific overpack pairings is not addressed in this method of determination of the bounding fuel baskets.
Furthermore, there is no indication of what action is to be taken for the introduction of a revised or new overpack.
In response to NRC staff concerns raised after the January 26 coordination meeting regarding the Beta parameter not addressing the dynamic response of the fuel basket in different overpacks, the applicant indicated that the magnitudes of basket maximum permanent deflection results are indicative of the basket dynamic response in different overpacks, where a higher permanent deflection value would reflect a higher dynamic energy input. Therefore, presumably based on this assumption, the applicant chose to reanalyze the baskets with higher permanent deflection values in the Version E overpack.
Holtec Letter 5018117 Attachment 2 Page 2 of 6 As a means of validating the applicants statement, the staff reviewed the basket deflection and stress results as well as the associated deceleration results for all basket/overpack pairings submitted by the applicant in November 2023. The staff has summarized the following observations, which should be addressed in justifying the bounding fuel basket determination.
- a. The baskets with the 10 highest maximum stress results are not associated with the MPC-32ML and MPC-37-CBS baskets, which Holtec chose as limiting based on the Beta value. Therefore, there does not appear to be a direct correlation between the magnitude of the Beta parameter and the resulting basket stresses.
- b. The two largest maximum permanent deflection values do appear to correspond to the two baskets chosen as limiting, however, there does not appear to be a direct correlation between these deflection values and the maximum reported stresses.
- c. The maximum deceleration values for the lids of the Version E overpack system appear to range from 48 to 69 gs, whereas those of the Version UVH appear to range from 86 to 90 gs, and those of the Standard version fall somewhere in between. It is noted that the maximum deceleration of the MPC-37-CBS basket in the Version E overpack, chosen as the limiting CBS-type fuel basket for stress determination, is 48 gs. Although the staff recognizes that the dynamic response of any one component of a more complex structural system is not intuitively predictable, the magnitude of the dynamic impact energy input to each unique cask system via the overpack during the tipover event is generally indicative of the impact energy levels imparted to the internal cask components, including the fuel baskets. Therefore, the magnitudes of the dynamic impact for each basket/overpack pairing should be further considered in choosing any limiting cask system for stress analysis.
Additionally, the applicant should add explicit instructions regarding the analytical requirements for the introduction of a new or revised overpack paired with an existing or new fuel basket.
Based on the items listed above, the staff concludes that the applicant has not provided sufficient justification to conclude that the basket/overpack pairings chosen by the Beta factor and maximum permanent deflection criteria reliably produce the most limiting stress results.
This information is needed to determine compliance with the regulatory requirements in 10 CFR 72.236(b).
Holtec Response:
As concurred during the February 28th meeting between Holtec and NRC staff, Holtec has (i) removed the discussion of Beta parameter from FSAR Chapter 2, (ii) explicitly analyzed a total of five MPC/overpack pairings to demonstrate that the primary stresses in the Metamic-HT fuel baskets are acceptable (with results included in FSAR Chapter 3 and Supplement 3.I), and (iii) added a licensing commitment in FSAR Chapter 3 that the enhanced LS-DYNA model must be used to evaluate primary stresses in the fuel basket for future tipover analyses.
Holtec Letter 5018117 Attachment 2 Page 3 of 6 Clarification Question 2:
Justify how the methodology for selecting the bounding baskets for detailed stress analysis accounts for changes to components of the storage system (e.g., the overpack) other than the basket panel slenderness and fuel assembly weight.
In SAR section 2.2.8, the applicant introduced a methodology for selecting the bounding baskets for detailed stress analysis. There are currently two screening criteria that determine if a basket is bounding and whether a detailed stress analysis must be performed using an enhanced FEA model.
After discussing the screening criteria, the SAR then states that this detailed stress analysis shall be performed for the storage overpack that caused the maximum permanent deflection of fuel baskets. The staff is concerned that the overpack could affect which baskets experience the most critical stresses, and thus the overpack should be considered in determining which baskets are bounding. By accounting for the overpack after determining the bounding basket, the applicant could be neglecting to analyze a basket and overpack combination that results in the most critical stress results.
For each basket/overpack pairing, an LS-DYNA model must be constructed and analyzed for the tipover condition to verify that the design criteria for the overpack, MPC, and fuel basket permanent deflection limit is met. It appears to staff that once this FEA model is created, the determination of the resulting basket stresses for each basket/overpack pairing would be readily available and provide a much more direct and accurate indication of which pairing produces the maximum basket stress than the maximum permanent deflection results (as discussed in Question 1). These maximum stress results would then provide a direct comparison with the design acceptance criteria of 90% of true ultimate strength. If the stress criterion is not met in this initial FEA model, the fuel basket would be analyzed using the enhanced model, as described in section G.2 of Report HI-2200503, revision 8.
This information is needed to determine compliance with the regulatory requirements in 10 CFR 72.236(b).
Holtec Response:
As concurred during the February 28th meeting between Holtec and NRC staff, Holtec has (i) removed the discussion of Beta parameter from FSAR Chapter 2, (ii) explicitly analyzed a total of five MPC/overpack pairings to demonstrate that the primary stresses in the Metamic-HT fuel baskets are acceptable (with results included in FSAR Chapter 3 and Supplement 3.I), and (iii) added a licensing commitment in FSAR Chapter 3 that the enhanced LS-DYNA model must be used to evaluate primary stresses in the fuel basket for future tipover analyses.
Holtec Letter 5018117 Attachment 2 Page 4 of 6 Clarification Question 3:
Describe the enhanced FEA model and the selection of the bounding basket in a prominent place in the SAR and calculations, presenting only the new analysis models used for the determination of basket stress results. Also, since some legacy figures of the original FEA model must remain to support the existing presented permanent deflection results, these must be labeled appropriately to distinguish them from any new figures.
Currently, the statements of intent in sections 2.2.8, 3.4.4.1.4, and 3.I of the latest revision of the SAR that rely on the current stress results of the limiting baskets, MPC-32ML and MPC-37-CBS, do not clearly convey that the existing stress (and strain) results for other baskets presented in previous SAR revisions are superseded by those of the limiting baskets. To eliminate confusion for current and future users of the SAR, this new information must be prominently and clearly communicated, and further clarifying language should be added to explain that the existing FEA models for all baskets are still used as the basis for reported permanent deflection results.
New figures depicting the basket temperature zones associated with the enhanced FEA model must be added to the SAR, and the existing ones supporting the legacy permanent deflection results must be clearly labeled to distinguish between them. The previous basket stress contours that are no longer valid must be removed from the SAR, and any tables presenting basket strain results must be corrected to remove these results, as they are no longer pertinent or valid (e.g., table 3.I.3.9).
Similarly, the design reports supporting the basket tipover designs must be revised for clarification purposes. Currently, only the report for the Version E overpack tipover analysis, HI-2200503, is revised and submitted to add appendix G. The other appendices of this report as well as the other design reports documenting the standard and Version UVH overpack tipover analyses (i.e., HI-2094353, HI-2166998, and HI-2210313) must also be revised to communicate the change in the method of basket stress determination, as well as remove existing erroneous and superseded results. Current examples of portions of HI2200503 that require further revision are the main body and appendices C, D and F.
These requested clarifications serve to prevent any confusion as to which tipover models and results form the licensing basis.
This information is needed to determine compliance with the regulatory requirements in 10 CFR 72.11(a).
Holtec Response:
The enhanced FEA model used to evaluate primary stresses in the fuel baskets is now described in subparagraph 3.4.4.1.4e of FSAR Chapter 3. In addition, the updated temperature zones for the MPC-32ML, MPC-89 CBS, and MPC-37 CBS, which are used in conjunction with the enhanced FEA model, are shown in Figures 3.4.52A through 3.4.52C, respectively. For all other basket types, the temperature zones are unaltered, and they remain as shown in Figure 3.4.12.
Holtec Letter 5018117 Attachment 2 Page 5 of 6 All stress results presented in the FSAR for the Metamic-HT fuel baskets are now linked to the enhanced FEA model. Earlier stress/strain results associated with the original FEA model, [
PROPRIETARY INFORMATION WITHHELD PER 10CFR2.390 ], have been deleted from the FSAR. The calculation packages (HI-2094353, HI-2166998, HI-2200503, and HI-2210313) have also been updated to clearly indicate that the stress results obtained using the enhanced FEA model supersede the stress/strain results from the earlier solutions, as discussed during March 20th meeting. The calculation packages also include discussions of secondary stresses, as well as localized element erosions, that arise in the fuel baskets due to non-mechanistic tipover event.
Clarification Question 4:
Correct or justify the true stress-strain curve values determined for Metamic-HT basket material at 180 °C.
Appendix B of report HI-2200503, revision 8, presents the determination of true stress-strain curves for the Metamic-HT basket material for use in the LS-DYNA model. For the material at 180 °C, it is stated that the input parameters are interpolated from the minimum guaranteed values presented in the Metamic-HT Source Book for 200 °C and ambient temperature. In Appendix B, ambient is defined as 30 °C, while the Source Book states that ambient is 40 °C.
This information is needed to determine compliance with the regulatory requirements in 10 CFR 72.236(l).
Holtec Response:
The true stress-strain curve for Metamic-HT material at 180°C has been updated in Appendix B of HI-2200503 to align with the Metamic-HT Source Book and use a 40°C ambient temperature for interpolation of strength properties. However, the LS-DYNA tipover solution presented in Appendix G for MPC-37 CBS inside the HI-STORM FW Version E still conservatively uses the previous true stress-strain curve, which is determined based on a 30ºC ambient temperature. This embedded conservatism is documented in note below Figure G.3(h).
Clarification Question 5:
Administrative errors that warrant attention, but are not necessary for the staff to make a regulatory finding:
- a. Provide a copy of the revised DS-331 document for staff review. SAR reference 2.2.12 indicates that the current revision of this document is Revision 2. However, Revision 3 was submitted for staff review in August 2023. In December 2023, the staff advised that several errors were present in this revision, specifically the definition of the true ultimate strength values for Metamic-HT. Please correct these errors and resubmit for review, as well as updating reference 2.2.12 to the correct revision number.
Holtec Letter 5018117 Attachment 2 Page 6 of 6
- b. In Table 2.2.5, the w value of 10.81 lbf/in entered for the MPC-32ML fuel basket should be 11.22 lbf/in.
Holtec Response:
- a. DS-331 has been revised to address the comments received from NRC staff in December 2023. The latest revision of this document is now Revision 4. SAR reference 2.2.12 has been updated accordingly, and a copy of DS-331 Rev. 4 is provided as Attachment 9 to this submittal for the staffs review.
- b. Table 2.2.5 has been deleted from FSAR Chapter 2 since the previous Beta parameter is eliminated. However, a similar table has been added in Chapter 3 (i.e., Table 3.4.22), which summarizes the key parameters for the existing Metamic-HT fuel baskets. The correct value for w is reported in Table 3.4.22 for the MPC-32ML.