ML21070A324
| ML21070A324 | |
| Person / Time | |
|---|---|
| Site: | 07201015 |
| Issue date: | 03/04/2021 |
| From: | Fowler W NAC International |
| To: | Document Control Desk, Office of Nuclear Material Safety and Safeguards |
| References | |
| ED20210033 | |
| Download: ML21070A324 (5) | |
Text
A NAC:
INTERNATIONAL March 4, 2021 U.S. Nuclear Regulatory Commission 11555 Rockville Pike Rockville, MD 20852-2738 Attn:
Document Control Desk Director, Division of Spent Fuel Management, Office of Nuclear Material Safety and Safeguards Atlanta Corporate Headquarters 3930 East Jones Bridge Road, Suite 200 Peachtree Corners, GA 30092 Phone: 770-447-1144 Fax: 770-447-l'JW
Subject:
10 CFR 72.242 Reportable Licensing Basis Thermal Evaluation Deficiency (Fuel Rod Assembly Effective Thermal Conductivity) for the NAC-UMS Dry Cask Storage Systems Docket Nos. 72-1015 72.242(d)(I) -Abstract Overview NAC has internally identified a licensing basis thermal evaluation deficiency for the NAC-UMS and MAGNASTOR dry cask storage systems. Specifically, the deficiency pertains to how some PWR thermal models use the fuel pellet size in the licensing basis. Correction to this parameter does impact the results of some licensing basis thermal evaluations. While the deficiency affects some licensing basis thermal evaluations and their results, NAC has determined there is no safety consequence on the ability of the storage systems to perform their licensing basis functions.
The corrections to these thermal model inputs does have a negative impact on the reported peak clad temperatures (PCTs) in the associated dry cask storage system's Final Safety Analysis Report (FSAR). Irt other words, some reported PCTs are currently lower than they should be. When these fuel pellet parameters are corrected, some of the PCTs for the NAC-UMS dry storage system are slightly higher than the licensing basis limits (i.e., 400 °C), but that is not the case for the MAGNSTOR dry storage system.
Therefore, NAC has determined this situation is reportable for the NAC-UMS system under 10 CFR Part 72.242( d) but not MAGNASTOR. However, the deficiency is associated with both systems which is why NAC is also including discussions on MAGNASTOR in this report.
This condition is reportable since the deficiency results in some PCTs being in excess of the licensing basis limits for the NAC-UMS system when the corrections are made to the licensing basis thermal evaluations, as presented in the FSAR. However, NAC has determined there is no safety consequence.
Since the NAC-UMS system is an older cask design, and it uses conservative thermal modeling assumptions than what are currently used for the MAGNASTOR system. When the more currently regulatory accepted modeling considerations are applied to the NAC-UMS system, the affected PCTs are well below licensing basis limits.
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U.S. Nuclear Regulatory Commission March 4, 2021 Page2 72.242(d)(2)
Description:
Some thermal modeling of dry cask storage systems requires the modeling of the fuel rod assembly to determine its effective thermal property when performing both steady state and transient thermal evaluations. For this reportable event, the fuel rod assembly effective thermal property was incorrectly calculated. Specifically, the diameter of the fuel pellet was modeled using the outside diameter of the fuel cladding (i.e., the fuel pellet was modeled larger than it actually was). This results in an extra amount of fuel rod assembly mass being part of the effective thermal property calculation.
This extra mass results in an increase in the fuel rod assembly's thermal capacitance and has an impact on the transient analyses. During transient analyses, a fuel rod assembly with more thermal capacitance will result in lower PCTs. By correcting the fuel pellet diameter and associated effective thermal property calculation, the PCTs for associated thermal evaluations will increase.
72.242(d)(2)(i) Date/I'ime o(Discovery:
On February 2, 2021, the fuel rod assembly effective thermal property issue was identified byNAC Engineering personnel who were working on a new project considering the storage of individual rods and not an intact fuel assembly. While working on this project, a department in Engineering re-quested support of another engineering department. This collaboration resulted in one department believing there was not enough physical space to support all the individual targeted fuel rod types to be loaded. This resulted in a conflict between the two departments and further investigation into why they were not in agreement.
Engineering then issued Self-Identification Report (SIR)21-001, which identified the issue as being an incorrect effective thermal property calculation for the fuel rod assembly. This effective thermal property calculation for the fuel rod assembly modeled the fuel pellet diameter as if it had the same. diameter as the fuel rod (i.e., cladding outer diameter). Subsequent to issuance of SIR 21-00 I, NAC Quality Assurance (QA) issued Corrective Action Report (CAR) 21-02 on February 5, 2021.
72.242(d)(2)(ii) Cause:
The cause of the issue goes back to June 18, 2001. Specifically, the origin is NAC calculation EA790-3006 for the NAC-UMS system. This calculation used an ANSYS macro to calculate the effective thermal property for a pressurized water reactor (PWR) fuel rod assembly. While the macro functions correctly and as intended, the macro incorrectly used the fuel rod diameter as the fuel pellet diameter.
Since the starting dimension for the macro is the fuel pellet diameter, the overall size of the fuel rod is larger than it should be because the pellet to clad gap and cladding thickness are subsequently added to the fuel pellet diameter. Thus, the issue is not with the macro itself but rather the input parameter into the macro (i.e., the fuel pellet diameter). This macro has been used since its inception in 2001, without modification of the input fuel pellet dimension, for not only for the NAC-UMS system but was also later used for the MAGNASTOR dry storage system thermal evaluations. However as previously discussed, herein, the errors associated with MAGNASTOR are not reportable under 10 CFR 72.242(d) since the PCTs are within licensing basis limits even when the fuel rod assembly effective thermal property is corrected. NAC's other dry cask storage system, which is the NAC-MPC system, was determined to not have implemented this ANSYS macro and is therefore, unaffected by this issue.
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U.S. Nuclear Regulatory Commission March 4, 2021 Page 3 72.242(d)(2)(iii) Failure Mode:
For undamaged PWR fuel, the fuel cladding is required to perform its fuel-specific or system-related function. This is, in part, accomplished by maintaining the PCTs below acceptable temperatures. For the NAC-UMS system, NAC has determined that for some licensing basis transient conditions the PCT will exceed the licensing basis temperature limits. However, this only occurs when using the original licensing basis method of evaluation (MOE) after making the correction to the fuel rod assembly effective thermal property.
The NAC-UMS system is an older NAC cask design and does not incorporate modeling currently accepted by the industry and regulator like more contemporary cask designs ( e.g., MAGNASTOR). The NAC-UMS system uses a very conservative assumption for the canister transfer operation vacuum drying phase transient analysis thermal models.
Like MAGNASTOR, when a trace amount of gas thermal conductivity is considered in those thermal models for the NAC-UMS transient analyses, the PCTs are once again below accepted licensing basis limits. Therefore, there is no safety consequence and the systems have and will continue to perform satisfactorily.
72.242(d)(2)(iv) Systems/Secondary FW1ctions:
There were no other NAC-UMS systems or secondary :(unctions affected by this deficiency. The deficiency is specific to the PWR fuel rod assembly effective thermal property that was calculated in the licensing basis. Its impact is directly focused on calculated PCTs.
72.242(d)(2)(v) Method o(Discovery:
As part of the response to 72.242(dX2)(i), provided herein, the method of discovery was discussed. See response to 72.242(dX2Xi) for the method of discovery.
72.242(d)(2)(vi) Model Number o(Each Component:
There is no manufacturer or model number associated with how the effective thermal property of the fuel rod assembly is derived. However, the fuel rod assembly modeled was a licensing basis PWR fuel rod assembly.
72.242(d)(2)(vii) Model/Serial Numbers o(Affected Casks:
The NAC dry cask storage system affected and reportable under 10 CFR 72.242( d) is the NAC-UMS dry cask storage system. The deficiency described, herein, is generic to the NAC-UMS FSAR PWR thermal evaluations and is associated with the following casks:
Duke Energy (McGuire and Catawba) - UMS-TSC-418- 001 thru-052 Arizona Public Service (Palo Verde)-NAC-UMS-TSC-407-001 thru-152 The NAC-UMS cask model is not currently being manufactured for any Independent Spent Fuel Storage Installations (ISFSis) in the United States (U.S.) nor are there any casks currently available to be loaded.
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U.S. Nuclear Regulatory Commission March 4, 2021 Page4 72.242(d)(2)(viii) List o(Licensees Affected:
With regards to the NAC-UMS system, the affected users are those loading under the generic PWR sections of the FSAR and Certificate of Compliance (CoC) Technical Specifications (fS). This includes Duke-McGuire, Duke-Catawba and APS-Palo Verde. Note, the NAC-UMS system also includes site-specific PWR fuel for Maine Yankee. Maine Yankee fuel loading is handled specifically within the TS and FSAR and is not part of the generic PWR sections. Maine Yankee is unaffected by this deficiency.
None of these sites are currently loading or plan to load additional NAC-UMS systems, because Duke-McGuire, Duke-Catawba and APS-Palo Verde have transitioned to MAGNASTOR and Maine Yankee is a standalone Independent Spent Fuel Storage Installation (ISFSD with all fuel currently in dry storage.
For MAGNASTOR, as previously discussed herein, the errors associated with MAGNASTOR are not reportable under 10 CPR 72.242( d) since the PCTs are within licensing basis limits even when the fuel rod assembly effective thermal property is corrected.
72.242(d)(3) Assessment o(Safety Consequences and Implications As part of the response to 72.242(d)(2Xiii), provided herein, the safety consequences and implications were discussed. As described in the response, there are no safety consequences or implications associated with the fuel rod assembly effective thermal property error. See response to 72.242(d)(2Xiii) for further details. Additionally, NAC has reviewed this issue against the reporting requirements of 10 CPR Part 21 and has determined it is not formally reportable under that regulation since there are no substantial safety hazards associated with this issue. However, NAC is always engaged with our system users and has already notified the user group of the current situation.
In addition, there are no operability implications for those NAC-UMS casks currently in service. Current NRC guidance on operability determinations allows the use of alternate methods of evaluations to those currently licensed (e.g., Inspection Manual Chapter (IMC) 0326, Section 08.04). For the NAC-UMS system, the application of a trace amount of gas thermal.conductivity in the system during the transient analyses is consistent with how the systems were loaded and is also consistent with the transient evaluations of more contemporary licensed storage systems (i.e., MAGNASTOR). When this modeling assumption is taken into consideration for the NAC-UMS system, peak clad temperatures remain below the licensed limit of 400°C. NAC considers this analytical approach consistent with the NRC guidance on implementing operability determinations. NAC will take final corrective action on this issue by obtaining an NRC amendment to the NAC-UMS system that formally accepts this modeling assumption of a taking credit for a trace amount of gas thermal conductivity in the system during transient evaluations ( e.g.,
canister transfer operation vacuum drying phase).
72.242(d)(4) Corrective Actions NAC's Quality Assurance Program (QAP) is handling the issue via CAR 21-02. The CAR contains the following corrective actions:
Perform extent of condition review; Determine the safety significance and evaluate the ability of the components to perform their intended safety function; ED20210033
U.S. Nuclear Regulatory Commission March 4, 2021 Page 5 Determine the impacted customer facilities and customer notification protocol Perform a root cause analysis (RCA) on the issue; and Identify and implement corrective actions to prevent recurrence NAC's evaluation of the ability of components to perform their safety function is based on numerous revised thermal model runs. These runs have been preliminarily checked and are in the process of being finalized per our QAP requirements. NAC has no reason to believe the conclusions made in this report will be invalidated by the finaliz.ation of these calculations.
72.242(d)(5) Previous Similar Events NAC is unaware of any other previously similar events within NAC or outside relative to an error in modeling the fuel rod assembly effective thermal property by applying an incorrect fuel pellet diameter.
72.242(d)(6) Contact Mr. Wren Fowler Director, Licensing Phone: 678-328-1236 Email: wfowler@nacintl.com Conclusion NAC is committed to nuclear safety and keeping the NRC informed on such matters. This reportable issue pertaining to how the effective thermal property of the PWR fuel rod assembly is calculated is for the NAC-UMS system. The MAGNASTOR system does not reach the threshold for reportability but since it is associated with this error, NAC has provided pertinent information relative to MAGNASTOR to keep the NRC fully informed on the scope of this error. NAC is continuing to work through the issue via NAC' s QAP and CAR 21-02. Should the Commission require further details regarding the condition described herein, please contact me.
Mr. Wren Fowler Director, Licensing Engineering cc.
Andrea Kock, Director, Division of Fuel Management Christopher Regan, Deputy Director, Division of Fuel Management John McKirgan, Chief, Division of Fuel Management, Licensing Branch Bernie White, Senior Project Manager, Division of Fuel Management, Licensing Branch ED20210033