ML23069A215
| ML23069A215 | |
| Person / Time | |
|---|---|
| Site: | 07201015, 07201031 |
| Issue date: | 03/10/2023 |
| From: | Baldner H NAC International |
| To: | Office of Nuclear Material Safety and Safeguards, Document Control Desk |
| References | |
| Download: ML23069A215 (1) | |
Text
Atlanta Corporate Headquarters 3930 East Jones Bridge Road, Suite 200 Peachtree Corners, GA 30092 Phone: 770-447-1144 Fax: 770-447-1797 ED20210029 March 10, 2023 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
Subject:
10 CFR 72.242 Reportable Licensing Basis Non-Mechanistic Tip-over Evaluation Deficiency for the NAC-UMS and MAGNASTOR Dry Cask Storage Systems Docket Numbers. 72-1015 and 72-1031 72.242(d)(1) - Abstract Overview NAC has internally identified a licensing basis deficiency for the NAC-UMS and MAGNASTOR dry cask storage systems. Specifically, a parameter used in the computation of bending stress in the finite element model used to structurally evaluate a fuel rod under the non-mechanistic tip-over accident condition was incorrectly specified resulting in the non-conservative calculation of stresses. More specifically, the calculation uses an ANSYS 2-D elastic beam element, BEAM3, to represent a single fuel rod. The BEAM3 element requires the input of various real constants that define physical properties of the element such as cross-sectional area, area moment of inertia, and height. The height real constant is used by ANSYS in the computation of bending stress in the element and is defined in ANSYS documentation to be the total height of the beam (i.e., rod outer diameter in this case). The input code in question incorrectly set the height real constant using a parameter equal to the outer radius of the rod, which is half of the correct value. Use of the correct value effectively doubles the computed bending stress.
NAC has completed an impact to safety evaluation and determined there is no Impact to Safety or concerns relative to the ability of the components to perform their intended safety function based on the error identified. The calculational error is specific to the non-mechanistic tip-over analysis. Both the MAGNASTOR FSAR and UMS FSAR provide evaluations which determined the loaded systems on the ISFSI pad do not tip-over within licensed conditions. Additionally, the safety functions of the VCC and TSC (e.g., passive heat rejection, environmental protection, TSC confinement, fuel assembly geometry control) are unaffected by the error in fuel rod modeling.
U.S. Nuclear Regulatory Commission March 10, 2023 Page 2 ED20230029 This condition is reportable since the deficiency when corrected results in the non-conservative calculation of fuel rod bending stresses exceeding the licensing basis limits for the NAC-UMS and MAGNASTOR. However, NAC has determined there is no safety consequence.
NACs other dry cask storage system, which is the NAC-MPC system, was determined to not have been affected by this issue.
72.242(d)(2)
Description:
Fuel rod side drop analyses are presented in UMS FSAR Sections 11.2.16.1 (PWR) and 11.2.16.2 (BWR) and MAGNASTOR FSAR Section 3.8.4 as bounding evaluations for the lateral accelerations experienced by the fuel during a non-mechanistic tip-over event. The 60g uniform lateral acceleration applied in these evaluations significantly bounded the maximum accelerations reported for the tip-over event of 33.4g and 26.6g at the top of the basket for UMS and MAGNASTOR, respectively. As noted above, correction of the input error doubled the bending stress and resulted in the storage tip-over evaluation margins being negative.
The calculational error presented is specific to the non-mechanistic tip-over analysis. Both the UMS and MAGNASTOR FSAR provide evaluations which determine the loaded systems on the ISFSI pad do not tip-over within licensed conditions. It is also important to note that the analyses performed are on fuel assemblies with up to 60 of unsupported fuel rod length (simulating a missing fuel grid). This analysis provides the basis for treating assemblies with partially damaged grid straps (corners missing, side damage, etc.) as undamaged and not requiring canning in damaged fuel cans. Because this missing fuel grid evaluation met the yield stress acceptance criteria, undamaged fuel is considered bounded.
Updated fuel rod structural analyses associated with the non-mechanistic tip-over, are in process. The updated analyses apply a revised method of evaluation (MOE) utilizing an acceleration load and a dynamic load factor (DLF) determination developed from the LS-DYNA cask tip-over time history.
These updated analyses will be provided in amendments to the affected system FSARs. Preliminary results of the updated analyses demonstrate stress in the fuel rod cladding remains below allowable yield stress under the non-mechanistic tip-over conditions and supports the existing criteria of less than or equal to 60 of unsupported rod length i.e., loss of grid strap support, allowing fuel assemblies with grid damage to not require placement into damaged fuel cans.
72.242(d)(2)(i) Date/Time of Discovery:
On March 29, 2022, an error was discovered during checking of a new calculation (423-2020 Rev. 0) being prepared for the fatigue and side drop evaluation for HBU fuel rods in an NAC-STC transportation cask. This calculation used the model (ANSYS input) from a preceding side drop analysis (71160-2139 Rev. 0, circa 12/2020) which itself references preceding ANSYS input files from calculation 71160-2025 Rev. 1, circa 8/2007.
Engineering then issued Self-Identification Report (SIR)22-002, which identified a parameter used in the computation of bending stress in the finite element model for the fuel rod 60g Side Drop analysis was incorrectly specified resulting in the non-conservative calculation of stresses.
U.S. Nuclear Regulatory Commission March 10, 2023 Page 3 ED20230029 The calculation error was documented per Standard Practice (SP) SP-116 (not part of the formal QA Program) as Self-Identification Report (SIR) No.22-002, Dated 03/29/2022, and remained in Engineering for resolution prior to formal QA notification on 02/28/2023. The SIR was immediately escalated to CAR 23-01. This process weakness is identified in Finding Report (FR 23-006).
72.242(d)(2)(ii) Cause:
The error has been traced back to calculation 71160-2025 Rev. 1 approved in 2007 and is the origin of the error. The error was then propagated by incorporating the incorrect ANSYS model into subsequent analyses.
72.242(d)(2)(iii) Failure Mode:
The potential failure mode would be the possibility of a beyond-design-basis event resulting in storage cask tip-over, which could cause fuel rod yielding in fuel assemblies with partially damaged fuel grids creating an unanalyzed condition.
72.242(d)(2)(iv) Systems/Secondary Functions:
The safety functions of the VCC and TSC (e.g., passive heat rejection, environmental protection, TSC confinement, fuel assembly geometry control) are unaffected by the error in fuel rod modeling. There were no other NAC-UMS and MAGNASTOR systems or secondary functions affected by this deficiency.
The deficiency is specific to the fuel rod cladding evaluated for the hypothetical cask tip-over event that were calculated in the licensing basis.
72.242(d)(2)(v) Method of Discovery:
The error was discovered during checking of a new calculation (423-2020 Rev. 0) being prepared for the fatigue and side drop evaluation for HBU fuel rods in an NAC-STC transportation cask.
72.242(d)(2)(vi) Model Number of Each Component:
There is no generic manufacturer or model number associated with the NAC-UMS or MAGNASTOR systems. Serial numbers for each of the affected TSC can be found below in response to 72.242(d)(2)(vii) 72.242(d)(2)(vii) Model/Serial Numbers of Affected Casks:
The NAC-UMS dry cask storage system is affected and reportable under 10 CFR 72.242(d). The error discussed above in Section 72.242(d)(2) was introduced in Amendment 5 which permitted the storage of high burnup PWR fuel (up to 60 GWd/MTU assembly average burnup). The deficiency described, herein, is generic to the NAC-UMS FSAR PWR tip-over evaluations. Those sites that have TSCs loaded to
U.S. Nuclear Regulatory Commission March 10, 2023 Page 4 ED20230029 Amendment 5 or after are affected. This includes sites that have recertified their systems to Amendment 5 or any amendment thereafter. The following casks are affected:
Arizona Public Service (Palo Verde) - UMS-TSC-407-001 thru -152 Those sites that have TSCs loaded prior to Amendment 4 or have not recertified their systems to later amendments are not affected by this error and are thus not reportable under 10 CFR 72.242(d).
NAC-UMS dry cask storage systems loaded at Duke Energy (McGuire or Catawba) were loaded under Amendments 4 and have not been recertified to a later amendment, and therefore are not impacted. This encompasses the following casks:
Duke Energy (McGuire) - UMS-TSC-418-001 thru -024 and 049 thru -052 Duke Energy (Catawba) - UMS-TSC-418-025 thru -048 NAC-UMS dry cask storage systems loaded at Maine Yankee are also not affected and also not reportable under 10 CFR 72.242(d) because they have a site-specific tip-over evaluation which demonstrates the Maine Yankee fuel rods are structurally adequate for the evaluated 60g side drop condition (FSAR Section 11.2.15.1.6).
Maine Yankee - UMS-TSC-790-001 thru -060 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.
The MAGNASTOR dry cask storage system is affected and reportable under 10 CFR 72.242(d). The deficiency described, herein, is generic to the MAGNASTOR FSAR PWR tip-over evaluations and is associated with the following casks:
Duke Energy (McGuire) - MAG-TSC-418-055 thru -068, -071, -072, -073, -075, -076, -098, -
099, -100 thru -114 Duke Energy (Catawba) -MAG-TSC-418-069, -070, -074, -077 thru -097, -119, -121, -123, -124,
-126, -128 Zion - MAG-TSC-215555-001 thru -030, MAG-TSCDF-215555-001 thru -30 and MAG-TSC-418-053 and -054 Kewaunee - MAG-TSC-30026-089-01 thru -12 and MAG-TSCDF-30026-184-13 thru -24 Three Mile Island Unit 1 (TMI-1) - MAG-TSC-30076-280-001 thru -023 and MAG-TSCDF-30076-265-001 thru -023 Arizona Public Service (APS-Palo Verde) - MAG-TSC-30032-280 -172 thru -174 and MAG-TSCDF-30032-265-153 thru -166, -168, -170, -171 The MAGNASTOR cask model is currently being manufactured for ISFSIs in the U.S. at Duke McGuire, Duke Catawba, and Palo Verde, and there are casks currently available to be loaded Duke McGuire, Duke Catawba and Palo Verde. The following cask have been delivered and are available to be loaded:
Duke Energy (McGuire) - MAG-TSC-418-115 thru -118
U.S. Nuclear Regulatory Commission March 10, 2023 Page 5 ED20230029 Arizona Public Service (APS-Palo Verde) - MAG-TSC-30032-280-175 and MAG-TSCDF-30032-265-167, -169 72.242(d)(2)(viii) List of Licensees Affected:
With regards to the NAC-UMS system, the affected users are:
APS - Palo Verde None of these sites are currently loading or plan to load additional NAC-UMS systems.
With regards to the MAGNASTOR system, the affected users are:
Duke - McGuire, Duke - Catawba APS - Palo Verde ZionSolutions - Zion KewauneeSolutions - Kewaunee Constellation - Three Mile Island Unit 1 (TMI-1)
Duke McGuire, Duke Catawba and APS Palo Verde are currently loading or plan to load additional MAGNASTOR systems.
72.242(d)(3) Assessment of Safety Consequences and Implications NAC has completed an impact to safety evaluation and determined there is no Impact to Safety or concerns relative to the ability of the components to perform their intended safety function based on the error identified. The calculational error is specific to the non-mechanistic tip-over analysis. Both the MAGNASTOR FSAR and UMS FSAR provide evaluations which determined the loaded systems on the ISFSI pad do not tip-over within licensed conditions. The safety functions of the VCC and TSC (e.g.,
passive heat rejection, environmental protection, TSC confinement, fuel assembly geometry control) are unaffected by the error in fuel rod modeling.
Additionally, NAC has reviewed this issue against the reporting requirements of 10 CFR 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.
There are no operability implications for those NAC-UMS or MAGNASTOR 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). 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 and MAGNASTOR systems.
U.S. Nuclear Regulatory Commission March 10, 2023 Page 6 ED20230029 72.242(d)(4) Corrective Actions This issue is documented and being tracked under NACs Quality Assurance Program (QAP) via CAR 23-01. The CAR contains the following corrective actions:
Perform extent of condition review; Determine safety significance, evaluate the ability of the components to perform their intended safety functions, and determine regulatory reportability; Determine impacted customer facilities and identification of components delivered; and determine customer notification protocol; Perform root cause analysis RCA; and Identify and implement corrective actions to prevent recurrence.
As noted above, NAC will bring the systems into compliance with the CoC by obtaining an NRC CoC amendment to the NAC-UMS and MAGNASTOR systems.
The RCA will also include assessment and appropriate corrective and preventative actions for the NAC SIR process and the apparent weakness in timely follow-through and tracking of SIR 22-002.
NACs extent of condition review will also consider transportation casks, and we will address any related 10CFR71 reportability under separate communication.
72.242(d)(5) Previous Similar Events NAC identified a similar event relative to an error in modeling the fuel rod assembly effective thermal properties in which the incorrect fuel pellet diameter was used in calculating peak clad temperatures in 2021 affecting the NAC-UMS system.
72.242(d)(6) Contact Mr. Heath Baldner Director, Licensing Phone: 678-328-1252 Email: hbaldner@nacintl.com
U.S. Nuclear Regulatory Commission March 10, 2023 Page 7 ED20230029 Conclusion NAC is committed to nuclear safety and keeping the NRC informed on such matters. This reportable issue pertains to an error for calculating the bending stresses in fuel rods during a non-mechanistic tip-over event. NAC has provided pertinent information relative to NAC-UMS and MAGNASTOR to keep the NRC fully informed on the scope of this error. NAC is continuing to work through the issue via NACs QAP and CAR 23-01. Should the Commission require further details regarding the condition described herein, please contact me.
Sincerely, Mr. Heath Baldner Director, Licensing Engineering cc.
Shana Helton, Director, Division of Fuel Management Jacob Zimmerman, Deputy Director, Division of Fuel Management Carrie Safford - Deputy Director, Division of Fuel Management Yoira Diaz Sanabria, Branch Chief, Storage and Transportation Licensing Branch Nishka Devaser, Project Manager, Division of Fuel Management, Licensing Branch Heath M. Baldner Digitally signed by Heath M. Baldner Date: 2023.03.10 14:55:42 -05'00'