ML24332A026
| ML24332A026 | |
| Person / Time | |
|---|---|
| Site: | Westinghouse |
| Issue date: | 11/26/2024 |
| From: | Ekaterina Lenning Licensing Processes Branch |
| To: | |
| Shared Package | |
| ML24332A025 | List: |
| References | |
| EPID L-2024-NTR-0005 | |
| Download: ML24332A026 (1) | |
Text
OFFICIAL USE ONLY - PROPRIETARY INFORMATION 1
OFFICIAL USE ONLY - PROPRIETARY INFORMATION REQUEST FOR ADDITIONAL INFORMATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION WCAP-18850-P/NP, REVISION 0, "ADAPTATION OF THE FULL SPECTRUM' LOCA (FSLOCA') EVALUATION METHODOLOGY TO PERFORM ANALYSIS OF CLADDING RUPTURE FOR HIGH BURNUP FUEL" WESTINGHOUSE ELECTRIC COMPANY DOCKET NO. 99902038 ISSUE DATE: 11/26/2024
=
Background===
By letter dated February 29, 2024 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML24060A160), Westinghouse Electric Company (Westinghouse) submitted Topical Report (TR) WCAP-18850-P/NP, Revision 0, Adaptation of the FULL SPECTRUM' LOCA (FSLOCA') Evaluation Methodology to Perform Analysis of Cladding Rupture for High Burnup Fuel (Proprietary/Non-proprietary) for the U. S. Nuclear Regulatory Commission (NRC) review and approval. The NRC staff has reviewed the TR WCAP-18850-P/NP, Revision 0, conducted a regulatory audit on October 22-23, 2024, and determined that additional information is required to complete the review. Westinghouse indicated via an email dated November 26, 2024, that the responses to the NRC requests for additional information (RAIs) will be submitted to the NRC within 60 days from the receipt of the RAIs.
Regulatory Basis The NRC staff is reviewing TR WCAP-18850-P/NP, Revision 0, to ensure compliance with requirements associated with the postulated loss-of-coolant accident (LOCA), including Title 10 of theCode of Federal Regulations (CFR)50.46 and General Design Criterion 35. The purpose of the WCAP-18850-P/NP, Revision 0, methodology is to assess margin to cladding rupture for high-burnup fuel rods. Assuring acceptable margin to cladding rupture for high-burnup fuel rods containing fuel susceptible to fragmentation and dispersal is a means of demonstrating compliance with existing requirements for the LOCA, including the requirement in 10CFR50.46(b)(4) to maintain a coolable core geometry.
Question 1 Figure 3.1-2 from WCAP-18850-P/NP, Revision 0, shows the WCOBRA/TRAC-TF2 gap conductance and gap width for an example pressurized-water reactor (PWR) high burnup fuel rod. This example appears to be discussed further in Westinghouses response to NRC RAI 20 for WCAP-18446 TR. The NRC staff seeks further information concerning the example presented by Westinghouse to assess its relevance for demonstrating acceptable predictions by the WCOBRA/TRAC-TF2 code for gap conductance and gap width for high-burnup fuel rods within the scope of WCAP-18850-P/NP, Revision 0.
- a. Please specify the fuel rod design and burnup used to produce this figure.
- b. Please clarify whether the location of the example rod on the periphery, as described in Westinghouses response to NRC RAI 20 for WCAP-18446, significantly affects the predicted behavior, since different fuel rod powers at the core interior and periphery would presumably affect fuel pellet temperature and gap conductivity predictions.
OFFICIAL USE ONLY - PROPRIETARY INFORMATION 2
OFFICIAL USE ONLY - PROPRIETARY INFORMATION
- c. Please explain why the WCOBRA/TRAC-TF2 codes predictions of gap conductance and gap width are justified in part by [
..........-................................ (... /...). ]
Question 2 Section 3.3.2 of WCAP-18850-P/NP, Revision 0, describes a [
.................................................................... ] for cladding rupture experiments is necessary to assure an acceptable validation basis for the WCOBRA/TRAC-TF2 code that Westinghouse has proposed to use to determine whether the cladding of high-burnup fuel rods would rupture during a LOCA.
- a. Please provide further explanation on how the [ ] is performed for data taken in the Westinghouse LOCA burst test apparatus and any other adjusted data sets used to develop or justify the burst temperature model. Specifically:
- 1. Provide further justification for the [ ] and the methodology used to determine an appropriate [ ] for each data point.
- 2. Show some examples of how the [ ] is calculated for test data from the different test databases.
- b. It is the NRC staffs understanding that Figure 3.3-5 shows the [ ] burst temperature curve. Please confirm this understanding and, in addition to Figure 3.3-5, please provide the
[ ] burst temperature curve in terms of cladding engineering hoop stress.
Question 3 In Section 3.5, WCAP-18850-P/NP, Revision 0, states that [
...../...... -...................................................................................] However, the available experimental databases discussed in WCAP-18850-P/NP, Revision 0, all use [
............-....... ] burnup when reporting and analyzing data. Furthermore, the NRCs Research Information Letter (RIL) 2021-13, which assessed data available through its time of publication, recommended a burnup threshold for fine fragmentation of 55 GWd/MTU local burnup. Given that non-uniform reactor core power distributions typically result in peak local burnups well in excess of [ ] it is clear that the criterion proposed in WCAP-18850-P/NP, Revision 0, would not predict fine fuel fragmentation at local burnup values considered susceptible to fragmentation in RIL 2021-13. The NRC staff further observes that uncertainty remains with regard to determining the threshold for fine fragmentation and that research results indicate the potential for a more complicated relationship with a fuel rods power history than is captured by an integral burnup value. Therefore, please justify the use of
[))))))
]
Question 4 Section 3.6 of WCAP-18850-P/NP, Revision 0, describes the LOCA transient fission gas release model. However, the TR description does not contain sufficient information concerning the
[ ] and applicability ranges. Assuring an adequate
OFFICIAL USE ONLY - PROPRIETARY INFORMATION 3
OFFICIAL USE ONLY - PROPRIETARY INFORMATION understanding of transient fission gas release and associated uncertainties is necessary to assure that relevant impacts on the determination of cladding rupture for high-burnup fuel have been adequately considered. Please provide additional information on how the model is used within the WCAP-18850-P/NP, Revision 0, methodology. Specifically:
- a. Explain how [ ] is performed.
- b. Explain how the model is used below [
] around the trendline could result in [ ] transient fission gas release.
- c. Describe any potential lower or upper burnup ranges for the applicability of the LOCA transient fission gas release model.
Question 5 Packing fraction data from the [...-............ ] described in Section 3.7 of WCAP-18850-P/NP, Revision 0, were not available during the original development of the FSLOCA evaluation model (EM). The experimental data now available from these tests [
.................................................................................... (.........,............),.................
................................] in WCAP-18850-P/NP, Revision 0. Determination of an appropriate range of packing fraction uncertainties based on currently available data assures that the best-available knowledge concerning the range of potential packing fraction outcomes for relocated fuel has been considered in the analysis.
Appropriately considering the range of possible packing fractions for relocated fuel is relevant to the potential for cladding rupture due to the influence of packing fraction on cladding temperature.
- a. Please provide additional justification for the [ ] sampling range proposed for WCAP-18850-P/NP, Revision 0. The current sampling range does bound [
........... ] but does not bound [...]
- b. WCAP-18850-P/NP, Revision 0, TR states that the packing fraction is sampled [
...................................... ] Please justify the choice of a [ ] distribution to represent the data.
Question 6 In Section 7.2 of WCAP-18850-P/NP, Revision 0, Westinghouse included a proposed limitation and condition similar to Limitation and Condition 11 in the NRC staff s safety evaluation (SE) on TR WCAP-16996-P-A, Revision 1. The proposed limitation and condition appears to be intended to address similar considerations associated with ensuring statistical fidelity of conclusions drawn from simulated realizations, while incorporating modifications that reflect differences between the two methodologies. However, the limitation and condition proposed in WCAP-18850-P/NP, Revision 0, does not address an important contingency discussed in WCAP-16996-P-A, Revision 1, namely, how to address a situation in which calculational inputs must be revised after being declared because insufficient analytical margin exists to satisfy applicable regulatory acceptance criterion. Thoroughly addressing this contingency in the calculational procedure would provide confidence that statistical analysis methods are not being repeatedly executed to achieve a passing result, which would result in degradation to the intended 95/95 statistical tolerance limits. Therefore, please describe how Westinghouse would address a situation where the results of a plant-specific design calculation do not satisfy or provide insufficient margin to the regulatory acceptance criterion. Please explain whether records would be maintained that would allow independent verification of the appropriateness of
OFFICIAL USE ONLY - PROPRIETARY INFORMATION 4
OFFICIAL USE ONLY - PROPRIETARY INFORMATION the as-executed calculational procedure for plant-specific analyses and its capability to preserve the intended 95/95 statistical tolerance limit.
Question 7 Section 9.6 of WCAP-16996-P-A, Revision 1, TR describes energy deposition modeling in the WCOBRA/TRAC-TF2 code, which is relied upon to support the WCAP-18850-P/NP, Revision 0, evaluation model. During audit discussions supporting the NRC staff's review of WCAP-18850-P/NP, Revision 0, Westinghouse indicated that energy transfer is [
....................................... ] While a review of Section 9.6 of WCAP-16996-P-A, Revision 1, TR found limited information consistent with the position Westinghouse expressed during the audit, the NRC staff did not find the information reviewed fully consistent with the audit discussions for two reasons. First, the NRC staff did not identify a clear high-level explanation of the energy deposition modeling approach within the FSLOCA EM in WCAP-16996-P-A, Revision 1, or WCAP-18850-P/NP, Revision 0. Second, the NRC staff identified a number of passages in Section 9.6 of WCAP-16996-P-A, Revision 1, suggesting that WCOBRA/TRAC-TF2 models energy redistribution [
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- a. Please provide a high-level functional description of the generalized energy distribution model, indicating the types of energy redistribution considered as well as the sources and potential targets for each type of energy redistribution considered in WCOBRA/TRAC-TF2 as applied to both the FSLOCA EM and the WCAP-18850-P/NP, Revision 0, evaluation model.
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OFFICIAL USE ONLY - PROPRIETARY INFORMATION
- b. Please explain how the citations in the body of this RAI from Section 9.6 of WCAP-16996-P-A, Revision 1, are consistent with the objective stated above in part a. of this RAI or provide an updated description of the generalized energy distribution modeling approach that more clearly reflects its application within the FSLOCA EM and WCAP-18850-P/NP, Revision 0.
- c. Please explain how the maximum power in the [
........... ] is determined and justify that the process used to determine this power level is acceptable. In particular, Section 29.4.1.2 of WCAP-16996-P-A, Revision 1, indicates that the [
.................... ;] however, for a condition where a lower-powered high-burnup assembly may be surrounded by hotter, fresh assemblies, higher [
........... ] than considered in WCAP-16996-P-A, Revision 1, could be necessary.
- d. Please clarify the approximate magnitude of the energy redistribution relative to the total power for a high-burnup fuel rod for conditions within the expected application range of WCAP-18850-P/NP, Revision 0, and justify that the proposed generalized energy distribution modeling approach for WCAP-18850-P/NP, Revision 0, is acceptable.
Question 8 The methodology described in WCAP-18850-P/NP, Revision 0, appears to focus largely upon two reactor coolant pump operating conditions:
cases where offsite power is lost, and the reactor coolant pumps are assumed to trip coincident with the reactor.
cases where offsite power is available, and the reactor coolant pumps are assumed to continue operating, presumably until procedural trip criteria are satisfied, with a five-minute delay.
WCAP-18850-P/NP, Revision 0, TR does not appear to thoroughly address a likely and possibly more limiting scenario where reactor coolant pumps would operate following a reactor trip, but for an effective duration less than that associated with a five-minute trip delay (e.g., due to an earlier manual trip or ineffectiveness due to voiding in the reactor coolant system) following a LOCA in the small-to-intermediate break range. Potential scenarios involving reactor coolant pump tripping at intermediate times (e.g., one-minute trip delay) were found to have the potential to produce more limiting results by both the NRC staffs confirmatory analyses and the licensed analysis methodology for one Westinghouse 3-loop reactor, although this conclusion was found to be design-dependent (e.g., see Section 3.2.2.4 of the NRC staffs SE in ADAMS Package Accession No. ML20325A088).
- a. Please confirm or correct the NRC staffs characterization of the reactor coolant pump trip assumptions that Westinghouse has explicitly analyzed within WCAP-18850-P/NP, Revision 0.
- b. Please justify that the cases considered in WCAP-18850-P/NP, Revision 0, encompass the limiting condition for the range of potential impacts of reactor coolant pump operation following LOCAs in the small-to-intermediate break range for all plants within the scope of the methodology or discuss any modifications to the calculational procedure to encompass other scenarios with different assumptions concerning reactor coolant pump operation.
- c. Please clarify whether the identification of other possibly more limiting reactor coolant pump trip scenarios than originally assumed in WCAP-18850-P/NP, Revision 0, could affect the results of any sensitivity studies considered therein.
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OFFICIAL USE ONLY - PROPRIETARY INFORMATION Question 9 Section 5.2.3 of WCAP-18850-P/NP, Revision 0, TR discusses Westinghouses proposal to remove two conservatisms associated with two-phase flow modeling that it had originally proposed in 2014 to support the Region I approach for analyzing small break LOCAs during the licensing of the FSLOCA EM. However, the submitted TR does not contain the full set of information necessary for the NRC staff to confirm that the proposed modeling changes would not adversely impact the capability to predict cladding temperature (and hence rupture potential) for high-burnup fuel rods. Therefore, the NRC staff requests further information to assess the expected impact of the changes on the Region I and Region IB regions of the break spectrum and assure that impacts on the validation of the WCOBRA/TRAC-TF2 code are acceptable.
- a. Please summarize or provide citation to the specific separate or integral effects experimental comparisons that Westinghouse credits with most effectively demonstrating the validation of the [.............................................. ] for WCOBRA/TRAC-TF2 and provide justification.
- b. As applicable to the break spectrum regions where new or modified treatments of [
] are proposed in WCAP-18850-P/NP, Revision 0, please provide results of available data comparisons or sensitivity studies demonstrating the impacts of ranging these uncertainty factors to their proposed limits and provide justification that the resulting uncertainty bands acceptably encompass relevant measured data.
- c. In Section 5.2.3.1 of WCAP-18850-P/NP, Revision 0, Westinghouse indicates that the NRC staff has reviewed numerous implementation requests for the FSLOCA EM without observing unphysical behaviors relative to the modeling of [ ] However, it is not clear that the limited information included in the licensing applications reviewed would provide definitive evidence to confirm or refute the existence of anomalous predictions.
Furthermore, the analyses submitted for review presumably involved the requisite [
] Therefore, please clarify to what extent Westinghouses experience performing plant-specific analyses with the FSLOCA EM is relevant to demonstrating the acceptability of the proposed new or modified approaches for modeling [
..] in WCAP-18850-P/NP, Revision 0.
- d. To the extent that simulations with previous versions of WCOBRA/TRAC-TF2 are relied upon to address this RAI, please justify that subsequent code changes have not adversely impacted the analyses and derived conclusions concerning model validation and uncertainty range.
Question 10 While the baseline treatment of breaks within Regions I and II of the approved FSLOCA EM has been well defined in WCAP-16996-P-A, Revision 1, the NRC staff did not identify a comparably complete and systematic description of the intended modeling practices for the intermediate break region (i.e., Region IB) added in WCAP-18850-P/NP, Revision 0. In this regard, the NRC staff observes that different modeling practices and uncertainty treatments are applied in Regions I and II for certain analytical parameters, such that the intended treatment for intermediate breaks, which have some characteristics in common with both small and large breaks, may not be apparent. Therefore, for relevant key phenomena and reactor design inputs, please describe the proposed modeling practices intended for the intermediate break region (i.e., Region IB) at a comparable level of detail to the descriptions provided for Region I and II breaks in WCAP-16996-P-A, Revision 1.
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OFFICIAL USE ONLY - PROPRIETARY INFORMATION Question 11 Westinghouse describes in Section 6 and other parts of WCAP-18850-P/NP, Revision 0, a number of sensitivity studies relied upon to support conclusions therein, especially with respect to which plant conditions may be limiting for the analysis of intermediate breaks using the WCAP-18850-P/NP, Revision 0, methodology. Please provide the following additional information regarding the sensitivity studies described in WCAP-18850-P/NP, Revision 0, so that the NRC staff can complete its assessment of the relevance of the sensitivity studies to Westinghouses proposed calculational procedure:
- a. Clarify whether the sensitivity studies described in WCAP-18850-P/NP, Revision 0, reflect insights from only a single simulation such that the reported results may be subject to significant variation, or whether they are informed by or representative of a larger population of simulations.
- b. Clarify whether the obtained results or derived conclusions from any sensitivity studies would be substantially affected by credible variations in the assumed reactor coolant pump trip timing.
- c. Clarify the extent to which any sensitivity study results are intended to be applied to other reactor design categories that were not explicitly analyzed (e.g., Combustion Engineering pressurized-water reactors).
- d. Clarify the extent to which the sensitivity results are sufficiently definitive and accompanied by adequate phenomenological understanding to assure that they may be considered fully applicable to the entire population of plants within the larger design category for which each sensitivity analysis was performed, or whether the results of some sensitivity studies (e.g.,
steam generator tube plugging, limiting break size, offsite power availability) may not be fully conclusive with respect to all individual plants within the category.
Question 12 Axial transport of gas within a fuel rod has significant potential to impact cladding rupture potential, and considerable uncertainty remains concerning this behavior. Section 3.6.3 of WCAP-18850-P/NP, Revision 0, describes [ ] Westinghouse performed to assess axial gas communication within a fuel rod during a LOCA. In describing these calculations, Westinghouse stated that [
.....................................................,&.....;.............................]
Westinghouse further stated that [
................................................................,............................................. ] Please clarify the following information concerning the fuel rod axial gas communication [ ]
so that the NRC staff can assure that potential impacts of transient fission gas release on cladding rupture have been adequately addressed:
- a. [
.....,............................................................................................ ]
- b. Please clarify in particular [
. ]
OFFICIAL USE ONLY - PROPRIETARY INFORMATION 8
OFFICIAL USE ONLY - PROPRIETARY INFORMATION
- c. Please clarify [
.......................................................................]
- d. Please clarify the statement indicating that [
....................................................... (..............................] and further explain why [
..................................(.).......................................................(.)...........................
...................... ]
Question 13 Assuring an acceptable nodalization for computational analyses is necessary to determine accurate and reliable results. The proposed extension of the FSLOCA EM to accommodate increased fuel burnup may involve nodalization considerations that have not previously been fully considered within WCAP-16996-P-A, Revision 1, and which are not fully addressed in WCAP-18850-P/NP, Revision 0. Please provide the following additional information concerning the fuel nodalization approach reflected in Section 3.2 of WCAP-18850-P/NP, Revision 0, to support the NRC staff's review concerning the acceptability of the proposed nodalization for high-burnup fuel:
- a. Please clarify whether the modeling of high-burnup fuel requires any modifications to the general fuel nodalization approach described in WCAP-16996-P-A, Revision 1.
- b. Please address in particular whether the hot rod associated with a high-burnup assembly could be in a more limiting position if situated at an exterior location (i.e.,assembly edge or corner) that is adjacent to fresh, high-powered fuel assemblies, rather than within the interior of the high-burnup assembly. If an exterior rod position has the potential to be more limiting, please describe how the methodology would address this condition (e.g., adjustments to the powers of the hot assembly rod or other rods).
Question 14 Westinghouse states in Section 3.3.2 of WCAP-18850-P/NP, Revision 0, that [
] data does not appear to be shown in the figures in Section 3 of WCAP-18850-P/NP, Revision 0, that illustrate AXIOM rupture data in terms of rod internal pressure. Please either (1) confirm the datapoints in question are included in the engineering hoop stress curve in Figure 3.3-2 of WCAP-18850-P/NP, Revision 0, (2) confirm that the datapoints are included in Figures C.2-1 and C.2-2 of WCAP-18446-P-A, Revision 0 (which appear to contain additional datapoints not included in Figure 3.3-2 of WCAP-18850-P/NP, Revision 0), or (3) provide a graph of the data.
Question 15 In Section 5.1.1 of WCAP-18850-P/NP, Revision 0, Westinghouse discusses the differentiation between the three break regions and the expected peak cladding temperature (PCT) results and trends for each of these regions. Westinghouse states that the Region I (small break) and Region II (large break) utilize insights from application of the FSLOCA EM for determining the
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OFFICIAL USE ONLY - PROPRIETARY INFORMATION limiting break size within these regions. Potential limiting conditions for the newly added Region IB (intermediate break) are discussed in this TR. For the staff to understand the PCT and potential burst behavior in Region IB, please discuss the following:
- What is the phenomenological demarcation used to distinguish breaks in Region I from Region IB?
- What parameters are dominant in maximizing PCT and hoop stress in region IB?
- Are the results for Region IB always expected [.........................] (as shown in Figure 5.1-9: Illustration of Regions for the Cladding Rupture Calculations)? If so, please explain why.
- In Figure 5.1-6: IBLOCA Break Spectrum Peak Cladding Temperature versus Break Diameter for a 3-Loop Westinghouse-designed PWR (Case B), the PCT results show [
.................................................................... ] Please discuss whether this large change in PCT is a result of physical or numerical phenomena and why this may or may not be expected and acceptable. Is Region I or Region II susceptible to this type of variation and why?
- Under what conditions will plant specific analyses be performed for Region IB?
Concurred via email OFFICE NRR/DORL/LLPB/
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BC NAME ELenning GGeorge DHarrison SKrepel DATE 11/26/2024 11/25/2024 11/25/2024 11/21/2024 OFFICIAL RECORD COPY