ML25188A053
| ML25188A053 | |
| Person / Time | |
|---|---|
| Site: | Saint Lucie  |
| Issue date: | 07/02/2025 |
| From: | Natreon Jordan NRC/NRR/DORL/LPL2-2 |
| To: | Florida Power & Light Co |
| References | |
| EPID L-2024-LLA-0057 | |
| Download: ML25188A053 (1) | |
Text
REQUEST FOR ADDITIONAL INFORMATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION Florida Power & Light Company St Lucie Unit 2 DOCKET NO. 05000389
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Background===
By letter dated April 30, 2024, (ADAMS Accession No. ML24122A727) Florida Power and Light Company (FPL) (the licensee) submitted a license amendment (LAR) request to change the St.
Lucie Unit 2 (SL2) Technical Specifications (TS) 3.7.15, Spent Fuel Pool Storage, and TS 4.3, Fuel Storage, to support updated spent fuel pool (SFP) and new fuel vault (NFV) criticality analyses which account for the impact of a proposed transition to 24-month fuel cycles on fresh and spent fuel storage at SL2.
Regulatory Basis In accordance with the licensees amendment request the regulatory requirements and guidance, which the NRC staff considered in assessing the proposed TS change, are as follows:
Title 10 of the Code of Federal Regulations (10 CFR), Part 50, Appendix A, Criterion 62, Prevention of criticality in fuel storage and handling, requires that criticality in the fuel storage and handling system shall be prevented by physical systems or processes, preferably by use of geometrically safe configurations.
Per 10 CFR 50.68(a), each holder of an operating license shall comply with either 10 CFR 70.24 or the requirements in 10 CFR 50.68(b). The licensee has elected to meet 10 CFR 50.68(b).
Accordingly, and as relevant to this license amendment request, the licensee must comply with the following 50.68(b) requirements:
(1) Plant procedures shall prohibit the handling and storage at any one time of more fuel assemblies than have been determined to be safely subcritical under the most adverse moderation conditions feasible by unborated water.
(2) The estimated ratio of neutron production to neutron absorption and leakage (k-effective) of the fresh fuel in the fresh fuel storage racks shall be calculated assuming the racks are loaded with fuel of the maximum fuel assembly reactivity and flooded with unborated water and must not exceed 0.95, at a 95 percent probability, 95 percent confidence level. This evaluation need not be performed if administrative controls and/or design features prevent such flooding or if fresh fuel storage racks are not used.
(3) If optimum moderation of fresh fuel in the fresh fuel storage racks occurs when the racks are assumed to be loaded with fuel of the maximum fuel assembly reactivity and filled with low-density hydrogenous fluid, the k-effective corresponding to this optimum moderation must not exceed 0.98, at a 95 percent probability, 95 percent confidence level. This evaluation need
2 not be performed if administrative controls and/or design features prevent such moderation or if fresh fuel storage racks are not used.
(4) If no credit for soluble boron is taken, the k-effective of the spent fuel storage racks loaded with fuel of the maximum fuel assembly reactivity must not exceed 0.95, at a 95 percent probability, 95 percent confidence level, if flooded with unborated water. If credit is taken for soluble boron, the k-effective of the spent fuel storage racks loaded with fuel of the maximum fuel assembly reactivity must not exceed 0.95, at a 95 percent probability, 95 percent confidence level, if flooded with borated water, and the k-effective must remain below 1.0 (subcritical), at a 95 percent probability, 95 percent confidence level, if flooded with unborated water.
In order to complete its independent assessment of whether the requested LAR provides reasonable assurance that the proposed changes will be compliant with 10CFR50.68 the NRC staff makes the following requests for additional information (RAI):
RAI #1 The SL2 SFP has three rack designs. Region 1 (R1) and Region 2 (R2) are identical except R1 has permanently installed stainless steel inserts. The third region is the cask pit rack (CPR) that is different in several aspects including the permanently installed neutron absorbing material (NAM) Boral'.
The proposal is for the SL2 SFP to have a total of 11 distinct storage configurations. R1 is to have two storage configurations, each identified by repeating 2x2 arrays of storage cells. R2 is to have four storage configurations, each identified by repeating 2x2 arrays of storage cells. The CPR is to have five storage configurations, each represented by repeating 2x2 arrays of storage cells. Of those 11 2x2 storage configurations only three are symmetrical; the 2F configuration in both the R1 and CPR and the 4S configuration in the CPR. In addition, the proposal includes another five temporary storage configurations that would only be used during Fuel Inspection and Reconstitution (Figure 3-7). Of those five storage configurations, all are symmetrical.
For each of the 11 distinct storage configurations the analysis modeled three eccentric positioning scenarios in 2x2 arrays. The three scenarios considered are: the fuel assemblies centered in the storage cells; the fuel assemblies moved to the center-most position of the 2x2 array; and the fuel assemblies moved to the same relative corner in their storage cells.
Additionally, an 8x8 array with all fuel assemblies moved to the center-most position of the array was analyzed. The analysis does not appear to consider eccentric positioning in the five temporary storage configurations. The results of the analysis are presented in HI-2230346 R1 Table 8-10 Reactivity Effect of Fuel Assembly Radial Positioning.
The application stated the updated criticality analyses follow the regulatory guidance provided in U.S. Nuclear Regulatory Commission (USNRC) Regulatory Guide (RG) 1.240, Fresh and Spent Fuel Pool Criticality Analyses, March 2021 (ML20356A127), which endorses with clarifications and exceptions, Nuclear Energy Institute (NEI) 12-16, Revision 4, Guidance for Performing
3 Criticality Analyses of Fuel Storage at Light-Water Reactor Power Plants, September 2019 (ML19269E069).
The guidance in Regulatory Guide 1.240/NEI 12-16 calls for eccentric positioning to be analyzed with a 4x4 array of storage cells. There are two basic reasons, one is the eccentric position analysis is an optimization analysis that may not be fully explorable with a 2x2 array, and another is because in some scenarios boundary conditions for 2x2 arrays means adjacent arrays are effectively being moved further away creating a modeling construct of a flux trap between neighboring 2x2 arrays that does not exist. This flux trap modeling construct can minimize, or obscure potential reactivity increases from the eccentric positioning. This can be seen in HI-2230346 R1 Table 8-10 where every 8x8 array with all fuel assemblies moved to the center-most position of the array is more reactive than the 2x2 array with all fuel assemblies moved to the center-most position of the array.
When performing the eccentric positioning analysis, it is common practice to consider five positions within the storage cell: centered and each corner. While there certainly are more, these five are reasonable unless there would be some atypical aspect of the rack or fuel design that would need to be examined. With five positions in each storage cell to be considered a 2x2 array with four fuel assemblies will have 625 possible combinations. Since the proposal is for eight such typical 2x2 arrays (one will have two FA and two will have three FA) the proposed SL2 SFP will have thousands of possible combinations. Symmetry in the storage configurations is important as it reduces the number of unique combinations. The applicant analyzed a tiny fraction of the possible combinations and did not provide adequate justification for the limited scope of its analysis.
Based on the above it is not clear whether the applicant has found the limiting eccentric positioning scenario for the St Lucie U2 SFP. Therefore, the NRC staff requests the applicant demonstrate it has identified and applied the limiting eccentric positioning scenario to the estimation of keff for the St Lucie U2 SFP.
RAI #2 The SL2 SFP has three rack designs. R1 and R2 are identical except R1 has permanently installed stainless steel inserts. The third region is the CPR is different in several aspects including the permanently installed neutron absorbing material (NAM) Boral'.
The proposal is for the SL2SFP to have a total of 11 distinct storage configurations. R1 is to have two storage configurations, each identified by repeating 2x2 arrays of storage cells. R2 is to have four storage configurations, each identified by repeating 2x2 arrays of storage cells. The CPR is to have five storage configurations, each represented by repeating 2x2 arrays of storage cells. Of those 11 2x2 storage configurations only three are symmetrical; the 2F configuration in both the R1 and CPR and the 4S configuration in the CPR. In addition, the proposal includes another five temporary storage configurations that would only be used during Fuel Inspection and Reconstitution (Figure 3-7). Of those five storage configurations, all are symmetrical.
In HI-2230346 R1 Sections 3.6.7, Incorrect Loading Curve (Multiple Misload), and 3.6.8, Incorrect Burnup (Multiple Misload), the applicant addressed the accidental misloading of
4 multiple fuel assemblies. In Section 3.6.7, the initiating action is the use of an inappropriate loading curve when developing the fuel handling instructions (aka move sheets) for a fuel movement campaign. This is an example of a planning or process error that leads to multiple fuel assemblies being placed into unallowed storage configurations. The applicant performed an analysis of this type of scenario. In Section 3.6.8, Incorrect Burnup (Multiple Misload), the applicant makes the statement that Typically, a single event resulting in multiple misloaded assemblies is the result of a planning or process error, which is considered non-credible due to a robust administrative control program at the SL2. This program includes QA validated software that prevents a common-fault error of selecting the wrong location for multiple fuel assemblies, visual color-coded SFP maps, pre-verification of planned fuel moves, post-movement verification, etc. Also, loading multiple fresh assemblies instead of irradiated assemblies is typically not considered a credible condition, since fresh (unburned) assemblies are visually different from irradiated burned assemblies. Therefore, as a defense-in-depth, an additional hypothetical scenario of a multiple misload condition is evaluated. The analysis then considered a different scenario of misloading multiple under burned fuel assemblies. The applicant did not consider scenarios of misloading multiple fresh fuel assemblies.
The application stated that the updated criticality analyses follow the regulatory guidance provided in U.S. Nuclear Regulatory Commission (USNRC) Regulatory Guide (RG) 1.240, Fresh and Spent Fuel Pool Criticality Analyses, March 2021 (ML20356A127), which endorses, with clarifications and exceptions, NEI 12-16, Revision 4. Therefore, the guidance in RG 1.240/NEI 12-16 should be applicable. The specific guidance is found in NEI 12-16 Section 6.3.5, Multiple Assembly Misload. The guidance is based on industry experience and considers a planning or process error leading to multiple misloading event credible. The information provided by the licensee to this point does not differentiate SL2 from the rest of the industry. The licensee hasnt described or provided any details about the analysis it performed of the SL2 program controls that led to its conclusion that a planning or process error leading to a multiple misloading event at SL2 is non-credible. Therefore, the NRC cannot make an independent assessment of whether the administrative control program at SL2 does make a planning or process error leading to multiple misloading events non-credible.
The NRC has accepted administrative controls to limit the extent of multiple misloading events that need to be considered. To date acceptance has not been based solely on a planning or process error being non-credible. Nor when fresh and irradiated fuel are comingled as at SL2.
Instead, those were based on a combination of robust administrative control programs to reduce the probability of a planning or process error and the plant operators, that are moving the fuel, being able to identify if a planning or process error has occurred and then acting to not follow the erroneous move sheets. The operators ability to identify a planning or process error while in the midst of moving fuel to date has been based on the ability to distinguish fresh fuel from irradiated fuel, fresh fuel is not co-mingled with irradiated fuel, the requirements for individual storage locations are not subject to change. The last two do not apply to the applicants initial proposal.
Based on the above the applicant has not established that multiple fresh fuel assembly misloading events are non-credible at SL2. In order for the NRC to perform an independent
5 assessment the NRC staff requests the licensee provide the details of the analysis it performed that led to its conclusion that multiple fresh fuel assembly misloading events at SL2 are non-credible.
RAI #3 In the Incorrect Loading Curve events the applicant considered multiple misloadings of under burned fuel assemblies. The analysis performed is described in HI-2230346 R1 Sections 3.6.7 and 8.13.4. The results of the applicants analysis are summarized in Table 8-27. Table 8-27 indicates that under the applicants current assumptions regarding multiple misloading events it will take 1882.7 ppm of soluble boron in the SFP to meet 10CFR50.68(b)(4).
The St Lucie U2 TS 3.7.14 Spent Fuel Storage Pool Boron Concentration requires the SFP to have a minimum of 1900 ppm soluble boron. This provides very little margin between the proposed amendment and the current St Lucie U2 TS 3.7.14. This margin prompts the need to consider factors not normally considered in a SFP criticality analysis, such as whether there is any bias or uncertainty in the applicants determination of compliance with SL2 TS 3.7.14.
Therefore, the NRC staff requests the applicant to provide justification that the current St Lucie U2 TS 3.7.14 requirement is adequate to ensure compliance with 10CFR50.68(b)(4).
RAI #4 The current LAR cites Reference [1] St. Lucie Unit 2 Criticality Analysis for EPU and Non-EPU Fuel, HI-2104753, Revision 4, Holtec International as precedence. St. Lucie Unit 2 Criticality Analysis for EPU and Non-EPU Fuel, HI-2104753, Revision 4 is the analysis that was used to support SL2s most recent SFP/NFV license amendment, NRC letter, St. Lucie Plant, Unit 2 -
Issuance of Amendment Regarding New Fuel Vault and Spent Fuel Pool Nuclear Criticality Analysis (TAC NO. ME8782), dated September 19, 2012, (ML12263A224). Sometimes citing HI-2104753, Revision 4 is the sole justification for some portion of the current analysis documented in HI-2230346 R1. Guidance for the use of precedents is found in NEI 12-16 R4 Section 1.5. While precedents can be useful, they are not without limitations. The guidance states, in part, Precedents may be used in whole or in part with technical justification. Any similarities or differences should be technically supported and demonstrated as appropriate.
HI-2230346 R1 Section 1.0 mentions some of the differences between the current proposal and the earlier HI-2104753, Revision 4, The difference between this analysis and the previous criticality analysis in [1] is an extended list of qualified fuel assemblies, SFR [spent fuel rack]
neutron absorber inserts, revised loading configurations (arrangements) as well as adjustments to the analysis methodology. Additionally, a new assembly type (Framatome 16x16) with essentially the same nominal dimensions is introduced in support of 24-month cycle. However, it leaves out a significant difference; that of an increase in U-235 enrichment. Despite mentioning differences between HI-2230346 R1 and HI-2104753, Revision 4, the current application does not provide a technical justification for using the earlier HI-2104753, Revision 4, as a precedent.
6 To complete its review of the current application the NRC staff needs to know whether HI-2104753, Revision 4 is an appropriate precedent for HI-2230346 R1 for the extent to which it is used in HI-2230346 R1. Therefore, the NRC staff requests the applicant provide the technical justification for the use of HI-2104753, Revision 4 as a precedent for HI-2230346 R1.
RAI #5 Request for Confirmation In HI-2230346 R1 Section 3.2.1 the applicant states that CASMO5 was used for the depletion portion of the analysis, however the application does not indicate whether it was used consistent with topical report SSP-14-P01/028-TR-P-A, Generic Application of the Studsvik Scandpower Core Management System to Pressurized Water Reactors, Revision 0 (ADAMS ML17279A985). Therefore, the NRC staff requests the applicant to confirm that CASMO5 was used consistently with the topical report, with identification and justification for any deviations.
RAI #6 Request for Confirmation HI-2230346 R1 Section 3.3 Analysis Methods states that minimum 10B loading of the BORAL' absorber material is used and minimum 10B loading of the B4C absorber material (CEA and METAMIC') is used However, it is not clear what is meant by minimum as there could be several different minimums that could have been used. The NRC staff requests the applicant to confirm that it is the minimum Certified 10B loading that was used when modeling the neutron absorbing materials.
RAI #7 Proposed Technical Specifications Change Regulatory Basis:
50.36(c)(2)(i) Limiting conditions for operation are the lowest functional capability or performance levels of equipment required for safe operation of the facility. When a limiting condition for operation of a nuclear reactor is not met, the licensee shall shut down the reactor or follow any remedial action permitted by the technical specifications until the condition can be met. When a limiting condition for operation of any process step in the system of a fuel reprocessing plant is not met, the licensee shall shut down that part of the operation or follow any remedial action permitted by the technical specifications until the condition can be met.
RAI The NRC staff has identified a concern regarding the licensees proposed use of the word can in place of shall as specified in the licensees combined TS. The word shall is prescriptive which impositions or enforces a regulation, such as 10 CFR 50.36. Since Figure 3.7.15-2, Allowable Region 2 Storage Patterns and Fuel Arrangements, is part of limiting condition for operation (LCO) 3.7.15, which requires a licensee to follow any remedial action permitted by the TS until the condition can be met, NRC staff determined that use of the word can in the
7 proposed change is not appropriate because it does not enforce the requirement in the LCO.
Further, St. Lucie TS Section 1.1, Definitions, (page 1.1-1) defines an ACTION as:
ACTIONS shall be that part of a Specification that prescribes Required Actions to be taken under designated Conditions within specified Completion Times.
Based on the NRC staffs position discussed above, the staff requests that the licensee provide the basis for the proposed change.