ML25016A152
| ML25016A152 | |
| Person / Time | |
|---|---|
| Site: | 05200050 |
| Issue date: | 01/16/2025 |
| From: | Shaver M NuScale |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| Shared Package | |
| ML25016A151 | List: |
| References | |
| RAIO-178471 | |
| Download: ML25016A152 (1) | |
Text
RAIO-178471 NuScale Power, LLC 1100 NE Circle Blvd., Suite 200 Corvallis, Oregon 97330 Office 541.360.0500 Fax 541.207.3928 www.nuscalepower.com January 16, 2025 Docket No.52-050 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk One White Flint North 11555 Rockville Pike Rockville, MD 20852-2738
SUBJECT:
NuScale Power, LLC Response to NRC Request for Additional Information No. 018 (RAI-10142 R1) on the NuScale Standard Design Approval Application
REFERENCE:
NRC Letter to NuScale, Request for Additional Information No. 018 (RAI-10142 R1), dated March 02, 2024 The purpose of this letter is to provide the NuScale Power, LLC (NuScale) response to the referenced NRC Request for Additional Information (RAI).
The enclosure to this letter contains the NuScale response to the following RAI question from NRC RAI-10142 R1:
15.4.6-2 is the proprietary version of the NuScale Response to NRC RAI No. 018 (RAI-10142 R1, Question 15.4.6-2). NuScale requests that the proprietary version be withheld from public disclosure in accordance with the requirements of 10 CFR § 2.390.
The enclosed affidavit (Enclosure 3) supports this request. Enclosure 2 is the nonproprietary version of the NuScale response.
This letter makes no regulatory commitments and no revisions to any existing regulatory commitments.
If you have any questions, please contact Amanda Bode at 541-452-7971 or at abode@nuscalepower.com.
I declare under penalty of perjury that the foregoing is true and correct. Executed on January 16, 2025.
Sincerely, Mark W. Shaver Director, Regulatory Affairs NuScale Power, LLC
RAIO-178471 Page 2 of 2 01/16/2025 NuScale Power, LLC 1100 NE Circle Blvd., Suite 200 Corvallis, Oregon 97330 Office 541.360.0500 Fax 541.207.3928 www.nuscalepower.com Distribution:
Mahmoud Jardaneh, Chief New Reactor Licensing Branch, NRC Getachew Tesfaye, Senior Project Manager, NRC Stacy Joseph, Senior Project Manager, NRC
- NuScale Response to NRC Request for Additional Information RAI-10142 R1, Question 15.4.6-2, Proprietary Version : NuScale Response to NRC Request for Additional Information RAI-10142 R1, Question 15.4.6-2, Nonproprietary Version : Affidavit of Mark W. Shaver, AF-178472
RAIO-178471 NuScale Power, LLC 1100 NE Circle Blvd., Suite 200 Corvallis, Oregon 97330 Office 541.360.0500 Fax 541.207.3928 www.nuscalepower.com NuScale Response to NRC Request for Additional Information RAI-10142 R1, Question 15.4.6-2, Proprietary Version
RAIO-178471 NuScale Power, LLC 1100 NE Circle Blvd., Suite 200 Corvallis, Oregon 97330 Office 541.360.0500 Fax 541.207.3928 www.nuscalepower.com NuScale Response to NRC Request for Additional Information RAI-10142 R1, Question 15.4.6-2, Nonproprietary Version
Response to Request for Additional Information Docket: 052000050 RAI No.: 10142 Date of RAI Issue: 03/02/2024 NRC Question No.: 15.4.6-2 Regulatory Basis 10 CFR 52.137(a)(4) states a standard design application must include [a]n analysis and evaluation of the design and performance of SSC with the objective of assessing the risk to public health and safety resulting from operation of the facility and including determination of the margins of safety during normal operations and transient conditions anticipated during the life of the facility, and the adequacy of SSCs provided for the prevention of accidents and the mitigation of the consequences of accidents.
GDC 10, Reactor Design, states that the reactor core and associated coolant, control, and protection systems shall be designed with appropriate margin to assure that specified acceptable fuel design limits are not exceeded during any condition of normal operation, including the effects of anticipated operational occurrences.
GDC 13, Instrumentation and Control, states that instrumentation shall be provided to monitor variables and systems over their anticipated ranges for anticipated operational occurrences as appropriate to assure adequate safety. It further states that appropriate controls shall be provided to maintain these variables and systems within prescribed operating ranges.
Issue The NPM-20 design relies on boron concentration limits in the US460 reactor building pool to ensure the reactor core remains subcritical during refueling. Internal flooding could result in the inadvertent addition of diluted or unborated water to the pool. Such an addition will reduce the pool boron concentration and may lead to inadvertent criticality, challenging specified acceptable fuel design limits described in General Design Criteria 10. In the refueling configuration, the reactor pressure vessel and containment vessel are disassembled and do not serve as fission product barriers.
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FSAR Section 15.4.6 evaluates inadvertent boron dilution during refueling (Mode 5) conditions.
The evaluation considers whether internal flooding sources add quantities of diluted or unborated water that induce criticality. The volume of diluted or unborated water added by each internal flooding source is assessed in the internal flooding analysis described in FSAR Chapter 3, which indicates that internal flooding sources are isolated by an assumed operator response.
NRC staff examined this analysis via audit and determined that margin to acceptance criteria is highly sensitive to the timing of operator action. However, the FSAR Chapter 15 safety analysis does not allow credit for operator action in the first 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> of design basis events as the US460 is a passive plant design.
Information Requested Provide an analysis of the Mode 5 boron dilution transient which demonstrates that subcriticality is maintained for the limiting source of unborated water without reliance on operator intervention. Operator intervention at any time within the 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> following the initiating event would constitute a reliance on that operator action to mitigate the event. Revise the FSAR with pertinent portions of the analysis, such as the limiting internal flood volume, flood sources evaluated, alternate flooding analysis assumptions, design features and SSCs credited in lieu of operator response to isolate the flood source, and assumptions pertinent to event timing and boron mixing throughout regions of the pool.
Alternately, to enable staff assessment of the use of operator actions, provide the following information in the FSAR: 1) identification and description of redundant instrumentation and control room alarms that alert the operators of an unplanned boron dilution, 2) for each flood source requiring operator intervention, a listing of the specific operator actions necessary to terminate the flooding events or otherwise maintain subcriticality, and the total time required (the event duration is considered to include the time from the initiating event through the following 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />), and 3) the event sequence and timing with the smallest elapsed time from the signal generated and control room alarm detecting the dilution event to the time inadvertent criticality would occur. Revise pertinent FSAR Chapters, such as 7, 15, 18, and 19, to include relevant details for control room indication and alarms, and assumed operator response during the 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> following the initiating event for the analysis of inadvertent boron dilution during refueling (Mode 5) conditions.
NuScale Nonproprietary NuScale Nonproprietary
NuScale Response:
Executive Summary:
The Mode 5 boron dilution event analysis provided in Final Safety Analysis Report (FSAR)
Section 15.4.6 is revised to demonstrate that subcriticality is maintained for the limiting volume of unborated water without credit for operator intervention. The analysis is performed in a conservative manner such that, in the limiting scenario, the useable inventory of two separate large externally-located water tanks totaling 600,000 gallons is pumped into the Reactor Building (RXB) ((2(a),(c) Even in this conservative scenario, there would still be a margin of approximately 370,000 gallons to a potential loss of shutdown margin. Reasonable assurance of adequate protection is provided for a boron dilution event in Mode 5 without credit for operator actions. Revised Analysis Results Summary: NuScale has performed an analysis of the Mode 5 boron dilution transient which demonstrates that subcriticality is maintained for the limiting source of unborated water without reliance on operator intervention as requested. Results of the updated analysis are shown in Table 1. The limiting scenario includes an assumed loss of power. As shown in Table 1, a margin of approximately 370,000 gallons is maintained to criticality even for the limiting scenario with no reliance on operator intervention. ((
}}2(a),(c) Details of the revised analysis are provided in the section following Table 1.
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Table 1: Mode 5 Boron Dilution Analysis Results Description Scenario with Power Available Scenario with Power Unavailable Initial volume of ultimate heat sink assumed (gallons) (( }}2(a),(c) 935,282 Maximum dilution volume before Technical Specification 3.5.3 pool level limits are exceeded (gallons) 117,609 117,609 Maximum dilution volume based on internal flooding analysis in FSAR Section 3.4.1 (gallons) 363,000 363,000 Maximum assumed dilution volume that is self-terminating (gallons) (( }}2(a),(c) 600,000 Dilution volume required for loss of shutdown margin (gallons) (( }}2(a),(c) 967,700 Minimum dilution volume margin to criticality (gallons) (( }}2(a),(c) 367,700 Revised Analysis Approach to Limiting Dilution Volume: In FSAR Section 15.4.6, Revision 0, NuScale calculated a limiting dilution volume and demonstrated it was greater than the maximum dilution volume before Technical Specification (TS) 3.5.3 pool level limits were exceeded (117,609 gallons per Table 1). NuScale believes this approach is a reasonable approach to demonstrate safety because licensee compliance with TS is a requirement of the operating license. In FSAR Section 15.4.6, Revision 1, NuScale calculated a limiting dilution volume and demonstrated it was greater than the maximum dilution volume determined by the RXB internal flooding analysis in FSAR Section 3.4.1 (363,000 gallons per Table 1). NuScale believes this approach is a reasonable approach to demonstrate safety because this volume represents the NuScale Nonproprietary NuScale Nonproprietary
largest allowable volume that could enter the RXB as identified elsewhere in the FSAR. NuScale also believes this approach is reasonable because it is consistent with the approach reviewed and approved by the NRC in the Design Certification Application (DCA) for the US600 design codified in Appendix G to 10 CFR 52, Design Certification Rule for NuScale. Despite NuScales belief in the validity of the previous two approaches identified above, NuScale has identified a more conservative method to identify a limiting dilution volume to satisfy the NRC request. The limiting dilution volume under this approach is dependent on the power availability assumptions.
Power unavailable - If power is unavailable, the ability of diluted water to reach the ultimate heat sink (UHS) is limited due to lack of pumps. The exception is the fire water system, which includes a diesel-powered pump to comply with fire protection requirements. A fire protection main line break that is left unmitigated could theoretically deliver the volume of both firewater storage tanks to the RXB. Since each tank has a useable volume of 300,000 gallons, this amounts to a total of 600,000 gallons. There are automatic makeup sources to the firewater storage tanks, but they would be unavailable due to the loss of power. After both firewater storage tanks are emptied, the diesel fire pump would fail and no further dilution volume could reach the RXB. Therefore, the maximum possible dilution volume with power unavailable is 600,000 gallons.
Power available - If power is available, there may be sources of inventory available in addition to those above because of the availability of other pumps. There is automatic makeup to the firewater storage tanks from the utility water storage tank that has a useable volume of 150,000 gallons. A maximum possible dilution source could therefore be 750,000 gallons. However, there is also automatic makeup capability to the utility water storage tank itself. Because the flow rate of the fire protection pumps is larger than the flow rate of the various makeup pumps, the fire protection pumps will eventually deplete the inventory of the firewater storage tanks and then fail. After the fire pumps fail, no further dilution volume could reach the RXB. The pump flow rates are used to calculate the duration of fire pump operation before failure and therefore the total inventory pumped to the RXB. (( }}2(a),(c) The two maximum dilution volumes determined as described above (600,000 gallons for power unavailable and (( }}2(a),(c) for power available) are significantly larger than those assumed in either FSAR Revision 0 or FSAR Revision 1. The two maximum dilution NuScale Nonproprietary NuScale Nonproprietary
volumes are also self-limiting in that the volumes are inherently limited by the plant design (e.g., the tank inventories). Revised Analysis Approach to Initial Mixing Volume: Along with the revised approach for calculating dilution volume above, NuScale revised the analysis to slightly modify the initial mixing volumes used. (( }}2(a),(c) Table 2 shows the impact of the change on the limiting power unavailable scenario. As indicated by Table 2, the change increases the assumed initial volume by less than 0.5 percent. Therefore, the change in the initial mixing volume approach is not significant. Table 2: Initial Volume Assumptions for Limiting Power Unavailable Scenario Description Units Previous Analysis Revised Analysis Initial volume assumed in limiting power unavailable scenario ft3 124,412 125,029 gallons 930,669 935,282 Revised Analysis Approach to Critical Dilution Volume: The section above identifies a minor change to the initial mixing volume. A more significant change was made to the analysis approach regarding the critical dilution volume. In the previous analyses, NuScale assumed that the volume remained at the initial volume throughout the dilution. An arbitrary flow rate of the dilution flow entering the UHS was assumed. Starting from event initiation, the calculation determined the amount of dilution flow entering (time step multiplied by flow rate). The volume was then perfectly mixed while the volume was held constant. The volume of pure water entering replaced an equal volume of highly borated water that was assumed to leave in that time step. The process was repeated by time step until shutdown margin was lost. The critical dilution volume was then determined as total time duration multiplied by the arbitrary flow rate. This calculation was similar to the Mode 1 NuScale Nonproprietary NuScale Nonproprietary
calculation process for dilution of the reactor coolant system where letdown was assumed equal to makeup so that inventory did not change. This approach, although conservative, generates a non-physical outcome; a significant quantity of water is added to the UHS but level does not increase. NuScale revised the analysis to be more consistent with the physical reality of the event. Specifically, the UHS level will increase as the dilution source enters the pool. The process is outlined below. 1. The initial water mass is determined from the initial volume (see Table 2). 2. The initial boron mass is calculated from the initial water mass and the initial (i.e., minimum) boron concentration. 3. The boron concentration at which shutdown margin becomes zero is determined from the initial shutdown margin and the minimum pool boron concentration. 4. The mass of water necessary to have the boron concentration in Step 3 with the boron mass in Step 2 is calculated. 5. The new (final) pool volume is determined from the mass in Step 4. 6. The critical dilution volume is the difference between Step 5 and Step 1. The revised approach results in critical dilution volumes that are larger than previously calculated. As a result, the larger assumed dilution volumes in the revised analysis (and shown in Table 1) can be accommodated with margin remaining. Conservatism in the Chapter 15 Analysis: Although NuScale has revised the analysis approach to reduce over-conservatisms as described above, the overall analysis is still conservative. A review of key conservatisms is provided in the list below. 1) (( }}2(a),(c) NuScale Nonproprietary NuScale Nonproprietary
2) (( }}2(a),(c) 3) Initial Pool Concentration is at Minimum Required Value - The TS LCO 3.5.3 also specifies limits on pool boron concentration. The analysis assumes that the pool starts at the minimum boron concentration allowed by TS LCO 3.5.3. 4) (( }}2(a),(c) 5) (( }}2(a),(c) 6) (( }}2(a),(c NuScale Nonproprietary NuScale Nonproprietary
(( }}2(a),(c) 7) (( }}2(a),(c) 8) (( }}2(a),(c) 9) (( }}2(a),(c) NuScale Nonproprietary NuScale Nonproprietary
10) (( }}2(a),(c) Compliance with TS: As identified in Table 1, a dilution volume of 117,609 gallons of water entering the UHS is sufficient to raise the pool level from its TS LCO 3.5.3 minimum limit to its TS LCO 3.5.3 maximum limit. For a dilution event where greater than 117,609 gallons of water enters the pool, it is guaranteed that the TS maximum limit would be exceeded and TS required actions would apply. The TS include immediate actions to address the level condition and the implication on refueling activities. The unborated volume of 363,000 gallons assumed to enter the pool based on the internal flooding analysis in FSAR Section 3.4.1 (also shown in Table 1) corresponds to a pool level increase of over 6 ft from its initial level. Similarly, an unborated volume of 600,000 gallons entering the pool (i.e., the maximum assumed level in the limiting case) corresponds to a pool level increase of over 10 ft from its initial level. Given the narrow range of pool level allowed by TS (i.e., nominal plus or minus one foot), the required attention to TS compliance by licensed plant operators, the immediate nature of the TS required actions, and the presence of plant personnel in the area to support refueling operations, it is very unlikely that operators would allow the pool level to increase by several feet without taking actions. Pool level is displayed in the main control room and has alarms as described in FSAR Section 9.2.5. Note that TS LCO 3.5.3 also contains immediate required actions for pool boron concentration being outside limits, although detection of a change in level is more obvious than a change in concentration. The dilution volumes associated with prompting TS required actions is provided to demonstrate the significant conservatism of the assumed maximum dilution volume. Regardless of the TS requirements, the analysis does not take any credit for operators taking any action based on changing pool conditions. Design Certification Application Margin Comparison: This request for additional information and associated audit questions have identified a heightened NRC interest in the Mode 5 boron dilution event compared to the NRC review of the DCA for the US600 design. In discussion with the NRC on clarification calls, the NRC identified the heightened interest was because of the reduced margin shown in the FSAR for the US460 NuScale Nonproprietary NuScale Nonproprietary
design compared to the DCA. The NRC stated that the reported margin in the DCA FSAR (determined by comparing DCA FSAR Table 15.4-19 to DCA FSAR Table 15.4-12) was large enough that further interest in the Mode 5 boron dilution was not warranted during the DCA review. Therefore, a comparison of the margin is provided in Table 3 below for reference. The US460 value reflects the output of the revised analysis discussed in this response. As shown in Table 3, the revised analysis for the US460 now has more margin than the DCA did when it was reviewed and approved by the NRC. Although the changes in analysis approach described in this response would likely cause the DCA margin to increase if they were applied to the DCA, the DCA analysis assumed a smaller possible dilution volume based on its FSAR Section 3.4.1 analysis. Therefore, the point remains that the US460 boron dilution analysis for Mode 5 now has more margin than that judged to be large during the DCA review. Table 3: Mode 5 Boron Dilution Margin Comparison Description DCA (US600) SDAA (US460) Margin to loss of shutdown margin in limiting power unavailable scenario (gallons) 244,650 367,700 Updates to Final Safety Analysis Report: Consistent with the NRC request, FSAR Section 15.4.6 is revised as indicated in the attached markups to incorporate the revised analysis. The level of detail provided in the markups is consistent with that previously provided in the FSAR as well as with that of the DCA FSAR. Highlights of the markups are provided below:
FSAR Table 15.4-12 identifies the minimum pool boron concentration and the boron reactivity coefficient assumed in the Mode 5 analysis.
FSAR Table 15.4-17 identifies the assumed initial mixing volume for the limiting Mode 5 scenario.
FSAR Table 15.4-17 identifies the assumed dilution source for the limiting Mode 5 scenario.
FSAR Section 15.4.6.2 identifies the mixing assumptions, including the impact of loss of power on the mixing assumption for Mode 5.
FSAR Section 15.4.6.3.2 identifies conservative assumptions applied to the Mode 5 analysis. NuScale Nonproprietary NuScale Nonproprietary
FSAR Section 15.4.6.3.1 summarizes the method used for the Mode 5 analysis. Updates to Topical Report: TR-0516-49416, Revision 4, Non-Loss-of-Coolant Accident Analysis Methodology, is also revised as indicated in the attached markups to describe the method for the Mode 5 analysis. Impact on US460 SDAA: FSAR Section 15.4.6 and Topical Report TR-0516-49416, Non-Loss-of-Coolant Accident Analysis Methodology, have been revised as described in the response above and as shown in the markups provided in this response. Note this response references a proprietary version of the topical report that is marked as containing export controlled information (ECI). However, the extracted pages of the topical report that are attached to this response do not contain ECI as submitted herein. Notwithstanding, any proprietary information included in the response and the attachment hereto shall be withheld per 10 CFR 2.390. Additional Information: The FSAR Section 15.4.6 markups include updates to the Mode 2 and Mode 3 analysis results that are not related to this RAI. Markups associated with Mode 1 are provided separately with the response to RAI 10142 Question 15.4.6-1. NuScale Nonproprietary NuScale Nonproprietary
NuScale Final Safety Analysis Report Reactivity and Power Distribution Anomalies NuScale US460 SDAA 15.4-18 Draft Revision 2 These MPS signals provide protection in Modes 1 through 3. No single failure could occur during a boron dilution of the RCS that results in a more severe outcome for the limiting cases. The diversity, redundancy, and independence of the MPS and CVCS isolation valves ensure the NPM is protected from a boron dilution of the RCS despite a single failure. 15.4.6.3 Boron Mixing, Thermal Hydraulic, and Subchannel Analyses 15.4.6.3.1 Evaluation Models RAI 10142 Question 15.4.6-2 Two calculation techniques are used to analyze the boron dilution event in Modes 1 through 3 that provide conservative boron dilution assumptions for the evaluation of both reactivity insertion and loss of shutdown margin. The first method evaluates the boron dilution by assuming an instantaneous perfect (complete) mixing model. The second method evaluates the boron dilution by assuming a slug flow or dilution front (wave front) mixing model. In the instantaneous perfect mixing model, unborated water injected into the RCS is assumed to mix instantaneously with the effective system volume. The change in core boron concentration with time is continuous and homogeneous, corresponding to the increasing amount of dilution water entering the RCS. In the dilution front model, unborated water injected into the RCS is assumed to mix with a slug of borated water at the injection point. The diluted slug is assumed to move through the RCS (i.e., through the riser, steam generators, downcomer, and finally though the reactor core). The change in core boron concentration with time depends on the location of the diluted slug. Audit Question A-15.4.6-1 The two calculation techniques provide the reactivity insertion rate due to the boron dilution. To ensure that the SRP 15.4.6 acceptance criteria are met, the reactivity insertion rate in Mode 1 operation is compared to the spectrum of reactivity insertion rates evaluated in the uncontrolled CRA withdrawal from a subcritical or low power startup condition and uncontrolled CRA withdrawal at power analyses in Section 15.4.1 and Section 15.4.2, respectively. The reactivity insertion rates are also used as input to NRELAP5 thermal-hydraulic analyses that provide reactor trip timing to determine when the MPS terminates the boron dilution event. The NRELAP5 model is based on the design features of the NPM. The non-LOCA NRELAP5 model is discussed in Section 15.0.2. Adequate shutdown margin must remain in the mixing model analyses at the time when the boron dilution event is terminated in the NRELAP5 analyses. For Mode 2 and Mode 3 operation, the boron dilution scenarios are evaluated at the time of DWS isolation to ensure that adequate shutdown margin remains at the time of automatic DWS isolation. Boron dilution of the pool is estimated using the perfect mixing equation (Equation 15.4-1).
NuScale Final Safety Analysis Report Reactivity and Power Distribution Anomalies NuScale US460 SDAA 15.4-19 Draft Revision 2 RAI 10142 Question 15.4.6-2 In Mode 5, the dilution scenario results in an increase in the Reactor Building pool inventory. The initial boron mass is calculated using the assumed initial pool volume and concentration. The boron concentration at which shutdown margin is lost is determined from the initial shutdown margin and boron worth. The total water volume having the initial boron mass and the boron concentration at which shutdown margin is lost is calculated. The assumed initial volume is subtracted from the total water volume to determine the dilution volume that would result in loss of shutdown margin. This dilution volume is then compared to the volumes of possible dilution sources. The boron dilution scenarios in Mode 5 operation are performed with conservative assumptions of water volumes to demonstrate that shutdown margin is maintained. 15.4.6.3.1.1 Boron Dilution Assuming Perfect Mixing The perfect (complete) mixing method evaluates the boron concentration of the RCS with the following equation: Eq. 15.4-1
- where, Qin
= dilution flow rate of unborated water (gpm). The maximum dilution flow rate is used for this parameter based on the ability of the makeup pumps to deliver water to the CVCS injection line. in = dilution water density (lbm/cu.ft). The density value at 14.7 psia, 40 degrees F (minimum temperature value) is used in all cases. The heat addition by the regenerative heat exchangers is not credited, so the analysis assumes that the recirculation pumps are not operational. Vr = effective water volume of the RCS (gal). A conservatively small value is used that removes the volume of the pressurizer. r = density of the water in the RCS (lbm/cu.ft), and C(t) = time dependent concentration of boron in the RCS (ppm). dC dt Q inin Vrr
C t( )
=
NuScale Final Safety Analysis Report Reactivity and Power Distribution Anomalies NuScale US460 SDAA 15.4-21 Draft Revision 2 15.4.6.3.2 Input Parameters and Initial Conditions The initial conditions and input parameters for the boron dilution of the RCS analysis are selected to ensure a conservative calculation. The shutdown margin threshold in this analysis for Mode 1 is when keff = 0.995. The shutdown margin threshold for Modes 2 and 3 in this analysis is when keff = 0.93. The shutdown margin threshold in this analysis for Modes 4 and 5 is when keff = 0.90. Therefore, the shutdown margin reactivity credited in this analysis is 503 pcm for Mode 1, 7527 pcm for Modes 2 and 3 and 11,112 pcm for Modes 4 and 5. For Mode 1 operation, the initial power levels considered for a boron dilution of the RCS include: hot zero power (HZP), 25 percent power, 50 percent power, 75 percent power and full (100 percent) power. The BOC and EOC conditions are also considered. The Mode 1 cases of HZP and full power at BOC are provided in this section. RAI 10142 Question 15.4.6-2 Maximum critical boron concentrations and boron coefficients are assumed because the rate of change of concentration and associated reactivity is greater for an initially higher concentration. The critical boron concentrations and boron reactivity coefficients assumed for each mode of operation are provided in Table 15.4-12. For Mode 5, minimum initial pool boron concentration is used to minimize initial boron mass. The makeup flow rates assumed in the analysis are 5 gpm and 25 gpm. A makeup flow rate of 50 gpm is assumed for critical boron concentrations below the limit for two pump operation in Table 15.4-12. The letdown flow rates are assumed to be equal to the makeup flow rates assumed in the analysis. A minimum makeup temperature of 40 degrees F is assumed for the analysis of boron dilution of the RCS. The minimum RCS flow rates are assumed to increase loop transit time, which increases the timing for detection and isolation. Audit Question A-15.4.6-1 A conservatively smaller pool volume is used to provide a limiting boron dilution for Mode 5. Allowances for instrument inaccuracy are accounted for in the analytical limits of mitigating systems in accordance with RG 1.105. 15.4.6.3.3 Results The results for a boron dilution of the RCS during Mode 1 operation are presented in Table 15.4-13 for hot full power and Table 15.4-14 for HZP. The tabulated results for the hot full power scenario demonstrate that the reactivity insertion rates are bounded by the range of the reactivity insertion rates that are evaluated in the uncontrolled CRA withdrawal at power analysis, presented in Section 15.4.2. The tabulated results for the HZP scenario demonstrate that the reactivity insertion rates are bounded by the range of the
NuScale Final Safety Analysis Report Reactivity and Power Distribution Anomalies NuScale US460 SDAA 15.4-22 Draft Revision 2 reactivity insertion rates that are evaluated in the uncontrolled CRA withdrawal from a subcritical or low power startup condition analysis, presented in Section 15.4.1. The tabulated results for the Mode 1 scenarios also demonstrate that shutdown margin is maintained at the time of reactor trip and DWS isolation. The results for a boron dilution of the RCS during Mode 2 operation and Mode 3 operation are presented in Table 15.4-15 and Table 15.4-16, respectively. The tabulated results for the Mode 2 and Mode 3 scenarios demonstrate that shutdown margin is maintained at the time of DWS isolation. Audit Question A-15.4.6-1 RAI 10142 Question 15.4.6-2 The results for a boron dilution of the RCS during Mode 5 operation are presented in Table 15.4-17 for the limiting case with power unavailable. The total dilution volume necessary to achieve criticality is greater than the largest volume of water that could be unexpectedly introduced to the Reactor Building pool from internal flooding sources. The tabulated results demonstrate that shutdown margin is maintained during Mode 5 operation. Although operator actions are not necessary to ensure shutdown margin is maintained in the limiting case, Table 15.4-17 shows that operator actions prompted by the technical specifications or associated with the Reactor Building flooding evaluation in Section 3.4.1 limit the dilution volume and preserve more shutdown margin. 15.4.6.4 Radiological Consequences The NPM conditions after the limiting decrease in boron concentration cases during Mode 1 operation are bounded by the uncontrolled CRA withdrawal from a subcritical or low power startup condition and uncontrolled CRA withdrawal at power analyses presented in Section 15.4.1 and Section 15.4.2, respectively. Based on the uncontrolled CRA withdrawal from a subcritical or low power startup condition and uncontrolled CRA withdrawal at power results, no fuel failures are predicted and radionuclide barriers maintain integrity during a decrease in boron concentration event. The results for the non-power modes of operation show that shutdown margin is maintained for a decrease in boron concentration event. The normal leakage related radiological consequences of this event are bounded by the design-basis accident analyses presented in Section 15.0.3. 15.4.6.5 Conclusions Audit Question A-15.4.6-1 The NPM conditions after the decrease in boron concentration cases during Mode 1 operation are bounded by the uncontrolled CRA withdrawal from a subcritical or low power startup condition and uncontrolled CRA withdrawal at power analyses presented in Section 15.4.1 and Section 15.4.2, respectively. Shutdown margin is also maintained for Mode 1 cases. The results for the non-power modes of operation (Modes 2 and 3) show that shutdown margin is maintained for a decrease in boron concentration event. The results of pool dilution during Mode 5
NuScale Final Safety Analysis Report Reactivity and Power Distribution Anomalies NuScale US460 SDAA 15.4-45 Draft Revision 2 RAI 10142 Question 15.4.6-2 Table 15.4-12: Bounding Critical Boron Concentrations and Boron Reactivity Coefficients (15.4.6 Inadvertent Decrease in Boron Concentration in the Reactor Coolant System) Operation Mode Critical Boron Concentration1,2,3 (ppm) Boron Reactivity Coefficient (pcm/ppm) Mode 1, 25% power 1600 / 600 -10 Mode 1, hot zero power 1900 / 1000 -10 Mode 2 600 -11 Mode 3 650 -12.5 Modes 4 and 5 1900 -11.5 Mode 5 1900 -11.5 1Where two values are provided for critical boron concentration, the two values correspond to BOC and EOC, respectively. 2Operation with two makeup pumps is prohibited when critical boron concentration is above 600 ppm. 3For Mode 5, the concentration is the minimum pool concentration rather than the critical boron concentration.
NuScale Final Safety Analysis Report Reactivity and Power Distribution Anomalies NuScale US460 SDAA 15.4-48 Draft Revision 2 Table 15.4-15: Mode 2 Results (15.4.6 Inadvertent Decrease in Boron Concentration in the Reactor Coolant System) Parameter Value Value Dilution rate (gpm) 5 25 Initial wave reactivity step (pcm) 207.37 979.34 Time to loss of shutdown margin (minutes) 16901751 334347 Time of DWS isolation (minutes) 15921077 2419 Shutdown margin remaining at DWS isolation (pcm) 2452201 13511944
NuScale Final Safety Analysis Report Reactivity and Power Distribution Anomalies NuScale US460 SDAA 15.4-49 Draft Revision 2 Table 15.4-16: Mode 3 Results (15.4.6 Inadvertent Decrease in Boron Concentration in the Reactor Coolant System) Parameter Value Value Dilution rate (gpm) 5 25 Initial wave reactivity step (pcm) 203.34 966.48 Time to loss of shutdown margin (minutes) 16601708 336342 Time of DWS isolation (minutes) 15621079 22143 Shutdown margin remaining at DWS isolation (pcm) 2312175 12751894
NuScale Final Safety Analysis Report Reactivity and Power Distribution Anomalies NuScale US460 SDAA 15.4-50 Draft Revision 2 Audit Question A-15.4.6-1 RAI 10142 Question 15.4.6-2 Table 15.4-17: Mode 5 Results, Limiting Power Unavailable Scenario (15.4.6 Inadvertent Decrease in Boron Concentration in the Reactor Coolant System) Parameter Value Assumed initial mixing volume (ft3gallons) 124,412935,282 Total dDilution volume required to reduce shutdown margin to zero (gallons) 366,686967,700 Maximum dilution volume causing violation of pool level technical specification limits (gallons) 117,609 TotalMaximum dilution volume offrom largest internal flooding source1 (gallons) 363,000 Maximum dilution volume from largest assumed internal flooding source2 (gallons) 600,000 1 Internal flooding of the Reactor Building is evaluated as described in Section 3.4.1. 2 The maximum flooding source in this scenario is the combined volume of the firewater storage tanks described in Section 9.5.1.
Non-Loss-of-Coolant Accident Analysis Methodology TR-0516-49416-NP Draft Revision 5 © Copyright 2024 by NuScale Power, LLC 634 RAI 10297 Question NonLOCA.LTR-31, 32, 46, 56, 65 7.2.16 Inadvertent Decrease in Boron Concentration The methodology used to simulate an inadvertent decrease in boron concentration for an NPM, and an evaluation of the acceptance criteria for an AOO listed in Table 7-4, are presented below. 7.2.16.1 General Event Description and Methodology The boric acid blend system incorporated into the NuScale plant design permits the operator to control the boron concentration of the reactor coolant via the charging fluid chemistry. While the NuScale plant design incorporates both automatic and manual controls, strict administrative procedures govern the process for adjusting the boron concentration of the reactor coolant. These administrative procedures establish limits on the rate and duration of the dilution. The primary means of causing an inadvertent decrease in boron concentration is failure of the blend system, either by controller or mechanical failure, or operator error. The event is terminated by isolating the source for the diluted water, i.e., by closing the demineralized water system (DWS) isolation valves. RAI 10142 Question 15.4.6-2 For Mode 1 plant operating conditions, the perfect mixing model and the wave front model are both evaluated. The perfect mixing model is evaluated for Mode 1 operating conditions because it provides a slower reactivity insertion rate, delaying detection, potentially allowing further loss of shutdown margin. The wave front model is physically conservative because it assumes the maximum amount of reactivity as the diluted slug of water sweeps through the core. This model does not assume any axial blending to ensure that this reactivity insertion rate is maximized. For all other operating modesModes 2 and 3 where boron dilution is allowed and limited mixing exists, a wave front model is used. These mixing Table 7-71 Representative sensitivity studies - control rod misoperation, dropped control rod assembliesNot Used (( }}2(a),(c)
Non-Loss-of-Coolant Accident Analysis Methodology TR-0516-49416-NP Draft Revision 5 © Copyright 2024 by NuScale Power, LLC 637 RAI 10142 Question 15.4.6-1 The reactivity insertion rates associated with these configurations are determined using both the perfect mixing model (Equation 7-1) and the wave front model (Equation 7-2 and Equation 7-3). The times of reactor trip and isolation of the dilution source via closure of the DWS isolation valves are obtained from the NRELAP5 results for these reactivity insertion rates. The calculations performed with the perfect mixing model are also used to determine the shutdown margin available after isolation of the DWS, and the time at which the shutdown margin would be lost if the dilution source is not terminated. The system responses for all other acceptance criteria, such as MCHFR and peak RCS pressure, are comparable to the uncontrolled control rod bank withdrawal at power event (Section 7.2.14) by comparison of the reactivity insertion rates. Mode 1 (Operations) HZP RAI 10142 Question 15.4.6-1 In this mode of plant operation, an inadvertent decrease in boron concentration causes a reactivity insertion that increases reactor power, but does not lead to a rise in coolant temperature, pressurizer level, or RCS pressure. A loss of shutdown margin would occur quickest for the highest reactivity insertion rate, considering the maximum dilution flow rate scenarios discussed above. The reactivity insertion rates associated with these configurations are determined using both the perfect mixing model (Equation 7-1) and the wave front model (Equation 7-2 and Equation 7-3). The time of reactor trip and isolation of the dilution source via closure of DWS isolation valves is obtained from the NRELAP5 results for these reactivity insertion rates. The calculations performed with the wave front model are also used to determine the shutdown margin available after isolation of the DWS, and the time at which the shutdown margin would be lost if the dilution source is not terminated. The system responses for all other acceptance criteria, such as MCHFR and peak RCS pressure, are comparable to the uncontrolled CRA bank withdrawal from subcritical or low power startup conditions event (Section 7.2.13) by comparison of the reactivity insertion rates. Mode 2 (Hot Shutdown) and Mode 3 (Safe Shutdown) In these modes of plant operation, the MPS protection logic ensures the DWS is isolated when the RCS flow rate is less than the low flow setpoint (1.7 ft3/s in the example Table 7-3). This protection scheme precludes the possibility for an inadvertent decrease in boron concentration. When the RCS flow rate is greater than or equal to the low flow setpoint (1.7 ft3/s in the example Table 7-3), the reactivity insertion from the maximum dilution flow rate of 25 gpm (1 CVCS pump) with unborated water causes an increase in reactor power (neutron population). The increase in neutron flux is detected by the MPS count rate protection signal and used to close the DWS isolation valves. The calculations performed with the wave front model (Equation 7-2 and Equation 7-3) determine the shutdown margin available after isolation of the DWS, and the time
Non-Loss-of-Coolant Accident Analysis Methodology TR-0516-49416-NP Draft Revision 5 © Copyright 2024 by NuScale Power, LLC 638 at which the shutdown margin would be lost if the dilution source is not terminated. Mode 4 (Transition) In this mode of plant operation, all CVCS connections to an NPM are disconnected, isolated, or locked out. Thus, the possibility of a design-basis inadvertent decrease in boron concentration is precluded. Mode 5 (Refueling) Audit Question A-NonLOCA.LTR-43 RAI 10142 Question 15.4.6-2 In this mode of plant operation, the Technical Specifications require the pool boron concentration to be sufficient to have appropriate shutdown margin. For some NPM designs, the Technical Specifications also require the pool level to be maintained within a narrow range. Surveillance of the boron concentration, and level if applicable, of the refueling pool is performed at appropriate intervals and is expected to prompt operator actions in accordance with Technical Specifications during an inadvertent dilution of the pool; such operator actions are not credited.to prevent significant inadvertent dilution from flow paths to the reactor pool, or proximate water sources such as fire mains or feedwater piping. RAI 10142 Question 15.4.6-2 Two methods can be used for assessing the dilution volume that, if allowed to enter the pool, could cause a loss of shutdown margin in Mode 5. If the pool volume is treated as constant, then the perfect mixing model described above for Mode 1 can be used to calculate the time to dilution for an arbitrary flow rate. The dilution time and arbitrary flow rate are then used to determine the dilution volume. Alternatively, if pool volume is treated as increasing during the event, the initial boron mass is calculated using the assumed initial pool volume and concentration. The boron concentration at which shutdown margin is lost is determined from the initial shutdown margin and boron worth. The total water volume having the initial boron mass and the boron concentration at which shutdown margin is lost is calculated. The assumed initial volume is subtracted from the total water volume to determine the dilution volume that would result in loss of shutdown margin. The dilution volume determined from either of these two methods is then compared to the volumes of possible dilution sources. The relevant acceptance criteria, SAF, and LOP scenarios are listed in Table 7-72.
RAIO-178471 NuScale Power, LLC 1100 NE Circle Blvd., Suite 200 Corvallis, Oregon 97330 Office 541.360.0500 Fax 541.207.3928 www.nuscalepower.com Affidavit of Mark W. Shaver, AF-178472
AF-178472 Page 1 of 2
NuScale Power, LLC AFFIDAVIT of Mark W. Shaver I, Mark W. Shaver, state as follows: (1) I am the Director of Regulatory Affairs of NuScale Power, LLC (NuScale), and as such, I have been specifically delegated the function of reviewing the information described in this Affidavit that NuScale seeks to have withheld from public disclosure, and am authorized to apply for its withholding on behalf of NuScale. (2) I am knowledgeable of the criteria and procedures used by NuScale in designating information as a trade secret, privileged, or as confidential commercial or financial information. This request to withhold information from public disclosure is driven by one or more of the following: (a) The information requested to be withheld reveals distinguishing aspects of a process (or component, structure, tool, method, etc.) whose use by NuScale competitors, without a license from NuScale, would constitute a competitive economic disadvantage to NuScale. (b) The information requested to be withheld consists of supporting data, including test data, relative to a process (or component, structure, tool, method, etc.), and the application of the data secures a competitive economic advantage, as described more fully in paragraph 3 of this Affidavit. (c) Use by a competitor of the information requested to be withheld would reduce the competitors expenditure of resources, or improve its competitive position, in the design, manufacture, shipment, installation, assurance of quality, or licensing of a similar product. (d) The information requested to be withheld reveals cost or price information, production capabilities, budget levels, or commercial strategies of NuScale. (e) The information requested to be withheld consists of patentable ideas. (3) Public disclosure of the information sought to be withheld is likely to cause substantial harm to NuScales competitive position and foreclose or reduce the availability of profit-making opportunities. The accompanying Request for Additional Information response reveals distinguishing aspects about the response by which NuScale develops its NuScale Power, LLC Response to NRC Request for Additional Information (RAI No. 10142 R1, Question 15.4.6-2) on the NuScale Standard Design Approval Application. NuScale has performed significant research and evaluation to develop a basis for this response and has invested significant resources, including the expenditure of a considerable sum of money. The precise financial value of the information is difficult to quantify, but it is a key element of the design basis for a NuScale plant and, therefore, has substantial value to NuScale. If the information were disclosed to the public, NuScales competitors would have access to the information without purchasing the right to use it or having been required to undertake a similar expenditure of resources. Such disclosure would constitute a misappropriation of NuScales intellectual property, and would deprive NuScale of the opportunity to exercise its competitive advantage to seek an adequate return on its investment. (4) The information sought to be withheld is in the enclosed response to NRC Request for Additional Information RAI 10142 R1, Question 15.4.6-2. The enclosure contains the designation Proprietary at the top of each page containing proprietary information. The information considered by NuScale to be proprietary is identified within double braces, (( }} in the document.
AF-178472 Page 2 of 2 (5) The basis for proposing that the information be withheld is that NuScale treats the information as a trade secret, privileged, or as confidential commercial or financial information. NuScale relies upon the exemption from disclosure set forth in the Freedom of Information Act (FOIA), 5 USC § 552(b)(4), as well as exemptions applicable to the NRC under 10 CFR §§ 2.390(a)(4) and 9.17(a)(4). (6) Pursuant to the provisions set forth in 10 CFR § 2.390(b)(4), the following is provided for consideration by the Commission in determining whether the information sought to be withheld from public disclosure should be withheld: (a) The information sought to be withheld is owned and has been held in confidence by NuScale. (b) The information is of a sort customarily held in confidence by NuScale and, to the best of my knowledge and belief, consistently has been held in confidence by NuScale. The procedure for approval of external release of such information typically requires review by the staff manager, project manager, chief technology officer or other equivalent authority, or the manager of the cognizant marketing function (or his delegate), for technical content, competitive effect, and determination of the accuracy of the proprietary designation. Disclosures outside NuScale are limited to regulatory bodies, customers and potential customers and their agents, suppliers, licensees, and others with a legitimate need for the information, and then only in accordance with appropriate regulatory provisions or contractual agreements to maintain confidentiality. (c) The information is being transmitted to and received by the NRC in confidence. (d) No public disclosure of the information has been made, and it is not available in public sources. All disclosures to third parties, including any required transmittals to NRC, have been made, or must be made, pursuant to regulatory provisions or contractual agreements that provide for maintenance of the information in confidence. (e) Public disclosure of the information is likely to cause substantial harm to the competitive position of NuScale, taking into account the value of the information to NuScale, the amount of effort and money expended by NuScale in developing the information, and the difficulty others would have in acquiring or duplicating the information. The information sought to be withheld is part of NuScales technology that provides NuScale with a competitive advantage over other firms in the industry. NuScale has invested significant human and financial capital in developing this technology and NuScale believes it would be difficult for others to duplicate the technology without access to the information sought to be withheld. I declare under penalty of perjury that the foregoing is true and correct. Executed on January 16, 2025. Mark W. Shaver}}