ML24101A194
| ML24101A194 | |
| Person / Time | |
|---|---|
| Site: | Calvert Cliffs  |
| Issue date: | 04/22/2024 |
| From: | Marshall M Plant Licensing Branch 1 |
| To: | Rhoades D Constellation Energy Generation |
| Marshall, Michael | |
| References | |
| EPID L-2017-LRC-0000 | |
| Download: ML24101A194 (1) | |
Text
April 22, 2024 David P. Rhoades Senior Vice President Constellation Energy Generation, LLC President and Chief Nuclear Officer Constellation Nuclear 4300 Winfield Road Warrenville, IL 60555
SUBJECT:
CALVERT CLIFFS NUCLEAR POWER PLANT, UNITS 1 AND 2 - CLOSEOUT OF GENERIC LETTER 2004-02, POTENTIAL IMPACT OF DEBRIS BLOCKAGE ON EMERGENCY RECIRCULATION DURING DESIGN BASIS ACCIDENTS AT PRESSURIZED-WATER REACTORS (EPID L-2017-LRC-0000)
Dear David Rhoades:
The U.S. Nuclear Regulatory Commission (NRC) issued Generic Letter (GL) 2004-02, Potential Impact of Debris Blockage on Emergency Recirculation During Design Basis Accidents at Pressurized-Water Reactors (Agencywide Documents Access and Management System (ADAMS) Accession No. ML042360586), dated September 13, 2004, requesting that licensees address the issues raised by Generic Safety Issue (GSI) -191, Assessment of Debris Accumulation on PWR [Pressurized Water Reactor] Sump Performance.
The stated purpose of GL 2004-02 was focused on demonstrating compliance with Title 10 of the Code of Federal Regulations (10 CFR) Section 50.46. Specifically, GL 2004-02 requested addressees to perform an evaluation of the emergency core cooling system and containment spray system recirculation and, if necessary, take additional action to ensure system function in light of the potential for debris to adversely affect long term core cooling.
On July 23, 2019 (ML19203A303), GSI-191 was closed. It was determined that the technical issues identified in GSI-191 were now well understood and therefore GSI-191 could be closed.
Prior to and in support of closing GSI-191, the NRC staff issued a technical evaluation report on in-vessel downstream effects (ML19178A252 and ML19073A044 (not publicly available, proprietary information)). Following the closure of GSI-191, the NRC staff also issued review guidance for in-vessel downstream effects, NRC Staff Review Guidance for In-Vessel Downstream Effects Supporting Review of GL 2004-02 Responses (ML19228A011), to support review of the GL 2004-02 responses.
By letter dated November 12, 2020 (ML20317A112), as supplemented by letter dated February 9, 2024 (ML24040A149), Constellation Energy Generation, LLC (Constellation, the licensee) provided its final updated response and stated that it will pursue Option 2 (deterministic) for the closure of GSI-191 and GL 2004-02 for Calvert Cliff Nuclear Power Plant, Unit Nos. 1 and 2 (Calvert Cliffs).
The NRC staff has reviewed the licensees responses and request for additional information supplements associated with GL 2004-02. The NRC staff finds the information provided by the licensee demonstrates that debris will not inhibit the emergency core cooling system or containment spray system performance following a postulated loss-of-coolant accident.
Therefore, the ability of the systems to perform their safety functions, to assure adequate long term core cooling following a design basis accident, as required by 10 CFR 50.46, has been demonstrated. Based on the evaluations, the NRC staff finds the licensee has provided adequate information as requested by GL 2004-02. Therefore, the NRC staff finds the licensees responses to GL 2004-02 are adequate and considers GL 2004-02 closed for Calvert Cliffs.
Enclosed is the summary of the NRC staffs review. If you have any questions, please contact me at (301) 415-2871 or Michael.Marshall@nrc.gov.
Sincerely,
/RA/
Michael Marshall, Senior Project Manager Plant Licensing Branch I Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket Nos. 50-317 and 50-318
Enclosure:
NRC Staff Review of Constellations Response to GL 2004-02 for Calvert Cliffs cc: Listserv
Enclosure U.S. NUCLEAR REGULATORY COMMISSION STAFF REVIEW OF THE DOCUMENTATION PROVIDED BY EXELON GENERATION FOR CALVERT CLIFFS NUCLEAR POWER PLANT, UNIT NOS. 1 AND 2 DOCKET NOS. 50-317 AND 50-318 CONCERNING RESOLUTION OF GENERIC LETTER 2004-02 POTENTIAL IMPACT OF DEBRIS BLOCKAGE ON EMERGENCY RECIRCULATION DURING DESIGN-BASIS ACCIDENTS AT PRESSURIZED-WATER REACTORS
1.0 INTRODUCTION
A fundamental function of the emergency core cooling system (ECCS) is to recirculate water that has collected at the bottom of the containment through the reactor core following a break in the reactor coolant system (RCS) piping to ensure long-term removal of decay heat from the reactor fuel. Leaks from the RCS, hypothetical scenarios known as loss-of-coolant accidents (LOCAs), are part of every plants design-basis. Hence, nuclear plants are designed and licensed with the expectation that they are able to remove reactor decay heat following a LOCA to prevent core damage. Long-term cooling following a LOCA is a basic safety function for nuclear reactors. The recirculation sump provides a water source to the ECCS in a pressurized-water reactor (PWR) once the primary water source has been depleted.
If a LOCA occurs, piping thermal insulation and other materials may be dislodged by the two-phase coolant jet emanating from the broken RCS pipe. This debris may transport, via flows coming from the RCS break or from the containment spray system (CSS), to the pool of water that collects at the bottom of containment following a LOCA. Once transported to the sump pool, the debris could be drawn toward the ECCS sump strainers, which are designed to prevent debris from entering the ECCS and the reactor core. If this debris were to clog the strainers and prevent coolant from entering the reactor core, containment cooling could be lost and result in core damage and containment failure.
It is also possible that some debris would bypass the sump strainer and lodge in the reactor core. This could result in reduced core cooling and potential core damage. If the ECCS strainer were to remain functional, even with core cooling reduced, containment cooling would be maintained, and the containment function would not be adversely affected.
Findings from research and industry operating experience raised questions concerning the adequacy of PWR sump designs. Research findings demonstrated that, compared to other LOCAs, the quantity of debris generated by a high-energy line break (HELB) could be greater.
The debris from a HELB could also be finer (and thus more easily transportable) and could be comprised of certain combinations of debris (i.e., fibrous material plus particulate material) that could result in a substantially greater flow restriction than an equivalent amount of either type of debris alone. These research findings prompted the U.S. Nuclear Regulatory Commission (NRC) to open Generic Safety Issue (GSI)-191, Assessment of Debris Accumulation on PWR Sump Performance, in 1996. This resulted in new research for PWRs in the late 1990s.
GSI-191 focuses on reasonable assurance that the provisions of Title 10 of the Code of Federal Regulations (10 CFR) Section 50.46(b)(5) are met. This deterministic rule requires maintaining long-term core cooling (LTCC) after initiation of the ECCS. The objective of GSI-191 is to ensure that post-accident debris blockage will not impede or prevent the operation of the ECCS and CSS in recirculation mode at PWRs during LOCAs or other HELB accidents for which sump recirculation is required. The NRC completed its review of GSI-191 in 2002 and documented the results in a parametric study that concluded that sump clogging at PWRs was a credible concern.
GSI-191 concluded that debris clogging of sump strainers could lead to recirculation system ineffectiveness as a result of a loss of net positive suction head (NPSH) for the ECCS and CSS recirculation pumps. Resolution of GSI-191 involves two distinct but related safety concerns:
(1) potential clogging of the sump strainers that results in ECCS and/or CSS pump failure; and (2) potential clogging of flow channels within the reactor vessel because of debris bypass of the sump strainer (in-vessel effects). Clogging at either the strainer or in-vessel channels can result in loss of the long-term cooling safety function.
After completing the technical assessment of GSI-191, the NRC issued Bulletin 03-01, Potential Impact of Debris Blockage on Emergency Sump Recirculation at Pressurized-Water Reactors (Agencywide Documents Access and Management System (ADAMS) Accession No. ML031600259), on June 9, 2003. The Office of Nuclear Reactor Regulation (NRR) requested and obtained the review and endorsement of the bulletin from the Committee to Review Generic Requirements (CRGR) (ML031210035). As a result of the emergent issues discussed in Bulletin 03-01, the NRC staff requested an expedited response from PWR licensees on the status of their compliance of regulatory requirements concerning the ECCS and CSS recirculation functions based on a mechanistic analysis. The NRC staff asked licensees who chose not to confirm regulatory compliance, to describe any interim compensatory measures that they had implemented or will implement to reduce risk until the analysis could be completed. All PWR licensees responded to Bulletin 03-01. The NRC staff reviewed all licenseesBulletin 03-01 responses and found them acceptable.
In developing Bulletin 03-01, the NRC staff recognized that it might be necessary for licensees to undertake complex evaluations to determine whether regulatory compliance exists in light of the concerns identified in the bulletin and that the methodology needed to perform these evaluations was not currently available. As a result, that information was not requested in Bulletin 03-01, but licensees were informed that the NRC staff was preparing a Generic Letter (GL) that would request this information. GL 2004-02, Potential Impact of Debris Blockage on Emergency Recirculation During Design-Basis Accidents at Pressurized-Water Reactors, dated September 13, 2004 (ML042360586), was the follow-on information request referenced in Bulletin 03-01. This document set the expectations for resolution of PWR sump performance issues identified in GSI-191, to ensure the reliability of the ECCS and CSS at PWRs. NRR requested and obtained the review and endorsement of the GL from the CRGR (ML040840034).
GL 2004-02 requested that addressees perform an evaluation of the ECCS and CSS recirculation functions in light of the information provided in the letter and, if appropriate, take additional actions to ensure system function. Additionally, addressees were requested to submit the information specified in GL 2004-02 to the NRC. The request was based on the identified potential susceptibility of PWR recirculation sump screens to debris blockage during design-basis accidents (DBAs) requiring recirculation operation of ECCS or CSS and on the potential for additional adverse effects due to debris blockage of flow paths necessary for ECCS and CSS recirculation and containment drainage. GL 2004-02 required addressees to provide the NRC a written response in accordance with 10 CFR 50.54(f).
By letter dated May 28, 2004 (ML041550661), the Nuclear Energy Institute (NEI) submitted a report (NEI 04-07) describing a methodology for use by PWR licensees in the evaluation of containment sump performance. This is also called the Guidance Report (GR). NEI requested that the NRC review the methodology. The methodology was intended to allow licensees to address and resolve GSI-191 issues in an expeditious manner through a process that starts with a conservative baseline evaluation. The baseline evaluation serves to guide the analyst and provide a method for quick identification and evaluation of design features and processes that significantly affect the potential for adverse containment sump blockage for a given plant design. The baseline evaluation also facilitates the evaluation of potential modifications that can enhance the capability of the design to address sump debris blockage concerns and uncertainties and supports resolution of GSI-191. The report offers additional guidance that can be used to modify the conservative baseline evaluation results through revision to analytical methods or through modification to the plant design or operation.
By letter dated December 6, 2004 (ML043280641), the NRC issued an evaluation of the NEI methodology. The NRC staff concluded that the methodology, as approved in accordance with the NRC staff safety evaluation (SE), provides an acceptable overall guidance methodology for the plant-specific evaluation of the ECCS or CSS sump performance following postulated DBAs.
Taken together NEI 04-07 and the associated NRC staff SE are often referred to as the GR/SE.
In response to the NRC staff SE conclusions on NEI 04-07 Pressurized Water Reactor Sump Performance Evaluation Methodology (ML050550138 and ML050550156), the Pressurized Water Reactor Owners Group sponsored the development of the following Westinghouse Commercial Atomic Power (WCAP) Topical Reports (TRs):
TR-WCAP-16406-P-A, Evaluation of Downstream Sump Debris Effects in Support of GSI-191, Revision 1 (not publicly available), to address the effects of debris on piping systems and components (SE at ML073520295).
TR-WCAP-16530-NP-A, Evaluation of Post-accident Chemical Effects in Containment Sump Fluids to Support GSI-191, issued March 2008 (ML081150379), to provide a consistent approach for plants to evaluate the chemical effects that may occur post-accident in containment sump fluids (SE at ML073521072).
TR-WCAP-16793-NP-A, Evaluation of Long-Term Cooling Considering Particulate, Fibrous and Chemical Debris in the Recirculating Fluid, Revision 2 issued July 2013 (ML13239A114), to address the effects of debris on the reactor core (SE at ML13084A154).
The NRC staff reviewed the TRs and found them acceptable to use (as qualified by the limitations and conditions stated in the respective SEs). A more detailed evaluation of how the TRs were used by the licensee is contained in the evaluations below.
After the NRC staff evaluated licensee responses to GL 2004-02, the NRC staff found that there was a misunderstanding between the industry and the NRC on the level of detail necessary to respond to GL 2004-02. The NRC staff, in concert with stakeholders developed a content guide for responding to requests for additional information (RAIs) concerning GL 2004-02. By letter dated August 15, 2007 (ML071060091), the NRC issued the content guide describing the necessary information to be submitted to allow the NRC staff to verify that each licensees analyses, testing, and corrective actions associated with GL 2004-02 are adequate to demonstrate that the ECCS and CSS will perform their intended function following any DBA. By letter dated November 21, 2007 (ML073110389), the NRC issued a revised content guide (hereafter referred to as the content guide).
The content guide described the following information needed to be submitted to the NRC:
corrective actions for GL 2004-02, break selection, debris generation/zone of influence (ZOI) (excluding coatings),
debris characteristics, latent debris, debris transport, head loss and vortexing,
- NPSH, coatings evaluation, debris source term, screen modification package, sump structural analysis, upstream effects, downstream effects - components and systems, downstream effects - fuel and vessel, chemical effects, and licensing basis.
Based on the interactions with stakeholders and the results of the industry testing, the NRC staff, in 2012, developed three options to resolve GSI-191. These options were documented and proposed to the Commission in SECY-12-0093, Closure Options for Generic Safety Issue - 191, Assessment of Debris Accumulation on Pressurized-Water Reactor Sump Performance, dated July 9, 2012 (ML121320270). The options are summarized as follows:
Option 1 would require licensees to demonstrate compliance with 10 CFR 50.46, Acceptance criteria for emergency core cooling systems for light-water nuclear power reactors, through approved models and test methods. These will be low fiber plants with less than 15 grams of fiber per fuel assembly.
Option 2 requires implementation of additional mitigating measures and allows additional time for licensees to resolve issues through further industry testing or use of a risk informed approach.
o Option 2 Deterministic: Industry to perform more testing and analysis and submit the results for NRC review and approval (in-vessel only).
o Option 2 Risk Informed: Use the South Texas Project pilot approach.
Option 3 involves separating the regulatory treatment of the sump strainer and in-vessel effects.
The options allowed industry alternative approaches for resolving GSI-191. The Commission issued a Staff Requirements Memorandum on December 14, 2012 (ML12349A378), approving all three options for closure of GSI-191.
By letter dated November 12, 2020 (ML20317A112), Exelon Generation (Exelon or the licensee) provided its final updated response and stated that it will pursue Option 2 (deterministic) for the closure of GSI-191 and GL 2004-02 for Calvert Cliff Nuclear Power Plant, Unit Nos. 1 and 2 (Calvert Cliffs).
On July 23, 2019 (ML19203A303), GSI-191 was closed. It was determined that the technical issues identified in GSI-191 were now well understood and therefore GSI-191 could be closed.
Prior to and in support of closing the GSI, the NRR staff issued a technical evaluation report on in-vessel downstream effects (IVDEs) (ML19178A252 and ML19073A044 (non-public version)).
Following the closure of the GSI, the NRR staff also issued review guidance for IVDEs, NRC Staff Review Guidance for In-Vessel Downstream Effects Supporting Review of Generic Letter 2004-02 Responses (ML19228A011) to support review of the GL 2004-02 responses.
The NRC staff summary is based on the review of the latest information provided by the licensee in response to GL 2004-02 and the associated RAIs (Final Response and Close-Out to Generic Letter 2004-02, dated November 12, 2020 (ML20317A112), RAI dated January 11, 2024 (ML24012A049), and RAI response dated February 9, 2024 (ML24040A149). Prior to the final response provided by the licensee and the associated RAIs and responses, the NRC staff also reviewed the licensees submittals, and responses to RAIs related to closure of GL 2004-02. However, those submittals and responses were superseded by the November 12, 2020, final response by the licensee and are only mentioned here for completeness. The following is a summary of the NRC staff review.
2.0 GENERAL DESCRIPTION OF CORRECTIVE ACTIONS FOR THE RESOLUTION OF GL-2004-02 GL 2004-02 Requested Information Item 2(b) requested a general description of and implementation schedule for all corrective actions. The following is a list of corrective actions completed by the licensee at Calvert Cliffs in support of the resolution of GL 2004-02.
Completed walk downs to sample and characterize latent debris, and identify other debris sources, e.g., labels, in September 2010.
Performed comprehensive debris generation analyses and debris transport analysis in accordance with NEI 04-07.
Completed as-built verification walk downs of insulation in containments in 2012.
Replaced simple geometry strainer with a complex geometry strainer with a filtering surface area of 6,060 square feet (ft2) and smaller openings in 2007 for Unit 2 and 2008 for Unit 1.
Completed head loss analysis for replacement strainers, and vortex testing and analysis in May 2009.
Completed bypass testing for replacement strainers in October 2007.
Completed head loss testing for the replacement strainers (including chemical effects) in January 2009 and October 2010.
Completed detailed structural analysis of new strainers in September 2014.
Completed high-pressure safety injection (HPSI) cyclone separator replacement and blockage testing in June 2008.
Removed or banded calcium-silicate (Cal-Sil) pipe insulation (March 2008).
Replaced trisodium phosphate containment buffering agent with sodium tetraborate in March 2009.
Installed blow-out panels in the reactor cavities ventilation ducting to allow early failure of the ventilation duct should it fill with water in March 2011.
Removed the telescoping aluminum ladder from the polar crane in containment in March 2013.
Replaced significant amounts of mineral wool and fibrous insulation with stainless steel reflective metal insulation (RMI) in March 2014.
Completed a comprehensive chemical effect head loss experimental and test program in November 2014.
Enlarged the reactor refueling cavity drains in March 2013.
Revised the emergency operating procedures (EOPs) to assure the flow rate through the strainer does not exceed 2,365 gallons per minute (gpm) prior to the recirculation mode of operation.
Completed a plant-specific simplified risk-informed analysis in August 2018.
In addition, in its November 12, 2020, submittal the licensee stated that it addressed large breaks using an alternate methodology. This methodology is described in the GR/SE and can be used for breaks larger than a defined alternate break size. The alternate break size is usually defined as the largest pipe connecting to the main RCS loop piping, usually the pressurizer surge line which for most designs is a 14-inch pipe with a break size less than 12 inches. The licensee defined the alternate break size for Calvert Cliffs as greater than those resulting from a break of 11.188 inches diameter. This sizing is consistent with the guidance in the GR/SE.
Breaks smaller than the alternate breaks size are designated Region I breaks while larger breaks are considered Region II. For Region II breaks the guidance allows licensees to use more realistic assumptions in some of the evaluation areas and allows operator actions to be credited in response to the LOCA. The licensee provided actions, already in place, that would be taken if the strainer could not support operation of the pumps in recirculation for Region II breaks. These actions can be used to initiate alternate injection, increase strainer submergence, or attempt to reduce strainer headloss to mitigate the effects of debris on the strainer.
In its implementation of the alternate methodology, the licensee stated that all Region I breaks were assumed to be double ended, even for the cases where single-sided breaks could have been assumed. This is conservative, but likely did not have a significant impact on the postulated Region I limiting debris loads.
The licensee stated that the limiting failure mode for the strainers is gas binding of the pumps due to deaeration of the fluid as it passes through the strainer. The conditions that could lead to this failure mode are only applicable for Region II breaks. The licensee described the defense-in-depth measures and plant design features that are available to mitigate this issue should it occur. Region I breaks are demonstrated to be mitigated without failure of the strainer or required operator actions.
Based on the information provided by the licensee, the NRC staff considers this item closed for GL 2004-02.
3.0 BREAK SELECTION The objective of the break selection process is to identify the break size and locations that present the greatest challenge to post accident sump performance. The term ZOI used in this section refers to the zone representing the volume of space affected by the ruptured piping.
NRC STAFF REVIEW:
The NRC staff review is based on documentation provided by the licensee on November 12, 2020.
The licensee used pipe break locations that could generate the maximum amount of fiber fines as the primary criteria to determine the limiting pipe break locations because head loss testing showed that debris bed strength, and in turn debris head loss, was primarily a function of the amount of fiber fines in the debris bed.
In its evaluation of the effects of debris, the licensee used an alternate break methodology described in the GR/SE. The methodology allows breaks larger than the transition break size to be treated more realistically than smaller breaks. The smaller breaks are defined to be in Region I and the larger breaks are defined to be in Region II. The transition break size is defined as the largest pipe attachment to the main loop piping in the RCS which is the pressurizer surge line and is assumed to be a 14-inch, schedule 160 pipe with an inner diameter of 11.188 inches, resulting in a break area of 196.6 square inches for a double ended break.
Therefore, breaks larger than this size may be evaluated using more realistic assumptions for some inputs. For example, operator actions may be credited, or single failures may not be assumed for these larger breaks. The relaxations are considered appropriate because the large breaks are very unlikely. Credit for Region II breaks will be discussed in the appropriate sections of this summary.
The licensee stated that the particulate load had less effect on head loss than fiber fines and did not vary significantly with break location because latent particulate and degraded coatings do not vary with break location and account for 90 percent of the Region I particulate and 80 percent of the Region II particulate. The licensee computed a cumulative qualified coating debris amount for the main loop piping for each loop. For example, the amount of qualified coatings that could be generated by a specific break on the 12B Cold Leg, is the combined amount of qualified coating debris that could be generated from a break anywhere along 12B Cold Leg and then summed together. The licensee stated that this conservative approach negates the need to evaluate a multitude of break locations to determine the limiting break location for qualified particulate debris.
The licensee stated that the amount of chemical precipitate (Sodium Aluminum Silicate) generated depends on the amount of aluminum in containment and the fibrous debris load. The aluminum load is a constant for all break locations. The fiber debris load in the sump pool correlates strongly with the amount of fiber fines generated by each break so that breaks that generate maximum fiber fines also generate maximum fiber in the sump pool. The licensee gave special attention to break locations with more mineral wool insulation since it has a greater effect on chemical precipitation than Electric-Glass.
The licensee stated that the highest head loss the debris bed could sustain was directly proportional to the amount of fiber fines in the debris bed and therefore, identifying break locations generating maximum fiber fines is the key criteria in identifying limiting break locations for strainer head loss.
The licensee stated that while Calvert Cliffs, Units 1 and 2, are mirror image plants of the same design, there are differences in the as-built insulation configuration. The debris generation analysis confirmed that the Unit 1 insulation configuration bounded the Unit 2 configuration with respect to GL 2004-02 issues.
The licensee stated that the debris generation analysis considered the following pipe break criteria prescribed by NEI 04-07:
Breaks in the primary RCS with the largest amount of potential debris within the postulated ZOI.
o The main criterion used in the debris generation analysis was identifying the break location generating the largest amount of fiber fines debris.
Large breaks in the RCS with two or more different types of debris.
o The licensee identified the break generating the maximum amount of mineral wool insulation during the break selection process.
Break locations with the most direct path to the recirculation strainer.
o The conservative simplifying assumptions made for debris transport negated the need to evaluate breaks based on whether they had a more direct path for debris to reach the recirculation strainer.
Break locations with the largest potential particulate to fiber mass ratio.
o The majority of the particulate load is constant for all break locations.
Break locations that could generate an amount of fibrous debris that after transport to the sump could form a uniform thin bed that could subsequently filter sufficient particulate to create a high head loss thin bed effect.
o Head loss testing consistently confirmed that the Control Components, Inc. (CCI) designed strainer for Calvert is not susceptible to the thin bed effect.
The licensee stated that the updated final safety analysis report (UFSAR) Chapter 14 safety analyses for main steam and feedwater line break accidents do not lead to the recirculation mode of operation and therefore, are not considered in this evaluation.
The licensee evaluated a hot-leg break at the connection to the steam generator in both the 11 and 12 loop compartments for both Region I and Region II. The licensee chose this location because of its proximity to the steam generator, which is the largest single source of fibrous insulation and its close proximity to other high volume insulation targets including the hot leg, cold legs (pump suction side), reactor coolant pumps (RCPs), and the surge line (11 compartment only).
The licensee also evaluated for both Region I and II, a pipe break on the 11A Cold Leg (pump suction side) at a point where the steam generator was still significantly impacted, but also impacted more of the surge line, and small-bore sample cooler and drain piping located at lower levels. This break produced 25 percent less fine fiber debris for Region I and 29 percent less fine fiber debris for Region II than the break at the steam generator.
The licensee also evaluated for both Region I and II, a pipe break on the 12B Cold Leg (pump suction side). The Region I break was assumed to be at a point where the steam generator was still significantly impacted, but also impacted more of the regenerative heat exchanger, and chemical volume and control system piping. This break produced 28 percent less fiber fine debris than the break at the steam generator. The Region II break was assumed to be at the point where the 12B Cold Leg connects to the suction of the 12B RCP. The licensee determined a double-ended guillotine break at this location would generate the maximum amount of mineral wool debris. The licensee stated that although this location generated the most mineral wool, the total mass of fiber fines was nearly 33 percent less than the break at the steam generator.
The licensee performed an independent check on the above by reviewing the results from a BADGER debris generation analysis to validate that the criteria used was sufficient and reliable in determining the limiting break locations for strainer head loss. The BADGER software uses a three-dimensional computer aided design model of the Calvert Cliffs containment building reflecting the as-built insulation configuration. Over 17,000 break location/sizes were evaluated.
The BADGER results confirmed that a hot-leg break at the connection to 11 steam generator generates the debris load most challenging to strainer performance.
NRC STAFF CONCLUSION:
For this review area, the licensee has provided sufficient information such that the NRC staff has reasonable assurance that the subject review area has been addressed conservatively or prototypically. Therefore, the NRC staff concludes that the break selection evaluation for Calvert Cliffs is acceptable. Based on the information provided by the licensee, the NRC staff considers this area closed for GL 2004-02 for Calvert Cliffs.
4.0 DEBRIS GENERATION/ZONE OF INFLUENCE (EXCLUDING COATINGS)
The objective of the debris generation/ZOI evaluation is to determine the limiting amounts and combinations of debris that can occur from the postulated breaks in the RCS.
NRC STAFF REVIEW:
The NRC staff review is based on documentation provided by the licensee on November 12, 2020. In addition, the NRC staff referred to RAI responses provided by the licensee on February 9, 2024.
In the staff summary for Calvert Cliffs, the ZOI represents a volume based on the break size.
For double ended guillotine breaks (DEGB), the GR/SE provides ZOIs for various materials.
More robust materials have smaller ZOIs. The ZOI is defined by a volume whose radius is a multiple of the break size. So, a DEGB of 10 inches is assumed to have a spherical ZOI surrounding the break at multiples of 10 inches depending on the material. For example, low density fiberglass (LDFG) has a ZOI of 17D, so for the 10 inch break all LDFG within a sphere of 170-inch radius is assumed to be damaged. A more robust material, like some RMI has a ZOI of 2D, so its damage zone would be a sphere with a radius of 20 inches. A single-sided break is represented by a hemisphere with the same radius so the damage zone or ZOI is half the volume of a DEGB. The GR/SE assumed a constant RCS pressure when developing the ZOI sizes. Some texts express the ZOI in terms of L/D. This is simply the length of the radius of the sphere expressed in terms of the diameter of the break. The size of a ZOI expressed as 10D is the same as one expressed as 10 L/D.
The licensee used a 2.0D ZOI for Transco RMI. Based on similarity of design and materials of construction, the 2.0D ZOI was used for the Performance Contracting Inc. RMI product.
The ZOI for Marinite board was based on testing. The test ZOIs ranged from 13.3D to 3.4D.
Only three of the tests generated visible debris (5.5D, 4.5D, and 3.4D). These references established a final ZOI of 5.6D. Because Marinite is similar to Cal-Sil, the licensee used the un-banded Cal-Sil ZOI of 6.4D for conservatism for Marinite. The licensee noted that although the NRC staff identified concerns about the nozzle size used in the jet impingement testing for Marinite, Calvert Cliffs does not have any Marinite within 3.4D of any break location.
The licensee used a 17.0D ZOI for Nukon insulation and based on similarity of design, used this ZOI for Thermal Wrap insulation as well. The licensee determined the debris characteristics consistent with the methodology of Appendix II of NEI 04-07. The licensee used an approach where air jet impact test (AJIT) data was used to compute refined size distributions for LDFG insulation types (i.e., Nukon, Thermal Wrap, generic fiberglass).
Consistent with NEI 04-07, the licensee used a 5.45D ZOI for banded Cal-Sil insulation and assumed 100 percent fines. The licensee stated that there is no Cal-Sil insulation within this ZOI for any of the limiting Region I or II breaks. Consistent with the methodology of Appendix II of NEI 04-07, the licensee used an analysis approach where it used AJIT data to compute refined size distributions for Cal-Sil insulation. The NRC staff noted that Table 2a on page 18 of the licensees submittal states that a 6.4D ZOI is used for Cal-Sil and provides a three-category size distribution. In its January 11, 2024, RAI, the NRC staff asked the licensee to clarify which ZOI was used for Cal-Sil. The licensee clarified that it used the NRC approved ZOI of 5.45D. The NRC staff found this response acceptable because it is consistent with approved guidance.
As specified in NEI 04-07, the licensee used a 11.7D ZOI for Temp-Mat. Consistent with the methodology of Appendix II of NEI 04-07, the licensee used an approach where AJIT data was used to compute refined size distributions for Temp-Mat.
The licensee stated that a small amount of mineral wool remains in the plants. The mineral wool is encapsulated in self-contained units of all-stainless steel construction including inner and outer casings and internal reinforcements. The licensee stated that the mineral wool cassettes are robustly constructed and are virtually identical to that of the original Transco RMI installed at Calvert Cliffs.
The licensee stated that the Transco stain steel cassettes are similar to those tested in the AJIT that resulted in an approved ZOI for Transco RMI of 2.0D. The licensee considered the destruction pressure for the Transco mineral wool cassettes and RMI cassettes as equal because the material within the cassettes contributes no strength to the cassettes. However, for conservatism, the licensee doubled the ZOI for the mineral wool cassettes to 4.0D.
The licensee stated that the generic fiberglass insulation at Calvert Cliffs is another LDFG like Nukon and Thermal Wrap and therefore, used a ZOI of 17.0D for it. The licensee stated that the generic fiberglass has a bulk density generally greater, but no lower than Nukon and Thermal Wrap and therefore, based on insulation density, assumed the same debris size distribution for generic fiberglass as that used for Nukon and Thermal Wrap. The licensee stated that because there is substantially less generic fiberglass than Nukon and Thermal Wrap at Calvert Cliffs, the assumed size distribution is acceptable. The licensee stated that because the generic fiberglass at Calvert Cliffs is shaped using resin and a steel jacket instead of a cloth jacket similar to those that the Nukon and Thermal Wrap insulations have, there are no intact pieces of insulation for generic fiberglass, and all the insulation that would be characterized as intact pieces is assumed to be large pieces.
The licensee stated that the lead shielding blankets potentially exposed to high energy line break jets are both free-hanging blankets hung adjacent to piping or wrapped around piping or components. Testing of lead blankets similar to those used at Calvert Cliffs has been performed with both air jets and steam jets. The air jet testing conducted as part of the AJITs is included in NEDO-32686-A, Utility Resolution Guide for ECCS Suction Strainer Blockage, Volume 1, October 1998 (ML092530482). The steam jet testing was performed by Wyle and is documented in WCAP-16727-P, Revision 0, Evaluation of Jet Impingement and High Temperature Soak Tests of Lead Blankets for Use Inside Containment of Westinghouse Pressurized Water Reactors, dated February 2007.
The licensee stated that the Wyle testing is used to show that open back (free hanging) blankets will be torn from their grommets and not generate fine debris when impacted by a jet and that open blankets are less likely to generate debris than those installed in a strong back configuration. The AJITs subjected rubberized cloth coated lead shielding blankets to a range of surface pressures up to a maximum pressure of 40 pounds per square inch gauge (psig). The licensee stated that no debris was generated in any of the AJITs. The licensee reduced the damage pressures by 40 percent to account for potentially enhanced debris generation in a two-phase jet, consistent with NEI 04-07. Therefore, the licensee applied a ZOI of 5.4D to the lead shielding blankets.
The licensee stated that although lead wool debris may be generated, it will not transport to the strainer due to the high density of lead. In the AJITs, no lead blanket debris was generated for surface pressures corresponding to a ZOI of 5.4D. The licensee used other tests performed as part of the AJITs to determine the size distribution for the lead banket cover debris which could potentially be generated inside of 5.4D. Based on the testing, the licensee considered lead blanket cover debris generated beyond 2.1D as large or intact pieces. Within a ZOI of 2.1D, the licensee considered the lead blanket covers on the break side of the lead wool in the blankets closest to the break to become 20 percent fines and 80 percent small pieces.
The licensee did not conduct destructive testing to determine the ZOIs for materials. Destructive test results that were used had been previously reviewed by the NRC.
The licensee provided debris generation quantities for the limiting Region II breaks with respect to fine fiber mass and particulate volume in Table 4a of its submittal. The licensee used a transport fraction of 100 percent for all fiber fines and particulate debris and 0 percent for small and large pieces of insulation debris. The licensee used an erosion fraction of 10 percent for the small and large pieces of fiberglass debris and 17 percent for small pieces of Cal-Sil, and small and large pieces of Marinite. Because the fibrous debris from lead blanket covers and RMI did not contribute to strainer head loss, only the bounding amount of these materials was provided in Table 4a.
The licensee provided debris generation quantities for the limiting Region I breaks with respect to fine fiber mass and particulate volume in Table 4b of its submittal. The licensee used a transport fraction of 100 percent for all fiber fines and particulate debris and 0 percent for small and large pieces of insulation debris.
The licensee used a spherical ZOI having the same radius as the hemisphere for all Region 1 breaks, even where a hemisphere could have been used. The licensee stated that using a spherical ZOI simplified the process of determining the limiting break location as well as provided considerable conservatism to the results.
The licensee did not evaluate the results of RMI in Region I breaks, since it is bounded by the Region II breaks and downstream effects are evaluated using the higher Region II debris loads.
The licensee performed walkdowns in the spring 2009 refueling outage (RFO) to ensure the amounts of labels, signs, placards, tape and tags in containment were identified. The walkdowns determined the total surface area of this type of debris is less than 300 ft2. The strainer design allows for 375 ft2 of sacrificial surface area. Applying the average mean packing ratio of 0.75 from NEI 04-07 to the 375 ft2, the total allowable surface area is 500 ft2.
The licensee further stated that valve tag labels are made of materials that will sink intact, and procedures require that all placards be chained to prevent transport to the sump strainer.
For the Marinite, the NRC staff concluded that the use of a 6.4D ZOI is conservative based on the test results which generated little debris, and a comparison of Marinite with Cal-Sil. Marinite is a more robust material, so use of the Cal-Sil ZOI is acceptable for the Marinite.
The licensee made refinements to Nukon, LDFG, including generic fiberglass, and Temp-Mat size distributions using sub-ZOIs within the approved ZOI for each material. The NRC staff found the refinements on size distributions acceptable because they are based on test results and these refinements have been used by many licensees and reviewed by the NRC staff in the past. The NRC staff found that the mass and size distribution of generic fiberglass were calculated correctly because the licensee accounted for the density of the material in its evaluations.
The NRC staff concluded that doubling the ZOI used to calculate the quantity of mineral wool debris is conservative since the cassettes are welded, like the cassettes used during destruction testing, except where repairs and field modifications were made using rivets. Doubling the ZOI provides adequate margin to account for riveted cassettes being less robust than the welded cassettes.
The NRC staff concluded that lead wool debris may be generated, but it will not transport to the strainer due to its high density and the low velocities approaching the strainer. The ZOI and debris characteristics for lead blanket cover material were derived from testing and included conservatism. The NRC staff has observed the material, including material damaged during destruction testing, and concluded that it is unlikely to transport to the strainer at Calvert Cliffs due to the low velocities approaching the strainer. Therefore, the assumptions used for lead blanket debris generation are acceptable.
The NRC staff concluded that the debris generation from Region I breaks for the hot-and cold-leg piping was evaluated conservatively because the licensee used a spherical ZOI instead of a hemispherical ZOI with the same radius for partial breaks. The method effectively doubles the required volume within the ZOI. This does not double the debris amount, but likely adds significant debris to most partial break scenarios. This method simplified the identification of the limiting break while adding margin to these debris generation scenarios. The Region II breaks were evaluated per NRC guidance without the conservatism applied to some Region I breaks.
NRC STAFF CONCLUSION:
For the debris generation/ZOI review area, the licensee provided information such that the NRC staff has reasonable assurance that the subject review area has been addressed conservatively or prototypically. Therefore, the NRC staff concludes that the debris generation/ZOI evaluation for Calvert Cliffs is acceptable. The NRC staff considers this item closed for GL 2004-02.
5.0 DEBRIS CHARACTERISTICS The objective of the debris characteristics determination process is to establish a conservative debris characteristics profile for use in determining the transportability of debris and its contribution to strainer head loss.
NRC STAFF REVIEW:
The NRC staff review is based on documentation provided by the licensee on November 12, 2020.
The licensee provided a detailed discussion of the size distribution assumed for Nukon, Thermal Wrap, generic fiberglass, Cal-Sil insulations, Temp-Mat, and lead shielding blankets in the debris generation section above. The licensee provided the debris size distribution assumed for the remining types of debris in Table 5 of its submittal.
The licensee provided the bulk densities of material and destroyed debris in Table 6 of its submittal, which were obtained from the NRC-approved methodology or vendor specific information (lead shielding blankets and coatings).
The licensee did not use specific surface areas for fibrous and particulate debris to determine debris head loss as head loss was determined via testing and therefore, did not provide those values in its response.
The licensee used the NEI 04-07 debris characterization assumptions for the debris generation, transport, and head loss analyses for Calvert Cliffs.
The size of particulates is consistent with 10 microns for coatings particulate. Coatings that were installed as qualified, but the licensee subsequently classified as unqualified based on inspections are assumed to have a failure distribution consistent with Keeler & Long PPG Report No. 06-0413, Design Basis Accident Testing of Coating Samples from Unit 1 Containment TXU Comanche Peak SES, dated April 13, 2006.
For downstream effect evaluations, the licensee assumed the size distribution of unqualified coatings to be that in the Linear Mass Fraction column of Table I-1 of WCAP-16406-P-A. The licensee assumed that qualified coatings in the ZOI, degraded qualified inorganic zinc (IOZ) coatings, and never-qualified coatings fail as particulate.
NRC STAFF CONCLUSION:
For the debris characteristics review area, the licensee provided information such that the NRC staff has reasonable assurance that the subject review area has been addressed conservatively or prototypically. Therefore, the NRC staff concludes that the debris characteristics evaluation for Calvert Cliffs is acceptable. The NRC staff considers this item closed for GL 2004-02.
6.0 LATENT DEBRIS The objective of the latent debris evaluation process is to provide a reasonable approximation of the amount and types of latent debris (e.g., miscellaneous fiber, dust, dirt) existing within the containment and its potential impact on sump screen head loss.
NRC STAFF REVIEW:
The NRC staff review is based on documentation provided by the licensee on November 12, 2020.
The licensee completed walkdowns to collect latent debris samples from the various surfaces of containment. The surface types included: containment liner, floor, stair grating, walls, horizontal and vertical cable trays, horizontal and vertical piping, horizontal and vertical ducting, and horizontal and vertical equipment. The licensee collected a minimum of four samples from each surface type. The licensee recorded the area of each sample, along with the weight of latent debris in the sample area. The licensee recorded the average and maximum weight per unit surface area and then multiplied the average by the horizontal and vertical surface areas that might have latent debris accumulate on them, which resulted in the latent debris loading for containment. The licensee stated that the latent debris load results used the maximum sample for each surface type with the exception of the non-stair grating where the licensee assumed that the latent debris load was the same as the floor.
The licensee assumed debris to be normally distributed for a given sample type, which was supported by the walkdown results. The licensee stated that averaging the latent debris for surface types having multiple samples is consistent with the sampling approach taken to estimate the amount of latent debris inside containment.
The licensee calculated a latent debris load of 150 pounds (lbs.) and described it as dust with no fiber in any sample. The licensee assumed that 15 percent of the latent debris is fibrous, and 85 percent is particulate.
The licensee accounted for latent debris (in the form of dust) in testing and analysis by including it in the test debris.
NRC STAFF CONCLUSION:
For the latent debris review area, the licensee provided information such that the NRC staff has reasonable assurance that the subject review area has been addressed conservatively or prototypically. Therefore, the NRC staff concludes that the latent debris evaluation for Calvert Cliffs is acceptable. The NRC staff considers this item closed for GL 2004-02.
7.0 DEBRIS TRANSPORT The objective of the debris transport evaluation process is to estimate the fraction of debris that would be transported from debris sources within containment to the sump suction strainers.
NRC STAFF REVIEW:
The NRC staff review is based on documentation provided by the licensee on November 12, 2020.
The licensee used a simplified transport evaluation that introduced conservatism into the analysis. The evaluation assumed that 100 percent of the debris that did not remain on the target transported to the containment pool. The licensee did not credit debris hold-up on structures, gratings, or in quiescent pools.
For the blowdown phase, the licensee assumed that 100 percent of the debris was blown into the recirculation pool. The licensee stated that this is conservative as it ensures that the entire debris load is available for production of chemical precipitates and transport during recirculation.
The licensee did not evaluate the washdown phase of transports since it assumed that all debris was blown directly into the recirculation pool. For the pool-fill phase of transport, the licensee determined that the potential for debris to transport to inactive cavities was not significant and therefore, did not credit debris transport to inactive cavities.
The licensee assumed, with the exception of lead wool, that all fines, including fines eroded from small and large pieces of fiberglass insulation, particulate, and precipitate debris transport to the sump strainer during the recirculation phase of transport. The licensee assumed small and large pieces of debris would not transport to the strainers as these sizes of debris were demonstrated to reduce strainer head loss in plant-specific strainer head loss testing.
The licensee stated that the erosion of LDFG insulation was the only area where the debris transport analysis deviated from the approved guidance which specifies an erosion fraction of 90 percent for fiberglass debris. Based on 30-day erosion testing, the licensee used a 10 percent erosion fraction for fiberglass debris except for that which is contained in an intact blanket/jacket.
The licensee included debris from permanent lead shielding blankets in the debris source term and assumed that debris would not erode in the recirculation pool because the jacket material is specifically designed for high temperature with improved resistance to abrasion, flexing, tear, and puncture. The licensee stated this assumption is consistent with approved guidance where jacketed pieces of fiberglass insulation are not susceptible to erosion in the recirculation pool.
The licensee did not use computational fluid dynamics codes and no credit was taken for debris interceptors.
The licensee stated that based on observations during head loss testing that fines from the cover on lead shielding blankets did not transport to the strainer even when using artificial agitation, it concluded that the cover material of the blankets would not transport to the strainer during sump recirculation. The licensee also stated that head loss testing showed that even when these fines were dropped right onto the debris bed, they were too course to assimilate into the fine fiber debris bed. The licensee did not credit the settling of any other fine debris in the transport calculations.
The licensee provided a summary of its debris transport assumptions in Table 7 of its submittal.
The NRC staff concluded that the use of an erosion fraction of 10 percent is acceptable based on proprietary testing conducted by Alion Science and Technology (ML101090490 (non-public))
and accepted by the NRC in letter dated June 30, 2010 (ML101540221).
The NRC staff determined that it was acceptable to assume that lead wool fines will not transport to the strainer due to the high density of the material and the low strainer approach velocity.
The NRC staff concluded that it is acceptable to assume that fines from lead wool blanket cover will not transport to the strainer based on plant-specific test observations.
The NRC concluded that the simplified transport analysis results in conservative estimates of transport to the strainer.
NRC STAFF CONCLUSION:
For this review area, the licensee has provided information such that the NRC staff has reasonable assurance that the debris transport has been addressed conservatively or prototypically. Therefore, the NRC staff concludes that the debris transport evaluation for Calvert Cliffs is acceptable. Therefore, the NRC staff considers this area closed for GL 2004-02.
8.0 HEAD LOSS AND VORTEXING The objectives of the head loss and vortexing evaluations are to calculate head loss across the sump strainer and to evaluate the susceptibility of the strainer to vortex formation.
NRC STAFF REVIEW:
The NRC staff review is based on documentation provided by the licensee on November 12, 2020. In addition, the NRC staff used information from RAI responses dated February 9, 2024.
The licensee provided in Enclosure 1-2.1 of its submittal a diagram of the ECCS and containment spray (CS) system-.
The licensee stated that the calculations for minimum containment flood level have demonstrated that, for small break LOCAs (SBLOCA) and large break LOCAs (LBLOCA), the emergency sump strainer will be completely submerged at the time of switchover to containment sump recirculation.
The licensee completed vortex testing and analysis. CCI performed generic vortex testing for Calvert Cliffs strainer design in March 2005. The licensee used the test results in the analysis which shows that vortexing is not possible at the design flows for the Calvert Cliffs strainers. For the clean strainer vortex analysis, the licensee computed a plant-specific Froude number of 0.0025, which is well below the Froude number of approximately 0.2 calculated at test conditions. Key conservatisms in the clean strainer vortex analysis included an assumed submergence level of 1.17 inches while the minimum strainer submergence is 16.2 inches and the vortexing limit was derived from test data based on results with substantially less than 1 percent air intake while 2 percent air intake is generally considered acceptable.
The licensee stated that based on CCI test results including Calvert Cliffs specific tests, the critical Froude number for borehole induced vortexing at Calvert Cliffs is 384.1. The licensee calculated the plant-specific Froude number as 4.7 for Region I breaks and 7.3 for Region II breaks. The licensee stated that the actual plant Froude number is significantly less than the critical Froude number, that was determined by testing, and stated that a key conservatism in this analysis is that it assumes a strainer submergence of 6.1 inches in determining the 384.1 critical Froude number. Using the actual minimum strainer submergence of 1.35 feet, the critical Froude number would increase to over 1000. Therefore, the strainer will not ingest air via vortices caused by boreholes.
The licensee also stated that during head loss testing flow sweeps it did not detect vortex formation, even at flows of 250 percent of the design flow rate.
The licensee used CCI, the strainer vendor, to perform the plant-specific strainer head loss testing. Although CCI performed four test programs, the licensee only credited the results from the last two in its response because the test program was revised based on experience gained in the first two test programs and NRC guidance on head loss and vortexing, dated March 2008.
The licensee conducted the third set of tests in late 2008, which used larger quantities of debris and were used to evaluate some of the limiting breaks from Region II. The licensee conducted the fourth set of tests in the summer of 2010. These tests were used to evaluate the limiting breaks from Region I and some of the Region II breaks. The licensee stated that in each test, one or more full-size strainer cartridges were placed in CCIs multi-function test loop in Winterthur, Switzerland, and loaded with the amount of debris computed to transport to this portion of the overall strainer. The licensee provided detail regarding the testing in its submittal, as well as figures and diagrams.
The licensee stated that the late 2008 testing used stone flour (mean particle size of 7.7 micrometer (µm)) as a coatings surrogate and actual epoxy particulate coatings debris. The summer 2010 testing used Silicon Carbide (mean particle size of 9.7 µm) as a surrogate for particulate debris. Degraded epoxy coatings debris were represented by epoxy coatings chips produced from the epoxy coating system used in containment. Marinite was represented by actual Marinite debris. The licensee stated that in both cases, the average surrogate size was less than the particle sizing requirement of 10 µm, and therefore acceptable.
The summer 2010 testing scaled the volumetric flow rate so that the average velocity through the test strainer cartridges corresponded to the expected average flow velocity through the strainer in the plant at the strainer design flow of 5,000 gpm. The test flow was reduced to 2,400 gpm prior to introduction of chemical precipitates to reflect EOP changes to have one CS pump secured prior to the onset of chemical precipitates. The recorded head loss at the test equivalent of 2,400 gpm is scaled to 2,365 gpm to reflect the maximum actual strainer flow.
The late 2008 testing used flow rates that resulted in average velocities higher than the average velocity through the plant strainer. The bulk of non-chemical debris was introduced at a flow rate corresponding to 2.7 times the strainer design flow of 5,000 gpm. Chemical debris was introduced at the expected average flow speed through the strainer corresponding to the strainer design flow of 5,000 gpm. The recorded head loss is scaled from 5,000 gpm to 2,365 gpm.
The licensee stated that the debris preparation and addition methods used by CCI for both sets of tests being credited for Calvert Cliffs are identical to each other, and to those witnessed by the NRC in spring 2008 at CCIs laboratory in Switzerland (Trip Report at ML081400706 (non-public)). The licensee provided detailed discussion of how the following debris were prepared:
fiber fines, fiber small pieces, Marinite, and degraded qualified epoxy coatings outside ZOI.
The licensee stated that particulate and fibrous materials were prepared and maintained separately. Particulate and fiber were not mixed together during preparation or addition. The licensee provided additional details regarding the debris additions, mechanical agitation, non-chemical debris additions, chemical precipitate formation, and chemical addition.
The licensee stated that seven head loss tests were performed in the summer of 2010, which included head loss for fibrous and particulate debris (conventional debris) as well as chemical precipitates. The testing was a test for success program based on multiple insulation replacement schemes. The licensee presented the results from these tests in Table 10a of its submittal. Adding the clean strainer head loss to the conventional and chemical debris head loss at 2,365 gpm yields a maximum strainer head loss for each test except for Tests 1 and 2 where head loss at 5,000 gpm with no chemical effects was limiting compared to that at 2,365 gpm with chemical effects. The licensee also provided graphs of the test results to illustrate how the head loss changed with varying debris loads and flow changes.
The licensee performed head loss tests in late 2008, which included head loss for fibrous and particulate debris (conventional debris) as well as head loss with chemical precipitates. Varying debris loads were considered, but in all cases the debris loads were significantly higher than those used in the summer 2010 testing. Because of test anomalies, flowrate control issues, chemical precipitate application, and use of excessive fiber quantities, some of the 2008 tests were considered invalid on non-prototypical. Only test 3 from 2008 is used to augment the summer 2010 testing for evaluating Region II breaks. The licensee presented the results from three of the 2008 tests (tests 3, 6, and 7) in Table 10b of its submittal and provided graphs of the testing.
The licensee stated that the pockets in CCIs strainer cassettes are designed to fill with debris with additional debris depositing outside of the strainer. The layout of the rows allows sufficient space for the debris to accumulate and completely envelop the strainer. The low flow velocity into the strainer prevents the pockets from becoming tightly packed with debris. Therefore, the licensee concluded that complete envelopment of strainer, as demonstrated in head loss testing, is acceptable.
The licensee stated that the strainer installed at Calvert Cliffs is CCIs pocket cassette type strainer. The geometry of the pocket filtration surface is such that it was not possible to have a uniform fiber bed on the filtration surface. The licensee demonstrated this during the 2008 and 2010 testing through several attempts to generate a thin bed. The thin bed testing included beds as thin as 1/16 inch up to a nominal full load of debris and included a wide range of simultaneous particulate loading to ensure that the maximum head loss condition for the CCI strainers and the Calvert Cliffs debris loads was identified. The licensee stated that maximum head loss occurs with maximum debris and no significant increase in head loss was seen in any thin-bed test. Therefore, the licensee concluded that Calvert Cliffs strainer exhibits no thin bed effect.
The licensee considered the following debris-related strainer failure modes: loss of pump NPSH margin due to excessive strainer head loss, structural failure of strainer due to excessive strainer head loss, release of dissolved gases (i.e., deaeration) inside strainer due to excessive strainer head loss, and vortexing.
The licensee found that adequate NPSH was the limiting strainer failure mode at elevated sump pool temperatures and deaeration was the limiting strainer failure mode at sump pool temperatures of 140 degrees Fahrenheit (°F) and below. The licensee provided the maximum allowable head loss for three different strainer failure modes and two LOCA sizes in Table 11a of its submittal.
The licensee provided a detailed discussion to demonstrate that the Calvert Cliffs containment sump strainer head losses will be below the maximum allowable values given in Table 11a of its submittal. The licensee compared the debris loads for each of the limiting breaks to the debris loads used in the head loss tests to identify which head loss tests can be considered to bound that break. The licensee then compared the head loss from these bounding tests to the maximum allowable head loss values of Table 11a. The tests that have a lower head loss than the Table 11a limits can be used to qualify that break case. The licensee did this evaluation for each of the limiting Region I and Region II breaks as well as for breaks smaller than 0.08 ft2, and also for sump pool temperatures greater than 140 °F, and sump pool temperatures less than or equal to 140 °F.
For all breaks, when the sump pool temperature is greater than 140 °F, the licensee stated that the strainer head loss is very small. The highest head loss for any test being credited for this condition is 0.444 feet, which is well below the limiting allowable head loss of 2.07 feet given in Table 11a of its submittal. The licensee stated that there are no head loss concerns at sump pool temperatures where chemical precipitates do not exist.
The licensee stated that the debris loads for the Region I breaks less than 0.08 ft2 are bounded by the thin-bed test debris loads. The licensee stated that the maximum head loss from the summer 2010 Test No. 2 thin-bed test is 0.322 feet, which is well below the limiting allowable head loss limit of 2.812 feet given in Table 11a for break sizes 0.08 ft2, and sump pool temperatures 140 °F. Therefore, the licensee stated that these breaks will not challenge strainer performance.
The licensee stated that comparing the summer 2010 test debris loads to the limiting Region I debris loads shows that each debris component of each of the four limiting Region I breaks is bounded by each of the five summer 2010 head loss tests. Comparing the head losses from the summer 2010 tests to the maximum allowable strainer head loss shows that all the summer 2010 head loss tests have a lower head loss than the maximum allowable head loss. Therefore, the licensee stated that any of the summer 2010 tests can be used to qualify a Region I break greater than 0.08 ft2 at sump pool temperatures less than or equal to 140 °F. The licensee used the summer 2010 and late-2008 head loss test results to qualify the Region II breaks. The four limiting Region II breaks were evaluated. The 11A cold-leg break was shown to be bounded by Test No. 3 of the 2010 test program. The licensee stated that the fiber fines, particulate, and chemical precipitate loads used in Test No. 3 of the summer 2010 tests each bound the predicted values for the 11A Cold Leg case. The head loss from this test is 3.219 feet, which is below the limiting allowable head loss of 3.227 feet. Therefore, the licensee stated that the head loss from this break location is acceptable.
The licensee stated that the fiber fines and particulate used in Test Nos. 5 and 7 of the summer 2010 tests bound the predicted values for the 12B Cold Leg case. The licensee stated that although the precipitate mass used in these two tests does not bound the precipitate mass predicted for the 12B Cold Leg case, these two test can still be considered bounding because additional precipitate would not have resulted in increased strainer head loss and the discrepancy in chemical precipitate is offset by the overage in particulate debris used in Test Nos. 5 and 7. The licensee stated that the head loss from these tests were 1.271 feet and 0.717 feet, which is below the limiting allowable head loss of 3.227 feet for breaks greater than 0.08 ft2 and sump pool temperature less than or equal to 140 °F. Therefore, the licensee stated that the head loss from this break location is acceptable.
The licensee stated that the fiber fines and particulate used in Test No. 3 of the summer 2010 tests bound the predicted values for the 12 Hot Leg break case. The licensee stated that although the precipitate mass used in this test does not bound the precipitate mass predicted for the 12 Hot Leg break case, this test can still be considered bounding because the additional precipitate would not have resulted in increased strainer head loss and the discrepancy in chemical precipitate debris is offset by the overage in particulate debris used in Test No. 3. The licensee stated that the head loss from this test was 3.219 feet, which is below the limiting allowable head loss of 3.227 feet for breaks greater than 0.08 ft2 and sump pool temperature less than or equal to 140 °F. Therefore, the licensee stated that the head loss from this break location is acceptable.
The licensee stated that the fiber fines, and chemical precipitate used in Test No. 3 of the late 2008 tests bound the predicted values for the 11 Hot Leg break case. The licensee stated that although the particulate volume used in this test does not bound the particulate volume predicted for the 11 Hot Leg break case, this test can still be considered bounding. The licensee stated that the discrepancy in particulate is offset by the excess in chemical precipitate and that further particulate/precipitate would not have increased strainer head loss. The licensee also stated that as shown from the tests, the strainer head loss increases as the tested fiber load increased, indicating that the fiber debris quantity is the foremost factor affecting strainer head loss. Therefore, the licensee concluded that the debris loads used in Test No. 3 of the late 2008 testing bound the debris loads predicted for the 11 Hot Leg break case. The licensee stated that the head loss from this test was 6.549 feet at a strainer flow of 2,365 gpm. The licensee stated that while this is above the limiting allowable head loss of 3.227 feet, in the case of a hot-leg break, containment response analyses show that by the time the sump pool temperature drops to 140 °F and below, the containment vapor temperature will have dropped well below the point where all CS flow can be secured, and containment cooling can continue with the containment air coolers alone. Without CS flow, the total strainer flow will drop to approximately 600 gpm.
The licensee stated that if the measured head loss from Test No. 3 of the late 2008 testing is scaled down to 1,150 gpm, it will be below all applicable head loss limits. The licensee stated additional defense-in-depth measures are available to mitigate this condition.
The NRC staff generally found the evaluation of the Region II breaks acceptable. A more detailed evaluation, including licensee responses to additional questions, is below in this section.
The licensee listed conservatisms used during head loss testing. Only fiber fines were used to conservatively bound head loss as it was observed that small pieces of fiber reduced debris bed head loss. Also, it was assumed that fiber fines produced by erosion arrive at the strainer at time t = 0, instead of hours or days later when flow margin is greater. The licensee also assumed that all fines transport to the sump strainer, even though testing demonstrated that the majority of the fines settled before reaching the sump screen. Head loss test procedures were designed to ensure uniform debris distribution on the strainer to maximize debris bed head loss, even though the design and physical layout of the Calvert Cliffs strainer promotes non-uniform debris distribution which is conducive to lower head loss. Also, debris materials that reduce strainer head loss such as lead shielding cover blanket and RMI were not included in testing.
The licensee also used chemical precipitates prepared and introduced in accordance with WCAP-16530 which includes conservatism.
The licensee determined that the clean strainer head loss was 0.064 feet at a flow rate of 2,365 gpm by scaling head loss with the square of flow based on the Affinity Laws. The head loss was determined by the licensee via testing, and calculations using industry standard equations and assumptions.
The licensee stated that the emergency sump strainer is fully submerged under all accident scenarios that include recirculation and that there is no vent above the water level. The licensee stated that the head loss testing did not credit near-field debris settling and test results were not scaled for temperature dependent dynamic viscosity. The licensee did not credit containment accident pressure in neither evaluating whether flashing occurs across the strainer surface nor evaluating whether deaeration occurs in the strainer. The licensee used a zero deaeration limit instead of the 2 percent in Regulatory Guide 1.82, Water Sources for Long-Term Recirculation Cooling Following a Loss-of-Coolant Accident Rev. 4, March 2012 (ML111330278), which is more conservative.
The NRC staff evaluated the licensees evaluation of the potential for air ingestion due to vortex formation. The staff noted that the licensee implied that it is acceptable to allow air ingestion of 2 percent and that the analysis indicates that significantly less than 1percent air ingestion will occur. The staff position is that any air ingestion at the pumps must be evaluated. However, the test conditions for vortexing were conservative compared to plant conditions and the plant-specific results indicate significant margin to the acceptance criteria. The NRC staff conclusion is based on test observations described in the licensees submittal and information provided by the licensee in response to an earlier NRC request for information (RAI) (RAI 17, ML102080089, Calvert Cliffs letter dated July 23, 2010). This information adequately demonstrates that vortexing will not occur for the Calvert Cliffs strainer.
The NRC staff concluded that velocity scaling corrections made by the licensee to the head loss results are acceptable because scaling was only to lower flows and was based on flow sweeps conducted during plant-specific testing.
The NRC staff concluded that the test results are applicable to the conditions to which they are applied by the licensees evaluation. The NRC staff also found that the maximum allowable head losses calculated for structural and NPSH limits as listed in Table 11a were determined correctly except that the staff identified that the NPSH section of the submittal listed a more limiting NPSH margin for the CS pumps at high temperatures (1.63 vs. 2.07 ft.). The NRC staff requested that the licensee clarify this potential discrepancy in the NPSH section. The licensee stated that the maximum allowable head loss of 2.07 ft. is not the same as the NPSH margin value of 1.63 ft. The maximum allowable head loss value includes both the NPSH margin and the tested head loss for the condition in question. Refer to the NPSH section of this summary for a more detailed description.
The NRC staff reviewed the test descriptions and verified that testing was performed in accordance with staff guidance. For all Region I breaks and Region II breaks with sump temperatures greater than 140 °F, the staff concluded that all the tests provide assurance that the ECCS and CSS functions will not be inhibited by the effects of debris on the strainer.
Therefore, Region I breaks were all evaluated acceptably. The Region II breaks are discussed below.
The NRC staff evaluated the licensees evaluation of crediting excess particulate to account for a lack of chemical precipitates in some of the Region II cases (12B Cold Leg and 12 Hot Leg).
Although the staff agrees that particulate debris and chemical debris cause increases in head loss, the chemical debris generally causes significantly higher head loss. For the plant-specific testing, the vast majority of the head loss was due to chemical precipitates. The effects of each type of debris are dependent on other factors in the test that affect the debris bed. Because only the Region II tests were deficient in chemical debris the NRC staff noted that the licensee could use more realistic test conditions to evaluate the head loss associated with the breaks. The NRC staff asked the licensee to justify the credit in RAI 3 dated January 11, 2024. The licensee responded on February 9, 2024, that the discussion of excess particulate in the test was to demonstrate defense-in-depth. The licensee stated that the head losses from the test program are representative of head losses that could occur in the plant and these test results show head loss is related to fibrous debris amounts. The head losses during testing were limited by the amount of fiber in the debris bed regardless of additional particulate or chemical precipitates.
The licensee stated that once the structural stability of the bed is disrupted by head loss, the head loss does not increase with added chemical or particulate debris. The NRC staff reviewed the licensees response and the test information provided and concluded that the licensees justification was acceptable based on the behavior of the debris bed during plant-specific testing.
Similar to the credit of excess particulate to account for a discrepancy in chemical debris in the application of head loss testing to the 11 hot-leg break case, the licensee stated that excess chemical debris may be credited to make up for a lack of particulate debris. In general, there is not a correlation to determine the amounts of particulate and chemical debris that would result in equivalent head loss because the effects of each are strongly dependent on the morphology of the debris bed and other test conditions. However, the NRC staff reviewed the licensees test program and the results of several tests and determined that in this case there is adequate evidence that the chemical debris overcompensated for the deficient amount of particulate in the test. Specifically, the NRC staff found that the chemical debris had a much larger effect than particulate on the final head losses in the test program. Also, the plant-specific testing resulted in bed break-through that limited the maximum head losses in the tests. Maximum head losses were associated with the amount of fiber in the tests. The NRC staff also noted that for cases above 140 °F, the head losses were much lower than the margin required to ensure system function. Because the evaluation that credited this substitution is for a Region II break, the NRC staff concluded that it is acceptable for this case.
The 11 hot-leg break case also credited reduced flow through the strainer to decrease the head loss. The licensee stated that by the time the sump pool temperature is reduced to 140 °F, and chemical precipitates can form resulting in higher head losses, the containment conditions will allow CS to be secured. The licensee stated that it would take more than 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> for the sump pool to cool to 140 °F so that the operators would have adequate time to secure the CS pumps and reduce flow through the strainer. The licensee scaled the head loss to a flow rate of 1,150 gpm and stated that the head loss was acceptable. The NRC staff requested the licensee to provide the head loss assumed for the lower flow condition in RAI 4 dated January 11, 2024.
The licensee provided the head loss in its RAI response letter dated February 9, 2024. The licensee stated that it no longer credits a flow rate of 1,150 gpm for this scenario. The updated flow rate is assumed to be 675 gpm which bounds the actual flow rate of 603.2 gpm. The licensee stated that the head loss at 675 gpm is calculated to be 1.856 ft. The NRC staff reviewed the licensees response and concluded that the head loss value provided was calculated acceptably because it follows guidance and used conservative assumptions. The NRC staff found that the licensees evaluation of the 11 hot-leg break case was acceptable including the time available for operators to secure the CS pumps and initiate additional defense-in-depth measures that have been implemented at the plant.
The NRC staff evaluated the licensees evaluation of deaeration of fluid as it passes through the strainer and debris bed. The staff questioned the maximum allowable head losses for deaeration in Table 11a because the licensee stated that the values were based on the midpoint of the strainer. The strainer submergence values for the top and midpoints of the strainer are also presented in Table 8. The staff concluded that for a determination of no deaeration the calculation should be based on submergence to the top of the strainer, not the midpoint. The NRC staff has allowed licensees to use the midpoint of the strainer (instead of integrating a value over the strainer height) as an estimate for total strainer deaeration when the deaeration occurs over the full height of the strainer. However, assuming that no deaeration will occur, calculations assuming the midpoint of the strainer could be non-conservative. The NRC staff recognized that there are some phenomena that may be treated more realistically, or timing aspects, that may be credited in the submergence and deaeration calculations for Region II breaks. The NRC staff asked RAI 2 dated January 11, 2024. The licensee responded in letter dated February 9, 2024. The letter provided updated allowable head losses considering the top of the strainer as the reference point for deaeration instead of the strainer midpoint. The licensee demonstrated that there is margin to deaeration for all cases listed in Table 11a by reducing the allowable head losses to account for using the top of the strainer as a reference point. The licensee determined that the limiting case is for a Region II break on the 11 Hot Leg after the sump cools to less than 140 °F. For this case the licensee assumed a lower flow rate than in the previous submittal to ensure that margin to flashing is retained. The licensee stated that the flow rate used is conservative with respect to the flow rate that would actually occur in the plant. The licensee also stated that the head loss for this case was calculated conservatively. The licensee also noted that the refueling water tank (RWT) could be refilled and additional water injected into the containment to suppress flashing by providing additional strainer submergence. The NRC staff reviewed the licensees response to the RAI and concluded that the updated information is consistent with NRC guidance. The NRC staff also noted that for the Region II breaks, which are the most challenging, the licensee could use more realistic assumptions and can credit operator actions (like refilling the RWT and injecting it into containment). Therefore, the NRC staff concluded that deaeration will not occur for the Calvert Cliffs plant-specific conditions.
Based on the information provided in the submittal, the NRC staff concluded that all R-I breaks were bounded by the test results from the 2010 testing. The results of the testing bounds all Region I breaks and all Region II breaks prior to the formation of chemical precipitates. Only the Region II breaks, at lower temperatures, were not specifically bounded by the testing. As discussed above, the licensee demonstrated that using more realistic analyses and performing a more holistic evaluation for Region II breaks provides reasonable assurance that the ECCS and CSS functions will not be adversely affected by debris collecting on the strainer.
NRC STAFF CONCLUSION:
For the head loss and vortexing area, the licensee has provided information such that the NRC staff has reasonable assurance that the strainer head loss and potential for air ingestion has been addressed conservatively or prototypically. Therefore, the NRC staff concludes that the head loss and vortexing evaluation for Calvert Cliffs is acceptable. The NRC staff considers this area closed for GL 2004-02.
9.0 NET POSITIVE SUCTION HEAD The objective of the NPSH section is to calculate the NPSH margin for the ECCS and CSS pumps that would exist during a LOCA considering a spectrum of break sizes.
NRC STAFF REVIEW:
The NRC staff review is based on documentation provided by the licensee on November 12, 2020. The staff also based its findings on RAI responses dated February 9, 2024.
The licensee stated that a single emergency containment sump supplies inventory to both trains of the safety injection and CSS. During recirculation, each train consists of a HPSI pump and CS pump. Maximum HPSI pump flow rate is 1,045 gpm (two pumps operating) and maximum CS pump flow rate is 3,450 gpm (two pumps operating). The maximum total strainer flow rate is 4,495 gpm. The design flow is 5,000 gpm.
The licensee stated that a low-pressure safety injection (LPSI) pump failure to stop at recirculation actuation signal (RAS) may result in a LPSI pump operating post-RAS. The licensee stated it is implementing procedure changes to assure that post-RAS LPSI flow is throttled to 600 gpm indicated to ensure decay heat removal requirements are met. The licensee stated that maximum HPSI flow is 645 gpm and therefore, for the failure of a LPSI pump to stop scenario, the maximum total strainer flowrate is (645 gpm + 3,450 gpm + 800 gpm
= 4,895 gpm), which is less than the strainer design flow of 5,000 gpm. The NRC staff requested the licensee to confirm that the procedure changes to ensure the flow rates assumed in the analysis had been completed in RAI 6, dated January 11, 2024. The licensee responded to the RAI on February 9, 2024, and stated that the procedure changes to the EOPs are complete. The procedure changes ensure that the strainer flow rate will be limited to 2,365 gpm prior to the potential for chemical precipitates to form and 603.2 gpm when the sump temperature reaches 120 °F. The NRC staff found the response acceptable because it directs flow rates to be reduced consistent with the assumptions in the analysis.
Prior to reaching sump pool temperatures of 140 °F plant procedures align the pumps so that only one CS pump will be operating (1,759 gpm maximum), one HPSI pump will provide core flush flow (173 gpm maximum), and one HPSI pump will provide cold-leg injection (430 gpm maximum). Therefore, the maximum total strainer flowrate at 140 °F and below is 2,362 gpm.
The licensee rounded the flow to 2,365 gpm and stated that it contains inherent conservatism.
The cold-leg injection flow required at this time is only 130 gpm and the CS flow rate of 1,759 gpm assumes a pump operating 10 percent above its pump curve. The licensee stated that a more realistic CS flow would be 1,600 gpm.
The licensee stated that the maximum post-RAS sump temperature is less than 212 °F and provided a list of post-RAS sump temperature values for various scenarios from the containment response calculation. If the post-RAS sump temperature exceeds 212 °F, the containment pressure is assumed to be saturation pressure. Therefore, the approach does not credit containment accident pressure in the NPSH available computation. The licensee provided the minimum sump pool water levels for the various break sizes and converted them to strainer submergences (discussed in the head loss and vortexing section above).
The licensee assumed the following in its analysis:
HPSI flow rate is throttled as directed by procedure to ensure decay heat removal and NPSH requirements are met.
CS flow rate is that predicted by a hydraulic flow model where the CS flow rate is 10 percent above the vendor pump curve, which bounds the tested performance of the pumps.
To increase NPSH requirements, the diesel generator is assumed to be operating 2 percent over-frequency, which bounds the performance of the emergency diesel generator.
The licensee stated that the NPSH required values are provided on the vendor pump curve as a function of flow rate. Original test data for the CS pumps were used to determine NPSH required. The licensee considered the NPSH required to be equal to the NPSH available value when a decrease in total developed head was observed. Plant-specific data for Calvert Cliffs HPSI pumps was not available. However, the HPSI pump vendor (Sulzer) had data for an identical pump that indicated that the NPSH required values on the pump curve are based on a 3 percent degradation in the pump total developed head. The 3 percent degradation point is a pump industry standard for reporting NPSH required.
The licensee stated that losses in the ECCS recirculation pump suction piping are computed using a hydraulic model of the ECCS piping. The NPSH available for the HPSI and CS pumps was obtained by subtracting the piping losses from the static head differential between the containment water height and the pump suction elevation and the sub-cooled margin available at the limiting fluid temperature.
The licensee stated that the ECCS consists of three HPSI pumps, two LPSI pumps, and four safety injection tanks (SITs). After a safety injection actuation signal (SIAS) is received a start signal is given to two HPSI pumps and both LPSI pumps. Each HPSI pump has an independent injection header (main and auxiliary) which branches into four separate lines. The four branch lines from each header are connected together prior to the cold-leg injection points. The LPSI pumps inject into a low-pressure header that also connects to the HPSI injection header at each of the cold legs. Each SIT injects into a single cold leg. The SITs automatically discharge when the RCS pressure decreases below the SIT pressure.
For a LBLOCA, two HPSI and both LPSI pumps are automatically started. The third HPSI pump can be manually started if one of the other two HPSI pumps does not start. HPSI pump flow to the core will not begin until the pressurizer pressure is approximately 1,280 psi absolute (psia),
and LPSI pump flow to the core will not begin until the pressurizer pressure is approximately 185 psia.
The CS pumps automatically start on a SIAS signal. CS flow to the containment occurs after the empty CS headers are filled and the spray control valve opens at a containment pressure of 4.75 psig. Containment air coolers also start at a containment pressure of 4.75 psig. The SITs automatically discharge to the RCS when the RCS pressure drops to about 200 - 250 psig.
Initially, the SI and CS pumps take suction from the RWT. When the RWT level reaches the low-low level signal setpoint level, a RAS is generated, and HPSI and CS pump suction switches from the RWT to the containment emergency sump. The LPSI pumps automatically stop when the RAS is generated. The HPSI flow is throttled post-RAS to a constant value.
Throttling of HPSI flow may be implemented to match the decreasing rate of decay heat. CS flow continues until the containment pressure decreases to 4.0 psig or less at which point one CS pump is turned off.
For a SBLOCA, the licensee stated that the same automatic actuations exist. However, if the pressurizer pressure remains high for an extended period, the operators may secure the LPSI pumps to avoid running on mini-flow recirculation for an extended period of time. Also, for SBLOCAs the SITs may be isolated prior to these tanks injecting into the RCS.
The licensee provided a table describing the operational status of the ECCS and CS pumps before and after the start of recirculation.
The licensee used three design basis cases to establish the design limits for pump operation and sump performance. Both a hot-leg and cold-leg break assuming the failure of a diesel generator (and therefore, the failure of a safety train) as a single failure case were analyzed.
The licensee also evaluated the loss of a train of the service water system which removes a train of containment air coolers from service resulting in reduced containment cooling.
The licensee identified an additional single failure case as the failure of one LPSI pump to turn off upon initiation of recirculation. This failure would cause additional flow through the strainer, the debris bed, and the suction piping from the strainer to the pumps. The licensee has implemented procedure changes to assure the LPSI pumps are either secured prior to RAS, or LPSI flow is throttled to 600 gpm and a HPSI pump secured if two are operating, as discussed above in this section.
The licensee determined the volume of water in the sump pool by summing all sources that contribute to the pool inventory and subtracting the volume of water in the hold-up volumes. The licensee provided details regarding the three sources of water that contribute to the containment sump pool inventory which are RWT, SITs, and RCS. The licensee also listed and explained the hold-up volumes used in reducing the sump pool inventory and provided water level values based on break sizes in Table 13 of its November 12, 2020, supplement.
The licensee stated that the containment water level is conservatively calculated by minimizing the sources of water and maximizing the volume of water holdups. The water sources were stated to be the minimum RWT inventory, the SITs, and some RCS inventory depending on the break location and size. The licensee also listed the inventory holdups assumed.
The licensee made the following assumptions to conservatively maximize the amount of water in the atmosphere: maximum containment free volume is used, initial humidity in containment is 0 percent, the containment atmosphere is assumed fully saturated post-accident, bounding temperatures are used, and maximum post-RAS temperature is used for all temperatures. The licensee assumed that CS pipe and other selected pipes are empty and are holdup volumes for the water level calculation.
The licensee investigated the hold up of water on horizontal surfaces and found that a 1/16-inch film would account for approximately 450 gallons. The licensee doubled this value to account for additional surfaces that might hold up water in a similar fashion. The total hold-up volume for horizontal surfaces is approximately 900 gallons. The licensee estimated the hold-up of water vapor released from the RCS and subsequently condensed onto surfaces as 1,370 gallons. The licensee estimated the moisture in the containment atmosphere to be approximately 7,500 gallons including droplets falling to the pool.
Calvert Cliffs modified the size of the line from each refueling cavity drain line to general area of containment from 1 inch to 8 inches. Increasing the drain line size eliminated potential water sequestration in the refueling cavities, raised the post-LOCA sump water lever, and provided additional margin for ECCS and CSS pump NPSH and strainer deaeration. The licensee installed a trash rack strainer over the drain entrance to prevent large debris from clogging the drain. The trash rack is approximately 3.5 feet long, 1.5 feet wide, and 2.5 feet high and has approximately 420 equally spaced square openings measuring 2-1/8 inches on each side.
The licensee considered the volume occupied by concrete pillars in the displacement of water in the sump water level calculation. The licensee also assumed some miscellaneous structures and components in the sump pool displace water.
The licensee considered the following water sources as contributors to the containment post-accident pool volume:
RWT - 49,945.16 cubic feet (ft3) of water assumed to empty to the lower level of containment based on conservative parameters SITs - for LOCAs greater than 0.08 ft2, it is assumed that inventory from four SITs inject the RCS (minimum volume per SIT of 1,113 ft3 assumed to inject into the core)
The licensee did not credit containment accident pressure in determining the available NPSH.
The sump water temperature for all LOCA cases is above 212 °F prior to recirculation, but none have temperatures above 212 °F at or after the start of recirculation.
The licensee stated that the sump pool temperature calculation assumes a single failure of one emergency diesel generator resulting in a loss of one ECCS train (including HPSI and CS pumps and containment air cooler), or the loss of one containment air cooler train and loss of one decay heat removal heat exchanger. Heat transfer to the containment basemat is not credited in the analysis, so there is no sump cooling. The sump pool temperature analysis also neglects cooling from the sump pool by means of evaporation. The licensee assumed a containment pressure of 14.7 psia and 100 percent humidity. The licensee stated that these factors result in a conservatively high prediction of sump pool temperature.
The licensee stated that the containment accident pressure is set at the vapor pressure corresponding to the sump liquid temperature for sump liquid temperatures greater than 212 °F.
The licensee used 1 psi below atmospheric pressure (i.e., 13.696 psia) as the containment pressure during a LOCA for the NPSH calculation, which corresponds to the minimum containment pressure that could exist at the start of an accident per technical specifications.
The licensee stated that the maximum sump fluid temperature for the NPSH margin analysis is 212 °F and NPSH available comes only from the static head of water. The licensee also considered the NPSH margin at 140 °F (temperature at which chemical precipitates are assumed to form). However, a sub-cooled margin of 27.235 feet exists at 140 °F.
The licensee stated that the increased NPSH margin at 140 °F due to the sub-cooled margin is much greater than the increased head loss due to the effect of chemical precipitates. Therefore, for NPSH margin the limiting condition is at maximum sump pool temperatures of 212 °F for HPSI, 120 °F for LPSI, and 212 °F for CS.
The licensee provided the limiting NPSH margins in Table 14 of its submittal. The minimum margins are 2.983 ft. for the HPSI pumps, 30.157 ft. for the LPSI pumps, and 1.630 ft. for the CS pumps. The NRC staff noted that the minimum allowable head loss value was listed as 2.07 ft. on page 71, in Table 11a. The staff requested that the licensee provide information that justifies that the minimum allowable head loss is 2.07 ft. when the NPSH margin for the CS pump is stated to be 1.63 ft. in RAI 5, dated January 11. 2024. The licensee responded to the RAI on February 9, 2024, that 2.07 ft. is not the NPSH value, but is the maximum strainer head loss that could occur before NPSH margin is lost. The NPSH margin value of 1.63 ft. includes the tested strainer head loss of 0.444 ft. If this value is added to the NPSH margin value, the result is 2.07 ft. The NRC staff concluded that the values in the submittal are correct, and the response provides adequate clarification to the use of the values in the submittal.
NRC STAFF CONCLUSION:
For the NPSH area, the licensee has provided information such that the NRC staff has reasonable assurance that it has been addressed conservatively or prototypically. Therefore, the NRC staff concludes that the NPSH evaluation for Calvert Cliffs is acceptable. The NRC staff considers this area closed for GL 2004-02.
10.0 COATINGS EVALUATION The objective of the coatings evaluation section is to determine the plant-specific ZOI and debris characteristics for coatings for use in determining the eventual contribution of coatings to overall head loss at the sump screen.
NRC STAFF REVIEW:
The NRC staff review is based on documentation provided by the licensee through November 12, 2020.
The licensee stated that the original primers used in containment during construction were Dimetcote No. 6 (D6) and Mobilzinc 7. The original topcoats were Amercoat 66 and Mobil 89 Series.
The licensee stated that the coatings used at Calvert Cliffs since construction are: Ameron D6 primer with Ameron 66 topcoat (primary coatings referenced) and Valspar 13F12 primer with Valspar 89 topcoat. The licensee stated that Valspar 13F12 is the same as Mobilzinc 7 and Valspar 89 is the same as Mobil 89 Series.
The current Service Level 1 coatings allowed in containment at Calvert Cliffs are Carboline Carboguard 890 metal primer, Carboline Starglaze 20115 concrete primer, and Carboline Carboguard 890 topcoat. The licensee stated that there are also multiple coatings in containment of unknown pedigree, most applied to equipment and small components by the original equipment vendor. The licensee tracks these unqualified coatings in a calculation.
The licensee assumed all coatings in the ZOI, all coatings of unknown pedigree, and degraded qualified IOZ coatings fail as 10 µm particles and transport to the strainer.
The licensee stated that epoxy coatings installed as qualified, but subsequently found to be degraded fail as chips. The licensee stated that during head loss testing, the chips were shown not to be drawn to the debris bed even when dropped in front of the strainer and agitated. This confirmed that the paint chips will not transport in sump pool velocities less than 0.2 ft/sec.
The licensee stated that degraded qualified coatings systems used at Calvert Cliffs are of comparable nuclear grade to those tested by Keeler and Long and evaluated in the Keeler and Long Report. IOZ primers are assumed to fail as particulates and the epoxy topcoats will fail as chips (greater than 1/32 inch).
The licensee performed independent testing with coating chips of 1-4 mm in size to investigate the transportability of coating chips under plant-specific conditions. The transport and settling tests used conservative velocities compared to those expected at the plant following a LOCA to confirm that coatings in chip form would not transport to the strainer surface. The transport and settling tests confirmed that coatings in chip form would not transport to the strainer surface.
The licensee stated that the head loss testing used scaled quantities of coatings as part of the strainer debris load. For the summer 2010 head loss testing, Silicon Carbide was used as a surrogate material for coatings particulates. For the late fall 2008 head loss testing, stone flour was used as a surrogate material for coatings particulates. Actual paint chips made of Carboline Carboguard 890 were used to simulate qualified, but degraded epoxy coatings. The size distribution of the paint chips was controlled by sieving.
Silicon carbide particles have a median sphere diameter of 9.3 +/- 1 m, and stone flour has a median sphere diameter of 7.7 m. Coatings that fail as particulate are assumed to fail at a size of 10 m. Since an equivalent volume of surrogate was used in head loss testing, and the surrogate has a smaller particle size to obtain an equivalent volume, a larger number of particles were used. The licensee stated that since it is the effect of individual particulate filling interstitial voids in the fiber bed that increases flow resistance and head loss, the use of these surrogates for coatings is conservative.
Calvert Cliffs followed NEI 04-07 guidance for determining the quantity of coating debris.
All coating (qualified and unqualified) in the ZOI will fail, All qualified (design basis accident-qualified or acceptable) coating outside the ZOI will remain intact, 100 percent of the unqualified coatings outside the ZOI will fail. Degraded-qualified IOZ coatings will fail as particulate, degraded-qualified epoxy coatings will fail as chips, and never-qualified coatings will fail as particulate.
The licensee used a 4.0 L/D ZOI for epoxy-based coatings and a 10.0 L/D ZOI for un-topcoated IOZ primer.
The licensee stated that they determined the volume of coatings debris by multiplying the surface area of affected coating by a measured or conservatively assumed dry film thickness (DFT). A DFT of 12 mils was assumed for degraded-qualified coatings, which is the maximum thickness permitted for replacement Service Level 1 coatings on steel substrate. The licensee stated that all degraded-qualified coatings are conservatively assumed to consist of an 8 mils epoxy topcoat and a 4 mils IOZ primer. All qualified coatings are assumed to consist of 4 mils IOZ primer and 6 mils epoxy topcoat, which are the maximum thicknesses allowed during construction.
The DFT of never-qualified coatings is variable and obtained from the containment assessments during the 2003 and 2004 refueling outages. The purposes of these surveys were to locate, identify, and quantify the never-qualified coatings within containment. The licensee obtained the coating thicknesses using a digital coating thickness gauge. The majority of these items have a DFT of 1 to 3 mils. For conservatism, the licensee applied a 15 percent margin to the Alkyd-based never-qualified coating quantities.
Calvert Cliffs conducts condition assessments of Service Level 1 coatings inside the containment at least once each refueling cycle. The licensee visually inspects all of the accessible areas within the containment. As the licensee identifies localized areas of degraded coatings, they are evaluated and scheduled for repair or replacement. The periodic condition assessments, and the resulting repair/replacement activities, assure that the amount of Service Level 1 coatings that may be susceptible to detachment from the substrate during a LOCA event is minimized and is identified and tracked by the plant coatings condition assessment program.
NRC STAFF CONCLUSION:
For this review area, the licensee has provided information such that the NRC staff has reasonable assurance that the subject review area has been addressed conservatively or prototypically. Therefore, the NRC staff concludes that the coatings evaluation for Calvert Cliffs is acceptable. The NRC staff considers this item closed for GL 2004-02.
11.0 DEBRIS SOURCE TERM The objective of the debris source term section is to identify any significant design and operational measures taken to control or reduce the plant debris source term to prevent potential adverse effects on the ECCS and CSS recirculation functions.
NRC STAFF REVIEW:
The NRC staff review is based on documentation provided by the licensee through November 12, 2020.
The licensee stated that coatings installed inside containment are controlled per a procedure which requires involvement of the coatings engineer who maintains the GSI-191 coatings calculations. This ensures only qualified coatings are installed.
Operations procedures ensure tags and signage installed in containment are non-floatable.
The licensee stated that it has procedures and practices in place to ensure containment cleanliness is maintained and that debris inside containment is identified and minimized prior to power operations. The licensee stated that site procedures require performance of specific inspections and documentation of loose debris prior to containment closeout. In addition, a search of containment for sources of loose debris, and corrective actions are required, prior to entering Mode 4. Another procedure assigns specific responsibilities for plant areas, including containment when accessible, and requires weekly cleanliness inspections and prompt actions to remediate.
The licensee has implemented several containment housekeeping actions. The licensee stated that Calvert Cliffs is not a low fiber plant, nor is the Calvert Cliffs strainer susceptible to the thin bed effect; therefore, no specific controls exist to maintain latent debris within analyzed limits.
However, latent debris is maintained at a relatively constant value by normal containment cleanliness initiatives.
The licensee stated that an Exelon fleet procedure contains guidance specifically addressing foreign material exclusion (FME) concerns in areas like the containment and the containment emergency sumps. It classifies the containment emergency sumps as a special FME area (FME Zone-1) and requires a plan for any entry into the sumps. This procedure requires FME training for all personnel working in containment. An Exelon fleet procedure contains requirements for verifying containment cleanliness through closeout inspections after containment entries.
The licensee stated that its design attribute review for the common design process contains a list of topics for which plant changes must be screened against to ensure all potential impacts of the change are properly assessed. The list of topics specifically identifies the introduction of materials into containment that could affect sump performance and the introduction of aluminum into containment as well as coating systems in containment.
The licensee stated that another site procedure provides maintenance controls for maintenance that could introduce new debris sources into containment. The procedure requires engineering reviews for impact to the containment sump for any maintenance activity that will install materials expected to remain in containment during Mode 4 or higher operations.
The licensee stated that is has a site procedure that requires effective implementation of the Maintenance Rule program. The procedure describes approved methods to monitor, trend, establish and modify goals for system, structures and components. Additional site procedures for integrated work management and integrated risk management provide specific guidance on risk assessment and scheduling of maintenance and temporary changes.
The licensee reduced the fibrous insulation debris that could be generated by a LOCA.
Reducing the amount of fiber decreases strainer head loss and reduces the chemical effects source term. Starting during the 2012 RFO, the licensee re-insulated multiple sections of insulated piping with RMI, which does not contribute to strainer head loss.
The licensee installed bands on 2-3/4-inch centers on Cal-Sil pipe insulation within 17 L/D of the RCS piping. Cal-Sil pipe insulation outside of 17 L/D was banded at 6 centers. The licensee replaced all Cal-Sil insulation within 3 L/D of the RCS piping with fiberglass insulation.
The licensee removed insulation from two pipes in Calvert Cliffs, Unit 1, during the 2010 RFO for gain margin. The pipes are a shutdown cooling line which was insulated with mineral wool insulation and the pressurizer relief valve line outside of the pressurizer compartment which was insulated with generic fiberglass insulation. The licensee stated that the corresponding pipes in Calvert Cliffs, Unit 2, are uninsulated.
The licensee removed the telescoping aluminum ladder from the polar crane during the Calvert Cliffs, Unit 1, 2012 RFO and the Calvert Cliffs, Unit 2, 2013 RFO to reduce the aluminum content in containment.
The licensee stated that its valve equipment tags are made of materials that will sink in water and will not transport to the containment emergency sump. In addition, the tags will not delaminate in a post-accident environment.
Calvert Cliffs has a coatings program that monitors and controls the quantities and types of coatings installed inside containment. This program conducts periodic condition assessments, typically each outage, to verify the adequacy of existing coatings and direct repair/replacement, as necessary. Calvert Cliffs implemented controls for procurement, application, and maintenance of qualified coatings used inside containment consistent with licensing basis and regulatory requirements. The licensee tracks the quantity of unqualified coatings that are inside containment. The licensee stated that this program is adequate in its current form to ensure coatings are properly controlled, and that future additions of unqualified coatings are quantified.
NRC STAFF CONCLUSION:
For this review area, the licensee has provided information such that the NRC staff has reasonable assurance that the subject review area has been addressed conservatively or prototypically. Therefore, the NRC staff concludes that the debris source term evaluation for Calvert Cliffs is acceptable. The NRC staff considers this item closed for GL 2004-02.
12.0 SCREEN MODIFICATION PACKAGE The objective of the screen modification package section is to provide a basic description of the sump screen modification.
NRC STAFF REVIEW:
The NRC staff review is based on documentation provided by the licensee on November 12, 2020.
Calvert Cliffs installed a CCI cassette pocket strainer with 6,060 ft2 filtration surface area in both units. The strainer perforation size is 1.6 mm (1/16 inch) with no more than 3 percent larger holes and no holes larger than 2 mm (0.08 inch). There are 33 strainer modules (each approximately 3 feet high) divided among three strainer rows. There are 324 pockets in 29 of the strainer modules, and 252 pockets in four of the strainer modules. The pocket dimensions are 84 mm x 90 mm in cross-section, and 200 mm deep. The strainer rows tie into a common duct which directs the flow to the existing containment emergency sump, sometimes referred to as the sump pit. The containment emergency sump is a concrete curb with a steel roof and contains the inlets to both recirculation headers.
The licensee modified a 16-inch feedwater pipe support on Calvert Cliffs, Unit 2, to allow clearance for one of the strainer rows. The licensee also modified a cable tray support on Unit 2 to allow clearance for the radial duct. These modifications were not required on Calvert Cliffs, Unit 1. In addition, the licensee notched the 6-inch curb around the emergency recirculation sump to allow for installation of the common duct to the sump.
NRC STAFF CONCLUSION:
For the screen modification package review area, the licensee provided screen location, configuration, and construction information such that the NRC staff has confidence in the design of the strainer. Therefore, the NRC staff concludes that the screen modification package information provided for Calvert Cliffs is acceptable. The NRC staff considers this item closed for GL 2004-02.
13.0 SUMP STRUCTURAL ANALYSIS The objective of the sump structural analysis section is to verify the structural adequacy of the sump strainer including seismic loads and loads due to differential pressure, missiles, and jet forces.
NRC STAFF REVIEW:
The NRC staff review is based on documentation provided by the licensee on November 12, 2020.
The licensee used classical and finite-element (ANSYS or ME035) methods to analyze the following parts of the strainer: standard cartridges (cartridge depth 200 mm), support structure and duct of a standard module, radial duct, sump cover, and sump back plate.
The American Society of Mechanical Engineers (ASME)Section III, 2004 Edition, 2005 Addenda, Subsection NF, Supports for Class 2 components was used for the analysis. The licensee chose these rules to provide a recognized standard for structural analyses; however, the strainer components are non-ASME code items, Seismic Category 1.
The licensee stated that the standard module analysis assumes an 18-cartridge design which bounds the smaller 14 cartridge design for structural analysis purposes.
The licensee used the following design inputs:
Total weight of modules (2 support structures, duct, cover plate, and cartridges) o 18 Cartridge Module 941.73 pound-mass (lbm) (427.16 kilograms (kg))
o 14 Cartridge Module 802.68 lbm (364.09 kg)
Total debris mass transported to sump = 10,783 lbm (4,891 kg) (bounding value used for structural analyses only)
Differential pressure due to head loss = 10.15 psi (700 millibar). (Based on head loss tests, actual differential pressure due to chemical effects will be much less)
Operating Basis Earthquake o Maximum Horizontal Acceleration 1.96 acceleration due to gravity (g) at 3 Hz o Maximum Vertical Acceleration 0.59 g at 10 Hz Safe Shutdown Earthquake o Maximum Horizontal Acceleration 2.75 g at 3 Hz o Maximum Vertical Acceleration 1.11 g at 10 Hz Additional load from shielding blankets = 885.91 pound-force (lbf) (3940.76 Newtons)
The licensee provided a summary of design load combinations. The licensee determined in the strainer structural analysis that load combinations 1, 7, and 8 enveloped the other load combinations. Therefore, the licensee only analyzed load combinations 1, 7, and 8. Load 1 is a load combination of the weight of strainers and supporting structures with the pool dry at a temperature of 280 °F. Load 7 is a load combination of the weight of strainers and supporting structures plus the weight of debris, safe-shutdown earthquake (pool filled), and differential pressure at a temperature of 70 °F. Load 8 is a load combination of the weight of strainers and supporting structures plus additional load caused by radiation shielding blankets at 70 °F. For the operating basis earthquake and safe-shutdown earthquake cases, the licensee also computed a sloshing load computed to account for the impact of water sloshing in the sump pool.
The licensee stated that the emergency recirculation sump strainer structure consists of two separate structures: the floor structures and the sump pit structures.
- 1) The floor structures consist of the strainer modules and a radial duct which channels the flow from the three rows of strainer modules to the sump pit. Each of the strainer modules/radial duct segments are anchored to the concrete floor via anchor plates at each end. A retaining structure provides the mounting frame for the radial duct segments and the interior duct of the strainer modules. The strainer cassettes attach to the strainer interior duct and are covered with a deck plate. The modules/ducts and retaining structures are connected or anchored with various sized bolting hardware.
- 2) The sump pit structure consists of cover plates which cover the sump pit and support beams. Figures of the modules, strainers, duct, and sump pit are provided in the submittal.
The licensee provided tables showing the ratios of design stress and corresponding allowable stress for various components of the emergency recirculation sump strainer structural assembly.
The licensee stated that the bending stress for the radial duct is above the Level C allowable stress, and the inner duct walls of segment 1 exhibited greater stress than the level C allowable.
However, the licensee performed plastic-elastic analysis on the radial duct and determined that the strain intensity is well below 2/3 of the collapse load, and permanent distortion of the side walls do not lead to loss of function of the duct segment. Similarly for segment 1 of the radial duct, the actual differential pressure is well below 2/3 of the collapse differential pressure.
Therefore, the licensee stated that Segment 1 will not collapse, and the radial duct will perform its function.
The licensee stated that Calvert Cliffs has approval to use leak-before-break methodology so that the dynamic effects of a LOCA do not need to be considered in the design of structures and components. ECCS sump recirculation is not required for breaks in other piping systems.
The licensee stated that the Calvert Cliffs emergency recirculation sump strainer design does not incorporate a backflushing strategy.
The NRC staff reviewed the licensees submittal and noted the licensee provided Table 16 Cartridges; Table 18, Radial Duct; and Table 20, Analysis of the Duct Structure of Radial Duct Segment 1. These tables showed that the calculated stress exceeded the ASME Code Level C allowable stresses in some instances. For the exceedances, the licensee stated that it performed a plastic-elastic analysis to demonstrate that the sump strainer components would not collapse and would continue to perform their intended safety functions. The NRC staff previously reviewed Calculation CA06768, Revision 2 to determine the licensee approach for addressing the exceedances (see Audit Plan at ML18334A233 (non-public)). The staff noticed that the licensee evaluated the exceedances using the plastic elastic analysis approach consistent with Subparagraph NB-3228.3, Plastic Analysis, of the ASME 2004 Edition with 2005 Addenda. The licensee provided its results in the tables listed above and demonstrated that the strain intensity and the actual differential pressure are well below two thirds of the collapse load and thus would not lead to structural damage that would impair strainer function.
The staff also noted that impacts due to a HELB are not applicable due to the use of leak-before-break methodology, and the licensee does not take credit for a backflushing strategy.
On the basis of its review of the structural sump analysis for Calvert Cliffs, the NRC staff concludes that the information provided by the licensee is acceptable because: (1) the licensees approach for addressing the exceedances meets the requirements in the ASME Code (the licensees existing code of record) and provides staff-approved acceptance criteria, (2) the licensee demonstrated that the sump strainers can withstand the assumed design-basis loads for which it is deterministically qualified, and (3) the analyzed loads and load combinations are consistent with NEI 04-07 guidance. The use of the ASME Code and the NEI 04-07 guidance provides reasonable assurance that the licensee used the appropriate design inputs and load combinations in its design of the sump strainers and its supports.
NRC STAFF CONCLUSION:
For this review area, the licensee has provided information such that the NRC staff has reasonable assurance that the subject review area has been addressed conservatively or prototypically. Therefore, the NRC staff concludes that the sump structural analysis evaluation for Calvert Cliffs is acceptable. The NRC staff considers this item closed for GL 2004-02.
14.0 UPSTREAM EFFECTS The objective of the upstream effects assessment is to evaluate the flow paths upstream of the containment sump for holdup of inventory, which could reduce flow to the sump.
NRC STAFF REVIEW:
The NRC staff review is based on documentation provided by the licensee on November 12, 2020.
The licensee stated that the lower level of containment is open and contains no compartment or choke point which could prevent water from flowing to the sump. The flow path of water from the CSS is from the containment dome area and falls into the refueling pool cavities and directly into lower containment. The refueling pool cavities drain to the sump through a drain line.
The licensee modified the drain flow path from the refueling pool cavities to the lower level of containment to increase the diameter of the line to 8 inches for the entire distance. The licensee installed a trash rack over the drain opening in the refueling pool cavities to prevent blockage of the drain line.
There are no curbs of sufficient dimension to impact water flow to the sump.
The licensee inspected the reactor cavity drains via camera and found them to be functional.
The licensee assumes that for breaks that originate in the cavity that debris blocks the drain.
The door to the reactor cavity is not watertight; therefore, the licensee assumes that the cavity water-level is the same as that in the rest of the lower-level of containment.
NRC STAFF CONCLUSION:
For this review area, the licensee has provided information such that the NRC staff has reasonable assurance that the subject review area has been addressed conservatively or prototypically. Therefore, the NRC staff concludes that the upstream effects evaluation for Calvert Cliffs is acceptable. The NRC staff considers this item closed for GL 2004-02.
15.0 DOWNSTREAM EFFECTS - COMPONENTS AND SYSTEMS The objective of the downstream effects, components and systems section is to evaluate the effects of debris carried downstream of the containment sump screen on the function of the ECCS and CSS in terms of potential wear of components and blockage of flow streams.
NRC STAFF REVIEW:
The NRC staff review is based on documentation provided by the licensee on November 12, 2020.
The licensee performed evaluations to address ex-vessel downstream effects in accordance with WCAP-16406-P-A and the NRC SE. A bounding debris source term was used. The licensee assumed all particulate debris transports through the system and does not deplete over 30 days. A conservative wear equation was used for the downstream components except pumps.
The licensee used the size distribution from Table I-1 of WCAP-16406 for unqualified coatings to determine the amount that passes through the strainer for the evaluation of pump wear. The licensee stated that the pump wear evaluation does not model any depletion over time for unqualified coatings. The licensee stated that the use of Table I-1 of WCAP-16406 for unqualified coatings is conservative when compared to using a uniform 10 µm distribution. The licensee performed calculations based on WCAP-16406-P-A, that demonstrate using the Table I-1 size distribution is conservative compared to assuming 10 µm debris for erosive wear of pump clearances. The pump wear evaluation assumed that 26.9 percent of the unqualified coatings pass downstream of the sump screen and do not deplete over time.
The licensee used conservative fine fiber debris load masses in the downstream effects calculations. The fibrous debris load used is two to three times that predicted for a double-ended guillotine break of the RCS hot and cold legs. The licensee stated that debris loads for the downstream evaluations are based on bypass testing of the CCI strainer. A post-installation examination inspected for gaps at all strainer interfaces/joints. The acceptance criterion is no gap greater than 1/32-inch can remain. These small openings ensure no large particles enter the downstream recirculation piping.
The licensee replaced all of the HPSI pump cyclone separators with a design tested at Wyle labs to ensure that the seal water would be adequately clean considering debris in the fluid. The licensee stated that evaluation of the HPSI and CS pump mechanical seals determined that testing was not needed.
The licensee performed impeller/casing wear evaluations of the HPSI, LPSI, and CS pumps.
The licensee evaluated the required piping and valves according to methods of WCAP-16406-P-A. The licensee stated that most piping components passed the evaluation criteria for plugging and wear and those that did not were shown to have no adverse effect on component function.
The licensee stated that replacement of the HPSI pump cyclone separators was the only design modification change required as a result of downstream effects evaluations. The only operational change was to revise the EOPs to limit the HPSI motor operated valves position to a minimum of 30 percent open to avoid excessive wear.
NRC STAFF CONCLUSION:
For the ex-vessel downstream effects review area, the licensee has provided sufficient information such that the NRC staff has reasonable assurance that the subject review area has been addressed conservatively or prototypically. Therefore, the NRC staff concludes that the licensees evaluation of this area is acceptable. Based on the information provided by the licensee, the NRC staff considers this area closed for GL 2004-02.
16.0 DOWNSTREAM EFFECTS - FUEL AND VESSEL The objective of the downstream effects, fuel and vessel section, is to evaluate the effects that debris carried downstream of the containment sump screen and into the reactor vessel has on LTCC.
NRC STAFF REVIEW:
The NRC staff review is based on documentation provided by the licensee on November 12, 2020.
The licensee evaluated downstream effects on the fuel and in the reactor vessel using the methods described in WCAP-16793-NP-A. The licensee used a bounding debris source term for this evaluation. The licensee addressed the 14 limitations and conditions presented in the SE on WCAP-16793 in the evaluation.
The licensee stated that the fuel used at Calvert Cliffs is Framatome (formerly AREVA)
Advanced CE-14 HTP/HMP Fuel with FUELGUARD lower end fittings. For this fuel design, WCAP-16793 establishes a 15 grams of fiber per fuel assembly (g/FA) limit to assure LTCC.
The maximum flowrate per FA at Calvert Cliffs is 6.9 gpm, which assumes a LPSI pump failed to trip and operating at 800 gpm, and a HPSI pump operating at a conservatively high flow of 700 gpm. The licensee stated that Calvert Cliffs has an in-vessel fiber load ranging from 7.6 g/FA to 14.4 g/FA depending upon whether CS flow split is credited for debris diversion from the core. Both fiber bypass quantities (7.6 g/FA and 14.4 g/FA) are based on a hot-leg break with cold-leg recirculation. The debris load would be less for a cold-leg break since some fiber will spill out the break before reaching the core.
The licensee stated that WCAP-16793 also presents a method for evaluating debris deposition on the fuel rods (LOCA deposition model or LOCADM). The Calvert Cliffs LOCADM evaluation demonstrated acceptable deposition thickness and cladding temperatures. The maximum cladding temperature was 365.2 °F resulting in a margin of 434.8 °F based on the allowable maximum cladding temperature of 800 °F. The maximum total LOCA generated debris deposition thickness was 872.2 microns or 34.3 mils compared to the allowable of 50 mils.
The strainer vendor determined the amount of fiber that transports to the reactor core by performing two scaled fiber bypass tests using a prototypical fibrous debris mix and a nominal strainer flow rate. The licensee used the more conservative test results for the evaluation.
The licensee stated that the same vendor performed a similar bypass test for the Salem Nuclear Generating Station (Salem). The NRC staff reviewed and accepted the Salem methodology for determining fiber bypass of the strainer. Calvert Cliffs used the same evaluation approach and submitted the fiber bypass calculation for NRC review (ML15096A012 (non-public)).
In 2018, the licensee updated the Calvert Cliffs fiber bypass calculation to reflect more recent data. The calculation methodology is nearly the same as the original version. The only notable difference is the licensee assumed that all fibers shorter than or equal to the largest capture screen dimension (0.44 mm) penetrate through the capture screen. Originally, the licensee assumed all fibers shorter than or equal to 0.50 mm penetrate through the capture screen. The licensee stated that while it is possible that some fibers longer than 0.44 mm may have passed through the fiber bypass capture screen this is offset by the fact that some fibers less than 0.44 mm will be filtered out on the fiber bypass capture screen.
The licensee stated that after adjusting the strainer bypass test results using the Salem methodology the resultant fiber bypass quantity is 14.4 g/FA. The licensee updated the adjustment factor to be correlated to the total captured mass in the fiber bypass test.
The licensee stated that this result conservatively assumes that all fiber which passes through the sump strainer transports to the reactor FAs (i.e., no credit is taken for CS flow split reduction).
The NRC staff reviewed the licensees evaluation of downstream debris on the fuel and reactor vessel and found it to be acceptable. NRC staff verified that the plant-specific calculated amounts of fuel deposits and maximum clad temperatures calculated using LOCADM met the acceptance criteria. The licensees total amount of fiber passing through the sump strainer, assuming no credit for fiber passing through the CS, met the fiber limit criterion in WCAP-16793-NP-A. The staff reviewed and approved WCAP-16793-NP and specified a fiber limit in the SE of WCAP-16793-NP-A that is unable to form a filtering bed capable of blocking flow to the inlet to the reactor vessel core.
NRC STAFF CONCLUSIONS:
For the in-vessel downstream effects review area, the licensee has provided sufficient information such that the NRC staff has reasonable assurance that the subject review area has been addressed conservatively or prototypically. Therefore, the NRC staff concludes that the licensees evaluation of this area is acceptable. Based on the information provided by the licensee, the NRC staff considers this area closed for GL 2004-02.
17.0 CHEMICAL EFFECTS The objective of the chemical effects section is to evaluate the effect that chemical precipitates have on head loss and core cooling.
NRC STAFF REVIEW:
The NRC staff review is based on documentation provided by the licensee on November 12, 2020. The licensee performed multiple head loss tests with chemical precipitate to evaluate the plant-specific chemical effects. Calvert Cliffs switched the post-LOCA buffering material from trisodium phosphate (TSP) to sodium tetraborate in order to minimize early chemical precipitation due to TSP interaction with Cal-Sil.
The licensee determined the chemical source term produced by interaction between the post-LOCA environment and plant materials using the WCAP-16530-NP-A base model approach. Sump pool temperature and containment vapor temperature were determined from the containment accident analysis. The NRC has reviewed and approved this topical report for chemical effects evaluations, including the calculation of the chemical source term.
The licensees WCAP-16530-NP-A spreadsheet predicted Sodium Aluminum Silicate precipitate would form for all debris cases. The NRC SE for WCAP-16530-NP-A found either Sodium Aluminum Silicate or aluminum oxyhydroxide acceptable for head loss testing of aluminum precipitates. All Sodium Aluminum Silicate precipitate that was used in head loss testing met the settlement criterion before being added to the test loop. The licensee added the entire precipitate amount predicted for the 30-day mission time to the test loop and the testing was completed once the test termination criteria were met. The licensee used the resulting peak head loss to evaluate NPSH margin at the earliest time of chemical precipitation.
The licensee used one refinement to the WCAP-16530-NP-A approach, by taking credit for aluminum solubility until the sump pool cools to 140 °F. The licensee calculated aluminum solubility data as a function of pH and temperature using the Argonne National Laboratory equation (ML091610696) for temperatures less than 175 °F. The licensee determined the aluminum concentration for the pipe break that produced the largest quantity of chemical precipitate and then doubled the aluminum concentration for conservatism before applying the Argonne National Laboratory equation. The precipitation temperature was calculated to be 138 °F and the licensee applied the chemical precipitate head loss when the sump pool temperature reaches 140 °F. The use of aluminum solubility in this case is acceptable to NRC staff since the licensee used conservative inputs for the aluminum concentration. In addition, the Argonne National Laboratory equation that was used to determine aluminum solubility has been shown to be conservative for the pH range of interest by subsequent autoclave testing, documented in WCAP-17788-P, Volume 5, (ML15210A668).
The NRC staff asked a question related to chemical effects (ML24012A049) since the licensee had stated that excess particulate could be credited for a couple Region II breaks where 100 percent of the calculated precipitate was not added to the test. The licensee acknowledged that chemical precipitates have a larger impact on strainer head loss than particulates and indicated that the statement was intended only to show defense in depth. The licensee stated 12B Cold Leg break case Tests 5 and 7 had debris bed break-through and peak head loss occurred before all the chemical precipitates were added to the test. The licensee also stated the amount of fiber in the debris bed determines the point where bed break-through occurs. The licensee also discussed other tests where the head loss increase with excess percentages of chemical precipitate did not produce similar percentage increases in head loss due to bed break-through. The staff found the response acceptable and has observed many strainer tests where the peak head loss is reached before debris bed break-through, even though multiple batches of chemical precipitate were added after debris bed break-through. In addition, the staff notes that the solubility calculation that determined the temperature at which chemical head loss was considered for Calvert Cliffs included a doubling of aluminum concentration as a conservative measure. If the actual calculate aluminum concentration was used to calculate solubility, the precipitation temperature would be lower and the amount of NPSH margin would increase at the lower temperature. This offsets any uncertainty related to whether a higher head loss would have occurred with addition of more precipitates to the head loss tests, as discussed a
bove.
NRC STAFF CONCLUSION:
For the chemical effects review area, the licensee has provided sufficient information such that the NRC staff has reasonable assurance that the subject review area has been addressed conservatively or prototypically. Therefore, the NRC staff concludes that the licensees evaluation of this area is acceptable. Based on the information provided by the licensee, the NRC staff considers this area closed for GL 2004-02.
18.0 LICENSING BASIS The objective of the licensing basis section is to provide information regarding any changes to the plant licensing basis due to the changes associated with GL 2004-02. The NRC staff review is based on documentation provided by the licensee on November 12, 2020.
The licensee updated the description of the containment sump strainer in the UFSAR to reflect the new design that was installed in response to GL 2004-02. The licensee will update the ECCS pump NPSH margin values once the response to GL 2004-02 has been completed.
The Chapter 14 UFSAR accident analyses that reference long-term ECCS cooling, and the containment sump (LOCA & containment response) do not require revision and are not impacted by GL 2004-02.
The licensee stated that all LOCA break sizes can be successfully mitigated using station procedures and standard accident recovery principles. Successful mitigation of the limiting Region II break size during LTCC may require use of existing operator actions to prevent ECCS failures when the sump pool is cooled below 140 °F. The licensee will update the UFSAR to summarize the response to GL 2004-02 and to reflect the use of NEI 04-07 as a new methodology to evaluate post-accident debris effects on the containment sump system. The licensee stated that no prior NRC approval is required for the changes discussed herein, and no need for exemption from regulations was identified.
NRC STAFF CONCLUSION:
For this review area the licensee has provided information, such that the NRC staff has reasonable assurance that the subject review area has been addressed conservatively or prototypically. Therefore, the NRC considers this item closed for GL 2004-02.
19.0 CONCLUSION
The NRC staff performed a thorough review of the licensees responses and RAI supplements to GL 2004-02. The NRC staff conclusions are documented above. Based on the above evaluations the NRC staff finds the licensee has provided adequate information as requested by GL 2004-02.
The stated purpose of GL 2004-02 was focused on demonstrating compliance with 10 CFR 50.46. Specifically, the GL requested addressees to perform an evaluation of the ECCS and CSS recirculation and, if necessary, take additional action to ensure system function in light of the potential for debris to adversely affect LTCC. The NRC staff finds that the information provided by the licensee demonstrates that debris will not inhibit the ECCS or CSS performance following a postulated LOCA. Therefore, the ability of the systems to perform their safety functions, to assure adequate LTCC following a design-basis accident, as required by 10 CFR 50.46, has been demonstrated.
Therefore, the NRC staff finds that the licensees responses to GL 2004-02 are adequate and considers GL 2004-02 closed for Calvert Cliffs, Units 1 and 2.
Principal Contributors: S. Smith, NRR A. Russell, NRR P. Klein, NRR M. Yoder, NRR S. Lai, NRR B. Parks, NRR Date: April 22, 2024
ML24101A194 OFFICE NRR/DORL/LPL1/PM NRR/DORL/LPL1/LA NRR/DSS/STSB/BC NAME MMarshall KEntz SMehta DATE 04/09/2024 04/19/2024 02/27/2024 OFFICE NRR/DNRL/NCSG/BC NRR/DORL/LPL1/BC NRR/DORL/LPL1/PM NAME SBloom HGonzález MMarshall (AKlett for)
DATE 02/27/2024 04/22/2024 04/22/2024