ML093310294
| ML093310294 | |
| Person / Time | |
|---|---|
| Site: | Robinson  |
| Issue date: | 12/03/2009 |
| From: | Orf T Plant Licensing Branch II |
| To: | Mccartney E Carolina Power & Light Co |
| Orf, Tracy J.; NRR/DORL 415-2788 | |
| References | |
| GL-04-002, TAC MC4709 | |
| Download: ML093310294 (9) | |
Text
UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555*0001 December 3, 2009 Mr. Eric McCartney, Vice President Carolina Power & Light Company H. B. Robinson Steam Electric Plant, Unit NO.2 3581 West Entrance Road Hartsville, South Carolina 29550 SUB~IECT: H. B. ROBINSON STEAM ELECTRIC PLANT, UNIT NO.2 - REQUEST FOR ADDITIONAL INFORMATION REGARDING SUPPLEMENTAL RESPONSE TO GENERIC LETTER 2004-02, "POTENTIAL IMPACT OF DEBRIS BLOCKAGE ON EMERGENCY RECIRCULATION DURING DESIGN BASIS ACCIDENTS AT PRESSURIZED WATER REACTORS" (TAC NO. IV1C4709)
Dear Mr. McCartney:
By letter dated March 7, 2008 (Agencywide Documents Access and Management System Accession No. ML080730290), Carolina Power & Light Company, now doing business as Progress Energy Carolinas, Inc. (the licensee), submitted a supplemental response to Generic Letter (GL) 2004-02, "Potential Impact of Debris Blockage on Emergency Recirculation During Design Basis Accidents at Pressurized Water Reactors," for the H.B. Robinson Steam Electric Plant, Unit NO.2 (HBRSEP). By letter dated ~Iuly 25,2008 (ML081900649), the U.S. Nuclear Regulatory Commission (NRC) provided the licensee with a request for additional information related to the licensee's submittal. On December 17, 2008 (ML083570469), the licensee provided a response to the NRC staff's letter of ~Iuly 25,2008.
The NRC staff has reviewed the licensee's submittal. The process involved detailed review by a team of 10 subject matter experts, with focus on the review areas described in the NRC's "Content Guide for Generic Letter 2004-02 Supplemental Responses" (ML073110389). Based on these reviews, the staff has determined that additional information is needed in order to conclude there is reasonable assurance that GL 2004-02 has been satisfactorily addressed for HBRSEP. The enclosed document describes these requests for additional information (RAls).
The NRC requests that the licensee respond to these RAls within 90 days of the date of this letter. However, the NRC would like to receive only one response letter for all RAls. If the licensee concludes that more than 90 days are required to respond to the RAls, the licensee should request additional time, including a basis for why the extension is needed.
E. McCartney - 2 Please contact me at 301-415-2788 if you have any questions on this issue, would like to participate in a conference call, or if you require additional time to submit your responses.
Sincerely, Tracy J. Orf, Project Manager Plant Licensing Branch 11-2 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. 50-261
Enclosure:
As stated cc: Distribution via Listserv
REQUEST FOR ADDITIONAL INFORMATION H.B. ROBINSON STEAM ELECTRIC PLANT, UNIT NO.2 RESPONSES TO REQUEST FOR ADDITIONAL INFORMATION REGARDING SUPPLEMENTAL RESPONSE TO GENERIC LETTER 2004-02:
"POTENTIAL IMPACT OF DEBRIS BLOCKAGE ON EMERGENCY RECIRCULATION DURING DESIGN BASIS ACCIDENTS AT PRESSURIZED WATER REACTORS" DOCKET NO. 50-261 Debris transport RAI1 &
RAI 4 Provide additional information to justify the credit taken for the retention of small fibrous fines and particulate debris in the upper containment and inactive holdup volumes or provide additional basis to demonstrate that the head loss impact of the debris is insignificant.
Discussion: In request for additional information (RAI) 1, the staff requested that the licensee provide justification for the assumption that 50 percent of small fines of fiber and 50 percent of fine particulate would be retained in the upper containment rather than washed down to the containment pool. The RAI stated that this assumption was not consistent with the conclusions expressed in NUREG/CR-6369, Volume 2, "Drywell Debris Transport Study: Experimental Work," and was considered nonconservative. The licensee responded that the significance of this item was reduced due to the assumption of only 10 percent of debris being blown to the upper containment and other conservatisms associated with debris capture in inactive holdup volumes. The licensee's response further stated that explicit consideration of the debris that was retained in upper containment would only result in a head loss increase of approximately 0.08 ft. Although the staff recognized some of the licensee's qualitative arguments, the staff did not consider the licensee's assumption that only 10 percent of these types of debris would be blown into upper containment as a conservatism that could address the issue with upstream retention, since the assumption of 90 percent blowdown transport to the containment pool likely led to an overestimation of the capture of this debris in inactive containment pool volumes. RAI 4 further stated the staff's view that it appeared unrealistic to assume that multiple levels of grating would be capable of ensuring that only 10 percent of fine fiber and particulate would reach upper containment. The staff also does not understand the basis for the licensee's predicted incremental head loss increase, and expects that the incremental increase associated with this retained debris could be greater than predicted and could potentially challenge the net positive suction head margin for the plant. The licensee's response did not address these issues.
RAt 2 Provide additional information in light of the discussion below to justify the assumptions of zero percent erosion and zero percent transport of large pieces of fibrous debris.
Enclosure
-2 Discussion: The staff requested that the licensee justify the assumptions of zero percent erosion and zero percent transport for large pieces of fibrous debris. The licensee responded that the neglect of erosion was consistent with the baseline debris transport guidance and that a qualitative evaluation of the transport of large fibrous debris transport had been performed. The staff does not consider this response to be acceptable for reasons stated in detail below.
(1) The licensee took credit for refinements in the transport analysis that did not conform to the baseline guidance (e.g., credit for small and fine debris retention in upper containment). The baseline guidance was considered holistically acceptable; although it had known nonconservatisms (such as the neglect of erosion), other conservatisms in the guidance were considered sufficient to compensate for them. The licensee's basis for removing conservatism from the baseline guidance without addressing nonconservative aspects of the baseline guidance, such as erosion, is not clear to the staff.
(2) The qualitative assessment of large debris piece transport does not appear to be sufficiently rigorous. In particular, the staff expects, based on the licensee's discussion, that some large-piece transport may occur at Robinson, were a more detailed evaluation of the containment pool flows to be performed. The staff's expectation is based on the following points:
- i. With realistic assumptions, such as channeled flow around obstacles and a nonuniform flow profile, the actual pool velocities toward the strainer would likely exceed the average velocity calculated by the licensee significantly, ii. It is unclear to the staff that high-velocity flows could not exit the crane wall near the strainers at a sufficient velocity to cause large piece transport, iii. It is not clear that the strainers located inside the crane wall were addressed by the licensee's qualitative transport assessment, iv. The licensee's transport metric for large pieces of fiber is based on a study that used regularly shaped, scissor-cut pieces of debris that led to higher incipient tumbling velocities than would be the case for irregularly shaped pieces,
- v. The transport of large pieces via floatation was not addressed, and vi. It is not clear to the staff that debris interceptors or trash racks installed upstream of the strainers would completely prevent the transport of large pieces (e.g., although steel bars are installed in crane wall flowpaths, it is not clear that their minimum clearances would completely prevent debris transport to the strainers in the annulus, or that they would be effective at stopping debris transport to the strainers inside the crane wall). Because the strainers are mounted only 2.5 inches above the containment floor level, it is not clear that the accumulation of large pieces of fiberglass on the strainers would not adversely affect the performance of the strainers.
-3 Head Loss and Vortexing RAI 5 Provide justification that the debris preparation for the head loss testing resulted in a conservative or prototypic head loss value for limiting plant conditions.
Discussion: The staff requested information regarding the size distribution of fibrous debris used during head loss testing and how it compared to the analytical transport of fibrous debris sizes. The licensee provided additional information regarding fibrous debris sizing. The submittal stated that 60 percent of the fibrous material was considered small fines and 40 percent large pieces. In the debris characteristics section, a table states that 60 percent of the fibrous insulation types are fines. It is unclear what percentage of the fibrous debris added to the testing was actually fines. Based on testing observations, the staff considers fines to be easily suspendable and has further defined fine fibrous debris by referencing NUREG/CR-6808 fibrous size definitions. The staff considers that fines should generally be classes 1-2 with some class 3 permissible in the mixture because it is evident that these classes meet the definition of easily suspendable, while others do not. In general, the licensee's strainer test vendor has tested with a generic fiber mix that consisted of fibrous debris that was shredded and mixed with water prior to addition. This practice does not ensure that the size distribution of fibrous debris reaching the strainer matches the assumptions made in the transport and debris characteristics evaluations. The amount of fine fiber added to a test can have a significant impact on how the debris bed forms and the resulting head loss. In addition, the use of a fiber size mixture that is coarser than that predicted by the other evaluations has been observed to result in the lack of a thin bed forming during testing when one may actually occur in the plant.
The response to RAI 5 stated that the debris was prepared to meet the size categories of 1-4 as defined by NUREG/CR-6808 and that the NUREG stated that the debris would undergo additional fragmentation when subjected to the inlet flow into the pool. The statement that additional fragmentation would occur was not justified technically, nor was the additional fragmentation quantified. The fibrous debris was stated to be sized ranging from individual fibers to approximately less than 1 inch clumps. The response stated that the fibrous debris sizing met the Nuclear Energy Institute 04-07 definition of small fines and was, therefore, expected to provide appropriate proportions of various size fiber classes. Because the debris was stated to be sized as classes 1-4 and this included all fibrous debris, it is possible that the sizing was adequate to produce realistic head losses during the full load test. The remaining issue is the sizing of the debris for the thin bed test. Because Robinson has a significant amount of fibrous material in containment and other problematic particulate insulation, a significant filtering bed could form with only fine fiber and the other particulate insulation debris (Cal-Sil). This bed may result in higher head losses than was measured during the previous head loss testing.
RAI 7 Provide information that justifies that any agglomeration of debris did not affect head loss test results nonconservatively.
Discussion: The staff requested information regarding the debris introduction techniques used during head loss testing to ensure that excessive agglomeration of debris did not occur during the testing. The licensee stated that the debris was
-4 combined with water and well mixed in buckets prior to introduction to the test tank. The introduction point was between the return flow sparger and the tank wall. This was intended to ensure mixing of debris in the test tank so that agglomeration did not occur and the debris remained entrained in the tank. The debris concentration in the buckets was not controlled. The staff has witnessed testing at the licensee's vendor, and at other test vendors, and has noted that some agglomeration of debris occurs during the debris introduction process when the debris concentration is not controlled and under other circumstances. The staff has also noted in the NRC Staff Review Guidance Regarding Generic Letter 2004-02 Closure in the Area of Strainer Head Loss and Vortexing (ML080230038) that nonprototypical agglomeration should be prevented during head loss testing.
RAI 8 Provide an evaluation that shows that any settling that occurred resulted in a negligible effect on head loss test results or that the settling was prototypical or conservative compared to the expected plant conditions.
Discussion: The staff requested information on debris settlement that occurred during head loss testing. The licensee responded that there was some settling away from the test strainers, but that the amount of this remote debris was insignificant compared to the total debris load. The response also stated that there was settling within a few inches of the strainers. The settling near the strainers was not described as insignificant. It is likely that some settling would occur under the strainers during a loss-of-coolant accident (LOCA). However, the extent of expected settling cannot be determined based on the information available. Ensuring that all debris collected on the strainers would have very likely resulted in a higher test head loss. The staff has not determined that the debris was prepared and introduced in a conservative manner, or that the flow around the test strainers was realistic or conservative when compared to the plant. As discussed in the staff trip report for observations of testing at the Alion Hydraulic Labs (ML071230203) the staff has observed settling that was considered to be nonprototypical of the plant condition. Without additional information regarding the Robinson testing, the staff cannot conclude that the plant-specific testing resulted in a prototypical or conservative result.
RAI10 Provide additional information on the vortex evaluation methodology so that the staff may fUlly evaluate the ability of the strainer system to prevent the formation of vortices.
Discussion: The staff requested additional information regarding the testing that was conducted to ensure that vortex formation would not occur during operation of the strainers. The licensee response provided additional information about the vortexing evaluation that was conducted for the strainers. Based on the information provided, the staff could not make a determination as to whether the vortexing evaluation was conducted conservatively. The evaluation seemed to consider the maximum approach velocity based on the entire surface area of the strainer. The staff believes that a more relevant velocity value would be the highest circumscribed velocity approaching the strainer. Some strainer-specific relationships were developed for the evaluation. Based on a review of the RAI response and the original supplemental response, the staff could not determine the basis for these relationships or whether they adequately modeled the potential for vortex formation. The staff was unable to understand the terms used for approach velocity in the explanation. The testing conducted with the vortex suppressors installed above the strainer seems to indicate that there is some margin to vortex
-5 formation, but adequate details on the testing were not provided. The staff could not determine whether the average approach velocity as used in the testing was a measure of the same term as used in the calculation. The staff could also not determine whether the test conditions were conservative with respect to the conditions in the plant.
RAI11 Provide the size distribution of the particulate insulation surrogate used during testing.
Discussion: The staff requested information regarding the size distribution of particulate debris added to the head loss tests. The licensee response provided additional information regarding the surrogate used for particulate debris in the head loss testing.
The information provided regarding the coatings and latent particulate surrogates was acceptable. However, no information was provided for the surrogate used for the particulate insulations. The supplemental response stated that the surrogate was destroyed as small fines. However, for particulate insulation this could be inferred to mean that the debris simply had to pass through a grating to qualify as small fines. The staff expects that any surrogate for Cal-Sil or similar particulate insulations would be rendered into powder prior to addition to the test loop.
RAI14 Provide justification that the debris bed head loss was not limited due to bed shifting or provide an evaluation based on the head loss that could occur should the head loss be limited as described above.
Discussion: The staff requested additional information regarding the potential for pressure-driven bed degradation limiting the head loss results attained during testing.
The licensee response did not directly address the question. The licensee provided information regarding the test results and stated that the strainer had adequate fibrous debris on it to capture chemical and particulate debris. The response stated that the exact mechanism that caused the head loss to reach a steady state, and not increase was beyond the scope of the testing conducted for the strainer. The staff believes that a possible cause of the relatively steady-state head loss is that the debris bed was only structurally sound enough to maintain a limited differential pressure. Based on the head loss graphs it is possible that head loss across the strainer reached this differential pressure threshold followed by debris bed shifting several times during the head loss tests. This phenomenon has been observed during several tests of Enercon strainers.
The debris bed shifting is not necessarily an undesirable occurrence. However, if the head loss is limited by debris bed shifting and is then corrected to a higher temperature, a nonconservative result will occur. The debris bed appeared to have a limited head loss of about 9-10 ft. The result of the test was corrected to about 2.57 ft at 212 of. If the debris bed head loss was limited due to bed shifting, head loss may have to reach 9-10 ft regardless of the fluid temperature prior to bed shifting. That is, the head loss may have been limited by a pressure-driven phenomenon that operates regardless of temperature.
Structural analysis RAI17 Provide an explicit and detailed summary of the quantitative results of the structural design qualification of the different components of the replacement sump strainer structural assembly (including trash racks, if any) under design loads and load combinations; and their comparison to the design code acceptance criteria. This
-6 information should include actual stresses, forces, displacements, etc. (as applicable),
the component material type, the corresponding code allowables, and interaction ratios for the limiting members of the various structural components of the replacement strainer assembly. Additionally, provide a more detailed list of all the components that were analyzed for structural adequacy, including a description of the components.
Discussion: The staff requested the licensee to provide a summary of the structural qualification results and design margins for the various components that make up the replacement sump strainers in order to verify the overall structural adequacy of the licensee's replacement strainers. In its response, the licensee provided a brief tabulated summary of the code allowable stress limits and margins of code/vendor design allowables to the ultimate strength for three broad component types of the replacement strainers, which include the structural steel, threaded stud and bolt, and concrete anchor bolts. The staff found the information provided in the RAI response to be a vague and generic repetition of design code provisions and that did not discuss code acceptance limits for different structural responses (e.g., axial, bending and shear) and their interactions. The licensee did not provide any quantitative results of structural design qualification of the different components of the strainer structural assembly and trash racks that would demonstrate to the staff the structural adequacy of components in relation to the corresponding code acceptance limits. In addition, the licensee's response did not address the additional structural components of the replacement strainers.
Net Positive Suction Head RAI16 Please address whether a single failure of a throttle valve to the open position in post-LOCA scenarios during recirculation where throttling credit is taken would result in increased flows through the residual heat removal (RHR) pump(s) that could result in a loss of net positive suction head margin or emergency core cooling system strainer structural limits being exceeded.
Discussion: The staff requested the licensee to identify the maximum sump flow rate for dual-train operation and to provide a basis for concluding that 3820 gpm is the bounding flow rate when both net positive suction head and debris bed head loss are considered. The supplemental response dated December 18, 2009, included the statement that throttling of the RHR heat exchanger outlet valves was credited in order to limit the RHR pump flow rates for some pump configurations during post-LOCA sump recirculation.
E. McCartney -2 Please contact me at 301-415-2788 if you have any questions on this issue, would like to participate in a conference call, or if you require additional time to submit your responses.
Sincerely, IRA!
Tracy J. Orf, Project Manager Plant Licensing Branch 11-2 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. 50-261
Enclosure:
As stated cc: Distribution via Listserv DISTRIBUTION:
PUBLIC LPL2-2 r/f RidsN rrDorILp/2-2 RidsNrrDssSsib RidsNrrPMRobinson RidsNrrLACSola RidsOgcRp RidsAcrsAcnw_MailCTR RidsRgn2MailCenter ADAMS Accession No* ML093310294 NRR-088 OFFICE LPL2-2/PM LPL2-2/LA DSS/SSI/BC LPL2-2/BC LPL2-2/PM NAME TOrf CSoia MScott
- TBoyce TOrf DATE 12/02/09 12/01/09 08/24/09 12/02/09 12/03/09
- Via email OFFICIAL RECORD COpy