TMI-11-075, Response to Request for Additional Information Related to Technical Specification Change Request No. 351 Maximum Allowable Power with Inoperable Main Steam Safety Valves
| ML111120003 | |
| Person / Time | |
|---|---|
| Site: | Three Mile Island  |
| Issue date: | 04/21/2011 |
| From: | David Helker Exelon Generation Co, Exelon Nuclear |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| TMI-11-075 | |
| Download: ML111120003 (9) | |
Text
10 CFR 50.90 TMI-11-075 April 21, 2011 u.s. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001 Three Mile Island Nuclear Station, Unit 1 Renewed Facility Operating License No. DPR-50 NRC Docket No. 50-289
Subject:
Three Mile Island Unit 1 Response to Request for Additional Information Related to Technical Specification Change Request No. 351: "Maximum Allowable Power with Inoperable Main Steam Safety Valves"
References:
(1) Letter from P. B. Cowan (Exelon Generation Company, LLC) to U.S. NRC, "Technical Specification Change Request No. 351: Maximum Allowable Power with Inoperable Main Steam Safety Valves" dated September 24, 2010 (2) Letter from P. Bamford (U.S. Nuclear Regulatory Commission) to M. J. PadUo (Exelon Generation Company, LLC), "Three Mile Island Nuclear Station, Unit 1
- Request for Additional Information Regarding License Amendment Request Proposing Changes to the Number of Required Operable Main Steam Safety Valves (TAC NO. ME4808)" dated February 14, 2011 (3) Letter from D. P. Helker (Exelon Generation Company, LLC) to U.S. NRC, "Three Mile Island Unit 1 Response to Request for Additional Information Related to Technical Specification Change Request No. 351: 'Maximum Allowable Power with Inoperable Main Steam Safety Valves'" dated March 18, 2011 (4) Letter from P. Bamford (U.S. Nuclear Regulatory Commission) to M. J. Pacilio (Exelon Generation Company, LLC), "Three Mile Island Nuclear Station, Unit 1
- Request for Additional Information Regarding License Amendment Request Proposing Changes to the Number of Required Operable Main Steam Safety Valves (TAC NO. ME4808)" dated March 25, 2011 By letter dated September 24, 2010 (Reference 1), Exelon Generation Company, LLC (Exelon),
requested an amendment to the Technical Specifications (TS) for Three Mile Island Nuclear Station, Unit 1 (TMI, Unit 1) to revise TS 3.4.1.2.3 to allow up to two (2) Main Steam Safety Valves (MSSVs) per steam generator to be inoperable with no required reduction in power level, and to revise the required maximum overpower trip setpoints for any additional inoperable MSSVs.
The USNRC staff formally requested additional information on February 14, 2011 (Reference 2) and Exelon submitted a response to the request on March 18,2011 (Reference 3). The USNRC staff provided draft RAI questions to Exelon on March 2, 2011 and then formally
U.S. Nuclear Regulatory Commission April 21, 2011 Page 2 requested the second round of additional information on March 25,2011 (Reference 4). Based on telephone conversations with the USNRC Project Manager, Exelon is submitting a partial response to the second round of USNRC questions in the Attachment to this letter. A supplemental response will be submitted to the USNRC by May 27,2011 finalizing Exelon's response to the second round of questions. The additional time to complete the RAI response is needed to procure AREVA analysis reports not previously provided to TMI, Unit 1 and to contract AREVA to generate additional plots/figures based on data not previously provided to TMI, Unit 1.
Exelon has determined that the information provided in response to this request for additional information does not impact the conclusions of the No Significant Hazards Consideration or Environmental Consideration as stated in Reference 1.
There are no regulatory commitments contained in this submittal.
In accordance with 10 CFR 50.91, "Notice for public comment; State consultation," paragraph (b), EGC is notifying the Commonwealth of Pennsylvania of this response by transmitting a copy of this letter and its attachment to the designated State Official.
Should you have any questions concerning this letter, please contact Ms. Wendy E. Croft at (610) 765-5726.
I declare under penalty of perjury that the foregoing is true and correct. Executed on the 21 st day of April, 2011.
Respectfully,
~~
David P. Helker Manager - Licensing and Regulatory Affairs Exelon Generation Company, LLC
Attachment:
Response to Request for Additional Information Related to Technical Specification Change Request No. 351 cc: USNRC Region I, Regional Administrator USNRC Project Manager, TMI, Unit 1 USNRC Senior Resident Inspector, TMI, Unit 1 Director, Bureau of Radiation Protection, PA Department of Environmental Resources Chairman, Board of County Commissioners, Dauphin County, PA Chairman, Board of Supervisors, Londonderry Township, PA R. R. Janati, Commonwealth of Pennsylvania
ATTACHMENT Response to Request for Additional Information Related to Technical Specification Change Request No. 351 Three Mile Island Nuclear Station, Unit 1 Renewed Facility Operating License No. DPR-50
Response to RAI Related to Technical Specification Change Request No. 351 Attachment Page 1 of 6 NRC Question 1 In the LAR the licensee states, "The design capacity of the MSSVs is based on the ASME
[American Society of Mechanical Engineers Boiler and Pressure Vessel] Code, Section 1/1."
Section 10.3.1.1 of the TMI-1 Updated Final Safety Analysis Report (UFSAR), states, "Safety valves on the main steam lines and branches to the emergency feedwater turbine are designed in accordance with ASME Code Section 1/1, Class A requirements." Further, UFSAR Section 10.7.4, "Overpressure Protection," refers to ASME Section III, 1989 edition for the determination of MSSV set pressures. The ASME Code, Section III, Paragraph NC-7410, "Set Pressure Limitations for Upset Conditions," states, "the stamped set pressure of at least one of the pressure relief devices [MSSVs] connected to the system shall not be greater than the design pressure of any component within the pressure retaining boundary of the protected system."
In the LAR, the licensee is requesting to revise TMI-1 TS 3.4.1.2.3 to allow TMI-1 to operate at full power with seven of the nine MSSVs operable per steam generator and at reduced power level with up to five MSSVs inoperable per steam generator. In Attachment 3 of the LAR, Section 7.0, the licensee provides suggested changes to TS 3.4.1.2.3, as determined by the AREVA analyses which is included in the LAR, based on the number of inoperable MSSVs.
The AREVA analysis includes a proposed conditional statement to ensure that the TS revision for TMI-1 would continue to meet the requirements of ASME Section III, NC-7410.
The proposed TS revision in the LAR does not include this conditional statement. The licensee states, "TMI has not adopted the suggested change in 86-9054640-002- Section 7.0, which states, "At leas[t] one (1) OPERABLE MSSV on each steam generator must have a nominal lift setpoint S 1050 [pounds per square inch gauge or psig]." The reason provided in the application is, in part, that the main steam piping is designed to [United States of America Standard or USAS] B31.1 and that there is no requirement for B31.1 piping to have at least one safety valve set at, or below, the piping design pressure.
The NRC staff requests that the licensee explain in more detail why the AREVA recommendation was not included in the application. This explanation should specifically address the requirements for the ASME Section 1/1 designed branches to the emergency feedwater pump turbine, as well as the ASME Section 1/1 design code for the MSSVs themselves, as described in the UFSAR. Also, the staff requests that the licensee provide a basis for the acceptance criteria that peak steam line pressure remains below 110 percent (%)
of 1050 psig.
Three Mile Island, Unit 1 (TMI, Unit 1) Response The conditional statement recommended in Reference 1, Attachment 3, Section 7.0, by AREVA "At least one (1) OPERABLE MSSV on each steam generator must have a nominal lift setpoint S 1050 psig" was not included in the application because it is not necessary to ensure compliance with the codes. The ASME Boiler and Pressure Vessel Code, Section 1/1 (ASME Section III) requirement to have at least one valve set below the design pressure of the component being protected is only applicable to the Once Through Steam Generators (OTSGs),
and is satisfied because ALL of the main steam line valves are set below the design pressure of the OTSGs (1150 psig). There is no similar requirement in United States of America Standard Power Piping Code B31.1 (USAS B31.1). Therefore, there is no requirement for at least one operable MSSV to be set below the design pressure of the main steam line piping, 1050 psig.
Response to RAI Related to Technical Specification Change Request No. 351 Attachment Page 2 of 6 There is no main steam system piping, including the branches to the emergency feedwater pump turbine, designed to ASME Section III. The TMI, Unit 1 Updated Final Safety Analysis Report (UFSAR) Section 10.3.1.1 states, "The main steam lines from the steam generator, out through containment up to and including the main steam isolation valves, are designed fabricated erected and inspected in accordance with the Power Piping Code USAS 831.1.0, seismic Category I. This includes piping to the emergency feedwater pump turbine." The TMI, Unit 1 UFSAR Section 10.3.1.1 statement, "Safety valves on the main steam lines and branches to the emergency feedwater turbine are designed in accordance with ASME Code Section III, Class A requirements" clarifies that the safety valves on the main steam lines and the safety valves on the branches to the emergency feedwater turbine are designed to ASME Section Ill.
The OTSGs are designed to ASME Section III. For overpressure protection, maximum pressure may not exceed the design pressure by more than 110%. The design pressure of the OTSGs is 1150 psig.
The code requirement for the main steam line piping is USAS 831.1 which allows pressure to exceed the design pressure by 20% for events which would occur less than 1% of the operating period. The design pressure of the main steam line piping is 1050 psig.
The acceptance criteria used in the Reference 1, Attachment 3 analysis (110% of 1050 psig or 1155 psig) is conservative. This limit ensures that neither the ASME Section" I code requirement for the OTSGs (110% of 1150 or 1265 psig) nor the USAS 831.1 code requirement for the main steam line pressure (120% of 1050 or 1260 psig) is exceeded.
NRC Question 2 of the LAR provides the AREVA analyses for all of the scenarios involving certain MSSVs inoperable at various power levels. The NRC staff requests that the licensee identify the basis for, and assumptions used, in the analysis for drift tolerance for each of the MSSV pressure setpoints. Also, the staff requests a description of the valves' accumulation behavior as illustrated in Figures 4-16 through 4-19, and clarify whether valve lift setpoint uncertainty and valve accumulation uncertainty are considered independently.
TMI, Unit 1 Response The Reference 1, Attachment 3 analysis models the MSSVs to lift at 3% above their nominal setpoints to account for setpoint uncertainty. The allowable as-found setpoint of (+/-) 3%
controlled by TMI, Unit 1 inservice testing (1ST) procedure 1303-11.3 "Surveillance Test and Set Main Steam Safety Valves." The TMI, Unit 1 Technical Specifications Section 4.2.2 states that "1ST of ASME Code Class 1, Class 2 and Class 3 pumps and valves shall be performed in accordance with the ASME Code for Operation and Maintenance of Nuclear Power Plants (ASME OM Code) and applicable Addenda as required by 10 CFR 50, Section 50.55a, except where specific written relief has been granted by the NRC." The ASME OM Code provides the direction that 3% of the valve nameplate setpoint is an appropriate acceptance criterion for as-found drift tolerance.
There is 3% accumulation built into the model. The valves are modeled to pop partially open at 103% of their nominal setpoints, and travel to fUlly open at 106% of their nominal setpoints.
Response to RAI Related to Technical Specification Change Request No. 351 Attachment Page 3 of 6 Valve lift setpoint uncertainty and valve accumulation uncertainty are considered independently, as describe above.
NRC Question 3 In Section 4.1 of Attachment 3 in the LAR, AREVA provides an analysis of allowing TMI-1 upset condition (turbine trip) at full power (100% with a 2% uncertainty, hence 102%) with two MSSVs out of service. The licensee indicates that the most limiting condition occurs on steam generator 'B' steam line 4, with the following two MSSVs out of service: MS-V17D and MS-V20D, each with lift setpoint pressures of 1050 psig. On Figure 4-13, AREVA shows the maximum pressures reached in the main steam lines were 1159.06 psia [pounds per square inch absolute] (1144.36 psig), in steam lines 3 and 4, and approximately 1140 psia (1125.3 psig), in steam lines 1 and 2. Also, in Figure 4-13, the maximum steam line pressure for all four steam lines appear to exceed 1115 psia (1100 psig) prior to 3 seconds during the turbine trip transient.
Regarding Figures 4-16, 4-17, and 4-18, MS-V19A with a setpoint of 1080 psig on steam line 1; MS-V20B with a setpoint of 1092.5 psig on steam line 2; and MS-V20C with a setpoint of 1092.5 psig on steam line 3, respectively, are depicted as not lifting in response to the turbine trip transient when the maximum pressures for steam lines 1, 2 and 3 are reached. Additionally on Figures 4-16 through 4-19, the following MSSVs are depicted as lifting well after 3 seconds into the transient:
- MS-V18A with a setpoint of 1060 psig on steam line 1
- MS-V18B with a setpoint of 1060 psig and MS-V19B with a setpoint of 1080 psig on steam line 2
- MS-V18C with a setpoint of 1060 psig and MS-V19C with a setpoint of 1080 psig on steam line 3
- MS-V18D with a setpoint of 1060 psig on steam line 4 Figure 4-13 appears to contradict Figures 4-16 through 4-19 in regards to the time in which the MSSVs actuate during the transient.
The NRC staff requests that:
- a. The licensee clarify whether MSSVs MS-V19A, MS-V20B, and MS-V20C open in response to the maximum steam line pressures exceeding the safety valves' setpoints during a turbine trip transient.
- b. The licensee clarify the times in which the other MSSVs, as described above, lift open in comparison to the pressure in each steam line during the turbine trip transient.
- c. For the limiting case, steam generator exit pressure is provided in Figure 4-13. The NRC staff requests a plot showing the pressure in the control volume for each steam line representing the pressure at the safety valves, and the parameters used to derive these values.
- d. Please provide a nodalization diagram of the main steam system, indicating the location of each safety valve.
Response to RAI Related to Technical Specification Change Request No. 351 Attachment Page 4 of 6 TM!, Unit 1 Response Reference 1, Attachment 3, Figure 4-13 plots the maximum pressure in each steam line, which occurs at the outlet from the OTSG. Each MSSV is modeled to open at 103% of nominal setpoint based on the pressure in the steam line at the MSSV. The pressure at the MSSV is lower than the pressure shown on Figure 4-13 due to the pressure drop from flow through the steam lines. The magnitude of the pressure drop through the steam lines is a function of the flow rate through the steam lines, and therefore varies for each steam line over the course of the transient.
For example, MS-V18A is shown as lifting at approximately 4 seconds on Reference 1, , Figure 4-16. On Reference 1, Attachment 3, Figure 4-13, at approximately 4 seconds 'B' OTSG (steam lines 3 and 4) outlet pressure is approximately 1120 psia. MS-V18A nominal setpoint is 1060 psig. MS-V18A is modeled to lift partially at 1107 psia (i.e., 103% of nominal). The pressure drop and readability of the graphs accounts for the difference. This assessment validates the description of the model.
In summary, the maximum steam line pressure might exceed the MSSV setpoint and the MSSV would still be closed because of the pressure drop through the steam lines. The pressure in the steam line at the MSSVs does not exceed the modeled setpoint for MSSVs MS-V19A, MS-V20B, or MS-V20C. Consequently, these MSSVs do not open although they are available and functional for additional steam relief.
In addition to the comprehensive response above, TMI, Unit 1 will provide a plot of the pressure in the control volumes at the safety valves and a nodalization diagram in the supplemental response that will be submitted to the NRC by OS/27/11.
NRC Question 4 Section NC-7511 of the ASME Section III Code states that the total rated relieving capacity shall be sufficient to prevent a rise in pressure of more than 10% above the design pressure of any component with the pressure retaining boundary of the protected system under any system upset conditions. In the TMI-1 LAR, the licensee states that the design capacity for the MSSVs is sufficient to limit secondary system pressure to less than or equal to 110% of design pressure when passing 100% of design steam flow. The main steam line design pressure for TMI-1 is 1050 psig (1064.7 psia); hence 110% of design pressure is 1155 psig (1169.7 psia).
In Section 4.1 of Attachment 3 in the LAR, AREVA provides an analysis of allowing TMI-1 upset condition (turbine trip) at full power with two MSSVs out of service. The licensee indicates that the most limiting condition occurs on steam generator 'B' steam line 4, where the MS-V17D and MS-V20D MSSVs are located, each with lift setpoint pressures of 1050 psig (1064.7 psia). The analysis shows the maximum steam line pressure does not exceed the limit of 1169.7 psia.
The AREVA analysis in Section 4.2 of Attachment 3 provides the results of three MSSVs (MS-V17D, MS-V18D, and MS-V20D) being inoperable in steam line 4 at full power. The analysis determines that this condition would exceed the maximum allowable steam line pressure limit of 1169.7 psia.
However, the NRC staff postulates that a more limiting condition could possibly exist with MS-V17B and MS-V18B MSSVs inoperable on steam line 2. Figure 4-17 in Attachment 3 depicts
Response to RAI Related to Technical Specification Change Request No. 351 Attachment Page 5 of6 MSSV MSN20B (setpoint of 1092.5 psig) not lifting during the transient. Based on AREVA's method of making the two lowest MSSVs in the steam line inoperable, the remaining relief capability on steam line 2 may be less than that of steam line 4 determined by AREVA as the most limiting case. Therefore, in comparison to the analysis in Section 4.2 showing only one MSSV lifting (MS~ V19B lifts with a setpoint of 1080 psig) on steam line 2, the maximum pressure reached could exceed the allowable steam line pressure limit at full power.
Therefore, the NRC staff requests that the licensee provide the results of the sensitivity study, as well as the inputs and assumptions, used to determine how the MS~V17D and MS~V20D MSSVs were selected as the most limiting case for the turbine trip transient.
TMI, Unit 1 Response For clarification, Reference 1, Attachment 3, Section 4.1 explains the sensitivity evaluation and analysis performed to determine the limiting configuration. The maximum reactor coolant system, OTSG, and main steam line pressures with MS~V17D and MS~V20D inoperable on steam line 4 were compared against the pressures for the case where MS~V17B and MS~V18B were inoperable on steam line 2 in a separate analysis. The sensitivity cases were analyzed and the submitted amendment request (Reference 1) is based on the most limiting case. The values for the more limiting case, MS~V17D and MS~V~20D inoperable, are reported in Reference 1, Attachment 3, Table 4~ 1. The results for the case with MS~ V17B and MS~ V18B inoperable were not provided despite being analyzed against the most limiting case.
In addition to the comprehensive response above, TMI, Unit 1 will provide plots for the case with MS~V17B and MS~V18B out of service in the supplemental response that will be submitted to the NRC by OS/27/11.
NRC Question 5 In Attachment 3 of the LAR, Section 5.3 evaluated a plant condition with four MSSVs inoperable at 62% power level. However, Table 4 in the proposed LAR shows in the "Analyzed Power Level (% of 2772 MWt)" column that TMI~1 can operate at a power level of 77% with four MSSVs per steam generator inoperable. In Section 5.5 of the AREVA report included with the application, the 77% case is discussed, referring to a separate AREVA analysis, which is not included in the application. The NRC staff requests that the licensee provide the analysis of four MSSVs inoperable per steam generator at 77% power for confirmation that the analysis supports the requested TS changes.
TMI, Unit 1 Response For clarification, Reference 1, Attachment 3, Section 5.2 documents that a maximum main steam pressure of 1187.43 psia was reached for the case of four MSSV's inoperable at 92%
power. As clarified by RAI 1 above, the USAS B31.1 code requirement for the main steam line pressure is 120% of 1050 or 1260 psig. Therefore, at 92% power with four MSSVs inoperable, the design code requirements of USAS B31.1 are satisfied to ensure piping integrity.
A proposed reactor power limit of 77% power is conservative. The Reference 1, Attachment 3, AREVA analysis developed this reactor power limit value to ensure the maximum main steam line pressure remained below 1169.7 psia using case results with four inoperable valves at 62%
Response to RAI Related to Technical Specification Change Request No. 351 Attachment Page 6 of6 and 92% initial reactor power assuming a linear relationship between maximum pressure and reactor power at constant relief capacity.
The use of the linear interpolation is not required because all code requirements are met with four MSSV's inoperable at 92% power. The proposed limit at 77% is conservative.
In addition to the comprehensive response above, TMI, Unit 1 will provide the analysis of the limiting four MSSVs out of service on an OTSG at 77% power in the supplemental response that will be submitted to the NRC by OS/27/11.
References
- 1. Letter from P. B. Cowan (Exelon Generation Company, LLC) to U.S. NRC, "Technical Specification Change Request No. 351: Maximum Allowable Power with Inoperable Main Steam Safety Valves" dated September 24, 2010