ML082730087
| ML082730087 | |
| Person / Time | |
|---|---|
| Site: | Watts Bar  |
| Issue date: | 09/26/2008 |
| From: | Riedl C Tennessee Valley Authority |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| TAC MD8395, TS-07-18 | |
| Download: ML082730087 (7) | |
Text
September 26, 2008 TS-07-18 10 CFR 50.90 U. S. Nuclear Regulatory Commission Mail Stop: OFWN P1-35 ATTN: Document Control Desk Washington, D.C. 20555-0001 Gentlemen:
In the Matter of ) Docket No. 50-390 Tennessee Valley Authority )
WATTS BAR NUCLEAR PLANT (WBN) UNIT 1 - RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION REGARDING ALLOWABLE VALUE FOR CONTAINMENT PURGE EXHAUST RADIATION MONITORS (TAC NO. MD8395)
References:
- 1. August 1, 2008 Letter from NRC to TVA - Request for Additional Information Regarding Allowable Value for Containment Purge Exhaust Radiation Monitors (TAC No. MD8395)
- 2. March 27, 2008 Letter from TVA to NRC - Technical Specifications (TS) Change - TS-07 Revision of the Allowable Value for the Containment Purge Exhaust Radiation Monitors - Limiting Condition for Operation (LCO) 3.3.6, Containment Vent Isolation Instrumentation This letter responds to NRCs request for additional information provided in Reference 1, concerning TVAs request to change WBNs TS as submitted in Reference 2.
There are no regulatory commitments associated with this submittal.
U.S. Nuclear Regulatory Commission Page 2 September 26, 2008 If you have any questions concerning this matter, please call me at (423) 365-1742.
I declare under penalty of perjury that the foregoing is true and correct. Executed this 26th day of September, 2008.
Sincerely, Original signed by C. J. Riedl Supervisor, Site Licensing Enclosure cc (Enclosure):
NRC Resident Inspector Watts Bar Nuclear Plant 1260 Nuclear Plant Road Spring City, Tennessee 37381 ATTN: Patrick D. Milano, Project Manager U.S. Nuclear Regulatory Commission Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation MS O-8 H4 Washington, DC 20555-0001 U.S. Nuclear Regulatory Commission Region II Sam Nunn Atlanta Federal Center 61 Forsyth St., SW, Suite 23T85 Atlanta, Georgia 30303 Mr. Lawrence E. Nanny, Director Division of Radiological Health 3rd Floor L & C Annex 401 Church Street Nashville, Tennessee 37243
ENCLOSURE Response to NRC Request for Additional Information Allowable Value for Containment Purge Exhaust Radiation Monitors Watts Bar Nuclear Plant, Unit 1 Tennessee Valley Authority Docket No. 50-390 By letter dated March 27, 2008 (Agencywide Documents Access and Management System Accession No. ML080870349), Tennessee Valley Authority (TVA) proposed a change to the Watts Bar Nuclear Plant Unit 1 (WBN 1) Technical Specifications (TSs).
The proposed change would revise the allowable value (AV) listed for Function 3, Containment Purge Exhaust Radiation Monitors, in Table 3.3.6-1, Containment Vent Isolation Instrumentation.
By letter dated August 1, 2008 (ADAMS Accession No. ML082130424), the NRC staff requested additional information. NRCs questions and TVA responses follow:
- 1. On page E1-3 of the Enclosure to the application, TVA states:
Because the monitors are not used to protect the reactor fuel or the Reactor Coolant System (RCS) safety barriers, the AV specified in the TS for this device is not a 10 CFR 50.36 limiting safety system setting.
The Nuclear Regulatory Commission staff does not agree with this statement, in that Title 10 of the Code of Federal Regulations (10 CFR) Section 50.36(d)(1)(ii)(A) states that Limiting safety system settings for nuclear reactors are settings for automatic protective devices related to those variables having significant safety functions. The instrumentation required by the TSs has been designed to assure that the applicable safety analysis limits will not be exceeded during anticipated operational occurrences and the settings must initiate automatic protective actions consistent with the design basis. Since the function of the containment purge exhaust radiation monitors is used to limit the radiological consequences of a design-basis small-break loss-of-coolant accident to within the limits of 10 CFR Part 100, Reactor Site Criteria, they perform a significant safety function. However, the NRC staff agrees that this function is not used to protect the reactor fuel or the RCS safety barriers, and therefore, does not have a safety limit limiting safety system setting (SL-LSSS).
TVA should revise its statement.
TVA Answer:
TVA will revise this statement to read:
E-1 of 5
Because the monitors are not used to protect the reactor fuel or the Reactor Coolant System (RCS) safety barriers, the AV specified in the TS for this device is not a 10 CFR 50.36 safety limit limiting safety system setting.
- 2. Provide a discussion, including the calculation, of the setpoint calculation methodology used for establishing the limiting trip setpoint (or the nominal trip setpoint) and the instrument setting tolerance (a range of values around the limiting trip setpoint) for the as-found and as-left setpoints as measured in periodic surveillance testing. Indicate the analytical limits and other limiting design values (and the sources of these values) for the limiting trip setpoint and setting tolerance range, including expected uncertainties in the instrument setpoint determination.
TVA Answer:
TVAs setpoint calculation methodology is defined in Technical Instruction EEB-TI-28, Setpoint Calculations, and is based on ISA-RP67.04.02-2000, Methodologies for the Determination of Setpoints for Nuclear Safety Related Instrumentation. Using this methodology, the limiting trip setpoint (LTSP), the acceptable-as-left (AAL) and acceptable-as-found (AAF) calibration tolerances, and the allowable value (AV) for the containment purge radiation monitors were determined in calculation 1-RE-90-130, Demonstrated Accuracy Calculation for Containment Building Purge Air Exhaust Monitors. As described in TVAs March 27, 2008 TS change request, the actual or nominal trip setpoint (NTSP) is determined by the plant Chemistry staff in accordance with the Offsite Dose Calculation Manual prior to every purge of containment.
The analytical limits (AL) for the containment purge exhaust radiation monitors were established in calculation WBNAPS3079, Safety Limit for the Containment Purge Exhaust Monitors. The calculation provides two analytical limits, both of which ensure that the monitors will isolate the containment purge exhaust before exceeding 10 percent of the offsite dose limits for a postulated small-break loss-of-coolant accident (LOCA). An AL of 1.87E-01 Ci/cc is applicable during movement of irradiated fuel assemblies within containment and credits the purge exhaust filters. The current TS AV of 8.41E-02 Ci/cc is based on this AL. A second AL of 4.3E-02 Ci/cc applies in Modes 1-4 and does not credit the filters.
The AV of 2.8E-02 Ci/cc in the proposed change is based on this second AL.
A conservative LTSP was selected for each case based on the AL, the required accuracy for the function and process uncertainty. Using the case for which the purge filters are not credited (the same methodology applies to both cases), the LTSP was determined as follows:
LTSP = {AL/(1 + total error/100)}/CF LTSP = (0.043 µCi/cc/2)/1.13 = 0.019 µCi/cc where:
E-2 of 5
Total Error = 100% or factor of 2. Total error is the required accuracy which was established in calculation WBNTSR038, Required Response Time, Range and Accuracy for the Containment Purge Exhaust Radiation Monitors. CF is a correction factor to compensate for pressure differences between the process and the detector. The correction factor was determined using guidance provided by the radiation monitor supplier.
As noted in TVAs March 27, 2008 TS change request, the methodology used to determine the AV is a conservative variation of method 3 of ISA-RP67.04.02-2000. This methodology results in an AV which is large enough to allow for drift and other uncertainties which are measurable during a surveillance test, but is small enough, considering the unmeasurable uncertainties, to ensure that the actuation will occur before the AL is exceeded. A summary of the calculation of the AV, as described in TVAs March 27, 2008 TS change request, is provided below:
The AV is determined based on the following equation:
(AL - (Adbe - Anf)) > AV > (SP + Anf) 6.055 - (0.499 - 0.242) > AV > (5.465 + 0.242) 5.798 V > AV > 5.707 V or 3.01E-02 µCi/cc > AV > 2.65E-02 µCi/cc where:
Adbe = design basis accident accuracy, the total uncertainty including both measurable and unmeasurable errors, Anf = normal measurable accuracy (AAF), and SP = limiting trip setpoint (LTSP)
The AV specified in TS Table 3.3.6-1 is set half way between these two values.
This assures that there is additional margin for non-statistical unmeasurables.
The Containment Purge exhaust radiation monitors initiate containment vent isolation to assure that releases are well within 10 CFR 100 limits. Because the monitors are not used to protect the reactor fuel or the Reactor Coolant System safety barriers, the allowable value specified in the TS for this device is not a 10 CFR 50.36 safety limit limiting safety system setting.
The values applicable to the two cases described above are summarized below.
The Ci/cc values are converted to an equivalent voltage for calibration measurements (the bistable output is 0-10 V), based on the following conversion equations from TVA calculation 1-RE-90-130, Rev. 15 Demonstrated Accuracy Calculation for Containment Building Purge Air Exhaust Monitors:
[((6/10)*(Volts))+1]
CPM = 10 or Volts = 10/6*[Log(CPM) - 1]
and E-3 of 5
1 CPM = 1E6 µCi/cc Purge Filters Credited Purge Filters Not Credited Ci/cc volts Ci/cc volts AL 1.87E-01 7.12 4.3E-02 6.055 AV 8.41E-02 6.541 2.8E-02 5.745 LTSP 5.0E-02 6.165 1.9E-02 5.465 AAL 1.01E-05 +/-0.1 1.01E-05 +/-0.1 AAF 1.39E-05 +/-0.242 1.39E-05 +/-0.242 The AAL value was established as 1% of the full-scale output of the bistable.
AAL = 0.01
- 10 V = 0.1 V The AAF value was calculated using the following equation:
2 2 2 2 1/2 AAF = (Ebs + OCTe + AAL + Ebd )
2 2 2 2 1/2 AAF = (0.007 + 0.01 + 0.1 + 0.22 )
AAF = +/-0.242 V where:
Ebs = Bistable Reference Accuracy, OCTe = Output Test Instrument Calibration Accuracy, and Ebd = Bistable Drift Accuracy TVAs March 27, 2008 TS change request TS-07-18 submittal identified the channel operational test procedures and calibration procedures which verify that the bistable performance meets the acceptance criteria for the AAL, AAF, and AV.
- 3. Since this setpoint has been determined not to be SL-LSSS, describe the measures to be taken to ensure that the associated instrument channel is capable of performing its specified safety functions in accordance with applicable design requirements and associated analyses. Include information on the controls TVA will employ to ensure that the as-left trip setting (after completion of periodic surveillance) is consistent with the setpoint methodology and calculation.
Also, discuss the plant corrective action processes (including plant procedures) for restoring channels to operable status when channels are determined to be inoperable or operable but degraded. If the controls are located in a document other than the TSs (e.g., plant test procedure), describe how implementation of the controls is ensured.
E-4 of 5
TVA Answer:
The results of the engineering calculations for analytical limits and demonstrated accuracy are applied to the development of the Nuclear Engineering Setpoint and Scaling Documents (in this case NE SSD 1-R-90-130 and NE SSD 1-R-90-131).
Values from the NE SSDs are then used to develop the site SSDs, which establish the loop setpoints, accuracy, and tolerances for the various elements of the instrument loop. For containment purge monitors, these are SSD 1-LPR-90-130 and SSD 1-LPR-90-131. Surveillance instructions 1-SI-90-5, 1-SI-90-6, 1-SI-90-7, and 1-SI-90-8 govern the quarterly and 18-month surveillances required by TS, and verify that the rad monitors are within specified tolerances and accuracies of the nominal setpoints established in accordance with the Offsite Dose Calculation Manual (ODCM). When out of specification values are found, SIs specify recalibration, and when calibration is unsuccessful, instrument maintenance personnel notify their supervision, who will initiate the appropriate corrective action in accordance with TVAs Corrective Action Program. In addition, the surveillance instructions and TI-49, Compliance Instruments identify these Containment Purge Radiation Monitors as Compliance Instruments. SPP-6.7, Instrument Setpoint, Scaling and Calibration Program, requires documentation and evaluation of out-of-calibration compliance instruments in accordance with SPP-3.1, Corrective Action Program, including consideration of effects of the condition on plant surveillance, inspection, and test requirements.
In addition to the allowable value definition in the SSDs, Procedure 1-ODI-90-15, Containment Purge Release, contains a limitation that the nominal setpoints for 1-RE-90-130 or 1-RE-90-131 cannot exceed the Limiting Trip Setpoint, 1.9E-02 Ci/cc, at all times, which will ensure that the radiation channels will trip before the process variables exceed the analytical limit.
E-5 of 5