ML20086D816
| ML20086D816 | |
| Person / Time | |
|---|---|
| Site: | Wolf Creek  |
| Issue date: | 11/07/1991 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20086D805 | List: |
| References | |
| NUDOCS 9111260270 | |
| Download: ML20086D816 (8) | |
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NO. 52 TO FACILITY OPERATING LICENSE NO. NPF-42 WOLF CREEK NUCLEAR OPERATING CORPORATION WOLF CREEK GENERATING STATION DOCKET NO. 50-482
1.0 INTRODUCTION
By application dated June 11, 1991, as supplemented by letters dated August 30, 1991, and September 20, 1991, Wolf Creek Nuclear Operating Corporation (the licensee) requested changes to the Technical Specifications (Appendix A to Facility Operating License No. NPF-42) for the Wolf Creek Generating Station.
The proposed changes would revise Technical Specification Tables 2.2.1, 4.3.1, and associated Bases to reflect the replacement of the existing resistant temperature detector (RTD) bypass system with an RTD thermowell system.
This design modification is intended to overcome drawbacks associated with the RTD bypass system such as pot 9ntial reactor coolant leakage from system components and radiation exposure of personnel performing work in proximity to the RTD bypass piping.
The engineering design and portions of the actual plant modi-fication are being provided by Combustion Engineering (CE) which has performed similar modifications at the Salem and Callaway plants.
The August 30, 1991, and September 20, 1991, submittals provided additional clarifying information and did not change the initial no significant hazards consideration determination.
2.0 BACKGROUND
2.1 Current Method The current coolant temperature measurement system design measures the primary coolant loop temperature by diverting a portion of the reactor coolant into the bypass manifolds.
The bypass manifolds utilite direct immersirn RTDs to measure the reactor coolant system (RCS) hot and cold leg coolant temperatures.
These measurements are input to the protection and control system logics which also calculate average and differential temperatures.
The currently installed bypass system is designed to account for streaming (non-uniform stratified temperatures) in the hot legs by having three hot leg sampling scoops, 120 degrees apart, protrude into each hot leg.
Each scoop has five holes which sample the hot leg flow along the leading edge of the scoop.
The flows from each hole mix to provide an average scoop temperature and then the flows from each scoop are combined to provide a representative average temperature for each hot leg.
The cold leg temperature is measured in a similar manner except 9111260270 9)1107 PDR ADOCK 05000402 P
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l. a single sample line is provided and a scoop does not protrude into the cold leg piping.
The cold leg sampling method is less complicated due to the uniforni temperature profiles in the pipe due to the mixing provided by the reactor coolant pumps.
After the temperatures are measured by the direct immersion RTDs in the hot and cold leg manifolds, the samples are returned to the RCS via a return line which penetrates the crossover piping between the steam generator and reactor coolant pump.
The bypass system valves and socket-welded pipes act as crud traps which increase the radiation exposure to personnel during plant maintenance and surveillance activities being conducted on both the bypass system and other nearby systems or components.
The bypass system also increases plant startup delays because of valve and flange leakage and potential RTD Swagelock fitting leakage and potential flow problems associated with the bypass system valves.
Therefore, due to the maintenance and ALARA ("as low as is reasonably acheivable" radiation doses) concerns, the bypass system is being removed and the RTDs are being housed in thermowells which are mounted directly into the RCS hot and cold legs.
2.2 New Method The proposed method for measuring hot and cold leg temperatures uses narrow-range, dual element, fast resp 3nse RTDs manufactured by the Weed Company.
One element of each RTD is used while the other is provided as an installed spare.
The RTDs are placed in thermowells to allow replacement without draindown of the RCS.
Three RTDs for each hot leg will be located within the existing sampling scoops.
Outlet ports will be added to each scoop to direct the flow past the thermowell and the sensing element of the RTD located inside.
The temperature measurement of the three RTDs in each hot leg are averaged electron-ically to arrive at a representative loop hot leg temperature.
A single dual-element RTO is planned for a thermowell in each cold leg since compensation for temperature streaming is not required.
3.0 EVALUATION The evaluation of the proposed replacement of the RTD bypass system with the thermowell system requires consideration of the potential impacts on measure-ment accuracies, system response time, system testing, system qualification (functional, seismic, and environmental), RTO failure detection ability, protection system setpoint determinations, safety ar alysis assumptions, and radiation exposure to plant personnel.
Each of these areas are addressed in the evaluations provided below.
3.1 Measurement Accuracies The new method of measuring each hot leg temperature with three RTDs installed in thermowells and obtaining an average by using electronic components has been analyzed to be slightly more accurate than the existing RTDs and the bypass system.
The accuracy of the Weed RTDs has been verified by CE to be an improve-ment over the existing RTDs.
This accuracy includes errors from hysteresis, repeatability, and drift over a 24 month period.
The measurement uncertainty I
for each hot leg RTD is also reduced in importance due to the averaging of the three RTDs to calculate cach hot leg temperature.
The additon nf electronic components to perform the averaging was accounted fcr in the uncertainty CdlCulations.
The accuracy determinations also accounted for temperature streaming and the limitations of the scoop and thermowell arrangeme ;.
A model test has been completed and calculations performed to ascertain that an accurate mixed mean temperature will be measured.
The model test prcvided information for the selection of the proper location of the RTD sensor in the scoop for accurate measurement and the expected temperature bias.
The licensee has committed to obtain confirmatory information on the mixed mean temperature accuracy.
This will be done by comparing pre-installation and post-installation calorimetric data for the RTD measurements at Wolf Creek for match;ng operating conditions.
As stated in the August 30, 1991 letter, the licensee will make this data available to the staff.
3.2 System Response Time The overtemperature delta-T (OTDT) reactor trip function response time is the time lag from when the hot leg temperature reaches trip conditions at the scoop until the control rods start to orop into the core.
As shown in Table 1, the OTDT response time for the proposed system has some gains and losses compared to the existing system, but the total response time of the proposed system is improved over the existing system (5.5 seconds vs. 6.5 seconds).
As shown in Table 1, the testable time delay Technical Specification (TS) limit is 6.0 seconds.
The testable time delay (excludes transport delay and thermal lag) for the existing system is 4.5 seconds and 5.25 seconds for the proposed system.
This makes the proposed testable system response time for the thermowell installed system slightly (0.75 seconds) longer than the existing system.
- However, the total time delay for the proposed system is compensated for by a reduction in the loop and scoop transient thermal lag response time, resulting in a lower first order lag for the proposed system versus the existing system (5.0 seconds vs. 6.0 seconds).
The electronic delay is 0.5 seconds for each system and therefore the total time delay for the thermowell system is 5.5 seconds which is less than the 6.5 second delay associated with the bypass system.
The response time used in the safety analysis is 8.0 seconds which provides ample margin between the safety analysis and the TS values.
3.3 System Testing The response time of the thermowell RTDs will be tested in place by using a Loop Current Step Response (LCSR) test during each refueling outage interval in accordance with TS 4.3.1.2.
The allocated response time for the RTDs includes a 10 percent error allowance for LCSR testing.
The LCSR method of response time testing is recommended by the NRC as detailed in NUREG-0809, " Safety Evaluation Report Review of Resistance Temperature Detector Time Response Characteristics." The LCSR method of response time testing uses an external electrical current (20-40 ma) to heat the RTD element and the temperature transient in the element is recorded.
From this transient, the response of
the RTD to changes in external temperature is inferred.
The Weed RTD is capable of being tested by the in-situ LCSR method and a continuous current of 20-40 ma will not damage the RTD.
3.4 System Qualification The proposed system will use a 4-wire Weed model N9004 RTD, fast response dual element RTD/thermowell assembly with the required fittings and sealed flexible tubing necessary to mate up with the quick disconnect assembly, junction boxes, splices, field cables, and containment penetration module assemblies.
It will be connected by field wiring to the existing 7300 process cabinets.
All of the above components and systems have been qualified pursuant to the requirements of the applicable IEEE standards and 10 CFR 50.49, 3.5 RTD Failure Detection A failed RTD would be detected by the loop delta-T versus auctioneered (high) delta-T alarm and/or the loop average temperature versus auctioneered (high) average temperature alarm.
In addition, each channel is checked once per 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> as required by TS 4.3.1.1.
When a failed RTD element is identified, the Action Statement would require the protection channel associated with the fa1:ed RTD to be placed in the trip condition.
Since the OTDT function utilizes a 2-out-of-4 logic, the failed RTD would not prevent safe operation or a safe shutdown of the plant.
The second element of each RTD is an installed spare which is connected to the master test cards in the 7300 process cabinets; therefore, this facilitates changing to the spare element as well as minimizing the tir,e that one channel would be in a tripped condition.
3.6 Protection System Setpoints Setpoint calculations were made for the OTOT and the overpuwer delta-T (0PDT) reactor trip functions using previously approved setpoint methodologies to secount for the new hot leg RTDs and the acced electronics.
The setpoint calculations also accounted for the various process uncertainties such as temperature streaming in the hot legs that could not be totally compensated for by the physical arrangement of the RTD thermowells.
Although the setpoints for the OTDT and OPDT functions did not change, the revised setpoint calculations did result in changes to the allowable values, sensor error and "Z" values provided in TS Table 2.2-1, " Reactor Trip System Instrumentation Trip Setpoints."
In addition to the protection system setpoints, the proposed changes to the RCS temperature measurecient system has the potential to affect the accuracy of the calorimetric measurements of reactor power and reactor coolant flowrate.
The revised uncertainties associated with the proposed RTDs were incorporated into an evaluation of the power and flow uncertainties and it was determined that the existing allowances remain conservative for the RTD thermowell system.
l
- 3. 7 Safety Analysis The impact of the RTD bypass elimination on USAR Chapter 15 non-LOCA accidents was evaluated by the licensee.
As previously mentioned, the RTD outputs are used by the OTDT and 00DT protection functions.
The OTDT reactor trip function is the primary trip credited, while the OPDT reactor trip provides backup protection against excessive power increases.
Since the temperature measurement response time and accuracy is not degraded by the replacement of the RTD bypass system with the thermowell system, the existing conclusions of the USAR transient analyses remain valid.
The replacement of the RTD bypass system has been found to not impact the uncer-tainties associated with RCS temperature measurement or the calorimetric measure-ments of reactor power or reactor coolant flowrate.
The removal of the RTD bypass system and installation of the thermowell system will not affect the LOCA analyses inputs and hence, the results of the existing analyses remain unaffected.
Therefore, the plant design changes due to the removal of the RTD bypass system are acceptable from a LOCA analysis standpoint without requiring any detailed reanalysis.
3.8 Radiation Exposure A motivating factor for the rer. cement of the existing RTD bypass system with the thermowell system is te '
inate the potential leakage and crud traps which result in increased alation exposures to personnel performing work on or near the RTD bypass system components.
An exposure savings of approximately 60-90 man-rem per refueling outage has been projected as a result of this plant modification. This is equivalent to approximately 2000 man-rem dose savings over the remaining life of the plant, assuming a 40 year operating license.
The exposure savings identified in the licensee's ALARA cost-benefit analysis is based upon the reduced radiation leveis, reduced maintenance requirements for the RTD bypass system, increased accessibility inside the bioshield, elimination of inservice inspection weld examination interferences, exposure associated with implementing the modification, reduced forced outage potential, and increased unit reliability over the life of the plant.
The licensee has taken several steps to ensure that the radiation dose experienced by those workers performing the replacement of the RTD bypass system is maintained as low as is reasonably acheivable.
Wolf Creek personnel observed and taped portions of the implementation of the plan $ modification to remove the RTD bypass system at the Callaway plant. Additionally, licensee personnel have participated in an RTD tooling / activity demonstration provided by the vendor.
These activi-ties have been factored into the ALARA techniques being considered for this modification which include:
a.
Mock up training for shielding installation and pipe demolition.
b.
Design and fabrication of special pipe handling tools for cutting, removing and transferring pipe from inside the bio-shield.
I
c.
Utilization of remote cameras to allow fire watches and health physics personnel to monitor modification activities from outside the bio-shield.
d.
Testing of various pipe cutting techniques for determining the best method of minimizing debris, improving the timeliness of cuts and minimizing the number of blade changes.
4.0 EVALUATION OF TECHNICAL SPECIFICATIONS As a result of the modifications associated with the removal of the RTD bypass system and the installation of the thermowell system, changes to the plant's Technical Specifications were proposed.
The following Technical Specifications were examined:
Change 1: Table 2.2.1 Table 2.2.1 is revised to reflect the changes to the values of the maximum allowable deviation from the setpoint, the sensur error (S), and the combina-tion of uncertainties included in the "Z" term for the OTDT and OPDT functions.
These changes are a result of the reanalysis of the measurement uncertainties and the differences in the various uncertainty contributors between the existing bypass system and the proposed thermowell system.
The table is also revised to remove a reference to the RTD manifold instrumentation.
These revisions have been reviewed by the staff and determined to be acceptable.
Change 2: Table 4.3.1 Table 4.3.1 has been revised to delete Note 13 from the instrumentation sur-veillance requirements.
This note requires that the RTD bypass loops flow rate be included in the surveillance performed each refueling outage. The note is not relevant after the removal of the bypass system and therefore the staff deems this change to be appropriate.
Change 3: Bases This change to the Bases deletes a reference to the transit delay associated with the temperature measurements by the RTD bypass system.
Since the modifi-r cation involves the removal of the RTD bypass piping, the time delays associated with the temperature measurement have changed as detailed in Table 1 and a revision to the Bases is required.
5.0 ENVIRONMENTAL CONSIDERATION
This amendment changes a requirement with respect to installation or use of a facility component located within the restricted area as defined in 10 CFR Part 20.
The NRC staff has determined that the amendment involves no signi-l ficant increase in the amounts, and no significant change in the types, of l
any effluents that may be released offsite, and that there is no significant l
t
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increase in individual or cumulative occupational radiation exposure. The Commission has previously issued a proposed finding that the amendment involves no significant hazards consideration, and there has been no public comment on such finding (56 FR 37595).
Accordingly, the amendment meets the eligibility criteria for categorical exclusion set forth in 10 CFR 51.22(c)(9).
Pursuant to 10 CFR 51.22(b) no environmental impact statement or environmental assessment need be prepared in connection with the issuance of tue amendment.
- 6. 0 CONCLUSION The Commission has concluded, based on the considerations discussed above, that:
(1) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, (2) such activities will be conducted in compliance with the Commission's regulations, and (3) the issuance of the amendment will not be inimical to the common defense and security or to the health and safety of the public.
Attachment:
Table Principal Contributors:
H. Balukjian, SRXB F. Paulitz, SICB W. Reckley, PDIV-2 Date:
November 7, 1991 I
l
k ei Table 1 Overtemperature Delta-T-Response Times Time Delays (seconds)
Safety Existing Proposed Analysis I.
First Order Lags
n/a
- b. Comoined RTD and Thermowell n/a 4.75*
- c. Bypass Piping and Thermal Lag 2.0 n/a
- d. Scoop Transport and included Thermal Lag in c 0.25 i
SU9f0TAL-First Order Lags 6.0 5.0
- 6. 0 II.
Pure Time Delays
- a. Electronics 0.30 0.30
- b. SSPS 0.001 0.001
- c. Reactor Trip Breakers 0.167 0.167 SUBTOTAL - Pure Delays 0.50**
0.50**
2.0 TOTAL - Testab'e Time Delays ***
4.5 5.25 TOTAL - Time Delays 6.5
- 5. 5 8.0 Includes 10% testing allowance for LCSR testing.
Existing 1
RTD response time makes use of margin available in OTDT l
analysis Delays total 0.468 seconds but were rounded to 0.50 seconds Technical Specification limit is 6.0 seconds (excludes l
transport delays and thermal lags) l
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