ML20086R429
| ML20086R429 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 12/20/1991 |
| From: | Stewart W VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.) |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| Shared Package | |
| ML20086R430 | List: |
| References | |
| 91-745, NUDOCS 9201020051 | |
| Download: ML20086R429 (8) | |
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YlltOINIA }U.1:CT I(I C A N D l'Oh 1:14 C OSI PANY lt ic u >t ox o, Voenix 4 u n u s,1 December 20, 1991 U.S. Nuclear Regulatory Commission Serial No.91-745 Attention: Document Control Desk NL&P/RMN Washington, D.C. 20555 Docket Nos.
50-338 50 339 License Nos. NPF-4 NPF 7 Gentlemen:
VIRGINIA ELECTRIQ AND POWER COMPANY NORTH ANNA POWER STATION UNITS 1 AND 2 PROPOSED TECHNLCAL SPECJFICATIQ1[_GHANGES Pursuant to 10 CFR 50.90, the Virginia Electric and Power Company requests ame idments, in the form of changes to the Technical Specifications, to Operating License Numbers NPF-4 and NPF-7 for North Anna Power Station Units 1 and 2, respec',:vely.
The proposed changes will revise the Technical Specifications to suppor1 the elimination of the reac*or coolant RTD bypass system and substitution of thermowells that extend into the main RCS piping.
The changes should be implemented concurrent with the hardware mod' cation. We plan to implement the design change during upcoming refueling outi ges which are currently scheduled for April 1992 for Unit 1 and Fall 1993 for Unit L. Although engineering to support the design change is not yet complete, we are submitting the change request at this time to permit adequate time for your review. We will inform you immediately if the final design results in any change to current assumptions or setpoints.
A discussion of the proposed changes is provided in Attachment 1.
The proposed changes are presented in Attachments 2 and 3 for Units 1 and 2, respectively.
Safety and Operating This request has been te, " ved by the Station Nuc
'r Committee and the Managt Safety Review Committe It has been determined that this does not involve ar.. reviewed safety question as defined in 10 CFR 50.59 or a significant hazards consideration as defined in 10 CFR 50.92. The basis for our determination that no significant hazards consideration is involved is presented in.
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I If you have any questions or require additionalInformation, please contact us at your earliest convenience.
Very truly yours,
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v W. L. Stewart Senior Vice President Nuclear Attachments i
1.
Discussion of proposed changes 2.
Prnoosed Technical Specification Changes for Unit i 3.
F;vposed Technical Specification Changos for Unit 2 4,
10 CFR 50.92 Evaluation oc:
U.S. Nuclear Regulatory Commission Region ll 101 Mariotta Street, N.W.
Suite 2P00 Atlanta, Georgia 30323 Mr. M. S. Lesser NRC Senior Resident inspector North Anna Power Station Commissioner Department of Health Room 400 109 Governor Street Richmond, Virginia 23219 1
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COMMONWEALTH OF VIRGINIA )
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COUNTYOF HENRICO
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The foregoing document was acknowled ed before '
- in and for the 0
County and Commonwealth aforesaid, today by W. L.
...u. art who is Senior i
Vice President - Nuclear, of Vir0 nia Electric and Power Company.
HL is duly authorized to execute and fiie the foregoing document in behalf of that Company, and the statements in the document are true to the best of his knowledge and belief.
Acknowledged before me this OO day of /ht & rn /u o, 1991 My Commission Expires:
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Discussion of Proposed Changes North Anna Units 1 and 2 Virginia Electric and Power Company
4 DISRUSSlQti OF Pfl0EOSED_C11AtLGES Introduction The proposed changes to the Technical Specifications increate the response time associated with Reactor Coolant System (RCS) temperature measurement. The specification change would be implemented in support of a modification to the temperature measurement system. This modification is planned for the Cycle 9 outages. These are current!y scheduled for the Spring of 1992 for Unit 1 and the Fall of 1993 for Unit 2.
Background
The North An a reactors each have three coolant inlets (cold legs) and three coolant outlets (hot lem '. The temperature is measLred at each leg in order to compute the average tr.mpt."lure arJ the cifference between hot and cold legs.
Temperature is measured using electrical devices called Resistance Temperat re Detectors (RTDs). The current method of measuring the hot and cold leg rent or coolant temperatures uses the RTD bypass system. Three mixing scoops are located in each hot leg,120 degrees apar1, to provide a representative sample. Each scoop has five orifices which sample the hot leg flow. The flow from the scoop is piped to a manifold where a direct immersion RTD measures the hot tog loop temperature. The cold leg temperature is measured in a similar manner with piping to a separate bypass manifold, except that no scoops are used. Temperatures in the cold leg are more uniform due to the mixing action of the RCS pump. The resulting system consists of nearly 240 feet of Reactor Coolant Pressure Boundary (RCPB) piping,21 associated valves, hangers which include 30 snubbers,6 sets of flanges and 6 RTD manifolds.
Plant experience has demonstra*ed two major drawbacks to the current design:
Lack of Reliability, Plant shutdowns have been required because of Forced Outages leakage (from valve packing or mechanical joints) or because of flow reductions due to valve problems.
High HEdiation Dose
- The RTD Bypass Piping (B/P) System is a significant contributor to man rom exposure bacause the numerous valves and socket welded p.ges are crud traps. Man rem is expended not only in maintaining and inspecting the RTD B/P System but in performing any work near the RTD B/P System such as Steam Generator and Reactor Coolant Pump maintenance.The removal of the RTD bypass manifolds is expected to reduce the collective exposure by about 60 man rom por unit per refueling cycle. In addition, forced outages will be avoided due to the avoidance of leaks and equipment failures, Total dose savings over the dves of the Units is estimated to be 4800 man rom.
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The modification WP! eliminate the RTD bypass loops and substitute thermowells thr.t extend into the RCS piping. A thermowell is a well with an RTD, extending into the pipe. The RTD will not be immersed in the coolant and it can be replaced without drnining the system.
The three mixing scoops in each hot leg will be retained. Holes will be added to the scoop so that the flow from the five inlet holes passes by the tip of the new thermowell.
The thermowell becomes part of the RCPD. The RTD assembly screws into the thermowell (three RTDs for each hot leg, one for each cold leg). The crossover leg connection, through which the RTD B/P System fluid is returned to the main RCS piping, will no longer be required and will be capped.
The modification will increase the total response time measured in accordance with the surveillance requirements because the sensor thermal response time in the redesigned system will increase. However, the accident analysis results of UFSAR Chapter 15 will not change. This is due to the fact that the increase in thermal response time is offset by a reduction in transport and thermal delay time associated with the sensor bypass piping, which is being eliminated. The bypass piping transport and thermal delay, which is of the order of two seconds, is reflected in the current safety analysis but is not part of the reactor trip or engineered sa%ty feature system response time defined and measured in accordance with the Technical Specifications.
Post modification testing will be performed to demonstrate that the total channel response time remains within that assumed in the safety analysis. The increase in response time listed in the Technical Specifications reflects the fact that a larger portion of the total response time will be directly measured than with the current system.
Weed Instrument Co., Inc. dual element RTDs will be used in the new design. Each RTD element will be shop tested inside a thermowell to ensure that the time response of both elements is within the required limi's. Response time of the RTDs will also be verified in the field. Since Weed RTD's are currently in use in the Bypass system there will be no loss of RTD accuracy in the new system. The spare RTD element will be wired to the 7300 Process Protection System cabinets so that switchover to the spare element can be done at the racks.
Each of the three hot leg RTDs per loop will be wired up to an RTD amplifier card and the three signals will be averaged to produce one hot leg temperature signal which will replace the loop's T Hot signal of the existing system. The added electronics will be identical to the existing 7300 electronic hardware now used.
Technical Spe_cl(( nation Qhangas G3Dalal With the proposed modification, the response time of the RTDs will be determined with the RTDs in the thermowell.
Therefore, the thermal lag times associated with the thermowell wEl be included in the RTD tested recponse time. The response time of the
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I proposed RTD/thermowellis 1.75 seconds slower than the existin0 direct immersion RTD's response time. This necessitates adding 1.75 seconds to the Technical Specification instrumentation response times for overtemperature AT in Table 3.3 2 and for low low T ave in Table 3.3 5.
Table 3.3 2 lists the :naximum time to automatically trip the reactor when the powe,
computed from the RCS temperature (provided by the RTD's), exceeds the limit. Table 3.3 5 lists the maximum times to automatically start certain engineered safety features when two out of three steam lines have excessive flow and the average reactor coolant temperature (provided by the RTD's) is below its " low low" limit. All of these response times would be increased by 1.75 seconds by the proposed changes.
The temperature time response has the following components:
Existina Pro.cosed Direct immersion RTD 3.0 sec.
N/A Combined RTD/Thermowell N/A 4.75 sec, Electronics / Electrical Delays 1.0 sec.
1.0 sec.
Subtotal (T.S. Response Time) 4.0 sec.
5.75 sec.
Loop or Scoop Transient and Thermal Lag 2.0 sec.
0.25 sec.
Total System Response Time 6.0 sec.
6.0 sec.
- The allocated time for RTD/thermowellincludes a 10 percent error allowance.
However, there is no change to any design bases since the total system response time remains unchanged.
The above Technical Specification response times do not include a 2.0 second delay, which is analytically established, to account for tho existing RTD bypass loop thermallag and travel time. With the proposed system, this component is reduced from 2.0 seconds to 0.25 seconds. The 0.25 secor ds account for the transport time and thermallag of the hot leg mixing scoop.
The temperature input to the engineered safety features (high steam line flow coincident with low low RCS Tavg) will be impacted in the same way as the reactor trip
_ input. The Technical Specification allowable response time should be increased by 1.75 seconds to reflect the fact that RTD thermal response time is being increased, and this increase is part of the measured response time covered by the surveillance. The i
concurrent elimination of the bypass loop transport offect (which is not measured in the -
- existing surveillance) ensures that the total system response time remains unchenged..
In addition, an administrative deletion of an expired footnote on Table 3.3 2 is
- proposed.
I Technical Soecification 3.31.1. Tablo 3.3 2 Item #7 Our1emperature AT: The 4 second response time is changed to 5.75 seconds.
Item #11 Pressurizer Water Level High: The double astorisk and the associated footnote is deleted. The footnote dealt with when to implement response time testing for
- Pressurizer Water Level High".
This response time testing bus been implemented. The footnote is deleted to remove unnecessary information.
Technical Soecification 3.3.2.1. Table 3.3-5 i
An additional 1.75 sec. response time is added to the following iterns:
ltem #5a Safety injection: Change 15.0 to 16.75 and 25.0 to 26.75.
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ltem #5b Reactor Trip: Change 5.0 to 6.75.
ltem #5c Feedwater Isolation: Change 10.0 to 11.75.
Item #5d Containment isolation: Change 20.0 to 21.75 and 30.0 to 31.75.
Item #5e Auxiliary Feedwater Pumps: Change 60.0 to 61.75.
Item #5g Steam Line Isolation: Change 10.0 to 11.75.
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