ML20059C213
| ML20059C213 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 12/27/1993 |
| From: | NRC |
| To: | |
| Shared Package | |
| ML20059C211 | List: |
| References | |
| NUDOCS 9401050017 | |
| Download: ML20059C213 (5) | |
Text
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. ?t UNITED STATES
[,, 21 k j NUCLEAR REGULATORY COMMISSION f
W ASH'NGTON D.C. 20555 0001 i
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION f
FACILITY OPERATING LICENSE NOS. NPF-4 AND NPF-7 VIRGINIA ELECTRIC AND POWER COMPANY OLD DOMINION ELECTRIC COOPERATIVE NORTH ANNA POWER STATION. UNITS NO. 1 AND NO. 2 DOCKET NOS. 50-338 AND 50-339
1.0 INTRODUCTION
By letters dated May 18 and July 16, 1992, Virginia Electric and Power Company (VEPC0 or the licensee), informed the staff of plans to perform extensive refurbishment activities for restoration of certain portions of the service water system (SWS) at North Anna Power Station, Units 1 and 2.
The July 16, 1992, letter also requested a temporary exemption from the requirements of 10 CFR Part 50, Appendix A, Criterion 2 (GDC-2), " Design bases for protection against natural phenomena," to allow excavation of certain portions of the SWS piping during Phase I of the project. The temporary exemption was necessary because the excavation exposed certain portions of the piping to missiles generated by natural phenomena, e.g., tornadoes. By letter dated December 3, 1992, the staff provided a safety evaluation and granted the requested temporary exemption to GDC-2 for Stage 1 of Phase I of the VEPC0 project.
Stage 1 mainly involved the SWS supply and return headers to the containment recirculation spray heat exchangers.
By letter dated July 30, 1993, the licensee requested another temporary exemption to GDC-2 to perform Stage 4 of Phase I of the SWS restoration project. This temporary exemption is necessary in order to allow excavation of portions of the auxiliary service water (ASW) lines with both units operating. The scope of Phase I, Stage 4, involves restoration of the 24-inch ASW supply lines from Lake Anna to the main SWS headers in the turbine building. This consists of approximately 900 linear feet of piping (450 feet per line).
Each 450 foot line consists of approximately 185 feet.of buried piping, 230 feet of concrete encased piping and 35 feet of exposed piping.
The primary goal of Stage 4 is to refurbish (clean, repair and coat) as much of the existing piping as possible, thereby minimizing pipe replacement. The concrete encased sections will be refurbished.
It is also planned to refurbish the buried piping with the exception of a small piece that must be removed for access.
If repair of the buried piping becomes impractical, it will be excavated and replaced.
For conservatism in evaluating the GDC-2 exemption, it was assumed that all of the buried piping will be excavated and replaced.
In addition to the restoration of the ASW supply lines, two 8-inch branch lines from the circulating water screen wash pump discharge up to the first isolation valves will be replaced.
94o1050017 931227 DR ADDCK 0500 8
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-The requested exemption is for a specified time period, specifically from January 3, 1994 through October 14, 1994.
2.0 EVALUATION The SWS can be supplied by either of two ultimate heat sinks (UHSs), the service water reservoir (SWR) via the SWS pumps or the Lake Anna reservoir via the ASW pumps. There are four 100% SWS pumps and two 100% ASW pumps. The SWR 1
is treated water and, therefore, the normal supply for the SWS.
Lake Anna _is an untreated source of water and is, therefore, only used_as a backup to the SWR and for makeup to the SWR. Both UHSs are safety-related sources and both are capable of independently providing at least a 30-day supply of water following any design basis accident. Both systems are designed to withstand all natural phenomena except for the SWR spray arrays which are not tornado missile-protected.
While producing some increase in missile interaction risk, excavation of the piping does not result in total vulnerability to missiles.
In the area of the-planned excavation, the lines are substantially below grade and are adjacent to the turbine building structure. This will prevent some lateral aspects of t
missile vulnerability. During the restoration of these lines, the seismic capability of the operable train will be maintained. Therefore, the only added vulnerability is from missiles or construction mishaps.
The risk from construction mishaps is considerably less than it was during Stage 1 because i
loss of the ASW system does not affect the ability of either unit to shut down or mitigate the consequences of an accident. The SWS would still be available with water being supplied by the SWR via the SW pumps. During Stage 1 the SWS headers were vulnerable to construction mishaps that could have affected both sources of water, since both UHSs use the SWS headers to supply cooling loads.
The risk from tornado missiles is also less for Stage 4 than it was for Stage I because in addition to missile damage to the excavated piping, the spray array to the SWR would also have to be rendered inoperable by missile damage.
Even this would not result in a loss of SWS flow but only in a reduced heat i
removal capacity of the SWR. A bypass is provided around the spray headers and is used during winter months for normal operation. The SWR volume is sufficiently large (originally designed for 4 unit operation) such that adequate heat dissipation exists for normal operation without spray flow during certain times of the year. To completely lose the spray capability due to missiles (flow blockage) would not immediately result in a loss of heat i
sink. The bypass around the spray arrays would still provide some heat removal capability following _ a loss of the spray system. Under these conditions the volume and surface area of the SWR still provides a certain amount of heat removal capacity (dependent on weather conditions). Hot standby conditions could be maintained for some undetermined amount of time l
since the significant heat loads come from reactor cooldown or accident heat loads.
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4 Even though the added risk is small, the licensee is providing contingency j
measures with compensatory actions to provide added assurance of safe operation of the facility during the exemption period. Although the exemption is requested only for missile protection, the risk also includes construction mishaps, although as previously noted, this risk is much-less than it was during Stage I because the SWS would still be operable from the SWR via the SW pumps. ~ However, since a construction mishap is a more probable event, the compensatory measures are geared toward preventing such mishaps in addition to minimizing the potential for missiles generated by severe weather. Also, during work in the turbine building valve pit (TBVP), where the potential for i
construction mishaps to damage one SWS line exists, additional contingency measures and compensatory actions will be provided. The contingency measures and compensatory actions include but are not limited to the following:
4 Electronic scanning and nondestructive locating methods will be used to accurately determine underground locations of piping, duct banks, and other buried utilities prior to excavation.
Machine excavation will be limited to an elevation that does not pose a threat to the ASW lines. The remaining excavation will be performed by hand operated power and manual tools.
i Physical barriers will be used to keep vehicles a safe a
distance from the excavation site.
Loose materials in and around'the excavation will be limited i
to only those absolutely necessary for activities in progress.
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Lifting and rigging components will be. inspected and load tested.
Lifting of equipment or construction materials over the excavation will be performed with adequate precautions in accordance with approved procedures, j
4 Direct verbal communication (using dedicated radios if necessary) will be maintained between equipment operators and supervisors / observers.
l Operable ASW lines will retain their seismic qualification.
.1 Engineering reviews will be performed for shoring and temporary supports.(if required) in the excavation area.
Gasoline powered equipment will be refilled using small containers and the number of containers within the excavation
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area will be restricted. This is in accordance with the existing station procedures.
Worker training and shift briefings will be conducted for all aspects of the project.
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i Severe weather procedures will be used to provide notification to clear the area of vehicles and loose materials in the event of a tornado watch or other high wind conditions.
Adequate wind protection and heating will be provided during freezing weather conditions.
The following additional actions will be used during work in the turbine building valve pit:
A temporary water supply from either the primary grade water or fire water systems will be available as a contingency.to 1
the charging pump coolers should the normal SWS supply be interrupted.
i Hotor-operated valves isolating the ASW lines from the SWS lines will be administrative 1y locked closed and additional i
blind flanges will be installed.
During the disassembly or reassembly of components, an operator will be stationed at the SW pump house with direct communications to the TBVP to isolate the affected SWS header i
immediately if the integrity of operable motor-operated valves is lost.
Communication between the control room and TBVP will be maintained at other times. Control room operators will be informed of the work in progress as part of the normal shift turnover.
Materials and tools necessary for performing emergency repair, i
of pressurized pipe will be staged near the TBVP.
Procedures will be developed and plant personnel will be trained in the use of these procedures and materials.
Lifting and rigging components will be inspected'and load tested.
Pre-job briefings will be conducted for the. intended route of removed or new pipe sections with the emphasis placed
'i on "important to safety" of keeping' these pipe sections. below-
-l the. pressurized piping.
l Pressurized ASW piping (only one ASW train is inoperable at' any given time) will be covered with.a protective blanket:to minimize the impact of a blow from a heavy object.
The staff has reviewed the licensee's proposed exemption to GDC-2, including l
the contingency and compensatory measures, and concluded that the likelihood of missile damage to the exposed ASW lines or other safety-related equipment banks is small' during the periods for which the requested exemption would apply. The staff has further concluded that with the contingency and-j
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.... - i compensatory measures provided, adequate assurance exists that the ability to bring the plant to a safe shutdown will be maintained following any natural i
phenomena, including tornadoes or other severe weather which could result in i
airborne missiles.
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3.0 CONCLUSION
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Based on the its review, the staff has concluded that the licensee's proposed i
temporary exemption to GDC-2 should be granted. This conclusion is based on l
the low probability of missile generation during the period of exemption l
coupled with the natural missile protection provided by the surrounding structures, and the contingency and compensatory measures that will be in place. The staff also concluded that the risk associated with the Stage 4 work is significantly less than that associated with the Stage I work which i
the staff has already approved and has been completed. This conclusion is based in part on the fact that redundant trains of SW (via the SWR) will i
remain operable throughout Stage 4 of Phase I.
i Principal Contributor:
W. LeFave
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Date: December 27, 1993
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