BVY-93-143, Provides Addl Info Re Plant Alternate Cooling Sys to Provide Assurance That Sys Fully Operable,In Response to NRC Request During 931202 Enforcement Conference

Provides Addl Info Re Plant Alternate Cooling Sys to Provide Assurance That Sys Fully Operable,In Response to NRC Request During 931202 Enforcement Conference

ML20058M817
Person / Time
Site:	Vermont Yankee
Issue date:	12/16/1993
From:	Wanczyk R VERMONT YANKEE NUCLEAR POWER CORP.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
BVY-93-143, NUDOCS 9312210123
Download: ML20058M817 (3)

Text

VERMONT YANKEE NUCLEAR POWER CORPORATION 7^q - P.O. Box 157. Governor Hunt Road j Vernon, Verrnont 05354-0157 i[p'j\I. (802) 257-7711 L' N' s /.].

December 16,1993 BVY # 93-143 U.S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, D.C. 20555

Reference:

a) License No. DPR-28 (Docket No. 50-271)

Subject:

Vermont Yankee Alternate Cooling System Operability This letter is submitted to provide additional information regarding the Vermont Yankee Alternate Cooling System to provide assurance that the system is fully operable. His information was requested during an Enforcement Conference on December 2,1993.

The Alternate Cooling System is designed and installed to provide an alternate source of cooling water in the unlikely event of a failure of the Vernon Dam. He Alternate Cooling System is not classified as an engineered safeguard system. The primary source of water corsists of a deep basin, located below the west Cooling Tower. Water from the deep basin is supplied to the suction of the Residual Heat Removal Service Water (RHRSW) Pumps and then pumped to the Emergency Diesel Generaiors, Residual Heat Removal (RHR) Heat Exchangers, and small cooling loads such as the RHRSW pump motor coolers and room coolers. Water is then returned to the cooling tower and latent heat is transferred to the atmosphere. The Residual Heat Removal System will provide cooling as if the plant were being supplied with river water and will function to safely remove the sensible and decay heat from the reactor and other heat loads. Additional design devas are provided in FSAR Section 10.8.

In order to place the Alternate Cooling System in service, there are several manual valves that must be repositioned prior to starting the RHRSW pumps. Dese consist of valves that must be opened to provide the proper suction and discharge paths for Alternate Cooling as well as valves which must be closed to isolate portions of the normal Service Water System, not required in this mode.

During the 1993 Refueling Outage, silt buildup in the deep basin ws identified and it was concluded that the Alternate Cooling System would not have performed its function since the suction path was plugged. Additionally, microbiologically induced corrosion was observed in the 24 inch diameter suction pipe from the deep basin. Both of these conditions were corrected during the outage. As a result of these findings, Vermont Yankee conducted a review of the Alternate Cooling System prior to startup from the outage to ensure there were no other portions of the system that were degraded or that could preclude system operation.

The evaluation assessed each valve and portion of piping in the system to verify that operability was ensured either by maintenance work done during the outage, or by routine operation and testing of applicable portions of the RHRSW and Service Water Systems. Valves required for Alternate Cooling that are not routinely operated or were not subject to maintenance performed during the outage were full stroke operated to ensure they could be manually operated if required. During the 1993 Outage, ten

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l U.S. Nuclear Regulatory Commission VERM ONT YAN KEE NUCLEAR POWER CORPORATION l December 16,1993  !

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valves in the alternate cooling system were inspected, cleaned, and coated with a corrosion inhibiting coating as part of our valve maintenance program. During these inspections, no conditions were noted that would have precluded the valves from operating properly. Additionally, six valves were replaced with new stainless steel valves as part of a design change upgrade to the RHRSW system.

During the outage, a new discharge pipe from the diesel generators was installed which improves )

diesel cooling. The new piping is stainless steel and a significant improvement in diesel cooling water flow and pressure loss was measured for this new arrangement. His results in a significant increase in cooling water flow during any postulated high backpressure scenarios. Additionally, prior to the outage l the station Air Compressors were replaced with air cooled compressors which makes the alt supply I system independent of the cooling water system, thereby decreasing the heat load on both the normal i Service Water System and the Alternate Cooling System. During outage maintenance, many portions of  !

the service water piping were inspected and cleanad or flushed and both RHRSW Heat Exchangers were l also cleaned and inspected. Any piping sections that had not been flushed as part of other outage I maintenance activities or are not routinely flushed by normal operation of RHRSW or Service Water were flushed prior to startup to ensure an unrestricted flow path for the Alternate Cooling System existed.

I FSAR Section 10.8 identifies the system cooling water flow design requirements as three RHRSW pumps at 2700 gpm each for a total of 8100 gpm. It should be noted that there is no requirement or anticipated condition where 8100 gpm would be required for an extended period of seven days in the Alternate Cooling mode of operation. In fact, assuming that flow is required to an RHR Heat Exchanger,  !

both Diesel Generators, the Corner Room Coolers and pump coolers, the design flow is approximately  !

5000 gpm, well below the 8100 gpm value identified in the FSAR.

As stated in LER 93-14, an engineering evaluation of the as found condition of the suction piping was completed to determine the potential effect of the microbiologically induced corrosion on RHRSW pump operation (NPSH). This engineering evaluation determined that the system would still have been able to fulfill its required cooling function. With the basin essentially empty, the pumps would have had adequate NPSH at 5000 gpm with the fouled portion of the suction line having a roughness equivalent to the worst case tubercule size found in the piping, and the remaining clean portion having a roughness three times that of clean pipe. With the basin full, the entire line could have had tubercules and there still ;

would have been adequate NPSH at 5000 gpm. The suction pipe was restored to a clean condition and {

chemically treated to reduce future corrosion prior to plant startup.  !

I The required NPSH for RHRSW pump operation is 24 feet (absolute). The available NPSH with the basin full, clean pipe, and three RHRSW pumps operating is 40.4 feet. With the basin essentially l empty, the available NPSH is 27.5 feet (the margin is 3.5 feet). At the calculated seven day loss level, approximately one additional foot NPSH is available.

A recent engineering review of the capability of the Alternate Cooling System discharge piping ,

to pass 8100 gpm, as identified in FSAR Section 10.8, has also been conducted. This review / analysis focused on the pressure drop across the diesel generators and the resulting pressure available to overcome l friction and elevation losses in the discharge piping and still pass 8100 gpm through the cooling tower riser to the top of the cooling tower. Expected pressure drops were determined at service water flows to the diesel generators ranging from 450 to 700 gpm. The loss coefficients in the piping segments from the RHRSW Pump Discharge to the base of the cooling tower riser were based on recent flow and pressure data gathered during tests. The portion of riser above water is normally drained and therefore is ,

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U.S. Nuclear Regulatory Commission VERMONT YANKEE NUCLEAR POWER CORPORATION December 16, 1993 Eage 3 considered to be clean. The results of the analysis indicated tha; even at the most extreme condition of pipe fouling considered, the flow to the diesel generators will be well above the minimum required flow, i All active portions of the alternate cooling system are routinely tested or operated. The RHR and ,

RHRSW System pumps and valves are tested in accordance with the Inservice Test Program. Diesel Generator testing is conducted per Technical Specification requirements. The cooling tower cell and fan are inspected and maintained once per year, prior to normal operation of the cooling towers each Spring.

Additionally, upgrades to the cooling towers have been implemented over several years to ensure a highly reliable and efficient heat removal system.

The only adverse findings during our review involved two (out of four) manual valves on the cooling tower distribution header for alternate cooling which were not free to close. ' Die valves were in I

the full open position, and their required position for Alternate Cooling mode is open, however, these valves could be used to throttle flow in each of the four headers to ensure a balanced distribution of spray

, within the cooling tower cell to maximize heat removal capability. Corrective maintenance was performed on these valves prior to plant startup to address this situation.  ;

I Based on this system review, successful cycling of valves, flushing of applicable piping sections, mechanical cleaning and treatment with corrosion inhibitor of the suction pipe, engineering evaluations i of the system, and thorough cleaning of the deep basin, we conclude that the Alternate Cooling System  !

is fully operable.

We trust that this information k c%ponsive to your request. Should you have additional questions or desire additional information, do not hesitate to call.

Sincerely, Vermont Yankee Nuclear Power Corporation d4.L hl Mc Robert J. czyk Plant Manager cc: USNRC Regional Administrator - Region i USNRC Resident Inspector, VYNPS l USNRC Project Manager, VYNPS I

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