ML20151H469

Loss of Salt Water Flow to Svc Water Hxs, Evaluation Rept

ML20151H469
Person / Time
Site:	Calvert Cliffs, San Onofre
Issue date:	04/25/1983
From:	Cintula T NRC OFFICE FOR ANALYSIS & EVALUATION OF OPERATIONAL DATA (AEOD)
To:
Shared Package
ML20151H460	List: IA-83-368, Loss of Salt Water Flow to Svc Water Hxs, Evaluation Rept ML20151H458 ML20151H469
References
FOIA-83-368, TASK-AE, TASK-E310, TASK-E311 AEOD-E310, AEOD-E311, NUDOCS 8305040150
Download: ML20151H469 (7)

Text

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AE0D ENGINEERING EVALUATION REPORT

• UNIT: Calvert Cliffs 2 EE REPORT N0.AE0D/E311 DOCKET N0.: 50-318 'DATE: April 25, 1983 LICENSEE: ' Baltimore Gas & Electric Co. EVALUATOR / CONTACT: T. C. Cintula NSSS/AE: Combustion Engineering, Inc./Bechtel Corp.

SUBJECT:

LOSS OF SALT WATER FLOW TO THE SERVICE WATER HEAT EXCHANGERS FOR 23 MINUTES AT CALVERT CLIFFS UNIT 2 EVENT DATE: July 20,1982 ,

SUMMARY

On July 20, 1982, Calvert Cliffs Unit 2 experienced a loss of both redundant trains of the safety-related service water system when a single nonsafety-related butterfly valve in the salt water system failed and blocked all salt water coolant flow. Since this event was not considered to be a credible accident during the safety reviews, the operating procedures did not include corrective measures for the possibility of system flow blockage. The licensee, however, diagnosed the problem and restered flow to one of the service water heat exchangers before any temperature alarms actuated. The actual conse-quences of the event to plant equipment were minimal.

Our review of the saltwater cooling system concluded that the system design could be improved from a safety standpoint by eliminating the source of potential single faGure which can cause loss of the system function, i.e.,

the nonsafety-related valves in the common discharge headers from the service water and component cooling water heat exchangers. The licensee is consider-ing this option.

A review of the FSARs of other operating PWRs indicated that Calvert Cliffs is not unique in having a single valve in the common discharge header of redundant safety-related heat exchangers.

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• This document supports ongoing AE00 and NRC activities and does not represent the position or requirements of the responsible NRC program office.

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DISCUSSION On July 20,1982, at.1:07 pm, with Calvert Cliffs Unit No. 2 operating at 85% power, salt water flow to the unit's two service water heat exchangers (SRWHX) was interrupted when a butterfly valve in the common SRWHX discharge -

header to the salt water discharge canal failed and system flow caused the valve disk to rotate to the closed position. The noise from the sudden valve closure was heard in the control room, and shortly thereafter, the operators reduced power to lessen the heat load on the service water system.

At 1:30 pm, the licensee was able to partially restore service water cooling by aligning salt water flow to one SRWHX and discharging to the emergency saltwater overboard line. This action also isolated one of the two component

. cooling water heat exchangers (CCWHX) (see Figure 1). A power increase was initiated at 2:05 pm.

No high temperature alams were received on equipment cooled by the service I water system. The peak outlet temperature for No. 22 SRWHX was 110*F.  ;

During the next day, the licensee verified that no additional damage had occurred from the sudden valve closure and resulting water hammer shock wave. The failed butterfly valve (2SW197) was replaced in-kind by a '

spare valve and salt water cooling flow was restored to both of the two service water and component cooling water heat exchangers at 10:40 pm on -

, the second day after the event.

The salt water system (SWS) for each unit at Calvert Cliffs supplies cooling water to both safety and nonsafety related components and consists of two redundant subsystems (No. 21 and 22) to assure safe shutdown of the unit assuming a single failure. Each SWS subsystem provides cooling for one service water heat exchanger, one component cooling water heat exchanger, i and an ECCS pump room air cooler. Each subsystem can supply 100% of the I,

cooling needs of the safety-related components and a third 100% capacity salt water pump (No. 23) can aligned to either subsystem.

The design basis for an assumed loss of flow in the SWS is to pemit

operation until the service water temperature reaches 120*F. This is postulated to occur approximately 21 minutes after a loss of salt water flow based on a initial service water temperature cf 95'F. The valve that failed, valve 2SW197, is downstream of the service water heat exchanger discharge valves in a nonsafety common heat exchanger discharge header that leads to the discharge tunnel. The salt water piping to the component cooling water heat exchangers is identical with the service water piping; it too has a single valve on the nonsafety common discharge header of the component cooling heat exchangers. The emergency saltwater overboard line provides the safety-grade flow path to the bay; however, its alignment permits only one-of-the-two service water and component cooling water heat exchangers to be in service because subsystem No. 21 cannot be used unless spool pieces are installed to cross connect the subsystems. The dashed line in Figure 1 shows the flow path of the salt water system when using the emergency over-board'line.

l Valve 2SW197 is a Henry Pratt Co., Triton XR-70, manually actuated 30" Butter-fly valve (Figure 2). The valve failed when two taper pins that connect the valve operator stem to the valve disk sheared. A third pin, the single pin that connects the disc to the bottom stub shaft, remained intact and fluid forces on the valve disk caused it to rotate to the closed position. The replacement valve was installed 90' rotated in the same plane from its original position in an attempt to reduce the flow induced closing rotation-al forces on the valve disk. The unit will continue to operate with the l_ __ _ . _ _ _ . _ _ _ _ . _ _ _ _ _ _ . _ _ _ _ _ _ _ _ _ _ _ _ _

replacement valve. The licensee is considering replacing this valve aiid its companion valve on the discharge of the component cooling water system with spool pieces, or removing the valve internals,' to allow unrestricted discharge to the bay. An action of this nature would obviate the need for the emergency overboard line.

FINDINGS The inspector observed that the two taper pins that failed'in valve 2SW197 showed signs of wear and fretting on the pin surface which indicated the pins were likely to be vibrating within the taper pin holes. A. metallurgical analysis to determine the cause of failure was inconclusive because the

, fracture surface was worn so severely. However, scanning microscopy and i chemical analysis indicated an abundance of sulfide inclusions which led to speculation that the grade of pin material (R-Monel) may have been selected for ease of machining properties rather than fatigue resistance.

The valve manufacturer contended that using the valve for throttling could accelerate failure; this valve, however, was maintain.ed open at all times.

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The manufacturer's statement that using valve 2SW197 for throttling could accelerate failure by increasing the velocity of the fluid led to the

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identification of an associated safety concern within the plant by the NRC inspector. The inspector noted that the valves immediately downstream of the service water (30" nominal pipe size) and the component cooling water (24" nominal pipe size) heat exchangers are also Henry Pratt Co., Triton XR-70, Butterfly valves. Each of these valves are throttled during nonnal power operation to prevent excessive cooling by the heat exchangers. In addition, each valve must cycle to either the "open" or " closed" position during a loss of coolant incident, and return to a throttled position if the presumed accident should cause a subsequent recirculation actuation signal.

Because of the NRC inspector's identification of this potential problem, the licensee's safety connittee has recommended further research on the app 11-cability of these valves for their intended use. A decision is scheduled for June 1,1983. It should be noted that valve 2CV5149, in the emergency salt water overboard line, is not capable of being throttled; the flow rate is controlled by an orifice at the end of this line.

The NRC inspector noted that licensee procedure A0P-2, LOSS OF SALT WATER COOLING, states that the loss of saltwater flow to the entire system is not a credible accident. The procedure discusses the failure of one subsystem caused by a single pump failure or a system rupture as credible failures, but fails to discuss the possibility of system flow blockage. Additionally, A0P-2 failed to address possible situations where the emergency saltwater overboard discharge line maybe needed. Based on these observations, the inspector considered A0P-2 inadequate in that it made incorrect assumptions and did not include reasonably expected failures or the use of portions of the saltwater systen designed for emergency use.

The SRWHX combined outlet valve (2SW197) is not a safety-related component and is locked in the open position. Because of these considerations the valve is not tested as part of the inservice test program (IST). The licensee was uncertain as to whether the valve had ever been cycled. Yalve 2CV5149, the safety-grade emergency overboard discharge valve to the bay, is a pneumatically operated normally closed valve and can be fully stroke tested in only Mode 5 (cold shutdown).

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Valve 2CV5149 will fail closed on a loss of instrument air. Since air is '

required to keep the valve open, a safety-grade air accumulator and check valve have been provided to ensure a pressurized air supply (perhaps 5-6

, cycles of operation) to the valve following a postulated loss of the instrument air system. The check valve is necessary to prevent the instrument air stored in the seismically designed accumulator from being lost through a break in the nonseismically designed instrument air system.

However, if the check valve leaks, or if there is leakage across the vent ports of the solenoid operated 3-way valve operators, the emergency over-board valve will drift closed with loss of instrument air. With movement of the valve disk, there is some possibility of the valve failing in i identical circumstances (taper pin failure and the valve disk rotating i to the closed position) as the SRWHX combined outlet valve (2SW197), or more simply, just by drifting to the closed position. The safety-grade check valve and solenoid valve operator are not tested as part of the IST. Given the failure of the butterfly valve 2SW197 which occurred, and assuming the unavailability of the nonsafety-related instrument air system, dependence is placed on the leaktightness of the check valve which isolates the accumulator and the 3-way valve' to prevent a total

! loss of service water system cooling due to the inability to open the i emergency overboard discharge valve 2CV5149. The saltwater overboard valve does not have a hand actuator to compensate for the valve closing on a loss of operator air pressure.

A review of the saltwater system design at Calvert Cliffs shows that total

system function (ability to accept heat from the various reactor and turbine components and transfer it to the Chesapeake Bay) is compromised by the failure 4 to provide true redundancy for the locked open valves in the common discharge piping to the discharge tunnel. In this design, salt water supply header No. 21 must be used as the emergency discharge to the Bay. Since the emergency overboard line is immediately downstream of the No. 21 salt water pump, the heat removal capabilities of this loop cannot be used.

i Since this event involved the failure of a single nonsafety-related component that disabled both redundant trains of the safety-related service water system, l we checked to find out if this piping configuration was unique to Calvert Cliffs 1 & 2. We subsequently reviewed the FSARs of some of the other operat-( ing PWRs for similar piping designs. We found that the service water systea at Point Beach 1 & 2 appears to be similar with a locked open 20" butterfly 4

valve, designated as valve 104, as the only discharge valve to the circulating water discharge from the component cooling water heat exchangers. Fort Calhoun has a single check valve designated as CW-188 in the 20" line to the discharge l

tunnel from the component cooling water heat exchangers.

CONCLUSIONS Our review of this event revealed no immediate safety concerns and we conclud-ed that the consequences of this event were minor. Operators re-established ,

flow through No. 22 service w'ater heat exchanger through the emergency over-board line well before the peak outlet temperature exceeded the design basis

' for a loss of flow in the system. This action was accomplished even though the procedures were inadequate and the event was not considered to be credible in the safety review. It would seem that the licensee's ability to realign the salt water system before any temperature alarms were received was en-hanced by the audible indication of the valve failure in the control room,

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a heat exchanger design with adequate mass of coolant to prevent rapid heat-up, and the licensee's familiarity with the plant rather than by reliance on a comprehensive emergency operating procedure..

The presumes increased potential for failure of the 'four Henry Pratt Co.

Trition XR-70 valves maintained in a throttled position downstream of the heat exchangers does not represent a credible event because at least two valves must fail during the presumed accident, and they must each fail in a position that blocks or severely restricts flow. However, the identi-fication of these valves' increased susceptibility for failure may lead to modifications to improve system reliability.

A previous AE0D case study, REPORT ON CALVERT CLIFFS UNIT 1 LOSS OF SERVICE WATER ON MAY 20, 1980, recommended in Paragraph 8.a.(2) that check valves and solenoid operated three-way valves in the seismically designed portion of the instrument air system be added to the IST program.

A review of this more recent event would tend to substantiate the merit of this recanmendation.

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In conclusion, the failure of a single nonsafety-r' elated component caused the disablement of both redundant trains of the safety-related service water system.

The event involved two fundamental aspects that should be considered in the design of safety-related systems:

1) Interaction between safety and nonsafety-related systems and components; and

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2) Common caused failure of redundant safety systems.

The licensee is now considering removal of the locked open nonsafety-related heat exchanger discharge valves in the salt water system. Their removal would obviate the use of the energency overboard line and, in turn, enhance complete system redundancy at all postulated operating conditions. We believe their removal should result in improved operational safety.

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