ML20084U917

AO 50-265/74-32:on 741231,MSIV Leak Rate Testing Revealed Excessive Leakage on Steam Lines A,B & C.Caused by Warpage Due to Washing of Seat Valve

ML20084U917
Person / Time
Site:	Quad Cities
Issue date:	03/10/1975
From:	COMMONWEALTH EDISON CO.
To:
Shared Package
ML20084U899	List: ML20084U896 ML20084U904 ML20084U917
References
AO-50-265-74-32, NUDOCS 8306290376
Download: ML20084U917 (2)

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REPORT NUMBER: A0-50-265/74-31 REPORT DATE: March 10, 1975 OCCURRENCE DATE: December 31, 1974 FACILITY:

Quad-Citics Nuclear Power Station Cordova, Illinois 61242 IDENTIFICATION OF OCCURRENCE:

Main steam isolation valve local leak rate testing revealed excessive Icakage on steam lines A, D and C.

CONDITIONS PRIOR TO OCCURRENCE:

Unit 2 was in cold shutdown for the current refueling outage.

DESCRIPTION OF OCCURRENCE:

On December 31, 1974 local leak rate testing was being performed on the Unit Two Main St.com isolation Valves. Prior to the test, all eight MSIV's were placed in the closed position from the control room.

The leakage measurements were taken by pressurizing the volume enclosed by the inboard and outboard MSIV's through a pressure test connecticn to a pressure of 25 psig. Leakage through the "D" MSIV's was determined to be 6.9 scfh. Therefore, the Technical Specification leakage limit criterion of 11.5 scfh per valve was met for that particular steam line.

The "A" steam line was pressurized between the inboard and outboard MSIV's, and the leakage was-measured to be 311 scfh at 25 psig test pressure. A similar test on the "B" steam line yielded a leak rate of 83.5 scfh. The "C" steam line yielded a leak rate of 33.9 scfh. All of these leakage values exceed the Technical Specification ilmit of II.5-scfh.for any one MSIV.

On December 28 and 29, the_ reactor water level was. Increased to flange level, and retesting of the MSIV's _ commenced so as to determine which valves on the "A", "B" and "C" steam lines were ~actually leaking _ excessively.

Since the water head inside the inboard MSIV's was greater than 25 psig cquivalent pressure, the. leakage to be measured through the MSIV's with-the reactor water icvel at the flange would be through the outboard MSIV's only. Leak test measurements yielded the following results:

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A0-50-265/74-91 March 10, 1975 VALVE LEAKAGE (scf h)

A0 2-203-1A Individual leak rates for these A0 2-203-2A . four valves could not be deter-mined because the enclosed volume A0 2-203-1B could not be drained when the A0 2-203-2B vessel level was increased.

A0 2-203-lc 6.6 (by subtracting 2C leakage from previous total)

A0 2-203-2C 32 3 A0 2-203-1D 6.9*

A0 2-203-2D 6.9*

• 6.9 scfh total for the two valves .

Valves IC, ID and 2D showed acceptable leakage values, but 2C exceeded the leakage limit. Because Individual leak rates could not be de*armined for the valves of the "A" and "B" steam lines, work requests were initiated to repair the inboard valves of these lines and valve 2C.

DESIGNATION OF APPARENT CAUSE OF OCCURRENCE:

Upon disassembly of the main steam isolation valves which experienced .

cxcessive leakage, washing of the valve seat was observed. Uneven heating produces a small warpage which eventually leads to a slight erosion of the stellite material . It is this slight amount of warpage which resulted in an inability to drain the volume enclosed by the MSIV's on steam lines "A" and "B" following the increase of reactor vessel water level to the flange.

' This was also the cause of the failure of valve 2C to meet the leak rate test criterion.

ANALYSIS OF OCCURRENCE:

In the event of a steam line break outside the primary containment, the

. total leakage possible through the "A" and "B" steam lines would be 156 scfh and 42 scfh respectively. A "C" steam line break would likewise result in a maximum leakage of 6.6 scfh, the value limited by the IC valve. There-fore, the total leakage through this particular steam line from the primary containment would be minimal under accident conditions. .

The excessive leakage rates did not in any way render the MSIV's inoperabic, nor was.the ability of the valves to perform their design function affected.

-Upon receipt of a Group i isolation signal, these valves would have shut in the required time, and performed the isolation function.

The Main Steam isolation -Valves utilize straight i lne flow to provide a good flow pattern and' upstream pressure to aid in valve closure at operating pressure by tilting the actuator toward the upstream side of the valve.

, The balancing _ feature of the valve makes it possible to take advantage I

of the' upstream pressure to aid in holding the valve closed and to have the

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