ML20079L719

Ro:On 910930,primary Coolant Isolation Valve V507A Took Longer than Normal to Close & Differential Pressure Indications Indicated Valve Failed to Seat Properly.Caused by Valve Stem Binding.Valve Will Be Replaced

ML20079L719
Person / Time
Site:	University of Missouri-Columbia
Issue date:	10/29/1991
From:	Mckibben J, Meyer W MISSOURI, UNIV. OF, COLUMBIA, MO
To:	Office of Nuclear Reactor Regulation
References
NUDOCS 9111070060
Download: ML20079L719 (4)

Text

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4 Resomch Reactor Facihty II nesevan rww cuumni w,som ccm UNIVERSITY OF MISSOURI-COLUMBIA October 29,1991 k""D'y ] Q Director of Nuclear Reactor Regulation U. S. Nuclear Regulatory Commission Mail Station Pi-137 Washington, D. C. 20555

REFERENCE:

Document 50186 University of Missouri Research Reactor License R-103

SUBJECT:

Report as required by Technical Specification G.I.h(2) concerning reactor operation with one reactor isolation valve in a degraded condition.

DESCRIPTION:

On September 30,1991 while shutt!ng down the primary cooling syste.n pumps and valves for scheduled maintenance day activitten. the Shift Supervisor noted that the primary coolant outlet isolation valve (V507A) seemed to take longer than normal to close and that differential pressure indications showed that one of the two primary coelant isolation valves had not seated properly.

Step (k) of the primary cooling system shutdown procedure (SOP IV.2.1) provides a check of the capability of the primary coolant isolation valves (V507A and V507B) to seal by their ability to maintain a pressure differential between the reactor side of the isolation valves and the remainder of the primary cooling system (pumps and heat exchangers). The Shift Supervisor noted that pressure had equalized across the isolation valves and reported this to the Reactor Operations Engineer and Reactor Manager.

Investigation of the primary cooling system components revealed that the primary coolant outlet isolation valve, V507A (a 12" diameter butterfly

-valve), was capable of closing to within a few degrees of its fully closed and se- ed positjon, but would not consistently seat to hold a pressure d rential as specified in SOP IV.2.1.k.

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Mrector of Nuclear Reactor Regubtion Octotx r 29,1991

- Page 2 Technical Specification 3.9(a) states that the reactor shall not be operated in Modes I (10MW) and 11 (5MW) unless the primary coolant isolation valves are operable. Operable is defined in Technical Speelfication 1,12 as follows: "A system or component is operable when it is capable of performing its intended function in a nonnal manner," Since the primmy coolant outlet isolation valve (V507A) was capable of closing, but was not able to consistently maintain differential pressure as specified in the Standard Operating Procedure, the valve may be considered operable but in a degraded condition, The primary system isolation valves operated properly on September 23,1991, the previous week's reactor and primary system l shutdown for maintenance, Therefore the reactor may have operated in

! Mode I for one week, at most, with one primary isolation valve in a degraded condition, Proper operation of this valve is not required for protection of fuel element integrity, This will be explained in the Analysis section below, ANALYSIS Reactor and Facility Operations staff perfonned an extensive investigation of tha V507A valve actuator and the three way air control solenoid valves that provide the opening force for the isolation valve, as well as, the air exhaust path during valve closure. The actuator for V507A has a spring to clos" the valve, The closing force of this spring was tested and the air exhaust p :h was verified clear in the course of investigating the

! problem.

After investigations showed that the valve actuator was functioning normally, it became apparent that the V507A valve required replacement, Reactor Operations staff prepared procedures for replacing the V507A valve,

while IIcalth Physics and Machine Shop staff staged themselves for the round the-clock maintenance effort. This effort resulted in the V507A valve being replaced at about 1500 on Tuesday, October 1. After testing and ,

recovery operations, the reactor returned to 10MW operations at 0307. on Wednesday, October 2. No attempt was made to work on the V507B valve-l since it demonstrated nonnal closure times and was seating properly.

During disassembly of the V507A valve for problem evaluation, the upper valve stem packing was found to be extremely hard and embrittled which is most likely due to age and radlauon effects. Once the packing was removed, the valve couki be actuated with nominal torque to its fully seated -

position The valve seat showed some signs of deterioration due to age, but the valve when placed in its closed and seated position was able to hold

' differential pressure of 40 psi for 30 minutes with no leakage. This testing indicated that the difficulty in consistently seating the V507A valve was due to valve stem binding 'on pieces of the embrittled stem packing.

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Director of Nuclear Reactor Regulation Octotrr 29.1991 Page 3 The original llazards Smmnary Report, Section 13.2.6 Runture of Reactor Loop, and llazards Sununary Report, Addendum 4, Loss of Coolant Analvsis describe the first line of protection against loss of core water resulting from rupture of the reactor coolant loop piping as being provided by the check valve (V502) on the reactor coolant inlet line and by the invert loop with siphon break system on the reactor coolant outlet line. The primary coolant isolation valves are described as flow stop valves installed in the reactor outlet and inlet line. The valves are located immediately outside the shield wall so that a line rupture between the core and the valves is improbable unless it occurs within the pool.

The primary system controls are designed such that the primary isolation valves (V507 A/B) automatically close and the antisiphon system valves (V543 A/B) automatically open when low pressure in the primary system is sensed by redundant pressure switches, PS 944A and B.

Technical Specification 3.0.(a) states that the reactor shall not be operated in Modes I and 11 unless the primary coolant isolation valves are operable. The basis for Technical Specification 3.9 (page 3 of 3) states:

The primary coolant isolatiott valves are located itt the inlet and outlet reactor coolant lines as close as practicable to the biological shield. Proper operation of these valves is not required for pro- tection offuel element integrity, however, their operation provides a meartsfor isolation of the in-pool portiotts of the reactor coolant system froin the balance of the system.

With a problem due to valve stem binding, the V507A valve was still capable of closing to within a few degrees of its fully closed and seated position, as evidenced each tune it was tested during problem investigation.

This position is fully capable of stopping primary flow. Therefore the V507A isolation valve, during the week of reactor operation from September 23 to

- September 30, was operable but in a degraded condition.

1 Operations staff, subsequent to the V507A replacement, continued to test and evaluate the reactor inlet isolation valve (V507B) and its actuator to verify that a generic problem did not exist. On October 14,1991, the actuator for the inlet isolation valve (V507B) was removed so that its i actuator could be tested mid its closing / seating torque could be measured.

The seating torque for 507B (~ 150 ft. Ibs) was greater than that of the new 507A valve (~ 75 ft. Ibs), but was less than the torque it took to seat-the old 507A valve (> 300 ft. Ibs) and well within the minimum torque capaelty of its actuator.

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L Director of Nuclear Itcactor llegulation October 29.1991 Pap,e 4 Evaluation of these findings, along with the normal closing times for V507D, have reinforced the engineering staff determination that a similar problem with V507B was not indicated.

CORRECTIVE ACTIOE The inunediate corrective action was the investigation of the reactor coolant outlet isolation valve (V507A) seating problem and, ultimately, the replacement of this isolation valve. Before resuming reactor operation, the Operations Engineering stalT evaluated the closing times of the reactor inlet isolation valve (V507B) to determine that it did not indicate a similar

. mble:n to that experienced with the V507A valve.

Reactor Operations will continue to monitor the V507B valve closely aen performing the step of the reactor cooling system shutdown pro-cedure that detected the V507A problem, SOP IV.2,1 k. Additionally, the closure time for the V507B valve will be measured at each weekly shutdown.

If either of these measures indicates a problem with the V507B valve, it will be replaced before further operation. Othenvise, replacement of this valve will be scheduled to coincide with other maintenance activities for reasons of AIARA.

Sincerely, ,

'hd ,

Walt A. Meyer, Jf Reactor Manager ENDORSEMENT:

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Reviewed and Approved ,

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,f J. C, McKibben (f Associate Director b, - -

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xc: Regional Administrator, NRC, Region Ill Reactor Advisory Committee ums'$[m$hsax camn Reactor Safety Committee m comssem to. un 14.im , ,

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