ML20086N110

Special Rept:On 911111,differential Pressure Indications Showed That One of Two Primary Coolant Isolation Valves Did Not Seat Properly.Caused by Valve Steam Falling Out of Valve Actuator.Actuator Key Replaced & Bottom Slot Recrimped

ML20086N110
Person / Time
Site:	University of Missouri-Columbia
Issue date:	12/10/1991
From:	Mckibben J, Meyer W MISSOURI, UNIV. OF, COLUMBIA, MO
To:	Office of Nuclear Reactor Regulation
References
NUDOCS 9112190004
Download: ML20086N110 (4)

Text

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Research Heactor FacAty w w, no 1 CtA4f fhet f.Ma tJf l [1]

UNIVERSi1Y OF MISSOURI-COLUMBIA *"Q, $$

December 10.1991 Director of Nuclear Renetor Regulation U. S. Nuclear llegulatory Commission Aall Station PI-137 Wr.shington D. C. 20555 itEFI; 'tENCE: Document 50-180 University of Missouri Research Reactor License R 103 SUlkJECT: Report as required by Technical Speelllcation G.I.h(2) concerning reactor operation with one reactor isolation valve inoperable.

DEscit1PTION:

On Nv/cmher 11.1991 while shutting down the primary cooling system pumps and valves for scheduled maintenance day activities. the

- control room operator noted that diderential pressure indleations showed that at least one of the two primary coolant isolation valves had not seated properly.

Step (k) of the pranary 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 abillB to maintain a pressure differential between the reactor side of the isolation valves and the remainder of the primary cooling system (pumps and beat exchangers). The ,

control room operator noted that pressure had equalized a n ass the isolation valves and reported this to the Shift Supervisor, who informed the Reactor Operations Engineer and the Reactor Manager, investigation of the primary cooling system isolation valves revcaled no problem with the primary coolant outlet isolation valve (V507A). but the primary coolant inlet isolation valve (V507H) was in the open positton, while its valve actuator was in the closed position. A square steel key which serves to engage the air actuator to the V507B-valve stem had fallen out, rendering the valve inoperable.

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,e, Director of' Nuclear Meactor Regulation i Da. ember 10.1991 Page 2 Technical Specification 3.9(a) states that the reartor shall not bc  ;

. operated in Moacs 1 (10MW) aud 11 (5MW) unless the primmy coolant isolation valves are operable.

The primary coolant system isolation valves operated properly on November 4,1991, during the reactor and primary system shutdown for '

maintenance. Therefore, the reactor may have operated in Mode 1, with one }

primmy valve not operable, for not more than one week, Proper operation 1 of this valve is rat required for protection of fuel element integrity as will bc  ;

explained below.

ANALYSIS The })rimary coolant inlet valve (V50713) is mounted such that its valve stem is vertical and below the coolant pl )c, The valve actuator for V50713 t mounts under the valve such that the "a ve stem, with a keyway slot, goes through the actuator coupling. The steel key which engages the valve actuator to the valve stem must have fallen out of the valve stem keyway sometime during the week of November 4.1991.

The bottom of the actuator keyway slot is crimped to ;)revent the key from falling out. In this case, the crimp failed to retain the tey. There has '

been only one other failure of this type in MURn 9perating history, in that instance. the actuator to valve stem engagement acy fell out of the keyway for the pool system isolation valve (V500). This failure was reported to the Director of Nuclear Reactor Regulation in a letter dated .lanuary 26,1970.

A : hat time, the crimp of the actuator keyways for V507A and V50713 were ,

m ried to be capable of retaining their valve engagement keys.  ;

The V50713 keyway crimp may have been disturbed by nmintenance  :

e.tivities that occurred on October 14, 1991. The valve actuator for V50713 was removed from the valve' as part of the investigation of V50713 val';c and actuator to ensure it did not have a generic problem similar to V507A, which failed on September 30 1991. The failure of V507A and the >

investigation of V507B condition are detailed in a report to the 1)lrector of Nuclear Reactor Regulation dated October 29,1991.  ;

The valve actuators had previously been removed from V507A and V507D for refurbishment. This occurred during October 20 to November 5.

1989, a one week scheduled maintenance period for replacing the beryllium' <

reflector. After that maintenance work, no problems were experienced w'th l the actuator keyways.

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.* Diroctor of Nuclerr !<eactor itegulation Decca.3er 10,1991 Page 3

, The original llazards Summary Report. Section 13.2.01hipture of Reactor Loop and llazards Sunnnary Report, Addendum 4, Loss of Coolant l

Analysis describe the first line of protection against loss of core water resulting fr om rupture of the reactor coolant loop piping as being provkled by the check valv( IV502) on the rcactor coolant inlet line and by the invert loop with siphon break system on the reactor coolant outline line. The prinmry coo ant isolation valves are described as How stop valves installed in the reactor octiet and inlet line. The isolation valves are located inunediately outside the shleid wall so that a line rupture between the core and the valves is improbable unless it occurs within the pool.

The basis for the Technical Specification [3.9(all dmt requires the primary coolant isolation valves to be operable during Mode 1 (10MW) ,

operation states; The primary coolant isolation valves are Lcated in i

. the inlet and outlet reactor coolant lines as close as e practicable to the biological shield. Proper operation '

of these valves is not required for protection qffuel clement -integrity, however, their operation provides ,

a means for isolation of the in-pool portions of the reactor coolant systemfrom the balance of the system.

The proper operation of the check valve (V502) on the reactor coolant ,

inlet line was repeatedly tested after V50713 was found to be inoperable. i The check valve seated each time tested with a reverse differential pressure (core to loop) and maintained the differential pressure. This indicates that it was capable of providing the first line of protection in the reactor coolant inlet leg against loss of core water in the case of ? rupture of the reactor coolant loop, i

CORRECTIVE ACTION -

The inunediate corrective action on November 11,1991 upon finding that the V50713 actuator to valve stem engagement key had fallen out was to test the V502 check valve on the reactor coolant inlet hne to ensure it was operable. After this test was completed satisfactorily, the V50713 valve to -

actuator key was replaced, the bottom of its actuator keyway slot was recrim >cd and V50713 was tested repeatedly to ensure it was operating proper y. The two other valves of similar design (V507A and V509) were-also checked to ensure their keys were in place and that their keyway slots were sufficiently crimped to prevent their <cys from falling out.

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i Director of Nuclear Itcactor llegulation  ;

Deceniber 10,19til l' age 4 On November 18.1991 (the r. ext week's innintenance day), a new design for retalning the actuator to valve stein engagernent key was installed on V507A, V50713 and V509. Thfs design attaches a part to the valve niems of V507A and V507B and to the actuator on V509 to physically block the key from dropping out of the 1.cyway. An annual preventive maintenance check has been set up to check the f 4ghtness of the screws that hoki the V507A, V50713, and V509 key retainers in place. Standard Operating Procedures for the primary system startup and shutdown that occur each week check the operability of V507A and V50713 and hence verify that thele valve to actuator keys are retained.

As additional information, the primmy coolant inlet isolation valve (V5078) was replaced as part of the November 18,1991 maintenance activities.

Sincerely. j' k

Walt A. Meyt! Jr.

Reactor Manager ENDORSEMENT: .

m Reviewed and Approved .

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J. C. McKibben Associate Director xc: Regional Administrator. NRC, Region ill _

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