ML20086S953

AO-S2-74-04:on 740617,during Startup as Reactor Approached 4% of Rated Power,Instrumentation Indicated That Main Steam Nonreturn Valve C Had Not Opened.Caused by Separated Disc Stuck in Valve Body.Valve Seats Lapped

ML20086S953
Person / Time
Site:	Surry
Issue date:	07/15/1974
From:	Stallings C VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.)
To:	Moseley N NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II)
Shared Package
ML20086S947	List: ML20086S953
References
AO-S2-74-04, AO-S2-74-4, NUDOCS 8403060052
Download: ML20086S953 (8)

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i Ur. Wornen C. Moseley, Dircctor Serial No. 203 Directorato of Regulatory Operations POMf/JTBaclw Unit.cd States Atonic Energy Co::cinaion Region II - Suita 818 Docket No. 50-281 230 Peachtreo Street, Northvcot Licenso No. DPR-37 Aticata, Goorgia 30303

Dear Mr. Moucicy:

Pursuant to Surry Power Station Technical Specification 6.6.3.1, the Virginia Electric and Pover Coc:pany hereby culmits ferty (40) cepion of' Abnoreal Occurrence Report No. AO-S2-74-04.

The cubstance of this rcport han been revicued by the Station Nucicar Safety e-nd Operating Con:nitteo cud will be placed on the agenda for the 3

next necting of tha Systca Nuclear Sciety and Operating Crm'itteo.

Very truly yourc,

&,t/ ,' ' lyt s't fn C. M. Stall.ings ,

Vico Procident-Pcvor Supply and Production Operationa l Enclosurco 40 copica of.A0-32-74-04  ! +

cc: Mr. K. R. Collor, Assistant Director v

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for Operating Reactors 6533

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i ABNORMAL OCCURRENCE REPORT 4

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REPORT NO. A0-S2-74-04 i

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MAIN STEAM NON-RETURN VALVES r

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O. O I. INTRODUCTION In accordance with Technical Specification 6.6.B.1 for Surry Power Station, Operating License Number'DPR-37, this report describes an abnormal occurrence which was identified on June 27, 1974. The Directorate

, of Regulatory Operations, Region II, was notified on June 28, 1974.

The occurrence reported herein is classified as an abnormal occurrence pursuant to Technical Specification 1.0.I.6 which states that "An abnormal occurrence is defined as: Engineered Safeguard System malfunction or other component or system malfunction which rendered or could render the Engineered Safeguard System incapable of performing its intended safety function." The occurrence described herein resulted in the failure of the three (3) main steam non-return valves (NRV's).

II.

SUMMARY

OF THE OCCURRENCE On June 17, 1974, following an extended shutdown period for maintenance and reactor coolant pump modifications, unit start-up commenced. As the reactor approached approximately four'(4) per cent of rated power, instrumentation ~ indicated that main steam non-return valve "C" had not opened. The unit was brought to a_ hot- _

shutdown condition to' free the valve. The main steam trip valves

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were closed and the' secondary-steam header pressure reduced using the steam dump valve. With a pressure differential of 225 psi.across' the valve, the valve opened. The "C" NRV was cycled utilizing.the motor operator and steam, pressure, and all parameters indicated the valve.was fully operational. Therefore,. start-up was resumed.' ,

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As reactor power increased, temperature; pressure and steam flow

. Indications from "B"' steam generator were different.than'those oft s team - generators "A" and "C.

'It appeared that1"B" NRV'was not

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fully opening with the steam flow. -Attempts were made to increase the flow through "B" NRV by manipulating the valve piston and to increase the differential pressure across the valve by increasing the power level. Both methods proved' unsuccessful. The unit was brought to cold shutdown condition and all three (3) non-return valves were dis-

mantled for inspection. The valve' manufacturer, Rockwell Manufacturing Company, was contacted and a representative was present when the valves were dismantled. As the piston / disc unit was removed from

"C" NRV, it was discovered that the disc had separated from the assembly. Subsequent investigation of "A" and "B" NRV's revealed similar failures. Following onsite preliminary evaluations, the disc and piston parts were shipped to the manufacturer.

111. ANALYSIS OF OCCURRENCE Three (3) non-return valves are' installed in the main steam a

lines between the main steam trip valves and the main steam header, and are located outside the containment in a missile-protected-enclosure. The internals of the _stellited disc are joined to the

, piston by means of a filleted butt weld at the disc. Tlie function 4

of the piston / disc' unit.is to prevent reverse steam flow. The valve

. ster is normally withdrawn to permit unobstructed movement of the' t

piston / disc, although, when-necessary, it is inserted against the disc'to' block steam flow.

i Following1 the removal of.the piston'_section from "C" non-return valve, station personnel performed a dye penetrant test to-determine -

j if additional damage' to the internals had occurred. An internal-t -

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rh y (v inspection of "B" non-return valve revealed that the separated disc had stuck at a point in the valve body. This had prevented full steam flow through the valve.

An independent testing laboratory was requested to perform an analysis of the valve failure. Their preliminary findings indicated that a crack had initiated at the inside surface of the piston assembly, above the weld that joins the disc to the piston. This crack propagated by fatigue, until failure occurred. In addition, it was noted that a thinning of the piston wall had occurred at and around the point of failure.

The valve manufacturer, after preliminary evaluation of the failed piston / disc assemblies determined that the failure was the result of nonuniform forces imposed on the disc during prolonged unit start-ups. The steam initially produced at start-up has a high moisture content. This low quality steam impinges on the disc, imparting a greater force than normally experienced with the production of high quality steam. Steam builds up beneath the disc until sufficient pressure has been produced to lift the disc / piston assembly off its seat. The lift force on the disc is greater at the point of steam exit than at any other point on the disc. This nonuniform force exerted on the disc is increased by the steam's high moisture content. During a unit start-up, this cyclic pressure buildup and release, or perking, may continue for several hours.

This perking of the valve disc, together with thermal gradient buildup, caused the failure.

IV. CORRECTIVE ACTION TO PREVENT RECURRENCE The damage to the "B" non-return valves body was corrected by grinding and the valve seats were lapped.

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As prenously stated, the valve manufacturer was contacted, and a

design modification was developed. The modification will consist of re-placing the Schedule 100 pipe of the present piston assembly with Schedule i 160 pipe, and installing a transition. piece between the assembly and the disc. The transition piece will afford the machining of a larger radius from the piston to the disc, thus reducing thermal gradients and lowering stress intensity. The larger radius and the increased wall thickness of the piston will enhance the dampening of nonuniform forces normally experienced during start-ups.

Three (3) piston / disc assemblies of the old design have been in-stalled on Unit Nos. 1 and 2. With the installation of the old designed internals, a maximum limit of 75 hours8.680556e-4 days <br />0.0208 hours <br />1.240079e-4 weeks <br />2.85375e-5 months <br /> " perking" time per -

valve has been established to prevent failure of the valves due to prolonged periods of stress. This limit is approximately 25 per cent of the " perking" hours experienced by the valves which failed. In the event that this limit is attained, prior tn refueling in September 1974 for Unit No. I and November 1974 for Unit No. 2,:the unit will i

be shutdown and ;he new piston / disc assemblies will.be installed.

The valves will be modified on Unit No. I and Unit No. 2 during the refueling outage, scheddled for September and November 1974,,respectively.

The non-return valves on Unit No. I were disassemblied and inspected, with no degradation detected. However, as a precautionary measure new piston / disc assemblies of_the old design were installed.

The licensee has experienced no previous failures of the' main steam non-return valves (Rockwell Manufacturing, Model 607 MY), and considers the proposed modification of the piston / disc. assembly to be 1

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V. ANALYSIS AND EVALUATION OF SAFETY IMPLICATIONS OF THE OCCURRENCE Each main steam line has a fast closing trip valve and a non-return valve. These six (6) valves prevent blowdown of more than one steam generator for any steam break location, even if one valve fails to close. The occurrence described herein could have rendered a portion of the engineering safeguards system incapable of performing its intended function. However, the main steam trip valves were operable the entire time and would have precluded the blowdown of more than one steam generator.

Based on the above, there were no significant safety implications associated with this occurrence.

V1. CONCLUSIONS The licensee concludes that:

1. The failure of "A", "B", and "C" non-return valves internals was due to cyclic fatigue stresses ex-perienced during prolonged start-up periods.

2. The occurrence described herein did not affect the safe operation of the station.

3. The occurrence described herein did not affect the health or safety of the general public.

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