ML19312D186

Forwards Turbine Disc Integrity Task Force Concensus Response to Generic Questions Contained in NRC 800225 Ltr Re Turbine Disc Integrity.Portions Withheld (Ref 10CFR2.790)

ML19312D186
Person / Time
Site:	North Anna
Issue date:	03/14/1980
From:	Schmerling J WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP.
To:	Eisenhut D Office of Nuclear Reactor Regulation
Shared Package
ML19312D185	List: ML19312D184 ML19312D186
References
NUDOCS 8003210426
Download: ML19312D186 (29)

Text

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LWestinghouse i Power Generation - N' "^

Electric Corporation - Group .e. t c eb : 0 -

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March 14,1980 Darrell G. Eisenhut, Acting Director

. Division of Operating Reactors

U.S. Nuclear Regulatory Commission Washington, D.C. 20555 1

Dear Mr. Eisenhut,

LYour letters of February 25,1980, to licensees with operating Westinghouse steam turbines requested ee. tain site specific and generic information relativc to turbine dis _c integrity. You urged in your letter that the lleensees address the generic questions and coordinate the responses through an owners' group.

2 Licensees with nuclear power plants and Westinghouse steam turbines have formed a Turbino_ Dise Integrity Task Force, with Mr. Wayne Stiede of Commonwealth Edison Company selected as Chairman. Westinghouse has been working with this .

Task Force to generate responses to your generic questions.

At a Task Force meeting on March 12 and 13,1980, the utilities present prepared and approved. consesus responses to each of your generic questions. The Task Force further directed Westinghouse Electric Corporation to transmit these re-sponses directly to you. The purpose of this letter is to transmit that information

= to you.

It is our understanding that Mr. Wayne Stiede, Chairman of the Task Force, will ,

also confirm to you by separate letter, the Task Force's decision to have Westing- 1 house transmit these responses direct to you. We.also understand that each utilit-y, in their specific response to your letter to that utility, will discuss the extent to which they agree with these consensus responses.

If you have any questions on these, please contact me.

Sincerely,

. M. Schmerling,

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Dise Integrity Prograr Manager '

ec: W.~J. Ross, Operating Reactors Branch -

USNRC, Washington D.C 20555

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. s t GENERIC QUESTIONS -TO BE COMPLETED IN 20 DAYS

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1. j Describe what Lquality c'ontrolland . inspection- procedures are used for the ~ disc j bore and keyways. ;

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ANSWER:

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s Chemical; analyses :aie~ made from each heat- ofIsteel. During ma_nufacture mechanical tests'are made from the disc bore regio'n. These include tensile and

Charpy.v-notch impact tests. Each disc bore region is subject to ultrasonic and imagnetic particle inspections.' On later units, the disc keyways are inspected

after machining, using liquid penetrant techniques.

For in-s'ervico-inspection two ultrasonic techniques, namely -the tangential aim-and radial aim scans, have been developed to detect and determine the depth.of

" disc: keyway and bore cracks. ' The in-service ultrasonic inspection does not re-quire unshrinking discs from the rotor.

. The tangential. aim scan is used to locate cracks. The technique requires sound .

energy to be coupled. and' directed ~ tangentially towards the keyway from a precalculated position on the hub. This is accomplished by means of a compound angled plexQalass wedge. The wedge is machined to provide a contoured face which makes complete contact with the disc hub,' while ' aiming the sound energy

> at the disc bore / keyway. Crack indications occuring _in the vicinity of the key--

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way apex and at the' bore will reflect the sound energy. The tangential aim scan is performed both in the clockwise and counterclockwise directions to permit

' locating crack indications with respect to the keyway apex.

A radial aim t'echnique'is used to. confirm cracks located by the tangential' aim

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scan.- The. technique is also'used to determine the crack depth by comparing the-time lapsed in obtaining a-ultrasonic reflection from the crack with the time to obtain a reflection from the keyway or bore.

cII. Provide; details of1 the -Westinghouse repair / replacement. procedures for faulty dises. -

? ANSWER:-

p qWhen cracks are found b an inservice inspection their severity is evaluated by 7, ;means of an: allowable ~ life ceiculation. The allowable life is relatable to the time required for the crack to grow to cHtical size for fracture., Based upon the

. results of, this calculation,' the following actions may be taken:- .

TAL 5If!the-affected dise has a calculated allowable life greater than zero a -

. reinspection of the'~ disc is= recommended at approximately one-half of the allowable life.;

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JB. Ilf the ~affected disc ~ $s an~ allowable life less than or close to zero, one Cor more of the following may be employed:

11e LThe affected disc is removed by " machining", and is replaced with ia collar and pressure drop baffle.

2.  ? Upstream keyways may be drilled oversize to remove cracks after.

.the downstream disc is removed.

< 3. The affected disc may be replaced . f This requires unstacking and restacking several discs on the rotor.

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III.A. . ' What immediate 'and long term actions are being taken by Westinghouse to mini-

.mize future stress corrosion problems with turbine discs?

rANSWER:

The following sh' ort range actions are being taken:

'-1, lThose discs'which have been observed to be most susceptible-to-stress' corrosion cracking are being redesigned. The new designs will achieve lower bore stresses and utilize lower yield strength -

material. These changes will increase the margin against stress corrosion' eracking.

2. Designs that will' eliminate spacers and bore keyways are being explored.

The following long range solutions are being examined:

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1. , - Bore Heating Ways and means to keep the dise keyways dry are

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being explored.-

2.- Sealing ~- ' Ways .of ' sealing the hub and bore from the steam environment are being studied.

'3. Coatings - Another method of sealing is to apply a protective

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coating. nWe are continuing to experiment with different coatings,-

but extensive' work is still required to develop processes for their

' application and to demonstrate their benefits.

- 4. Partial Integral Rotors - Since one piece forgings cannot be pro-cured:at this time,twe are exploring the possibilities of. partial integral rotors 'wh'ere;the first two or three discs qre made a part

- of the shaft. 'Only the last few discs will'have to be shrunk on. '

w 5.- - Integral Rotors -[A welded rotor design is being evaluated as a means to produce'an integral rotor.

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- , ;III.B.' . LWhat Lactions are being recommended .to utilitiesito ' minimize stress corrosion

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. icracking? :

m ANSWER:1 (Westinghouse has developed recommended limits for steam purity. When these.

" limits are1 exceeded corrective actions should be taken.

- MIV.A; fidentify the' impurities lknown.to cause cracking in the low pressure turbines, and

, itheir sources.-

JANSWER:-

(The main [ chemical:speel'es known to 'cause or contribute to stress corrosion of steam turbine materials in steam environments are:-

Sodium. hydroxide

-Sodium chloride .

Sodium sulfate

Oxygen i

< ' The sources of these impurities are under study.

lIV.B. 1 Discuss the relationship between steam gererator chemistry and steam chemistry

relative to ~ the introduction ~of corrosive impu-ities into the ' turbine, including aphosphate,' AVT, and BWR chemistry.

ANSWER:

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. Analyses of material within LP disc cracks from PWR units shows the. presence -

of Na, K,-Ca, SliC1, OH, and C together with~Fe, Co, V, Al and Ni ions.

!In1PWR. units .with recirculating steam generators, the total carry-over of non-s volatile dissolved solids, such as NaOH_ and Nacl depends mainly on.the mechani-cal carry-over. . However,' where ammonia is used for pH control such as with the

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_ all volatile water treatment, carry-over of anions may increase due to a forma-tion of volatile' ammonium salts..

J in the PWR: units with once-through steam generators, the high pressure turbine

' steam. p.urity-is similar to the feedwa+ :r purity. Most impurities entering the .

steam generator are carried directli .ato the turbine.

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- The publishedinformation on.BWR systems indicates the concentration of oxygen tin the:steamlis'in the' range of 10 to 30. ppm. :With respect to other elements,

• . ?however, it-is likely that.high steam' purity standards will be maintained for con'-

trolLof radioactivity.WTo achieve this, BWR' reactor ' water is generally double . -

fdemineralized. -

. . _ i IV.C.t : Discuss the1 mechanism of deposition'of these impurities that can lead =to'their- l

$~ . concentratio'n in certain areas of keyways and bores.

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' ANSWER:

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The impurities from' steam can get into shrunk-on disc bores and keyways in sev -

- eral possible ways:1

~1.  : Af ter ' deposition._in the. steam' path during ~ operation, corrodents can wash into disc keyways during layup due to moisture condensation.

J 2. - 1In:the wet ~ steam regions, the moisture can dry on hot metal surfaces.

( 3. LAs .long as the disc 1 retains. its shrink fit we are not aware of any-mechanism 'which can concentrate impurities on the bore.

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V 1.' :What 'r' ole does the refluxing action in the s_ team separation portion of the steam generator have on scrubbing corrosive. impurities from the steam?

- ANSWER:n

' Two modes of transport of corrosive impurities from the steam generator. to the 1 turbine are mechanical entrainment and volatility.

The, non-volatile. chemical species are transported by mechanical entrainment which is normally. expected to be small.

.The steam generator scrubbing equipment has minimum effectiveness in preven-1 ting the transport of volatile impurities, such as ammonium chloride, to the' tur-bine. The' concentration of volatile impurities in turbine steam is determined by

, l their concentration in the steam generator bulk water and their specific volati-

.lity coefficient which differs with each species.

VL - To what extent can the buildup of corrosive impurities-in the LP turbine be alle-

'viated? What would be' the effects of the following action:

- A. - Pumping moisture separator condensate to condenser?

ANSWER:

Pumping moisture separator condensate to the condenser would be beneficial in-units with condensate polishing . In units without condensate polishing, there will

- be no effect.-

B. -

Periodically: moving (the) point-of condensation to prevent locallized. buildup of

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corrosive impurities.

3 ANSWER:. _

l 1 Conceptually, dilution'of contaminants by increased levels of moisture and their i subsequent. transport to the condensate system: could substantially' reduce the -

buildup of ' impurities. -However,(the . cffectiveness of this technique and. the means-for_ successful control of the local environment of particular turbine parts lmust be developed and experimentally verified.

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~:Several of the less volatile active corrodants,~such as sodium chloride and sodium 1 m i sulphaWprecipitate ss cone'entrated liquid solutions in' a_ region.slightly above

the.. equilibrium'~ saturated vapor.lin'e of pure water. This region occurs locally-

-within 'a given. stage 'during normal operation 'and m.igrates .toward the turbine

2. exhaust'as load reduces. iControl of the zone' can be affected by changes in load -

L and nioisture separator ' reheater (MSR) outlet' temperatu're.

-- VIl ~' . Describe? fabrication 'and heat; treatment sequence for' discs, including thermal.

4 exposure during shrinking operation.

6 ANSWER:

- [The typical sequence for producing a disc forging includes the following opera-

<- L tions, not all of which are necessarily applicable to any given disc.

! A. Melting and casting of Ingot. Most discs manufactured since the early 1960's 1are.made using basic electric furnace steel which is vacuum stream degassed or

vacuum-carbon-deoxidized.-- .

LB.; Forging The ingot is heated to forging temperature, block forged and cut into 2 to 4 pieces from which the individual _ disc forgings are made.

I" C.: Preliminarv Heat Treatment This step consists of austenitizing and t.emper- .

ing the forging' to promote structure uniformity, grain refinement, and good machineability.

D. . Preliminary Machining The forging is machined to the disc contour.

. E. : 2 Preliminary . Ultrasonic Inspection Typically the supplier makes a partial ultrasonic inspection of the forging to assure that the quality warrants ~ continued !

manufacturing effort.

. F. JHeat Tree' tent for Propertig The forging is austenitized and tempered at

. approprir , ..mperatures to achieve the desired mechanical properties. Cooling from the austenitizing treatment is achieved by water quenching. After temper-ing the forging is. cooled in the furnace at a controlled rate.

G.. Mechanical Properties Tensile properties _are tested to determine if the re-quired strength level ~has b'een achieved. Since about 1960, Charpy v-notch im-

_ pact tests are made on each forging.

H. NDE Inspection _ The forgings'are rough machined to the Westinghouse draw--

ing requirements and'an ultrasonic inspection of the flat surfaces of the hub, Lweb, and rim _of the disc is performed.

LI.S Stress' Relief This treatment is requir'ed when a.significant-amo~unt of metal is t -

, machined off of.the forging'after it has been heat' treated for properties. Thei

~ stress relief.' treatment is 50-100 0, F. below the tempering temperature. ' Cooling -

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< is accomplished by a controlled furnace cool. ~

' lJ.EMechanical Properties'- When a stress relief is.used, the mechanical properties-

-are tested aft er the stress relief treatment. -(Reference Step G)'

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(KNDimensional ChecklThe forging is machined to a cle'an surface, the balance

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of; test prolongations- are removed, and the' dimensions checked.- The' forging is -

Jthen shipped.to.Wastinghouseifor final machining and ass.embly onto the rotor.

m L 'NDE Inspection-l A fluorescent magnetic: particle inspection is performed Tafter finish machining (This inspection.was n,t applied during the early 197.0's.)

M.' Shrinking Discs On the Rotor Shaft Thel assembly operation consists of four-parts; namely, preparation.of the shaf t, preparation of the discs, assembly of the rotor and pinning of the discs to the shaft. :

1 ; ' Preparation of the Shaft? After final shaft machining and inspec-tions artcomplete the shaft is cleaned:with degreaser and dry -

-_ lint-free cloths, and is mounted in a vertical position. The surface

- of the: rotor that,will.be .in contact-with the disc is coated with-lubricant

2. - : Preparation of the Disc. After final machining and inspections'

are complete. the disc surfaces and blades are cleaned to remove fore ~ ign material. Prior.'.to ' heating ior . assembly : the : disc bore

! diameter 11s measured and compared to that of the drawing to-assure :a correct shrink fit. The disc is placed on an assembly

, fixture, leveled and~ loaded into a furnace which is at '3000 F or -

_ -less.

!3; Assembly of the Rotor The disc.is slo.wly heated to the required-shrink temperature between'6000 and.750 0 F. When the shrink temperature'is reached-the disc is removed from the furnace and lowered onto the shaft.

4.- Axial Aligning and Pinning: of Discs. Liners are placed at. the-

' exhaust face of each disc to assure the proper axiallocation. The keyways.are then drilled. Since the early 1970's, a penetrant in-spection is performed in the keyway prior _ to inserting the key.

/ VIII)  : Discuss the 'effect of any local residual stresses on the cracking mechanism.

< ANSWER:

1. < D'epending'on their' nature'and magnitude,' residual surface stresses can have'an L T . effect on crack initiation.~ Proper contro1~ exercised in the selection of machining K ~_  : parameters results in compressive stressesiwhich .are~usually beneficial. LAt the

. apex of-the keyway,;the residua 1' stresses may. be influenced by local yielding as'a result _of the ' stress .cencentrating action o,f the key'way. -

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