ML19095A244

Letter Response to Request for Additional Information on Summary of Quality Control on Loose Steam Generator Tube Plugs

ML19095A244
Person / Time
Site:	Surry
Issue date:	08/21/1978
From:	Stallings C Virginia Electric & Power Co (VEPCO)
To:	Harold Denton, Schwencer A Office of Nuclear Reactor Regulation
References
Download: ML19095A244 (21)

Text

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1, VIRGINIA ELEc~i.n~.-AND PowkR C~M~~~; -*

RICHMOND,VIBGINIA 23261 August 21, 1978

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Serial No.>4-~0A PO&M/DLB: a.:;~~::;

~r. Harold R. Denton, Director Office of Nuclear Reactor Regulation Attention; Mr. A. Schwencer, Chief c,.)r -...-:--

Docket No.::L:~i(t-'281 u A,

S Operating Reactors Branch No. 1 U. S. Nuclear Regulatory Commission Washington, D. C.

20555 License No. ~fR-37

Subject:

Steam Generator Tube Plugs Surry Unit No. 2 C)

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Our letter of July 27, 1978 summarized the occurrence of two loose steam generator tube plugs on Surry Unit No.* 2.

This letter provides additional in-formation as requested by Hr. J. D. Neighbors and others of your staff during a meeting on this subject held on August 3, 1978,.

The following information is attached:

1.

2.

3.

"S.G. Tube Plugs *In Vesseln Analysis prepared by Westinghouse Electric Corp. to evaluate po-tential for mechanical damage due to loose steam generator tube plugs in the reactor vessel.

This study was prepared for another utility.

The first sentence which states that eleven.plugs.are missing does not apply to Surry Power Station.

nPlant Operation with Foreign Material *in the RCSn Westinghouse evaluation of. unit operation with* one or more steam generator tube plugs loose in the RCS.

nPossibility of a Steam Generator Tube Plug Lodging in Reactor Coolant Pump Casing 11 Analysis performed by Westinghouse.

Note that the Attachments referenced in this study are not included.

These attachments, which include related calculations, are available upon request.

"Summary of Remarks Concerning Quality Control Hade to :NRG on August 3, 1978, at Bethesdan A summary of Vepco Quality Control practices relative to steam generator tube plugging at Surry Power Station.

Prepared by

e VIRGINIA ELECTRIC AND POWER COMPANY TO Mr. Harold R. Denton Page No. 2 Surry Power Station QC personnel.

As explained on page 3 of this summary, certain referenced attachments have already been submitted and therefore are not included.

A summary of the effects of loose plugs on the consequences of a steam break was also requested.

The drop. out of a loose plug would expose a poten-tially deteriorated tube to the differential pressure *transient experienced during a steam break accident~

The effects of a steam break and LOCA acci-dents on deteriorated steam generator tubes.have been addressed in two prior submittals.

1.

Vepco letter to Hr. B. G. Rusche*, Director. of Nuclear Reactor Regulation, dated I1arch -25, 1977 :(serial No. 031B/011977).

2..

Westinghouse Report WCAP 7832 nEvaluation of Stea111 Generator Tubes, Tubesheet and Divider Plate Under.Combined LOCA Plus SSE Condition.s, 11 Attachments cc:

Mr. James P. O'Reilly Very truly yours, C. Ivi. Stallings Vice President - Power Supply and Production Operations

COMMONWEALTH OF VIRGINIA CITY OF RICHMOND

)

) s. s.

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Before me, a Notary Public, in and for the City and Common-wealth aforesaid, today personally appeared C~ M. Stallings, who being dt..ily sworn, made oath and said (.1) that he is Vice President-Power Supply and Production Operations, of the Virginia Electric and Power Company, (2) that he is duly authorized to execute and file the fore-going Amendment in behalf of that Company, and (3) that the statements in the Amendment are true to the best of his knowledge and belief.

Given under my hand and notarial seal this <J 5 r day of A.v311/>f

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• My Commission expires jeiVJverv 2 ~, )&J$J

(.SEAL)

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SUMMARY

OF REMARKS CONCERNING QUALITY CONTROL MADE TO NRC ON AUGUST 3, 1978 AT BETHESDA, }ID.

Purpose:

To describe the participation by Vepco Quality Control in steam gen-erator tube plugging evolutions at Surry Power Station.

QC personnel at Surry participate as follows during a steam generator (S/G) tube plugging evolution:

o receipt inspect personnel and material.

o verify leaking tubes(s) on initial hydrostatic test after opening generator.

o verify location and marking of leaking tube (s).

o verify marking of preventive plugging pattern.

o verify proper placement of plugs.

o verify cleanliness, final hydrostatic test, and close-out of S/G after plugging is completed.

o provide surveillance of the explosive powder mixing operation.

It is a regular QC function to perform a receipt inspection of services to assure an adequate qualification of the personnel and material involved in work on a safety-related system.

In the case of the tube plugging operation, Westinghouse services are receipt inspected during each outage their services are utilized.

A repre-sentative receipt inspection package is contained in Attachment l.

Prior to April 1978, Surry QC personnel were not permitted by Westing-house to participate or observe explosive powder mixing due to the "Company Proprietary" classification attached to the procedure by Westinghouse.

Vepco strongly requested declassification of the Westinghouse procedure.

Since de-

r -

classification, QC has provided coverage of the mixing ope~ations during the May - July 1978 outages of Units 1 and 2.

Results of the June 1st and 5th in-spections are contained in Attachment 2.

After we experienced the f.irst "loose plug 11 event last year, QC specula-ted that a probable cause might be attributed to an incomplete burning of the powder in the plug after detonation.

Since we were unable*to observe the pre-paration of the powder, we could not verify its quality.

(For example, if the drying of the nitrostarch had not been in accordance with the procedure, then the subsequent burning would not be complete, and an unsatisfactory bonding of the plug to the tube could result.)

Now that the mixing and plug loading inf or-*

mation is available, we are inspecting the operation to assure its accuracy.

We also feel that the conditions under which Westinghouse persor.nel per-form the mixing and plug loading operations have an effect on quality.

Accord-ingly, VEPCO has provided a cinder-block house for the handling of the powder.

Photographs of the mixing facility and the magazine storage areas are contain-ed in Attachment 3.

A comprehensive check list of events occurring during tube plugging evo-lution has been developed.

This Addendum I to 1~1P-C-RC-67, and a copy is in-cluded in Attachment 4.

A photographic technique to record the condition of the tube sheets was developed by Mr. R. W. Cephas, a Surry QC Staff Member.

A copy of ap article describing this technique is included in Attachment 5.

Representative photographs of the steam generator tube sheets to show the marking verification are contained in Attachment 6.

Close-out photomaps of another steam generator are included in Attachment

7.

This set of photographs contains an error in plug location.

The error was detected by QC during the review of the final photographs.

The plugging of an

-.)-

e incorrect 'tube in this case was attributed to an incorrect marking.

• Westing-house personnel did plug the tube which was indicated:

the error in marking was not discovered during the verification process by the Polaroid cameria technique.

Representative Polariod photographs are also contained in Attach-ment 7 *.

An irregularity occurred during the plugging of Row Column 4C54 in the hot leg of 2C steam g_enerator.

This is shown in the photographs contained in Attachment 8, and a statement of acceptability is also attached.

Quality Control personnel at Surry have been involved in steam generator tube plugging operations since this operation began several years ago.

How-ever, with the recent development of the photomapping technique and*the release of the powder mixing proprietary information several months ago, we feel there is an increased confidence level that the entire tube plugging operation is being accomplished in a satisfactory manner.

We feel that the probability of the incidence of a "loose plug 11 event is greatly reduced for the plugs that are being installed subsequent to May 1978.

The attaclunents included in this sub-mission are to portray our participation and involvem~nt which contribute to the assurance of the safe and reliable operation of this nuclear safety-related equipment.

Attachments:

1.

Recei].Jt Inspection Package for P. 0. 397 30 dated 11/ 18/77.

2.

QC Inspection Guide for Explosive Powder Mixing.

3.

Explosive Powder Mixing Facilities.

4.

Tube Plugging Check Off Sheet.

5.

"Steam Generator Photomapping Technique" an article prepared by by R. W. Cephas, QC, Surry Power Station.

6.

Photomaps of explosive plug irregularity.

(Attachments 1, 2, Li-8 have been submitted separately to Mr. D. Neighbors)

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-1 ATTACHMENT 3

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-*.-.*r EXPLOSIVE POWDER MIXING--AND STORAGE FACILITY SURRY POWER STATION UNIT l ----1!~

UNIT 2 ---._l!!:,.

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PO'iIDER MAGAZINES

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. HITRODUCTI OH:

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. I Eleven steam generator tube plugs ar~ missing and believed to be in the reactor vessel.-!' Th~re are t'im possible consequences that could arise from the ex*istence

@(th;se.mi;~ing tube plugs in the.reactor vessel:

Loose pieces impacting upon* the lower internals components

1.

(2)

Some or all of the plugs b~coming wedged between clearances and causing high forces during peri~ds of relative thermal growth The first consequ~nce w~s examined using known or estimated flow velocities in**~~--

the lower vessel plenum and.was judged to not be a serious*problem, provided the plugs ar~ not left in indefinitely. The plugs are too large to enter either the core region or the drive line areas and the chance of small pieces breaking off and.migrating upward is judged to be remote.

for the secon.d consequences severa 1 areas were i den ti fi ed where. the p~ss i bil i ty of tube plugs becoming wedged was consid~red.

• Of tho_se studied~ only one area.

was subsequently judged to have any rea.1 probab1lity of o'cc*tfrring *- and that is the area at the bottom of the vessel i where a clearance exfsts between the vessel and.the* secondary core support base p*1ate (see Figure rt~.

A close clearance exists between the underside of the base plate, at its. per-1hery>> and the reactor vessel bottom head.

This *c1earance is 1.00 inch (nom.)

cold. At the end of normal heatup, this gap has closed to 0.375 inch, and eventually stabilizes at 0.500 inch during steady state operation. Therefore, 1f one or more steam generator tube plugs {approx. O.Z2 inch*at solid end)-

~ere to become \\'tedged bct\\*:een the base plate and vessel bei:o!-'.'.e-or-duri ng hea tup s the constriction aga'inst thermal grm-,th \\':ould cause high, and potentially excessive forces to exist.

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. RESULTS OF J\\W\\LYSIS.

e A stud_y \\*1as peryormcd _to dctermi ne:

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0) Th~ magnitude of forces produce9 *bj a number of weqged tt.ibc plugs 1 *.

(2) ThQpQssible consequences of these forces upon the reactor vessel

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rand interna1s.

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fl..-test was* performed at Forest ~rills~ Pa. to determine.the forc~s ltJ,at would exist

~t given <leflections. For this te~ti the solid end of a tube plug was~com-

/'pressed betv,een two fl.at 304 SS.plates in a load machine.

The attached Figure

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• /. 2 indicates that fore-es qf _49 1 000 1bs. and 72~000 lbs. \\*muld exist at deflections.

of 0.250 inch and 0.375 i~ch, respectively. The 0.250 inch deflection repr~sents

~ :~. the remaining_ vertical growth of the internals *(.relative to the vesse1) once

~1--*-* contact has been ~ade with the tube pl~g; while 0.375 inch is for the end of

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. heatup condition. Correcting for operating temp~rafure reduc~s the above loads

.... -->_'_:.*_*t_o 42~0001bso and.62,000 lbs., per wedged plug.

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• : _i.-._.-.<*.; If more than one ph1g were wedged bet\\*1een the vess~ 1 ~nd *base pl ate, the 1 oad

• *~enerated would i~crease accordingly. Thus if, in the worst cas~, all eleven

.

• tube plugs were 1*1edged beneath the base plate, *the maximum theoretical' load

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• that c.oul d be generated wou*1 d be 682,000. 1 bs. i based upon the. resul ~s of the

• * ** test performed at Forest Hi 11 s.

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0 To assess the possib1e ~onsequences of these forcesi the forlowing areas were

{l) Stresses in vessel bottom head (2) loa.d capacity of.secondary core support energy absorber*

• (3) load capacity of internals hold dOl-m spring.

(4) LongitudinJl stress in vessel shell (5) Stresses in internals core supportf core !Jarrel and core support columns

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a RESULTS OF ANALYSIS_

e A stud_y \\*1as per:formed to determine:

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(1)

Th_e magnitude* of forces produce9

• b_y a number of wedged tube p 1 ugs >>.

(2) Th~ pQssible consequences of these forces upon the reactor vessel rand i nter.na 1 s.

0 A,*test was* performed at Forest Hi11 s ~ Pa. *to determine *the forces itJ,at would exi ~t it given d~flections. For this test, the solid end of a tube plug 0as.com~

/"pressed beb1een ti,,:o flat 304 SS.plates in a* load machine.

The attached Figure

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• /. 2 indicates that fore-es of _49,000 lbs. and 72JOOO lbs. \\*t0u1d exist at deflections of Do250 inch and 0.375 irich~ respectively. The 0.250 inch deflection repr~sents _

-~. the remaining verfic~l growth of the internals *(relative to the vessel) once

• -*--contact has been ~ade with the tube pl~g~ while 0.~75 inch is for the end of -~----~

. heatup condition. Correcting for operating temp~rafure reduc~s the above loads

• • .".:' to 42~000 1bs~ and.62,000 lbs 09 per wedged plug.

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i-.-*-_,:*_~ If more than one p1ug were \\'tedged bet.ween the vessel and *base plate, the load

• *~enerated would increase accordingly. Thus if, in the worst case, all eleven

.* tube plugs were wedged beneath the base plates *the maximum theoretical* load

_ *

• tha.t_c_ould be generated \\*wuld be 682,000. lbs.; based upon the. resul~s of the

• - test performed at Forest Hil'ls.

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_ To assess the possible ~onsequences of these-forcesp the following areas were

• .-. studied:

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{1) Stresses in vessel bottom head

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(2) load capacity of secondary core support energy absorber*

(3)

Load capacity of internals ho1d dmm spring.

(4) longitudinJ1 stress in vessel shell (5) Stresses in internals core suprort, -core barrel and co1*c support columns I -

~~ these five areas p on1A1e first three.were (ound to. ~ignificantly aff cct"cd.

An analysii of the vessel bottom head was p~rforrned using the methods of Bijlaard and Roark *. *This analysis indicates that the 3 *.srn (me_mbr~ne-plus-be~ding) stress limit is reached at a forte of somewhere between 450,000

. and soo~ooo lbs.

Analysis of the secondary core support energy absorber assembly indicates that

assemblyi comprising fotir energy absorber columns, will yield at forces in the I

r~nge of 430,000 to 590,000 lbs., depending on the actual yie1d strength prop-ert*ies of the material tJsed.

Any one column would yield at betwe_en 107,500 and l4?s500 lbs. Yielding of the energy absorber assembly would impair its ability to limit th~ fore~ produced by a postulated core drop accident (core barrel

-.(ailure) to within prescribed *values. :Yielding *of a single energy absorber

---.-column (as t resu1t of eccentric loading under the.base plate.), \\*1hile not ***--**---

• ~ :: de~irablef is not viewed to be as serious as the g~neral yielding case...

. \\'\\\\>.T~e 1oad capa~ity of the *internils hold dovm.s.pring \\*Jas examined for two con-

.*. ditions - steady state operation (mechanical design flow) and the hot pump overspeed ccndition.* During steady state operation a contact force of 512>>000 lbs. exists between the core barrel fl~nge and the vessel ledge

• (Figure 3), while a contact force of 396,000 ib~. exists during hot pump

• overspeed_ (Figure 4). Current design practice is to consider _lOOiOOO lbs: of the contact force as margin against uncertainties, which* leav~s 412,000 lbs.

• and *296,000 lbs. as reserve contact force during steady state operation and

!hot p(imp overs peed~ respectively. Any force.acting upward through the base

~*late."(due to \\tedged tube p1ugs) 'would act to reduce the reserve contact force described above.

If this contact force were overcome by the upward force of the wedged stea~

.. generator tube plugs, the consequences to the internals could ~e serious.

As contact is lost at the vessel c6re support ledge, flow through the resulting

~ap would tend to equalize the pressure above and below the ~ore barrel flange.

This in turn m'ight cause the 101*1cr* int1~rn<1ls to slam d01*m upon the ledge, where the procc~s could repeat

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_.: ~s 412>>000 lbs. If the hot pump overspeed condition

• limiting load reduces to 296~000 lbs.

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The forces *generated by wedged**tube plug's are summarized below.

• .: Condition

\\

* ** *;**.*.*Steady state operation:

Hot pump overspeed:

End of normal heatup:

. Heatup ~ pump overspeed:

Al 1 owab*1 e Force

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412~000 lb. t 42s000 lb.

296~000 lb. f 42j000 lb.

412t0QQ lb. I 62,000 lb.*

296,000 lb.* t 62~000 lb.

..

• Allo\\'table No.

• of Pluos t:.a

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= 9o80 plugs

~ 7.04 plugs

= 6.64 plugs

= 1.77 plugs From the. ~bove it can be seen that if a.hot pump overspee~ cori~ition is con-

.* sidered to occur at the end of a normal heatup~ no more than four (4) wedged

. ;**. steam generator tube plugs can be tolerated beneath the energy absorber base

- *plate,* If the above~postulated t;ansient is not considered viable, then the number of wedged tube plugs that can be tolerated increases to six (61. *

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'I CONCtUSI011S e

The loads discussed above are*surnmarized in*the following*table.

fomponcnt

. i o: Vessel bottom head 2G Energy*absorber assembly

'o 3." Hold do\\*m contact force

\\

I Allowable Load-

'. 450s000 lb. to 500~000 lb *.

430~000 lb. to 590,000 1b.

296~000 lb. to 412inoo lb.

Thus, during steady state operation the limiting load that can be tolerated

is 412>>000 lbs. If the hot pump overspeed condition is cbnsidered~ the

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• 1 imi ting 1 oad.reduces to 296,000 1 bs.

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"'II The forces *generated by wedged*"tube p1uos are summarized below.

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• Condition

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-*~*< Steady state operation:

~

Hot pump overspeed:

End of normal heatup:

Heatup ~ pump overspeed:

A11owab1e Force 412~000 lb. + 42~000 lb.

296t000 1b. 1 42~000 lb.

412~000 lb. + 62>>000 1b.

296,QQQ lb.* f 62iQQQ lbc

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• A'llo\\*table No.

£f Plugs

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.= 9.80p1ugs g; 7.04 plugs

= 6.64 plugs

= 4.77 plugs From the. ~bove it can be seen that if a.hot pump overspeed condition is con-

.* s1dered to occur at the end of a normal heatup 9 no more than four (4) wedged

. ;** steam generator tube plugs can be tolerated beneath the energy absorber base

- *plate~.* If the above-postulated t~ansient is not considered viable!) then the, number of ~tedged tuqe p 1 ugs that ca~ be to 1 era ted increases to l)_iX ( 6 }'.

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PLANT OPERATION WITH FOREIGN MATERI.L\\L IN THE RCS Plant operation with one or more steam generator tube plugs mis-placed in the RCS has been investigated. Potential effects on the plant ~afety or integrity have been evaluated. Specific recommendations have been made in some cases to prevent potential problems from arising.

Tube**p1~,~~)whi'cll enter the RCS from the outlet (cold leg) side of.the steam generator cai1 remain in the steam generator channel head or be pushed aiong by the reactor coo 1 ant fl ow through the crossover 1 eg p*i ping.

No :measureab le

,,impediment to flow is expected *by the presence of one or several jube plugs in the flow path~ Flow velocities in the piping is sufficient to carry plugs along to the reactor vesse1.

Passage of a plug through a reactor coolant pump during operation was also considered. Experience on canned motor pumps in the EMO shop has shown that if material which is *relat-ively large (compared to the size of the impe11er/dfffuser vanes) 5 breaks from the system and passes through or lodges in the pumps that the most noticeable effects will be higher vibra-tion level (ff an object lodges in a vane of the impeller) and vane denting or gouging~ usually very localized. Sfoce in this case the plug relative size and.weight is tiny, no significant damage to the impeller or diffuser would be expected in the event a plug were to pass through the pump hydraulics.

A loose plug bounding around the bottom of the casing will have no detri-mental effect on the casing interior sui*faces.

The casing *itself is 7-1/2 inches thkk (minfo1um) stainless steel and is unclad so there is no problem of cladding abrasion. Furthermore~ there is no delicate instrumentation or fragile h,trdl:\\1are within the volute which has any chance of befog damaged by a moving plug~

Once "in the vesse*t, the p1ug w-ill follow the path of the cold water and end up in the Tower plenum.

The presence of the plug in the lower plenum presents two potentia*i problems.

The first aspect considered was the potentia'!ity of loose pieces *impacting the lower internals components.

Due to th2 low velocities present only insignificant effects of such impacts can be expected. The plugs are too large to enter either the core region or the drive line area.

The chance that the plug will fracture and small pieces will migrate into the core region is believed to be remote due to the shape of the plug and material used.

A second potential problem which may occur is wedging of the tube plugs in close clearance areas during start-up after a cold shutdown.

Several areas exist 1'ihere wedging may occur~ however 9 only one a1~ea had any rea*1 p1~obabil *ity of occurdng.

The identified area is the clearance between the vessel and the secondary core support base plate. Should a plug become wedged between the base plate and vessel during start-up from cold shutdown 3 the constriction against thermal grm*1th would cause forces to exist. The existance of such

• f e

• forces acting with the normal hydraulic forces could cause movement of the vessel internals.

A sensitivity study indicates that for the most con~

servative conditions more than four plugs would need be wedged between the vessel and the baseplate.

The forces generated by any lesser number of-wedged plugs will not *be large enough to potentially cause movement

.of the internals.

While it is generally not good practice to operate with significant amounts of foreign materials in the RCS9 recovery opera-

... 0tfons should be in"itiated if more than four plugs are postulated to exist loose in the RCS.

The potential effects of loose plugs on the fuel performance and integrity 0was also evaluated. Since it was previously mentioned that no plugs could entei the core through the lower internals, no nuclear 9 thermal, hydraulic, or mechanical concerns exist.. Even if plugs v1ere found on the top surface*

. of the core no nuclear, thermal, hydraulic~ or mechanical damage is expected:;

When operation commences any plugs on the top of the core would probably be swept dowri the hot leg and into a channel head where it woul1 come to rest in a low flow area.

The clearances between fuel pins and assemblies are too small to allow movement of a plug into those areas.

As previously demonstrated, only the wedgirig of a plug between the secondary core sup~ort base plate and the vessel is i potential problem of operation with loose plugs in the RCS.

However,*it is not good practice to operate with foreign material in the RCS and, therefore, recovery of loose material should be attempted.

'(

. t J

POSS~ITY OF A STEAM GENERATOR TUBE~UG LODGING IN THE REACTOR COOLANT PUMP CASING The question of a loose tube plug lodging in the reactor coolant pump casing may be broken into two parts. First, can the plug reasonably

'be expected to reach a relatively 10\\'I velocity ar*ea of the casing as it travels through the pump? Second, if the plug somehow ends up at the location of minimum flow velocity~ the casing bottom, will it stay there?

The first question will be addressed by cons*idering the trajectory of a plvg as it leaves the diffuser and travels with the flow toward the

• pump discharge.

In the worst case, the plug will have to travel nearly the fu1l circumference of the 80-inch diarneter casing before* reaching the discharge nozzle. It is assumed that the plug would leave the diffuser traveling horizontally at the 30 fps diffuser exit velocity.

Shortly beyond the diffuser exit the plug would quickly decelerate *to.

the 15 to 20 fps average tangential velocity of the water circulating in the casing and begin falling toward the casing bottom.

At 15 fps it is calculated (see Attachment 2) that the trip around the casing to 1

the discharge would take 1.4 seconds.

However, at the lowest free fall velocity of 3.6 fps (see Attachment 1) the plug would hit the bottom of the casing prior to reaching the discharge nozzle.

Since it has been shown that a plug can reach the casing bottom, the answer to the second question is now obviously important. Considering a.plug lying on the casing bottom near the suction adapter, then it is apparent that if the local velocity is sufficient to create the small fl ow drag force ( approxirna te ly 2 oz.) necessary to overcome the friction between the casing at the p 1 ug, then the p 1 ug wi 11 move.

For the end-on.

(worst case) orientation to flow, it is calculated that only 5.3 fps flow velocity is necessary to move the plug (see Attachment 3)._ It is believed that the flow velocities at all locations near the casing bottom exceed this value; so the plug would at least mo~e on the horizontal surface.

Once moving, the plug would spiral outward on the flat casing bottom and start up the shallow slope of th~ bowl shaped portion of the casing bottom.

As it move~ up the slope, increasingly higher flow drag forces would be necessary to overcome gravity. But since there would be a tendency for the plug to roll on the casing surface, the plug orientation

{perpendicular to flow) would tend to maximize the drag forces and the

• plug could be easily moved by a very low flow velocity- (as litt*le as 2 fps) and the velocity would tend to be higher at larger diameters and higher elevations. Since the bottom of the discharge nozzle intersects the casing wall only about 12 inches up from the casing bottom where the casing wall slope is a maximum of 45 degrees, it is concluded that the plug would readily reach the nozzle and be carried out by the high velocity flow there. It should be pointed out that the analysis done in support of this conclusion was done assuming hot water and normal pump output flows.

For cold, one pump operation the flow vel6cities in the pump and drag forces on the plug would be significantly larger.