ML19337A776

Guide Thimble Inset Design, Nonproprietary Version

ML19337A776
Person / Time
Site:	Millstone
Issue date:	09/30/1980
From:	NORTHEAST UTILITIES
To:
Shared Package
ML19291C718	List: ML19337A773 ML19337A776
References
TAC-11348, TAC-11561, TAC-12505, TAC-42846, NUDOCS 8009300183
Download: ML19337A776 (15)

Text

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4 DOCKET NO. 50-336 l

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-ATTACHMENT (2)

MILLSTONE NUCLEAR POWER STATION, UNIT NO, 2 i

! GUIDE THIMBLE INSET DESIGN i

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TABLE OF CONTENTS Section Title Page .

1.0 Background and Description 1-1 .

2.0 Test Basis and Description . 2-1 3.0 Conclusion 3-1 p

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LIST 0F TABLES Table Title .

1 Lateral Wear Test-Summarization of Results .

LIST 0F FIGURES Figure Title 1 Location of Normal Force in Wear Comparison Test 2 Westinghouse Thimble / Instrumentation Tube Insets 3 Westinghouse Inset Sectional View With Control Rodlet 4 Wear Scar Geometry I

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WESTINGHOUSE PROPRIETARY CLASS 2 WESTINGHOUSE GUIDE THIMBLE INSET DESIGN

1.0 BACKGROUND

AND DESCRIPTION The Millstone Unit 11 core contains 217 fuel assemblies, 81 of which are During its first located under control element assemblies (CEA's).

refueling outage (December 1977), fuel inspection revealed full through-wall penetrations of the guide thimble and instrumentation tube wall in a number of assemblies located under CEA's.

Observations of the CE thimble and instrumentation tube wear scar and the location of maximun wear in fuel assemblies near the outlet nozzle 1 -

led to the conclusion that the wear was caused by flow induced lateral vibration of the Control Element Assembly (CEA) rodlet rubbing against the tube. This has since been confirmed by later transmittals from CE to the NRC, and was based upon metallographic analysis and out-of-pile .

testing.

Whether the control rod excitation is caused primarily by crossflows near the upper core plate or by the flow velocity in the guide and instrumentation tubes or a combination of both, it is control rod lateral vibration that creates the guide and instrumentation tube wear.

The greatest wear has occurred at the tips of CEA rodlet for the fol-lowing reasons:

1. The greatest normal force occurs at the tip

2. The tip experiences the greatest lateral motion

3. The CEA rodlet tip makes point contact with the tube wall To reduce the wear it is necessary to reduce the effects of the above mentioned phenomenon. Presently, this is accomplished through the use of guide tube sleeves which are inserted into the guide and instrumenta-tion tubes. The guide tube sleeves provide a secondary surface on which the CEA contacts thereby mitigating wear on the guide tubes.

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, .o The Westinghouse design to mitigate guide tube wear during Cycle 4 at Millstone Unit No. 2 consists of both guide tube sleeves and guide tuDe insets. The Westinghouse guide tube sleeve design was previously docketed in the W. G. Counsil letter to R. Reid dated October 9, 1979.

Four lead test assemblies utilizing guide tube insets will be located under CEA's during Cycle 4. The inset design is illustrated.in Section A-A of Figure 1.

The guide and. instrumentation tube insets are [ ]a,c inch long and

[ 3a,c inch wide rectangular deformations that reduce the original tube diameter locally from 1.035 inch to [ ]a,c inch. Four indivi-dual insets are located at two axial elevations of the guide and instru-mentation tube as shown in Figure 2.

The inset design reduces wear in the following manner: .

1. The locally reduced diametral clearance limits the lateral motion of the tip and and also reduces the im" pact loading of the tuoe due to the rodlet motion.

2. The insets are located above the region of the rodlet where the point contact occurs which forces the rod into more of a line con-tact wearing mode of the inset.

3. The four inset geometry influences the rod to a two point (or 2 line) support which is a more stacle stgte of equilibrium than the single point sphere on cylinder, or singld line cylinder on cylinder

-contact. This is a much more favorable condition of wear for the l

support of a stiff rod.

4. The insets design does not allow the tip of the rod to touch the tube wall which' precludes point contact wearing as shown in Figure 3.

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Also shown in Figure 3 is a dashed line which represents the control i

rodlet touching the undeformed section of the guide thimble.

Although we do not expect any significant inset wear to occur, this is mentioned only to show that there still exists margin to wear It through at the insets even if wear to the original 10 scce.vcd.

is expected that after the control rodlet tip touched the original tube 10, the wear rate would greatly decrease since a much greater surface area would be in contact.

A wear comparison testing program, as explained .in the following

' section has confirmed that the guide and instrumentation tube insets mitigate the wear.

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_2.0 TEST BASIS At:0 DESCRIPTION A test se, tup and procedure was designed to reproduce the wear pattern observed on CE guide thimble tubes (see Figure 4) which had bee'n located ,

Since little thermal-hydraulic under control rods in the reactor core.

information concerning the cause of the wear in CE reactors was pro-vided, no attempt was made'in this test to duplicate the actual ther-mal-hydraulic conditions in the core. The test was strictly a wear comparison test in which Westinghouse tube samples with inset' geometric

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features were subjected to the same environmental and loading conditions which had produced the observed wear pattern in a standard guide tube sample. Wear depth and volume in the two types of saiples were then compared to determine if the Westinghouse inset design reduces guide thimble wear for the same test conditions.

Zircaloy 4 guide and instrumentation tube test samples and Incone'l 625 i

simulated control rods were used to present the actual incore material '

1 interface. With the exception of total. length, all dimensions on these pieces were the same as those in the reactor. The length of insertion of the rod into the test piece matched the length of insertion of the real control rod into the guide thimble tube when the CEA is in its parked position. Since the most severe wear observed is known to have occurred when the control rod is at thit. location, the test simulates the actual relative positions of the control rod and guide thimble tube.

All test runs were performed dry at atmospheric pressure. To accelerate 0

the wear rate the temperature was kept at 600 F which is typical of 4 the temperature in the operating reactor, j The test setup contists of a shaker which vibrates a simulated control l rod inside a G andard guide tube test bicce that is held staticnary in much the s we manner as a guide thimble tube is held in a fuel assem- .

bly, l.ateral and axial vibration tests were conducted.

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I Lateral Wear Test lateral icst runs were performed with the simulated control rod vibrat-na"a+

ing at [ j,b,c one of its natural frequencies. A constaat of ener'JV was input to the rod for each test run. This parameter was held constant'because the mechanism which causes vibration of a particu-

' lar control rod in the reactor is not expected to change with' time.

A total or three standard guide tube samples and three Westinghouse samples with inset were tested in the lateral vibration test. Table 1 gives a brief des'cription of the results of these test runs.

As Tablo 1 indicates, samples C2 and W2 were tested with no initial side load; l.c., the control rod was initially centered in the test piece.

The remainder of the test samples were subjected to an initial normal force at the location indicated in Figure 1 (i.e., control rod is forced against the side of the tube wall and remains in'that location while vibratino). This preload was designed to represent the ' locking' phenomena as reported by CE which is believed by CE to be a contribJting factor to the type of wear observed in CE fuel assemblies. It also representv a lateral. force due to any cause, hydraulic or mechanical, which could cont ibute to accelerated wear. The magnitude of the normal forces thed in the test runs were based on calculations made from actual wear ob.civations.

r Axial Wear Test The axial wear test was designed to demonstrate the ability of the

1. Westin9h euse inset design guide and instrumentation tube to withstand j

repeatra insertion and retraction of the control rod throughout its '

lifetin.. Each test' piece was subjected to a total control rod travel

]b,c inches. This is equivalent to over [ ]b,c years Of [-

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6 (assuming 90 percent availability) of a [ ]b,c 7 jj insertion and full retraction of the CEA. Control rods at Millstone typically travel less than 500 in/ year (ref. CE letter to the NRC -

1. LD-78-001). Therefore, the Westinghouse test duration includes a factor of safety of well over [ ]b,c for a three year period. The equivalency stated is also conservative because the wear in the test pieceswasconcentratedovera[ ]b,c inch length rather than the full 137 in. stroke of the actual control rod. Standard guide thimble tube sampics were also included in the axial wear test to confirm the wear produced by axial control rod motion is not characteristic of. the wear which was found in CE fuel assemblies.

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3.0 CONCLUSION

By laterally vibrating a " locked" simulated control rod inside standard .

guide and instrumentation tube test pieces, Westinghouse was able to reproduce wear scars characteristic of those discovered at Millstone Unit II. When subjecting Westinghouse inset design test pieces to the same environmental and loading conditions which had produced the observed result in standard guide tubes, the Westinghouse tube typically exhibited approximately one tenth of the wear depth found in the stan-dard guide tubes. Comparing the size and shape of the wear scars obtained on the two types of semples, it is evident that the volume of material removed during the wearing process is considerably greater in

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the standard guide tube.

Altnough it appears that the actual wear is produced when the control rod is forced against the tube inside diameter, the tests performed demonstrate that the Westinghouse inset design also greatly reduces wear depth and volume in the case where the control rod is centered in the tube.

The tests performed indicate that the Westinghouse inset design guide and instrumentation tubes will withstand amounts of axial wearing motion over[ ]b c times greater than they are expected to be subjected to.

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, LATERAL WEAR TEST - SUW.ARIZATION OF RESULTS I' Test Nomal Total No. Maximum Piece Force of Cycles Physical Appearance of Wear Scar Wear Depth

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• W2 C3 W3 C4 W4

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C indicates a regular standard guide tube test piece

.l W indicates a W type tes+. piece with insets .

+ Test teminated early due to equipment failure ,

Represents the depth of scratches, not lateral wea'r '

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1 POSITION OF THE POSITION OF THE CONTROL ROD

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O INSET SECTIONAL VIEW WITH CONTROL RODLET C E A RODLET

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