ML19329D520
| ML19329D520 | |
| Person / Time | |
|---|---|
| Site: | Davis Besse, Oconee, Arkansas Nuclear, Crystal River, Rancho Seco, Midland, Crane  |
| Issue date: | 04/21/1976 |
| From: | BABCOCK & WILCOX CO. |
| To: | |
| References | |
| NUDOCS 8003160120 | |
| Download: ML19329D520 (4) | |
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REPORT ON REACTOR VESSEL SURVEILLANCE CAPSULE 110LDER TUBE IIOLDDONN MECHANISM.
This report has been written pursuant to 10 CFR 50.55(e) on significant deficiencies, and documents the determination as to whether this constitutes a caportable event.
Description of the Problem The design pf the reactor vessel surveillance capsule holder tubes is described in Section 2.5 of B&W Topical Report BAL'-10100A(1)
The surveillance capsules are attached to at:d secured within the holder tube by a spring-loaded push rod assembly holder train as shown in Figure 2-8 of Reference (1).
The plenum flange _ compresses the push rod assembly spring cartridge as the plenum is lowered into the _sre support shield.
The spring-loaded push rod assembly his four 3-piece spacers (see Figure 2-10 of Reference (1)) counted along its exial length to provide lateral positioning of the push rod.
The top two of these spacers are located in that portion of the axial length of the holder tube which is within the shroud tube.
The third spacer is axially located in a portion of the holder tube with a thicker wall and within a journal bearing at the lower end of the shroud tube, mounted to the core support shield.
The fcurth spacer is approximately in the center of the neee bend in the holder tube.
A' female fitting on the lower end of the push rod mates with the top end fitting of the upper surveillance capsule as shown in Figure 2-11 of Reference (1).
The bottom end fitting of the lower surveillance capsule mates with a cpacial fitting in the base of the holder tube which restrains the capsules from rotation.
The holder tubes themselves are contained within a shroud tube extending from the plenum flange to a location corresponding to the third push-rod spacer.
Tha lower portion of the holder tube is counted on two pintles which allow ratction of the tube against the thermal shield for removal or insertion of the racctor internals.
Rotation of the holder tube during.peration is prevented by the pin locking mechanism on each pintle and bearing, and the holddown force cpplied by the capsule holder train.
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- During the week of February 29, 1976, while attempting to remove a sur-vrillance capsule train from.its holder tube at the Three tule Island, Unit 1 Facility, wear to the holder tube was observed.
Subsequent removal of the ercond and third surveillance capsule train indicated similar wear to a second tube.
The holder tube was severed at the axial location of the second push rod epicer from the top, thus separating approximately the top six feet of the holder t ube.
One of these two tubes was also severely worn at the axial location of the first push rod spacer f om the top cud became separated at that location during capsule removal.
Remote video techniques have been used to inspect for further evidence of internal wear on two tubes at TMI-1 (the intact tube and one severed tube) and all three tubes at Oconee 1.
In addition, an external video inspection has been conducted of portions of the surveillance holder tube at TMI-1.
Additic m1 holder tube inspections have been recommended by B&W and are either in process or are being planned at the other B&W operating' plants of similar design.
The results of the video inspections show evidence of. wear on the internal curface of the specimen holder tubes at the axial locations o" the spring cartridge, each push-rod spacer, at the survcillcnce cepsule rings, and at locations in the ogee bend where the push rod itself may contact the holder tube.
In addition, wear of the holder tube journal bearing has been observed.
. Based on the inspections at Oconee 1 and TMI-1, B&W concludes that the cause of the holder tube wear is flow-induced relative motion of the surveillance c:psule holder train.
This motion is causing mechanical wear at the areas of c:ntact between the holder tube and the capsule holder train.
Evaluation It-is concluded that the surveillance capsule holder tube holddown mechanism (Push Rod Assembly) design represents a deficiency which requires notification of NRC per 10 CFR 50.55(e).
Although the surveillance capsule holder tube and surveillance capsule h21ddown mechanism do not perform a direct safety function, the surveillance cep:ules themselves provide monitoring of the fract, re toughness properties of u
tha reactor vessel beltline region materials.
This monitoring program permits th2 determination of the reactor vessel operating conditions which assure adequate 11r 1 _
fr cture toughness throughout the vessel service life.
The function of the curveillance capsule holder tube is to support the surveillance specimen easechlics (capsule =) in the annulus between the thermal shield and the reactor vessel wall.
It can be postulated that extremely severe wear of the surveillance c:psule holder tubes at locations below the shroud tube could lead to eventual loss of support to the capsules.
Redesign is considered necessary to establish the adequacy of the surveillance capsule holder tubes.
Extensive wear of the lower portion of the holder tube would also have the adverse effect of allowing parts of the holder tube and holddown mechanism to become loose parts in the annulus between the thermal shield and reactor vessel wall'.
The smaller of these parts could also contact the lower ve,ssel head and lawar reactor vessel internals.
Damage to the reactor vessel clad, incore instru-ment guide tubes and lower reactor vessel internals structures could result if no corrective action were taken.
This damage would not represent an imminent threat to public health and safety, but could require extensive repair to assure there structures remain serviceable for the life of the plant.
Plants to Which Applicable Crystal River Unir 3 -
CP-Davis-Besse Unit 1
- CP J Midland Units 1 & 2
- CP Oconee 1, 2, 3
- Operating Arkansas Nuclear 1, Unit 1 - Operating Rancho Seco
- Operating Three Mile Island, Unit.1
- Operating s
NOTE: This report is not applicable to Three Mile Island, Unit 2, since it _ utilizes a surveillance capsule holder tube of different design.
Corrective Actions B&W is presently modifying the design of the surveillance specimen capsule holder tubes.
This design modification, when completed, will be described for NRC review in a B&W Topical Report.
Until this design can be completed and parts i
procured, plants in operation are presently either removing all three surveil-lance capsule holder tubes and their holddown mechanisms, or where the sur-veillance capsule holder tubes are in serviceable condition, they are removing l
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e the holddown mechanism and replacing it with n modified spririg-loaded cartridge.
This replacement ccrtr1dge is designed to restrain the holder tube with the wear csusing portions of the holddown mechanism removed.
Operation without the sur-vaillance specimens in the-reactor for a period of time has been evaluated, and dots not reduce.the ability of the present surveillance programs to perform
.their intended function.
Plants soon to be in operation (Crystal River 3, Davis-Besse 1) will have the present surveillance capsule holder tubes removed prior to reactor operation.
The modified design holder tubes will be installed at all applicable plants on c schedule consistent with maintaining an effective reactor vessel material aurveillance,. program.
REFERENCE I
( )B&W Topical Report BAW-10100A, " Reactor Vessel Material Surveillance Program -
Confromance to 10 CFR 50, Appendix H, for Oconee Class Plants", February 1975.
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