ML19329E771
| ML19329E771 | |
| Person / Time | |
|---|---|
| Site: | Midland |
| Issue date: | 05/12/1976 |
| From: | Howell S CONSUMERS ENERGY CO. (FORMERLY CONSUMERS POWER CO.) |
| To: | Volgenau E NRC OFFICE OF INSPECTION & ENFORCEMENT (IE) |
| Shared Package | |
| ML19329E767 | List: |
| References | |
| HOWE-77-76, NUDOCS 8006170930 | |
| Download: ML19329E771 (6) | |
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Stephen H. Howell
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Vwe Presodent General Omces: 212 West MicNgan Avenue, Jackson, uscNgen 49201 May 12, 1976-Hove-77-76 Dr. Ernest Vogenau, Director Office of Inspection and Enforcement US Huclear Regulatory Co mission Washington, DC 20555 MIDIAND PIANT SURVEILIANCE CAPSULE HOLDER PRORIS4 - DOCKETS NOS 50-329 AND 50-330 The report attached to this letter was prepared by The Babcock & Wilcox Company to describe a problem with their reactor vessel surveillance capsule holder tube holddown =echanism which was first discovered at the Three Mile Island Unit #1 facility. This report is an interim report submitted per 10 CFR 50 55(e)..The sabcock & Wilcox Company is in the process of redesign-ing the holddown mechanisms on a generic basis and that redesign vill be used for both Midland units. Thi, redesign vill take considerable time; therefore, a final report per 10 CFR 50 55(e) vill be prepared when the generic redesi n 5
is completed. Either another interim report or a final report vill be sent to yo1 by Septe=ber 30, 1976.
c CC: Director, Office of Management Information and Program Control, URC, Esshington, IC (w/ene)
JGKeppler (2 ene), HRC, Region III
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REPORT ON REACTOR VESSEL SURVEILLAMCE CAPSULE IIOLDER TUBE !!OLDDOWN MECl!ANISM This report has been written pursuant to 10 CFR 50.55(c) on significant deficiencies, and documents the determination as to whether this constitutes a reportable event.
Description of the Problen The design of the reactor vessel surveillance capsule holder tubes is described in Section 2.5 of S&W Topical Report BAW-10100A( }.
The surveillance capsules are attached to and secured within the holder tube by a spring-loaded push rod assechly holder train as shown in Figure 2-8 of Reference (1).
The plenun flange compresses the push rod assecbly spring cartridge as the plenum is lowered into the core support shield.
The spring-loaded push rod assembly has four 3-piece spacers (see Figure 2-10 of Reference (1)) mounted along its axial 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 fourth spacer is approximately in the center of the ogee 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 special 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.
The lower portion of the holder tube is mounted on two pintles which allow rotation of the tube against the thermal shield for removal or insertion of the reactor internals.
Rotation of the holder tube during operation is prevented by the pin locking mechanism on each pintle and bearing, and the holddown force
. applied by the capsule holder train.
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During the ucek of February 29, 1976, while attempting to remove a sur-
'veillance capsule train from its holder tube at the Three Mile Island, Unit 1 Facility, wear to the holder tube was observed.
Subsequent removal of tlw second 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 spacer from the top, thus separating approximately the to'p six feet of the holder tube. One of these two tubes was also severely worn at the axial location of the first push rod spacer from the ' top and 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 THI-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 t6be at TMI-1.
Additional 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 surface of the specimen holder tubes at the axial locations of the spring cartridge, each push-red spacer, at the surveillance capsule 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 jo'urhal bear'ing 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 capsule holder train. This motion is cassing mechanical wear at the areas of contact between th,e 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 holddmns mechanism do not perform a direct safety function, the surveillance capsules themselves provide monitoring of the fracture toughness properties of the reactor vessel beltline region materials.
This monitoring program permits the det.crmination of the reactor vessel operating conditions which assure adequate m.
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fracture touchness throughout the vessel scrvice life.
The function of the
. surveillance capsule holder tube is to support the surveillance specimen assoc 51ics (capsules) in the annulus between the thermal shield and the reactor vessel wall.
It can be postula*ted that extremely severe ucar of the surveillance capsule holder tubes at locations below the shroud tube could lead to eventual loss of " support to the capsules.
Redesign is considered *necessary to establish
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the adequacy of the surveillance capsule holder tubes.
Extensive wear of the lower portion of the holder tube would riso have the adverse effect of allowing parts of.the holder tube and holddown mechanism to become loose parts in the annulus between the thermal shicid and reactor vessel wall.
The smaller of these parts could also contact the lower vessel head and lower reactor vessel internals.
Damage to the reactor vessel clad, incore instru-ment guide tubes and l'ouer reactor vessel internals structures could result if no corrective' action we're taken.
This damage would not represent an imminent threat te public health and safety, but could require extensive repair to assure these structures remain serviceable for the life of the plant.
Plants to Which Applicabic Crystal River Unit 3 - CP CP
. Davis-Desse Unit 1
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CP Hidland Units 1 & 2 Operating Oconee 1, 2, 3
-Arkansas Nuclear 1, Udit 1 - Operating Operating Rancho Seco Three FEle Island, Unit 1 - Operating NOTE:
This report is not applicabic to Three FEle Island, Unit 2, since it utilizes a survcillance capsule holder tube of different design.
, Corrective Actions i
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 L&W Topical Report.
Until this design can be completed and parts
- procured, plants in operation are presently either removing all three surveil-lance capsule holder. tubes and, their holddown mechanisms, or uhcre the sur-veillance capsule. holder tubes are in serviccable condition, they are removing
the holddown mechanism and replacing it with a modified spring-loaded cartridge.
This replacement cartridge is designed to restrain the holder tube with the wear causing portions of the holddoun mechanism removed.
Operation without the sur-i veillance specimens in the reactor for a t criod of time has been evaluated, and does not reduce the ability of the present surveillance programs to perform
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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 a schedule consistent with maintaining an effective reactor vessel material surveillance program.
i REFERENCE (1)B&W Topical Report BAW-10100A, " Reactor Vessel Material Surveillance Program -
Confromance to 10 CFR 50, Appendix li, for Oconee Class, Plants", February 1975.
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