ML20085B800

Ro:During Investigations & Repairs to Control Rod Drive Mechanism,Difficulties Experienced in Removing Nuts from Reactor Vessel Studs.Caused by Corrosion on Stud Shank & Threads.Reworded Procedures Listed

ML20085B800
Person / Time
Site:	Palisades
Issue date:	12/17/1971
From:	Haueter R CONSUMERS ENERGY CO. (FORMERLY CONSUMERS POWER CO.)
To:	Morris P US ATOMIC ENERGY COMMISSION (AEC)
Shared Package
ML20085B789	List: ML20085B800 ML20085B805
References
NUDOCS 8307080421
Download: ML20085B800 (4)

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s Dr. P. A. Morris, Director En- Docket 50 255

' Division Reactor Licensing -

License No DPR-20 i United States Atomic Energy commission ,

Washington, DC 20545 l

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B2ar.Dr. Morris:

- The purpose of this letter is tA'iormally report the difficulty experienced in removing the nuts from the reactor vessel' studs at our Palisades Plant. -

During the investigations of and repairs to the control rod drive mechanisms (reference our Special! Report No 1 dated December 3, 1971), it was decided to remove >the reactor vessel head in order to inspect the hanger sectione of the control blades.

However, there were difficulties experienced in removing the nuts from the reactor vessc3. studs.

These difficalties were reported to the Division of Com-pliance represer.tatives who subsequ6ntiy reviewed and witnessed the inspection and rework program on these reactor vessel studs.

The cause[cf this problem, the results of subsequent in-vestigations and the nature of the rework of these studs follow.

The reactor, vessel head removal process revealed that 41 of the 54 r6 actor vessel stud nuts were frozen to the studs to the e-rtent that they could not be re toved using the stud tensioner. The stud tensioner has a very low mechanical advantage for turning the nu ts . The studs and nuts were removed with the use of bolt heaters and a special slugging wrench under the supervision of Combustion

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, Engineeriyg, Ina personnel. The difficulty experienced in unbolting

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i thehead'vsssttributedtocorrosionand/orcorrosionproductson

, the stud shank and threads. Corrosion produ'c ts were also observed i

'on the head flange stud holes head flange surface, reactor flange f c! ~

surface," exterior of nuts and inner circle Cf'the stud washer.

[ld- AJ1 studs were sent to the stud manufacturer for assessment

'S C, of corrosion'and rework. Two nuts accompanied the studs. These nuts I were selected as visual examples of the most corrosion. The corrosion b,

on the studs was eentered on the shank area and the first few threads

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o o 9-Dr. P. A. Morri's 2 December 17, 1971 adjoining the shank on each end of the shank. Approximately three (3) threads on the bottom (vessel end) of th'e shank showed heavy corrosion while approximately six (6) threads were affected on the top end of the stud. From visual appearance of the corroded threads, it was ob-vious that not more than one (1) to two (2) corroded threads were en-gaged in the nut or the vessel flange. The balance of the threads was unattacked. They exhibited a coating of lubricant with some heavy lubricant buildup in the thread roots. The lower end of the stud shank showed a corrosion band on all studs that was approximately 1-1/2" wide.

When lower end thread corrosion was added to shank corrosion, a band ofapproximately1-3/4"wasnoted. This corrosion was fairly uniform over all studs. This band width corresponds to a countersunk area on the vessel flange.

Corrosion products were either a low-temperature aqueous red oxide believed to be oFe00H or a high-temperature hard gray deposit believed to be oFe20 3 . The stud shanks had general corrosion. In many cases, there was a streaming pattern indicating corrosion from a vapor phase since the streaming patterns were frm the bottom of the stud to the top. The two nuts that were sent were Numbers 51 and 52.

They had a heavy deposit of salt (believed to be boric acid salt) on the exterior flats. Only minor corrosion was evident on the nut thread leads. There was corrosien on the inner circle of the washers, the vessel flange and the head flange. Stud holes in the head flange were corroded and showed evidence of streaming like that found on the stud shank. The threads in the vessel flange were only slightly affected by the corrosion. There was only minor corrosion to the thread lead and the bottom of the stud holes showed sme slight evidence of cor-rosion products.

Rework procedures were developed to refurbish the studs as follows:

I. Removal of Corrosion Products A. Machine heavily corroded areas on shank next to threads on both ends. Permission was received from CE, Chattanooga, to reduce the shank diameter by a maximum of 0.020" below present drawing limits. The stud manufacturer planned to remove 0.015" maximum.

B. Grind the corroded threads to clean them. Permission was re-ceived from CE, Chattanooga, to reduce the minor diameter by a maximum of 0.010".

C. Remaining corrosion spots were to be hand polished.

D. Corrosion product cleanup and the stripping of the Parkerizing were to be completed by blasting with steel shot.

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Dr. P. A. Morris g A V 3 December 17, 1971 II. Nondestructive Testing The studs were to be magnetic-particle tested by the "magnaglo" method after shipping. Testing is done to AS'INE-138-63 and CE Specifications.

III. Parkerizing A magnesium phosphate Parkerized coating is to be applied to the studs.

IV. Nut and Washer Work I

The only work required on the nuts and washers was removal of the corrosion products.

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The rework has been completed and tbe studs returned to the Palisades Plant for reinstallation. During the nondestructive testing phase of the rework of the studs, 20 of the 54 studs exhibited several 3

-linear indications. All indications were l'. the shank portion of the 4

studs and were in a circumferential orientation. Two of the studs 4

showing these indications were shipped to Chattanooga, Tennessee, for metallurgical investigations to determine the cause of these linear indications. A description of the metallurgical investigations con-ducted and their results are contained in Attachment I.

From the metallurgical investigations performed, it was verified conclusively that the circumferential "magnaglo" indica-tions on these studs were not defect indications. The sensitive "magnaglo" test revealed indications resulting from magnetic permeability

' variations probably caused by slight variations in surface cold work induced by machining and possibly intensified by heating incurred in phosphate coating and in service.

It has been concluded that the corrosion in all cases re-sulted either directly or indirectly from borated water finding its way to the vessel flange (outside the sealing rings). All signs in-dicate that the aqueous colution had lain on the flange and countersunk areas around the stud holes. The solution vaporized upon vessel heatup and/or evaporated up through the head flange to corrode the area at the top of.the studs and the washer. There is no evidence to indicate that the solution attacked the studs by getting underneath the washer or nut.

All evidence indicates the corrosion to be due to attack by boric' acid solution. The Parkerized coating offers no special corrosion protection in this case.

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The source of the borated water was primary coolant which accumulated on?the vessel flange during venting operations conducted l after filling ~the primaryj ystem.

a Venting equipment at the Palisades I

Plant is being modified such that there is no possibility of getting e

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U Dr. P. A. Morris 4 December 17, 1971 borc.ted water on the reactor vessel flanges during venting operations in the future.

The effects of the reduction in the cross cection of the studs (resulting from refurbishing) were considered during the de-velopmental stages of the rework procedures. This analysis indicated an acceptable change. The final Stress Report will be supplemented to include the changes due to refurbishing. The two studs that under-went further metallurgical investigations will be documented separately in the final Stress Report.

The stud tensioning procedure has been reevaluated in light of the slightly reduced stud cross sections. No revisions to this procedure are necessary since the differences in shaft extension during tensioning is negligible.

During discussions with Division of Compliance personnel, Consumers Power Company agreed to perform additional inspections of )

the vessel studs and nuts during the first scheduled refueling outage.

This inspection will be performed in accordance with Item 1.8 of Table 4.3 1 of the Technical Specifications for the Palisades Plant. At least ten (10) studs will be inspected. The surface examinations re-quired will be performed by the magnetic particle method. The first s refueling outage is currently scheduled for September 1973 In addition, CE has agreed to establish a laboratory test I program to further evaluate the compatibility of CE normal phosphate coatings and super moly-lubricant in the design environment of the studs. The details of this laboratory test program are currently being developed.

The cause of the corrosion to the reactor vessel studs has ,

been eliminated. The thorough metallagraphic examinations conducted revealed that the studs are in good condition. All studs have been refurbished to eliminate similar future difficulties in removal.

Yours very truly, Robert L. Haueter (Signed)

RBS/dmb Robert L. Haueter Electric Production CC: Boyce H. Grier, Superintendent - Nuclear USAEC ,

c-o o ATTACHMENT 1 REPORT OF INVESTIGATION OF CLOSURE HEAD STUDS CONSIMERS POWER COMPANY PALISADES PLAIR C-E COIERACT 2966A December 12, 1971 l

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