ML20086E202

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AO 50-267/74/3A:on 740225,during Initial Fuel Loading, Orifice Assembly of Control Rod Drive Serial 30 in Penetration 11 Failed to Engage Top Metal Clad Reflector Elements.Caused by Design Error.Guide Pins Redesigned
ML20086E202
Person / Time
Site: Fort Saint Vrain Xcel Energy icon.png
Issue date: 04/09/1974
From: Swart F, Warembourg D
PUBLIC SERVICE CO. OF COLORADO
To:
Shared Package
ML20086E200 List:
References
AO-50-267-74-3A, NUDOCS 8312060266
Download: ML20086E202 (10)
v • d • e


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. O O DATEL April 9, 1974 ABNORMAL OCCURRENCE X FORT ST. VRAIN NUCLEAR GENERATING STATION PUBLIC SERVICE COMPANY OF COLORADO P.O. Box 361 PLATTEVILLE, COLORADO 80651 REPORT No. 50-267/74/3A Interim Final X l

IDENTIFICATION OF 1 OCCURRENCE:

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l This report is issued to finalize Interim Report 50-267/74/3, dated January 14, 1974, concerning manufacturing debris found in the metal ,

clad keyed top reflector elements. ,

This event is considered to be an abnormal occurrence as defined in The Technical Specifications, paragraph 2.1F.

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! l CONDITIONS PRIOR TO OCCURRENCE: Steady State Power Routine Shutdown l l

Hot Shutdown Routine Load Change

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Cold Shutdown Other-(specify)

Refueling Shutdown Initial Fuel Loading Routine Startup The major plant parameters at the time of the event were as follows:

Power RTR. N/A Mth ELECT. N/A We Secondary Coolant Pressure N/A 'psig-Temperature N/A~ -

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~ 8312060266 740114 Flow .N/A #/hr.

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l Primary' Coolant Pressure N/A psig

' Temperature N/A ___'F Core Inlet 4 N/A 'F Core Outlet Flow N/A DESCRIPTION OF OCCURRENCE:

i 4 At approximately 11:00 AM on Monday, February 25, 1974, entry of the Prestressed Concrete Reactor Vessel (PCRV) was made to facilitate manual insertion of the control rods of region 32. The work presently being accomplished is an internal inspection of the Control Rod Drive (CRD) manual i

retract gear. (See Abnormal Occurrence Report No. 50-267-74/4, dated February 12, 1974). e .- ;; >

l At this time it was noted.that the orifice assembly of CRD i

2 Serial No. 30 in Penetration No.11 had not properly engaged the cop metal-clad reflector elements. This assembly had been inserted in the penetration-i on February 20, 1974. Closer examination disclosed the orifice assembly to be resting on the reserve shutdown material guide tube on top of the center reflector element. The orifice assembly' appeared to be offset'approximately 3/8" - 1/2" to one side with the bottom of the' reserve shutdown guide tube

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resting on the edge of the top of the reserve shutdown channel in the reflector block. The centering guide pin had properly engaged the handling. hole,- but '

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the tapered guide fingers located at the bottom of the orifice valve had not

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. yet engaged the edge'of the reflector blocks to' guide it into the proper position to assure engagement of both control rod and reserve shutdown guide tubes.

j The valve rested in'such a positionLso as to form an annular opening approxi-

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' mately one' inch high between the bottom of the~ orifice assembly and the top.

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50-267/74/5A

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of the surrounding reflector.

All other CRD orifice assembly engagement was inspected and no problems found.

See Attachment "A."

APPARENT CAUSE OF OCCURRENCE: X Design Unusual Service Cond.

Including Environ.

Manufacture Component Failure Installatica/Const. Other (specify) 1 Operator Procedure ANALYSIS OF OCCURRENCE:

Because the CRD orifice valve assembly is designed to rest on top of the core and is not supported by the CRD, the CRD was not prevented from -

being properly inserted into the penetration in the PCRV top head. The control rods had been inserted and successfully tested for functional operability.

The orifice valve misalignment did not, in any way, prevent the control reds from being inserted properly and to the proper elevation in the core. At no time was the operation of the control rods hampered.

The operability of the reserve shutdown system was,not impaired either. Even with the bottom of the guide tube resting on top of the reflector, the passageway through the guide tube was in no way restricted to prevent proper insertion of the reserve shutdown material.

GA (Ge.naral Atomic) as clie supplier of the CRD's completed their analysis of the occurrence with the following results:

A study of the control tolerances of the CRD orifice valve assembly and the control plenum element drawings revealed that, for worst-case conditions (all tolerances being maximum and additive), it would be

() () 50-267/74/SA Page 4 possible to experience a maximum interference of 0.106 inches between the reserve shutdown guide tube in the plenum reflector block and the mating tube of the CRD with the guide pin partially inserted in the plenum block hole. As a result of this study it was decided that the existing guide pin diameter vould have to be increased to eliminate the interference problem.

The initial concern with designing a new guide pin was that the diameter of the pin had to be increased significantly to ensure elimination of the interference problem, but yet the diameter of the pin could not be increased to an extent that would limit orifice valve movement for maximum core offset conditions.

The new guide pin was ed' signed with a stepped taper as indicated in Attachment "B" with consideration to the need to eliminate the inter-ference problem and also with consideration to the requirement that the orifice valve must be free to tip in the maximum core offset condition.

4 The stepped taper design provides an adequate diameter at the point of contact with the plenum block to eliminate the interference problem .

and at the same time provides adequate taper for insertion and provides sufficient radial clearance when fully seated to allow for the core offset conditions. 3 CORRECTIVE ACTION: -

l Based on the redesign of the guide pin, 44 new guide pins were- f.

manufactured and delivered'to the site.. A controlled work procedure was developed by PSC for- the guide pin replacement work. All guide pins were -

i removed and replaced with all materials accounted for and documented. .

{} 50-267/74/5A Page 5 Prior to the installation of all guida pins a test pin was

' installed on CRD No. 30. The CRD was inserted into the core five different times with personnel in the core pushing the CRD to simulate teaximum mis-alignmentconditio$s. Iri avery test the CRD was successfully inserted. The test guide pin was then. removed and the old guide pin reinstalled. Upon normal CRD insertion into the core the CRD orifice mechanism hung up on the reserve shutdown guide tube. All CRD's modified with the new guide pin have

'been s'uccessfully inserted into position with no evidence of interference.

FAILUREDATA/SI!ILARREPORTEDOCCURRENCES:

No occurrences of this type previously experienced.

PROGRAMMATIC IMPACT:

This occurrence could have caused some delay in the preparation for continuing with low power physics testing had this item been on the critical path. As it turned cut, however, other areas requiring longer term scheduling were.the controlling items.

CODE IMPACT:

No codes involved!

Approved:

Prepared by: __

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frederic E. Swart Superintendent, l Fort St. Vrain Nuclear A%

Don Warenbourg Generating Station 'I Director,QualityAssu[rance

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