ML20213G060

RO 50-20/1986-3:on 861027,shim Blade 6 Could Not Operate at full-out Position Due to Mechanical Interference.Caused by Misalignment of Blade Proximity Switch Tube.Switch Tube Reinstalled Properly

ML20213G060
Person / Time
Site:	MIT Nuclear Research Reactor
Issue date:	11/06/1986
From:	Lisa Clark, Kwok K NUCLEAR REACTOR LABORATORY
To:	Haverkamp D, Murley T NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I)
References
50-20-1986-3, NUDOCS 8611170305
Download: ML20213G060 (5)

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I l r NUCLEAR REACTOR LABORATORY ,

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MASSACHUSETTS INSTITUTE OF TECHNOLOGY O K. HARLING 138 Albany Street Cambridge, Mass. 02139 L. CLARK, JR.

Director (617)253-4211/4202 Director of Reactor Operations November 6, 1986 Dr. Thomas E. Murley, Administrator Region I U.S. Nuclear Regulatory Commission Attn: Mr. D. Haverkamp, Project Engineer 631 Park Avenue King of Prussia, PA 19406

Subject:

Reportable Occurrence 50-20/1986-3, License R-37 Mechanical Interference with a Shim Blade Gentlemen:

Massachusetts Institute of Technology hereby submits this ten-day report of an occurrence at the MIT Research Reactor in accordance with paragraphs 7.13.2(d) and 1.15.3 of the Technical Specifications. An initial report was made by telephone to Mr. Robert Sommers of Region I on 28 October 1986.

The format and content of this report are based on Regulatory Guide 1.16, Revision 1.

1. Report No: 50-20/1986-3 2a. Report Date: 6 November 1986 2b. Date of Occurrence: 27 October 198C

3. Facility: MIT Nuclear Reactor Laboratory 138 Albany Street Cambridge, MA 02139

4. Identification of occurrence:

Mechanical interference with shim blade #6 at the full-out position such that this blade could not have been operated, at the full-out position, in accordance with Technical Specification 3.9.3, which requires that the time from the initiation of a scram signal to 80% of full insertion be less than 1.0 second.

It should be noted that Technical Specifications 3.9.2 and

3. II.2(c) allow operation of the reactor with one control blade inope rable if that blade can be maintained at or above the aver-age shim bank position.

8611170305 DR 861106 I ADOCK 05000020 I

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5. Conditions Prior to Occurrence:

The reactor had been shut down over the weekend. Full power mechanical and ins trument startup checklists had been completed in preparation for the usual Monday-Friday operating week. Reac-tor startup had not yet begun.

6. Description of Occurrence:

The reactor was being prepared for startup in accordance with approved operating procedures. A blade drop time test was con-ducted on shim blade #6. After the signal initiation for the blade drop time test, the blade was observed not to drop until its drive was driven in for about one-half inch. This test was repeated three more times on blade #6 with the same result each time. Drop tests were subsequently performed on all the remain-ing control blades with no abnormality found. A visual inspec-tion was then made of blade #6. A mechanical interference was found between the guide rod which was connected to the absorber section of blade #6 and the tube which houses the proximity

. switch for blade #6. This tube had moved out of its slot, with the resul t that it mechanically prevented the guide rod of blade

1. 6 from dropping if that blade were withdrawn more than 20.5 inches from the fully inserted position. (Note: Allowable range of blade travel is 0.0"-21.0".) On receipt of the initiation signal from the rod drop timer, the drive moved in and pushed the blade to below the height of interference. The blade then dropped freely to the fully inserted position.

7. Description of Apparent Cause of Occurrence:

This occurrence was apparently caused by the misalignment of the blade #6 proximity switch tube. The misalignment evidently occurred on 20 October 1986 when, as part of a scheduled proce-dure, the proximity switch for blade #6 was replaced. During the installation of the new switch, a evagelock fitting was tightened l onto the proximity switch tube. The torque generated from the I tightening of this fitting evidently caused the tube to twist out l

of its slot. (Note: The tube cannot normally be twisted because it is tackwelded to a mounting bracket which is bolted to the core tank. The tack weld on the bracket of the blade #6 proximi-ty tube was either broken or broke while the switch was being replaced.)

Tests were done following the installation of the new proxim-ity switch on 20 October to verify its operation. However, these tests did not require that the blade be taken to its full-out position. Blade #6 was never operated at a position higher than 14.05 inches throughout the week and was therefore always fully operable prior to the occurrence.

Each control blade is attached by a webbed of fset plate to a weighted guide rod that moves vertically in a guide tube machined

, to a 1/16 in. clearance (see Figure 1). An armature, with a flat upper surface, sits atop the guide rod. The blade may be raised l

a a Dr. Thomas E. Murley )

Paga 3 I or lowered by mating the armature to the lower surface of an electromagnet that is connected to the drive mechanism. Control blade position is indicated by a magnetically actuated proximity switch at the fully inserted position. Continuous indication of blade position is provided by signals taken f rom the drive mecha-nism. The proximity switch indicates only the fully inserted position of the absorber section of the control blade.

Subsequent to the determination that the tack weld on the support bracket of the blade #6 proximity tube was not holding the tube securely, all tack welds on the support brackets for the other blades were inspected. They were found to be holding the tubes properly.

8. Analysis of Occurrence:

The reactor was operated for one week in a condition in which blade #6 could not have satisfied Technical Specification 3.9.3 if it were withdrawn to 20.5 inches or higher. The furthest withdrawn position for blade #6 during that week was 14.05 inches. Blade #6 was therefore always fully operable and would have dropped to its full-in position in less than the required time of one second if it had been required to do so.

The shutdown margin in excess of that required by the Technical Specificaitons for the week of 20 October was 2.27 beta, which is more than the integral worth of blade #6. Therefore blade #6 could have been declared as inoperable with sufficient shutdown margin remaining in accordance with Technical Specifications 3.9.2. and 3.11.2(c). No damage to the reactor has or could have resulted from this occurrence.

9. Corrective Action:

The corrective action consisted of:

a) Performance of drop-time tests on the other five shim bl ade s . All data were within specifications.

b) Inspection of the tack welds on the support brackets of the proximity tubes for the other five shim blades. All tubes were found to be securely held.

c) Removal and leak testing of the proximity switch tube for blade #6, fitting, tack-welding the tube onto the bracket securely, again leak testing, and reinstalling the tube properly.

d) Es tablishment of a requirement to perform a blade drop time test after the replacement of a proximity switch.

e) Evaluation of other maintenance items which may cause simi-lar interference to control blades. Magnet and control rod drive change procedures were identified, but both already have detailed checklists which require mul tiple drop times after each drive or magnet change.

.r Dr. Thomas E. Murley Page 4

10. Failure Data: There have been no previous instances of blade inter-ference caused by a proximity switch tube. However, as discussed in ROR-80-2, a shim blade did previously become stuck in its slot due to foreign material.

Sincerely, Kwan S. Kwok

'C Assistant Superintendent Reactor Operations L M d W '-

Lincoln Clark, Jr.

Director of Reactor Operations KSK/LC:DKE cc: MITRSC USNRC-0MIPC USNRC-DMB

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