ML20132B424

Ro:On 850904,one Control Blade Failed to Drop Fully Into Core on Demand from 64% Withdrawn Position.Probably Caused by Bearings Supporting Blade/Shaft Coupling or Magnesium Shroud housing-to-blade Clearance

ML20132B424
Person / Time
Site:	05000083
Issue date:	09/17/1985
From:	Vernetson W FLORIDA, UNIV. OF, GAINESVILLE, FL
To:	Grace J NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II)
Shared Package
ML20132B430	List: ML20132B424 ML20132B452
References
NUDOCS 8509260189
Download: ML20132B424 (4)

Text

f sa r u. "- NUCLEAR FACILITIES DIVISION (m ca. %IRNET504, RE ACTOR M4N AGE R c ,n eq@g .

n NUCtEAR REACTOR BUILDING UNIVERSITY OF FLORD\ 'O .V ? .\

camwius.rtonion inii h . .

-- m mim nuu.m g N...#

Septembdr , 1985 Nuclear Regulatory Commission Suite 2900 101 Marietta Street, N.W.

Atlanta, Georgia 30323 Attention: J. Nelson Grace Regional Administrator, Region II Re: University of Florida Training Reactor Facility License: R-56, Docket No. 50-83 Gentlemen:

Pursuant to the reporting requirements of paragraph 6.6.2(3)(c) of the UFTR Technical Specifications, a description of a potential abnormal occurrence as defined in the UPTR Technical Specifications, Chapter 1 is described in this interim report to include NRC notification, occurrence scenario and proposed solutions. The potential abnormal occurrence involved the failure of one of the UFTR contral blades (Safety Blade #3) to drop fully into the core on de-mand from a 64% withdrawn position.

NRC Notification The Executive Committee of the Reactor Safety Review Subcommittee reviewed this occurrence on September 4, 1985 and concluded that it is a potential ab-normal occurrence as defined in UFTR Technical Specifications, Chapter 1. The RSRS then instructed NRC notification as per Section 6.6.2 of the UPTR Tech Specs. This notification was carried out by both telephone to Mr. Paul Frederickson and a following telecopy on September 4, 1985 (See Attacament I).

This interim report represents the 14 day followup report as required in UPTR Tech Specs, Paragraph 6.6.2 ( 3 ) ( c ) .

occurrence Scenario 8509260 PDR AD  % h hPDR 83 S

As indicated in the telephone conversation with Mr. Paul Frederickson, Section Chief , Region II, and a following telecopy on 4 September 1985 ( Attachment I),

one of the reactor control blades (Safety-3) on the University of Florida Training Reactor failed to completely insert on demand from a 64% removed po-sition. This faili.re (sticking about 31% removed) was discovered by a Reactor Operator as he commenced a power increase from the 1 watt critical position where a complete set of readings are required to be entered into the daily operations log. The operator had accidentally raised the Safety-3 about 20 units instead of the Regulating Blade for this power increase; in returning it to the normal 640 unit position he felt the response was sluggish and so he attempted to drop the blade from 640 units withdrawn to check it. Following clutch current release the blade stopped at the 310 unit position and was sub-sequently driven in with the other three blades to shut the reactor down. The Facility Director and then the Reactor Manager were notified immediately of this occurrence. 'k EQUAL OPPORTUhsTV/ AFFIRM AflVE ACTION EMPL0tta

t Nuclear Regulatory Commission

. September 17, 1985 Page Two

.It should be noted that the most recent blade drop times performed on June 21, 1985 showed a slightly increased drop time from the values determined in March following previous maintenance work. However, the blade was dropped four times with consistent and successful drop times on each check. In addition, several trips, both unscheduled and for training, showed proper S-3 drop response over the several months prior to this occurrence.on September 3, 1985.

Immediate checks (with all other control blades fully inserted) involving sub-sequent removal to various heights showed this sticking problem to be inter-mittant and to center in the 290-315 unit range but with some possible slug-gishness in the drop from other higher and lower heights. It should be noted that this is essentially a recurrence of the event reported by our facility in a letter dated January 28, 1985 with subsequent followup in an interi.n report dated February 9, 1985 and closed out in a report dated March 26, 1985.

As indicated to Mr. Frederickson on September 4, 1985 and again on September 16,1985, .the need to formulate plans, make various checks 'and, as entry 'into the core region is required, to let the core and structure cool for a period, prevents a final report on this occurrence at 'this time. However, Mr.

Frederickson did advise the submission of an interim 14-day report and recom-mended including an update on the status of the problem. This update is pro-vided in this report.

Evaluation Evaluation and determination of the methods for alleviating this problem of a sticking control blade as well as preventing recurrence were discussed by the UPTR staff on September 4, 1985.

Essentially this event represents a recurrence of the previous sticking blade event so the staff reviewed and expanded upon those items considered at the January 31 staff meeting following the original S-3 problem where it was de-cided that potential blade drag points would include:

1. Inside gear boxes and/or bearings (previously identified as the cause of the January 28 problem),

2. Inside the blade shrouds perhaps due to tailed rivets, buckling or warpage of the shroud or the control blade,

3. shifted blade shaft / pedestal or bearing,

,4 . shifted blade shaft / drive unit or bearing, 1

l 5. Mechanical drag of ' the . blade shaf t in its guide channel.

It was agreed that all of'these possiblities.should be investigated in a sys-tematic program until the cause of the sticking blade is isolated, corrected and prevented from recurring.

I

. . * -- .rm., ,, -

w_ f --y- . - + . ,

o_ -

+ , , -# e., , c , .-~~w,. ..,--g,m,#--

Nuclear Regulatory Commission

! September 17, 1985 f Page Three t

The Executive Committee of the Reactor Safety Review Subcommittee (RSRS) was apprised of this occurrence on the day it happened and met to evaluate it on September 4, 1985. The decision was also made to put the UPTR on administra-tive shutdown with. limitations noted in Attachments II and III. As indicated, they recommended reporting the event. The entire RSRS considered the event in more detail at .its regular meeting on September 6,1985. In both cases, the RSRS concluded in -agreement with the facility administration that this poten-tial abnormal occurrence did not compromise the health and safety of the public. The blade has always responded properly to drive in to allow reactor shutdown. Required shutdown margin has always been available.

The RSRS agrees that all the possibilities listed above should be investigated in a systematic program to assure the cause of the sticking blade is isolated, corrected and prevented for recurring.

Work Progress To Date Following staff and RSRS evaluations, this sticking S-3 blade problem is being

' addressed in a series of planned maintenance / inspection checks beginning with the right angle gear box, drive motor, magnetic clutch, etc. external to the biological shield (designated ex-core meaning essentially environmental back-ground radiation levels) and working in toward the core regions where rela-tively high radiation levels are expected. Each planned maintenance / inspection activity or series of activities is described in a procedure or instruction discussed by the UPTR staff and administration prior to performance. It is then being reviewed and approved by the Executive Committee of the RSRS con-sisting of the RSRS Chairman, 'the Radiation Control Officer and the Reactor Manager prior to the start of work.

As of this date, the following maintenance has been performed:

1. Right angle drive gear box inspection has been performed with all compo-nents found to be functioning properly.

2. Bearings in right angle drive unit were inspected.and replaced since a small amount of rough operation was noted in or.e bearing. This roughness was not considered sufficient to be the source of the problem.

3. The shaft and connecting bearing were uncoupled from the blade drive unit and removed from the control blade. Both were inspected for scar marks with no significant problems noted.

4. The shaft penetration was cleaned - some oxidation and carbon products were removed but not considered sufficient to have been the cause of the problem.

5. The bearing and shaf t were reinstalled and recoupled, tte potentiometer was repositioned and blade drop and timing checks made.

,~ . .- _ - . . . _ . _.

l o .

1 I

Nuclear Regulatory Commission September 17, 1985 Page Four

6. Preliminary checks indicated the sticking problem is not cleared.

Conclusions To Date The problem has been isolated to within the biological shielding ,in the vicinity of the core reflector; most likely causes are a problem with the bearings supporting the blade /shaf t coupling or with the magnesium shroud housing-to-blade clearance either warpage, misalignment or loose rivets.

1 j Consequences As concluded by the RSRS Executive Committee, the full RSRS Committee and UFTR administration, this potential abnormal occurrence did not compromise the health and safety of the public. This occurrence was discovered at a low power condition. The Safety-3 blade drive system was always functional; and even with the S-3 blade at ~30-35% withdrawn, the UPTR core has a shutdown margin of ~347% Ak/k.

Followup Since further work will involve considerable radiation dose commitment, the core and structure has been allowed to cool while the above checks and main-tenance efforts were completed. A preliminary procedure to address the remain-ing in-core maintenance checks is nearly complete and will~ be presented to the RSRS Executive Committee for approval on September 18. Work to check and in-spect for the possible in-core sources of the problem and to perform main- ,

tenance where necessary and approved is expected to begin later this week.

Casutituent The UPTR administration with concurrence of the RSRS is committed not only to clear the sticking blade problem but also to obtain a significant reduction in

the S-3 drop time. This reduction is considered necessary to preclude recur-rence of this . event. In addition, the UFTR administration has committed .to the RSRS to clear _any restart with NRC Region II prior to removing the facility 4 from the -current administrative shutdown.

( 0 Anl  % l /f William G. Vernetson D6te Acting Director of Nuclear Facilities 4

WGV/ps Attachments cc: P.M. Whaley Reac' tor Safety Review Subcommittee.

. . _ . - _ _ , . _ _ _ _ _ _ _ . _ ~ .- . . , , , . . . _ _ _ . . _ - - . . . .