ML20062C015

Ro:On 781022,control Blade B Unexpectedly Dropped from Drive Magnet & Hung Up Approx 7 Inches Above Full Insertion. Probably Caused by Broken Outer Race of One of Eight Vertical Guide Bearings

ML20062C015
Person / Time
Site:	University of Missouri-Columbia
Issue date:	10/31/1978
From:	Mckibben J MISSOURI, UNIV. OF, ROLLA, MO
To:
References
NUDOCS 7811060076
Download: ML20062C015 (3)

Text

.... a o-E

'[ x 5 Research Reactor Facility UNIVERSITY OF MISSOURI Researen Park Columbia Missouri 65201 October 31, 1978 Teiephone (314) sa24211 Director of Nuclear Reactor Regulations U. S. Nuclear Regulatory Comission ,

Washington, D. C. 2055

Reference:

a) Docket 0-186 Universitrof Missouri -

License R--103 1

Subject:

Bearing Failure on Control Blade Offset Mechanism Description .

On October 22,1978 at 0756, control blade "B" unexpectedly dropped frcm the drive magnet and hung up approximately seven inches above being fully inserted. At the time of the dropped rod, a reactor startup was in progress following a scheduled shutdown for changing flux trap samples; and the reactor power was being held at SMW for the primary coolant system tempera-tures to stabilize. The blade hanging up seven inches above full in is a deviation from Technical Specification 3.2.

Analysis After shutting down the reactor and lowering the pool to refuel level;

' the control blade was inspected for foreign objects preventing the control blade from going full in. Next, the control blade was checked to see how slight the resistance was holding the blade up; with a little additional force required over less than the first half of an inch of travel, the blade went in the rest of the way with no binding or dragging. After going full in, the control blade could be raised and lowered with no noticeable problem. The pull weight was then checked and found to be 40-50 lbs. a little above the expected 37-43 lbs.

The offset mechanism was removed and brought closer to the pool surface for inspection. A piece of metal could be seen in the bottom of the guide tube (see ai tached drawing). It could not be retrieved for positive r identification due to the activation level of the offset mechamism, but looked like part of the outer race of the vertical alignment bearing.

O P, wm ml Ol g j4 a COLUMBIA KANSgCITY ROLLA ST. LOUIS s\

\

h an eaus opportunity institution ,

e

. +

Director of fluclear Reactor Regulations October 31, 1978 Page 2 It is postulated that the outer race of one of the eight vertical guide bearings broke and jamed between the guide block and guide tube.

This will be investigated further when the offset mechanism has had sufficient decay time and can be removed from the pool for inspection.

MURR technical specifications and Hazard Summary Report are based on the assumption the control blade with the most reactivity worth is stuck full out; therefore, there is no unreviewed safety questions..

Corrective Action The spare offset mechanism was inspected and all the bearings were replaced with 'tew bearings of the same model as previously used. A new control blade was mounted on the offset mechanism, and the mechanism was installed in the control blado "B" position. The maintenance requirement sheet will be changed to require replacing all the bearings when the offset mechanism is disassembled.

Sincerely, J. . McKibben Reac:or Manager JCMK:vs Enclosure Endorsement: ~

Reviewed and approved s Don M. Alger  !

Associate Director cc: Directorate of Regulatory Operations-Region III Reactor Advisory Committee

a r t nne a c ,. 3 e .M .

m. . u [...-:,)

IE  !

a; y !e 7-lh i  ! 'il M

+ r e  !! j, ai j Ts

! h I N~L31  !! jf i l IA j

1 i! i [lI

~~

II R-I

~l"II a 1 I

) 8 1 I!

e -

r i

~-

1 ' -

. -i

.8 }.

.I on t > -

1 - .

- 11 s *g  % E;g M In 8!

I

,-- 5:  !,  ! ,

1 1 7 EMI crq.

.i e u i(6 gg: -

5, ,

g .sfg  ;

- t a <> 1 _1 a

n. u

!. I f t

i 1 Is e -

8 a 3 y a e s . 'g_ '

vg ' '

l

' c3 -

l tt 3 "E r =o ii ,I, i, . gl i 1-u_

E 1I y ) 5, l;'

11 li  !! .

i ' '

11 er ff \

~

=

a  ? '

l -o l, *,

e 4 gc

• % ='

T ( .

N l * .

a i , ~

s l,

'. g I

Z...

g ./

3:

3 O!

/ .\ b:

3. :*

~

.~ .

/ .

. r ~'~

h! .

g i ........_.;

g a , l,,

- }_.g1 i' , .

. E

.!I.)

n-I 3

,