ML19341B520

Ro:On 810120,rods 1 & 4 Dropped to Zero Causing Reactor to Shut Down.Cause Not Determined.Console Returned to Normal. Reactor Returned to Power.Event Will Be Further Investigated

ML19341B520
Person / Time
Site:	05000142
Issue date:	01/26/1981
From:	Zane A CALIFORNIA, UNIV. OF, LOS ANGELES, CA
To:	Office of Nuclear Reactor Regulation
References
NUDOCS 8102020439
Download: ML19341B520 (2)

Text

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• RC SCHOOLOF ENGINEERING AND APPLIED $CIENCE LOS ANGELES. CALIFORN!4 9mir24 January 26, 1981 Director, Division of Operating Reactors USNRC Washington, DC 20555

Dear Sir:

The UCLA R-1 Reactor experienced a possible abnormal (reportable) occurrence on January 20,1981 at 0850 when rods number 1 and 4 dropped to zero causing the reactor to shut down (from 100 kw). The reactor operator touched nothing and allowed the power to drop so that the reactor supervisor could better assess the problem.

The reactor supervisor noted the following:

1. Rods 1 and 4 were at the zero position and the down limit lights were lit.

The red LED that signifies than an inhibit had caused rod 4 to drive down was extinguished showing that no inhibit had occurred. Rods 2 and 3 were still at their set position of 48%.

2. All magnet currents were 60 ma or greater.

3. An-attempt to drive rod 1 up and down showed that the rod functioned normally.

4. The reactor operator stated that the horn had not sounded, and no reset was required.

5. A scan of the console showed everything normal except that the power was decreasing with the reactor in manual (normal when rod 4 hits either the 100% or zeio limit).

A drop rod scram was then initiated and the remaining rods dropped to zero.

The safety amplifier was checked; all voltages were normal, and all tubes satis-factory. A maintenance calibration was done and the unit returned to service.

No problems could be identified. All rod drives were checked for operability in both directions, inhibits were checked in the up direction for all rods and found to'be functioning properly.

Tests were conducted to ascertain the current level that would cause the [o2O y magnetic clutches to release. Rods 1 and 4 began slipping at approximately 15 ma whereas rods 2 and 3 did not slip even with the current adjustment at its lower limit of 13-15 ma. Since the normal magnetic current is 60 ma, the /

magnet power supply would have had to decrease to 25% of its normal operating voltane of 160 V. Close examination of the charts showed no glitches in any of the charts at the time of scram. There are no common conditions that would cause two out of four rods to drop, either all four drop or 'one independently fails.

Nectern ENEacy LAsonArony ivrs cArros. DIRECTOR 1102020 %

=_

- 2 After several operational checks, the probicm could not be identified or reproduced, and the console was normal, the reactor was returned to power and the run completed.

The problem was further investigated on the following day with attempts being made to induce a momentary interruption of the magnet AC power supply to examine the effect on the rods. The 4 rods were withdrawn to 40% and the magnetic AC power supply interrupted momentarily. All rods moved indicating that a failure from this mode could not have caused rods 1 & 4 to fall with 2 & 3 remaining at their original position.

The voltages across each magnetic clutch were measured at the specified 60 ma magnet current and found to be well within tolerance. 1 Circuit checks of the console revealed all circuits functioning normally.

Since no safety system failed to perform its safety function, the reactor is considered operational. The staff will continue to seek a solution to the event and report it if found.

Sincerely, A. Zane Reactor Supervisor CC: RUC: I. Catton /!,

R. Conn J. Garrick J. Hornar Wegst Zane Faw.senberry Region V l

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