ML20084U481

RO Re Bending of Nut Capture Devices & Continuous Outward Driving of Control Rod Drive Mechanism Motor Package.Caused by Control Blade Interference & Cable Termination Degradation,Respectively.Nuts Straightened & Wires Modified

ML20084U481
Person / Time
Site:	Palisades
Issue date:	12/16/1971
From:	Haueter R CONSUMERS ENERGY CO. (FORMERLY CONSUMERS POWER CO.)
To:	Morris P US ATOMIC ENERGY COMMISSION (AEC)
Shared Package
ML20084U483	List: ML20084U481
References
NUDOCS 8306290074
Download: ML20084U481 (8)

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General Officos: 212 W est Michigan Avenue, Jackson, Michigan 49201. Araya Code $17 788-0550 December 16, 1971 Dr. P. A. Morris, Director Re: Docket 50-255 Division of Reactor Licensing License DFR-20 United States Atomic Energy Co:enission Washington, DC 2051+5

Dear Dr. Morris:

This is to inform you of two unusual problems that cccur-red during t.nd as a result cf the maintenance activities that have been performed during the previous month at the Palisades Flant.

Tuese two problems are the bending of nut capture devices on the Type A fuel bundles and the continuous outward driving of a control rod drive mechanism (CRDM) motor package while it was disconnected from the remainder of the CRDM.

In order to perform the inspectiona of the control blade upper end fittings described in Special Report-1, " Description of Cause and Correction of Damage to Control Rod Drive Mechanisms at the Palisades Nuclear Plant" submitted December 3, 1971, it was necessary to remove the control blades frem the core through the upper guide structure to the refueling canal. This work was per-formed using the auxiliary hook on the polar crane complete with a load cell to detect any binding of control blades that might occur during removal or reinstallation. This work proceeded smoothly until it was noticed that one blade wac binding slight'ly as it van being reinstalled in the core.

The control blade was removed and inspected. It showed <

very light scratching of the stainless steel control blade claddin6 ;f Further inspections revealed that the cause of the binding to be _l the guide rod nut capture devices on two Type A fuel bundles adjacent id to this control blade being bent such that they interfered with the Y

( control blade during its reinstallation.

4 A review of the design of the locking tabs revealed that these nut capture devices are raised slightly e.bove the upper end of N. the threaded portion of the Type A fuel assemblies' guide rods. The

Pif same tabs on the Type B and Type C fuel have the upper end of the

' j 'b threaded portion of guide rod inserted through the hole in the nut 4 capture device (refer to attached sketch, Figure 1). '

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. December 1971 As this control blade did not show any sign of scratching during its inspection prior to being reinstalled in the core, it was concluded that the damage to the nut capture device occurred when reinstalling the control blade. The work was being done with the auxiliary crane and it was very difficult to obtain the required alignment such that the control blade could be lowered without catch-ing" on the edges of fuel assemblies. The nut capture devices are

) .060 inch thick and bend very casily.

The upper guide structure was removed and all nut capture devices were inspected. Twenty nut capture devices,were found bent, all on Type A fuel bundles. These 20 nut capture devices were straightened and visually inspected at 2X magnification for any signs of cracking. There were no signs of cracking.

- This type of problem will not reoccur during plant operation because the control rod blades cannot be withdrawn far enough to allow the tips to engage the nut capture devices. If further maintenance or core alteration activities take place prior to discharge of the Type A fuel assemblies that have the potential for bending these nut ,

capture devices, they wi.'ll be inspected at the completion of these

!. activities pricr to returning the plant to service.

The continuous outward driving of CRDM No 18 motor package occurred while the motor packade was removed to rezero primary rod po-

. sition indication synchros following rework of the clutch by the manu-

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i facturer. Electrically, the motor package was connected as it would be during normal plant operations. During the zeroing of this synchro-nous unit, the following sequence of events occurred:

1.

From the control room the motor was driven-in the raise direction starting from zero indication. Manual individual J control was selected for CRR4 No 18, Group B.

2. The control room operator released the " raise" control switch but observed that raise motion was still indicated. Motion was confirmed by electricians on the reactor head observing l CRDM No 18 package. The control cable was disconnected at

the CRDM and motion stopped.

s 3 The control cable was reconnected at the CRDM. Indicating

[ lights were restored but no motion occurred.

4. The control room operator again drove CRDM No 18 in the raise l

direction. Upon releasing the raise control switch, up motion again continued. This was interrupted by disconnecting the cable at the reactor head once again. ,

5 No other CRDM package could be made to continue to drive upwards after releasing ~the " raise" control switch.

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6. The control cable between CRDM No 18 and the junction box at 649' level was replacca by a spare cable.

7 CRDM No 18 now performed normally when driven up or down.

No unintentional motion could be made to occur.

8. The suspect control cable terminations were inspected re-vealing the following (refer to attached Figure 2):

a. No grounds detected.

b ._ CRDM end termination: No shorts between pins and no broken connections.

c. At junction box termination (straight end), the insu-lation on Pins L and G was either deteriorated or mis-sing such that the bare conductors and a piece of solder had caused a short circuit between these pins.

The effect of a short circuit from Pins L to G was studied to determine if it cculd cauce continuous driving of the motor package in the outward direction. This study assumed the conditions were the same as when the continuous driving occurred: namely, the mode selection switch was positioned to MI ranual indiviaual.; the group selection switch to Group B; and rod selection switch to No 18; the circuit is made up from the manual rod control switch, through Pin L; LS-1 (the upper limit switch) and B 18 raise contactor coil to neutral.

1. . A manual raise signal from the control room will energize B 18 " raise" contactor coil. This switches the 120-volt a-c supply to the CRDM windings, with a series capacitor in one circuit. The reversing interlock contact in the " lower" contactor coil circuit is opened. The motor will start and run up to speed, driving in the " raise" direction.

2. The short circuit between Pins L and G places in parallel with the circuit through LS-3 and B 1B raise contactor coil, a second circuit thz ough the control cable back to C-15, through the 6 pF capacitor and the No 3 contact of the " raise" contactor, throuch the c.ctar fuse to the 120-volt supply neutral.

3 This capacitive shunt across B 18 raf ce coil certainly did not prevent it fram remaining energizied.

h. The same short circuit from Pins L to G means that with the upper electrical limit Switch LS-1 c1:osed (rod anywhere from "O" to 131"), the " raise" contactor Cbil B 18 has scaled it-self in through the Motor Winding T3 - Th in series with the raise coil.

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5 Releasing the manual raise control switch thus will not stop raise motion. The sealed in coil holds open the re-versing interleck contact preventing a " lower" signal for B 18. It also holds closed the No 1, No 2 and No 3 contacts of the " raise" contactor cupplying full volts to the T6 -

T7 motor winding and the T3 - T4 phase shifted winding.

Motion will continue'until the LS-1 limit switch opens at inches withdrawal.

6. The anergency off selector switch will interrupt this sneak circuit to stop raise motion.

Th'e cause of the degradation of the cable termination was at-tributed to the many times the cable had been handled in the past. This handling coupled with the stray piece of solder allowed the cable in-sulation to deteriorate and a sneak circuit to develop.

A complete single failure analysis of the cable terminations was perfonned. This analysis assumes that pin-to-pin sneak circuits could develop chnilar to the one described above. The results of this single failure analysis revealed that the L to G sneak circuit is the only failure that allows outward motion of CRH4 motor that cannot be stopped with the manual rod control switch. This metion is not self-initiating; it only commences if the rod control switch is positioned to raise. Shorts between other adjacent pins could cause the CRDM to respond in a variety of ways, including a dropped rod, continuous in-ward motion after releasing the rod control switch, wrona direction _of rgd motion or no effect. There were no defects that could cause the failure of a CRDM to trip and only two (both previously discussed) which would allow rod motion that an operator would not be able to stop with the manual rod control switch. This motor motion can be halted by the

" emergency off" switch.

Wiring modifications are being made that relocate the Pin L function to Pin C, which was previously unused. This eliminates the

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two single failure sneak circuits that result in continued rod motion after releasing the rod control switch. A study of the modified wiring was made assuming that a single failure was again to occur. It was concluded from this study that further single failures of the nature described above would not cause any occurrences that were detrimental to reactor safety.

The equipment involved in this failure involved multiconductor cable,16AWG,25-1/C,1X3/CtriplexedandshieldedcableandaCannon MS 3106 E-32-7P. J Even if a failure of the nature experienced above were to occur with the CRDM assembled and the reactor operating, the control room op-erators would have been quickly alerted to the continued CRDM motion by the two rod deviation alarms (Rod Position Deviation Lo Limit); one when the affected CRDM deviates four inches from its group average and #

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. December 16, 1971 the second alarm (Rod Position Deviation Lo Lo) at an eight-inch de-viation. This would allow the operators to stop the continued rod motion very quickly with the " emergency off" switch. The uncontrolled rod withdrawal accident has been analyzed from a low initial and high-power level assuming the operator takes no corrective action. These analyses are presented in the Final Hazards Summary Report and show that in the unlikely event of a control rod withdrawal incident with the conditions varying from source level to full-power operation, the core and primary coolant system is protected by the high neutron flux trips, the high pressurizer trip, the thermal margin trip or the manual trip. The minimum DHB ratio is greater than 1.3 in all cases.

Based on the foregoing, it is concluded that the continued driving of the CRDM motor package does not involve an unreviewed safety question. In addition, the potential for recurrence through a similar failure has been eliminated for all CRDM by the wiring modifications described earlier in this letter.

Yours very truly, Robert L. Haueter (Signed)

RBS/dmb Robert L. Haveter Electric Production CC: Boyce H. Grier, Superintendent - Huclear

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