ML20199C537

Ro:On 971030,shim Safety Number 2 Was Found to Be Binding & Became Stuck in Withdrawn Position & Control Rod Failed to Scram.Caused by Incorrect Spacing Between Hold Down Foot & Bundle top-handle.Procedure SOP-II-I Modified

ML20199C537
Person / Time
Site:	05000128
Issue date:	11/11/1997
From:	Reece W TEXAS A&M UNIV., COLLEGE STATION, TX
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
97-0284, 97-284, NUDOCS 9711200048
Download: ML20199C537 (12)

Text

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A l TEX AS EN GIN EEltlN G EX PERI.t \ ENT STATION TEXAS A&M UNIVERSITY

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_L NUCLEAR SCIENCE CENTER 409'845 7551 November 11,1997 97-0284 Document Control Desk Nuclear Regulatory Commission Washington, DC 20555 SUlUECT: Final Repert of the RepNtable Occurrence of October 30,1997 REF: Texas A&hi University Nuclear Science Center (NSC), Facility License R 83, Docket 50-128

Dear Sirs:

On Thursday, October 30, during an evening Nuclear Engineering Lab (Reactor Calorimetric), Shim Safety No. 2 was found to be binding and became stuck in the withdrawn position The control rod failed to scram; therefore this was considered a reportable occurrence in accordance with the R 83 Technicel Speci0 cations. NSC hianagement informed hir. h1arvin hiendonca (NRIUDRPhi/PDND) of the NRC on October 31. The reactor was inspected and returned to normal operations.

The following is a description of the events and the conclusions.

SecuenccSUhents The reactor was being taken to a power of 500kW for a Nuclear Engineering 405 calorimetric experiment. The SRO instructed the RO to start up the reactor to 500kW.

Upon movement of the shim safeties in " gang," the HO noticed that the Rod Down light never cleared, This meant that the control rod was not coupled to the rod drive and was not withdrawing with the other three shim safeties. On Tuesday, October 28,1997, the control rod drive for shim safety No.2 had been replaced for scheduled annual maintenance. Knowing this, the SRO and RO decided to swap the control rod drives back because the old contial rod drive was known to have worked before the initial swap. ,

Aner the rod drive assembly replacement, startup continued and all indications were normal,

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- Pag'e 2 The reactor was at 500kW and the control rods were in the process of being banked. The control rod positions were at approximately 50.5% and were going to be banked at 60.0%. When shim safety No.2 wa: being moved to 60.0%, the " engaged" light went out and the" rod down" light did not actuate. The RO thought that the wiring behind the light (LED) mi ght be at fault. The shim safety No.2 was driven down and the " engaged" light re energired at approximately 59.8%. A second attempt was made to raise shim safety No.2 to 6( .0%. Shim safety No.2 was raised to M.0% with normal indications. The

" engaged" light blinked ofTand then back on whi. . the RO was taking log readings following the reactor startup.

The SRO veriGed that shim safety No.2 was still in the raised position at the Reactor Bridge. An attempt was made to individually scram shirn safety No.2 to determine its location. The " engaged" light went out momentarily, but the " rod down" light never actuated. The SRO directed to shutdown the reactor and abort the experiment. During the shutdown, shim safety No.2 was found to scram with the " rod down" light actuating between 20.0% and 30.0%. Shim safeiy No.2 was tested with the reactor shutdown. It was determined that the control rod scrammed at a maximum height of approximately 40.5%.

Aner these events, the Director and Assistant Director were notined and the appropriate comments were made in the shin change and RO logs.

SubcauenLEXtats On Friday, October 31,19')7, it was decided that the reactor would be partially unloaded to examine shim safety No.2 and the respective bundle for damage. The Health Physicist obtained a reactor pool sample to determine if there had been any damage to the bundle, fiieled follower or poison section. The results were negative and no fission products detected. The core was examined prior to unloading. It was discovered that the hold down foot, attached to the bottom of the control rod barrel (see attached Ogures), was not positioned properly on the bundle top handle. There was indication that the hold down foot was rotated approximately 45" It was also determined that the top of the control rod drive assembly at the bridge level was higher than the other control rod drives. The control rod was repositioned until the drive motor housing heights matched. The core was re-examined and it was found that the control rod hold down foot was still not properly positioned even though the height was correct. The hold down foot was found positioned on the side of the bundle tor handle and not directly over the handle. The core was then partially unloaid to the point where shim safeties No.2 and No 3 could be removed and inspected. Shim safety No.2 and the respective bundle were inspected and no abnormalities were found. The core was reassembled and the control rods returned to the same locations. The core and control rods were veri 0ed as being properly assembled by the Director and Assistant Director. Scram times were measured on both control rods that were removed. The times were 0.69 and 0.65 seconds for shim safetics ho.2 and

97.Q284 Page 3 No.3 respectively. The Director then gave written permission in accordance with the facility Technical Specifications for a retura to regular reactor operations.

Ro0LCause of thdcPm1Ahic_Qcnincace A root cause analysis meeting was held on November 4 with a representative of the NRC (Mr. Stephen llolmes)in attendance. The cause was identined as incorrect spacing between the hold down foot and the bundle top handle. The fueled control rod followers and hold down feet were installed in the reactor in 1972. The desit ;n was for the hold down foot to be 0.125 inches above the top handle to accommodate possiHe thermal expansion of the fuel bundle. The NSC procedure (SOP 111) for the installation of the control rod calls for the rod to be raised a/ynor/mately 0.250 inches via the control rod barrel support adjusting nuts. This distance was chosen to prevent measurement errors in a limiting direction. A 0.250 inch clearance was intended to insure the 0.125 inch distance while not lifling the hold down foot over the top handle (0.5 inch diameter).

The Shim Safety No.2 rod had been removed during a partial core unload in early October. Scram time was measured as 0.67 seconds at that time. There was no prior indication of rod binding until the rod became stuck on October 30. It is believed that the spacing was incorrect at that time but the rod was correctly aligned. During the change of Shim Safety No.2 drive assemblics the technician (a qualified RO) may have slightly turned the lower rod drive assembly. This misalignment then caused the rod to bind when it was partially withdrawn.

Conchision!LaniIrconnendations The SRO and RO made the correct response to the indications of a stuck rod. The failure of the single control rod to scram did not violate reactor safety. The procedure for removal and insenion of rod drives will be modified to prevent a reoccurrence of the incorrect spacing . The following is the draQ changes to SOP-il 1 to be reviewed and approved at the next Reactor Safety lloard meeting (compare to attached procedure).

a. Removal and Installation of the Shim Safety Rod (1) Disconnect the electrical connector . ) the rod drive unit.

(2) Using a ruler, determine and record the height, to the nearest 1/16 inch, of the rod drive unit top relative to the upper suppon plate.

(3) Attach the chain liQing device or a .

97-0284-

. Page 4 To install the rod, reverne the above applicable steps, raaking sure that the bottom of the hold down assembly is properly positioned and secured over the fuel

'oundle top handle.

1. Lower the assembly until the hold down foot of the hold down assembly rests on the fuel bundle top handle.-

2.. Adjust the barrel support rods for a clearance of between 0.125 to 0.250 inches between the hold down foot and the bundle top handle.-

3. Measure height of rod drive unit above upper support plate.

Height should be within 1/16 inch of previous measurement.

4, The control rod should be tes'.ed for operability as per SOP 11 1-6.d.

All Operators have been counseled on attention to procedures and maintaining a questioning attitude during normal and abnormal operations.

- Please contact Mr. Sean O' Kelly or Dr. Dan Reece if you require additional information.

' Thank you for your assistance in this matter. The NSC stafrand management appreciated the concern shown by the NRC in light of this reportable occurrence and the assistance that Mr. Ilolmes provided during his visit.

Respectfully,

/g O'y Dr. W.D. R(e Director Attachments xc: 12212/ Central File 17124/NRC Reportable Occurrence Dr, Ted Michaels, NRC.

Mr. Stephen liolmes, NRC Mr. Blaine Murray, NRC Region IV-Reactor Safety Board

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l NSC Section Page Fcrm 112A Nuclear Science Center II 9 STANDARD TERATING PROCEDURES l Prepared by Subject Procedure Rev,

{ . BFA' REACTOR CORE MANIPULATION I 1 Any11mg a control rod is to be removed, two other scrammable rods (if available in the core) will be cocked at 50%.

a. Removal and Installation of a Shim Safety Rod During the performance of these procedures, the shim safety rod being removed will be in the scrammed condition.

(1) Disconnect the electrical connector to the rod drive unit.

(2) Attach the chain lifting device or a 1/4 inch nylon lifting rope, with approximately a 3 ft.

loop, to the upper end of the rod drive unit.

(3) Secare the rope to the overhead crano.

(4) Remove the bracket which secures the rod drive unit to the control rod mounting plate.

(5) Carefully raise the control rod assembly approximately one inch using the crane.

(6) Remove the nuts from the top of the control rod barrel support. Remove the barrel support and tie it off to the control rod drive.

-(7) Raise the rod assembly until the shim eniety rod clears the fuel bundle.

NOTP: DO NOT use the overhead crane to phycically extract the rod assembly from the core. Manually lift the assembly from the core and use the crane to support the weight of the assembly during pauses in the lifting process.

(8) Make all necessary entries in the Fuel Log and Reactor Operations Log.

To install the rod, reverse the c.bove steps, making sure that the bottom of the hold down assembly is properly positioned and secured over the fuel bundle top handle. This is done by first lowering the assembly until the crow's foot of the hold down assembly touches the fuel bundle top handle ind then adjusting the barrel support rods for a clearance of approximately 1/4 inch between the crow's foot and the bundle top handle. The control rod should then be tested for operability as per SOP II-I-6.d. i Approved:

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lloid Down Tube A standard TRIGA control rod operates within a guide tube that currounds the rod and has holes for proper cooling. A fueled-follower control rod can not operate in a guide tube becauce of restrictive cooling. It was necessary to design and fabricate new hold down tubes for the control elements with fueled followers. In the absence of a guide tube, a hold down foot was designed as shown at the top of Figure 7 The foot fits over the top handle cross bar and the blade of the foot extends high enough for clearance of the rod when it is in the full up position. The blade is t2ttached to the sido cf a tube that houses the control rod extension. When the rod drive unit is secured to the reactor support structure, a 1/8" clearance is provided notween the foot and the fuel element top handle cross bar. This clearance permits small thermal expan-sion of the iuel without vertical restriction.

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