United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 94-40: Failure of a Rod Control Cluster Assembly to Fully Insert Following a Reactor Trip at Braidwood Unit 2

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                     OFFICE OF NUCLEAR REACTOR REGULATION
                         WASHINGTON, D.C.  20555-0001

                                 May 26, 1994


NRC INFORMATION NOTICE 94-40:  FAILURE OF A ROD CONTROL CLUSTER ASSEMBLY TO
                               FULLY INSERT FOLLOWING A REACTOR TRIP AT
                               BRAIDWOOD UNIT 2


Addressees

All holders of operating licenses or construction permits for pressurized-
water reactors (PWRs).

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to a potential for one or more rod control cluster
assemblies to fail to fully insert following a reactor trip due to unwelded or
failed cap welds of in-core thermocouple column funnel nozzle pins.  It is
expected that recipients will review the information for applicability to
their facilities and consider actions, as appropriate, to avoid similar
problems.  However, suggestions contained in this information notice are not
NRC requirements; therefore, no specific action or written response is
required.

Description of Circumstances

On April 5, 1994, Braidwood Unit 2 was operating at 99-percent power.  At      
3:39 p.m.(CST), the turbine-generator tripped as a result of an internal fault
in the 2E main power transformer.  The turbine trip generated a reactor trip. 
Following the valid trip signal, the control rod (rod control cluster assembly
and drive shaft) located in core position K-2 inserted only to step 210 from 
step 231, as indicated by the digital rod indicator system.  Further
evaluation confirmed that the rod control cluster assembly had indeed failed
to fully insert.  All other equipment responded as expected.  

A series of special tests indicated that an obstruction under the K-2 spider
body vane No. 14 at guide card No. 7 prevented the rod control cluster
assembly from inserting past step 210.  Inspection with a fiber boroscope
showed a cylindrical, metallic object, approximately 7.94 mm (5/16-inch) in
diameter and 22.23 mm  (7/8-inch) long.  The K-2 rod control cluster assembly
was removed to the spent fuel pool and the foreign object was removed.  The
object recovered from the top of guide card No. 7 was identified as an in-core
funnel pin from the head nozzle of a thermocouple column in the reactor
vessel.  


9405020070.                                                            IN 94-40
                                                            May 26, 1994
                                                            Page 2 of 3


Discussion

The vessel head has five in-core column nozzles and each nozzle has a funnel 
to aid in-core column and vessel head alignment during assembly.  Each in-core
thermocouple column reactor vessel head nozzle funnel is threaded onto the
nozzle and pinned to prevent the funnel from rotating (see Figure 1).  The
design calls for the two pins per funnel, installed during original
construction, to be secured by a cap weld.  However, the cap weld for the pin
that fell into the core upper internals appeared to be missing or may have
failed.  The funnel pin caps were welded on site by Nuclear Installation
Services Corporation (NISCO).  Additional inspections found that the pin
adjacent to the missing pin was loose and that a pin from another funnel was
missing.  The second missing pin was recovered from the top surface of the
reactor upper internal components. 

The failure or lack of the weld was an intermediate cause of the stuck rod
control cluster assembly.  Because the pin was apparently not welded, it
worked loose from the nozzle/funnel assembly, dropped into the guide tube
access hole for control rod K-2, and came to rest in a position where it
prevented the rod control cluster assembly from inserting past step 210.  The
most probable root cause of the failure of control rod K-2 to fully insert is
improper installation or other personnel error; the pin cap appeared not to be
welded, as required by design drawings, and the condition remained undetected
during subsequent inspections.  

The licensee performed initial acid etching that suggested that the loose
funnel pins had never been welded.  The licensee is having additional tests
performed on the affected pins to more narrowly define the root cause, such as
lack of sufficient weld material, failure of the weld, or missing weld.  

The licensee reworked all Unit 2 in-core thermocouple column nozzle funnels by
adding welds in three places around the top of the funnel to nozzle interface 
circumference.  In addition, all of the original cap welds were reinforced to 
preclude their failure, should the root cause subsequently be determined to be
weld failure.  

Video tapes from the last refueling outage on Braidwood Unit 1 indicate that
the funnel pins in the instrumentation nozzles were in place; however, the
licensee has committed to a direct inspection of all instrumentation nozzle
funnel attachments during the next refueling outage.

Failure of the cap welds on in-core thermocouple column funnel pins or failure
to perform the welding during original construction may lead to loose parts 
in the reactor coolant system which have the potential to jam rod control
cluster assemblies or cause impact wear on fuel and other system components.  
.                                                            IN 94-40
                                                            May 26, 1994
                                                            Page 3 of 3


This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact
one of the technical contacts listed below or the appropriate Office of
Nuclear Reactor Regulation (NRR) project manager.


                                     original signed by S.H. Weiss

                                    Brian K. Grimes, Director
                                    Division of Operating Reactor Support
                                    Office of Nuclear Reactor Regulation

Technical contacts:  Rolf A. Westberg,  RIII  
                     (708) 829-9732

                     Edward D. Kendrick, NRR 
                     (301) 504-2891

Attachments: (se file IN94040.WP1 for Figure 1)
1.  Figure 1 - In-Core Thermocouple Column
Page Last Reviewed/Updated Friday, November 15, 2013