United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 96-25: Traversing In-Core Probe Overwithdrawn at Lasalle County Station, Unit 1

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

                                April 30, 1996


NRC INFORMATION NOTICE 96-25:  TRAVERSING IN-CORE PROBE OVERWITHDRAWN AT  
                               LASALLE COUNTY STATION, UNIT 1


Addressees

All holders of operating licenses or construction permits for nuclear power
reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to a traversing in-core probe (TIP) that was
overwithdrawn at LaSalle County Station, Unit 1.  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 October 31, 1995, while LaSalle Unit 1 was operating at 96-percent power, a
failure occurred on the 1B traversing in-core probe machine.  The licensee was
running a test for nuclear instrument calibrations.  During the insertion of
the 1B TIP detector, the operator received indications that the detector was
withdrawing.  An attempt by the operator to stop the TIP was unsuccessful. 
The TIP withdrew past its shielded storage location to the drive unit itself. 
A reactor building area radiation monitor (next to the drive units) pegged
upscale (1 rem/hour) and alarmed in the control room.  At the surface of the
platform supporting the TIP drive machines, radiation surveys showed 7 Rem/hr. 
Licensee calculations showed potential dose rates of 250 Rem/hr one foot away
from the unshielded detector.  The operators entered emergency operating
procedures for secondary containment control (because of high radiation levels
at the 740-foot elevation level of the reactor building).  Personnel were
warned to stay clear of the area and an ALERT was declared based on the high
radiation levels.  The operational support center and the technical support
center were activated.  The licensee subsequently established high radiation
and contamination boundaries for restricting access controls in the reactor
building.  With the plant condition stabilized and the radiological boundaries
established, the licensee terminated the ALERT.  There was no impact on the
operation of Unit 1 or Unit 2, and they remained at full power.  The event
involved no radiological releases and no exposures of personnel.

Licensee personnel performed an inspection of the 1B TIP on November 7, 1995. 
The inspection revealed that the drive chain between the drive motor and the
feed and takeup reel had separated at the master link.  This separation caused


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the takeup reel to recoil due to a tensioner spring on the reel and to pull
the TIP detector all the way out onto the reel, an unshielded area.  The root
cause investigation as to why the master link separated is ongoing.

Discussion

The NRC staff discussed this event and the general design of the TIP with a
General Electric representative.  Because of the design of the TIP, when the
drive chain between the drive motor and the feed and take-up reel separates at
the master link, the fail-to condition is to recoil the take up reel and pull
the TIP detector all the way out onto the reel, which is usually unshielded. 
The kind of event could occur again because of the way the TIP is designed. 
The design has been modified to eliminate this type of event at newer designed
BWR plants.  

Irradiated components, such as BWR TIP and attached drive cables can create
substantial radiation fields in accessible RB areas.  Without timely worker
and control room actions in response to local and remote reactor building area
radiation alarms, the local radiation fields outside shielded rooms resulting
from an inadvertent activated TIP withdrawal has the potential for inadvertent
worker exposures in excess of regulatory limits.

Related Generic Communications and Correspondence

The following generic communications and correspondence discuss previous
related events:

Information Notice 88-63, "High Radiation Hazards From Irradiated Incore
Detectors and Cable," dated August 15, 1988.

IN 88-63, Supplement 1, dated October 5, 1990.

IN 88-63, Supplement 2, dated June 25, 1991.

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                                                            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
the technical contacts listed below or the appropriate Office of Nuclear
Reactor Regulation (NRR) project manager.


                                                signed by

                                    Brian K. Grimes, Acting Director
                                    Division of Reactor Program Management
                                    Office of Nuclear Reactor Regulation

Technical contacts:  Egan Y. Wang, NRR          
                     (301) 415-1076             
                     Internet:eyw@nrc.gov       

                     James E. Wigginton, NRR
                     (301) 415-1059
                     Internet:jew2@nrc.gov
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