United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 96-12: Control Rod Insertion Problems

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

                               February 15, 1996


NRC INFORMATION NOTICE 96-12:  CONTROL ROD INSERTION PROBLEMS


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 recent events during which rod control cluster
assemblies failed to insert fully.  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

South Texas Project

On December 18, 1995, with South Texas Unit 1 at 100 percent power, a pilot
wire monitoring relay actuation caused a main transformer lock out which
resulted in a turbine trip and reactor trip.  While verifying that control
rods had inserted fully after the trip, operators noted that the rod bottom
lights of three control rod assemblies were not lit; the digital rod position
indication for each rod indicated six steps withdrawn.  Boration of the
reactor coolant system was occurring with the charging pump suction having
been transferred to the refueling water storage tank.  One rod did drift into
the fully inserted rod bottom position within one hour, and the other two rods
were manually inserted later.  During subsequent testing of all control rods
in the affected banks, the rod position indication for the same three
locations as well as a new location indicated six steps withdrawn.  As
compared to prior rod drop testing, no significant differences in rod drop
times were noted before reaching the upper dashpot area for any of the control
rods.  Within an hour after the rod drop tests, two of the rods drifted to rod
bottom position and the other two were manually inserted.  All four control
rods were located in fuel assemblies that were in their third cycle with
burnup greater then 42,880 megawatt days(MWD)/metric ton uranium(MTU).


9602090161.                                                            IN 96-12
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Wolf Creek

On January 30, 1996, after a manual scram from 80-percent power, five control
rod assemblies at Wolf Creek failed to insert fully.  Two rods remained at six
steps withdrawn, two at 12 steps, and one at 18 steps.  Three of the affected
rods drifted to fully inserted within 20 minutes, one within 60 minutes, and
the last one within 78 minutes.  After the scram, the licensee initiated
emergency boration, as required, because all rods did not insert fully.  The
five rods were all in 17x17 VANTAGE 5H fuel with burnup greater than 
47,000 MWD/MTU.

Discussion

At both South Texas units, a 14-foot active fuel length core design is used. 
Several differences between the standard 12-foot active fuel design and the
14-foot one are as follows:  the 14-foot fuel design is approximately 76.2 cm
[30 inches] longer than the standard fuel assembly design, it has 10 mid grids
compared to 8 and the dashpot region is 25.4 cm [10 inches] longer and
comprises a double dashpot.  The control rod radial clearances above and in
the dashpot region of the 14-foot fuel assembly are similar to those of the
standard design.  The South Texas core contains three different 17x17 fuel
types--Standard XL, Standard XLR, and VANTAGE 5H--all of which are designed
and fabricated by Westinghouse.  This was the first operating cycle with
VANTAGE 5H fuel.  The core also contains 57 silver-indium-cadmium rods.  The
four affected rods were found in twice-burned Standard XLR fuel assemblies.

During subsequent testing, the rod drop traces revealed no significant change
in dashpot entry time; however, the affected rods did not show recoil on the
rod drop trace.  Recoil is a dampening affect that is normally seen in the 
traces due to control rod assembly spider hub spring contact against the fuel
assembly.  When similar rods in Unit 2 were tested, the results revealed no
adverse indications.  One rod did show the "no recoil" effect but inserted
fully into the core.

At Wolf Creek, subsequent cold, full flow testing of all of the control rod
assemblies indicated that eight control rods, including the five control rods
that did not fully insert following the January 30, 1996, reactor trip, did
not fully insert when tripped.  One control rod, H2, paused at 96 steps,
stopped at 90 steps, and slowly inserted to 30 steps over the next 2 hours. 
The control rod was then manually inserted.  The 7 other affected rods stopped
at various heights in the dashpot region, 5 of which fully inserted within 
22 minutes.  One of the other two drifted to the bottom within one and a half
hours while the remaining rod needed to be manually inserted.  The remaining
45 rods fully inserted when dropped, although a number of the rods did not
exhibit the expected number of recoils.  Of the total 53 control rod
assemblies, H2 (the only rod slowing outside the dashpot region) is a hafnium
control rod, while the remaining are silver-indium-cadmium control rod
assemblies.  The licensee retested all rods that stuck, as well as those rods
that failed to recoil more than twice, and the results were similar to the
previous testing.
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                                                            February 15, 1996
                                                            Page 3 of 3


Westinghouse, Westinghouse Owners Group, and the respective licensees are
pursuing the root cause identification of these events.  Possible root causes
are as follows:  debris (foreign matter), control rod or drive line
degradation, corrosion products, thimble tube bow, fuel assembly bow and/or
twist, reduction in thimble tube diameter, adverse alignment of guide tube
cards, and/or design tolerances.

Some foreign reactors have also experienced slow and/or stuck control rod
assemblies.

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 project manager.


                                          signed by

                                    Dennis M. Crutchfield, Director
                                    Division of Reactor Program Management 
                                    Office of Nuclear Reactor Regulation

Technical contacts:  Margaret Chatterton, NRR
                     (301) 415-2889
                     Internet:  msc1@nrc.gov

                     Stephen Koenick, NRR
                     (301) 415-2841
                     Internet:  ssk2@nrc.gov
Page Last Reviewed/Updated Thursday, November 21, 2013