United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 97-44: Failures of Gamma Metrics Wide-Range Linear Neutron Flux Channels

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

                                 July 1, 1997


NRC INFORMATION NOTICE 97-44: FAILURES OF GAMMA METRICS WIDE-RANGE LINEAR
                              NEUTRON FLUX CHANNELS


Addressees

All holders of operating licenses or construction permits for test and
research reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to inform addressees of failures that have occurred in Gamma Metrics
wide-range linear neutron flux channels that have the potential to defeat the
scram function of the channels.  It is expected that recipients will review
the information for applicability to their facilities and consider action, 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

The non-power reactor at North Carolina State University (NCSU) has two Gamma
Metrics wide range linear neutron flux channels.  One unit is used as a linear
channel for power monitoring and control and provides a high-power-level
scram.  The other unit is used as a safety channel that provides a high-power-
level scram.

The channels used at NCSU can be operated in two modes for ranging and two
modes for bistable scram functions.  For ranging, these units can be operated
(1) in an auto-ranging mode in which the unit changes scales automatically, or
(2) in a manual range-change mode in which the reactor operator changes the
range displayed on the channel to follow the reactor power level or leaves the
channel set on one range.  These units can also be operated with the bistable
set (1) in the standard mode in which the scram function is present on each
range at a set percentage of range, or (2) in a smart mode in which the scram
function is present at a set percentage of range on one specific range
determined by the operator and all ranges above it.  The emphasis of this
information notice is on the auto-ranging and smart mode combination.

The initial problem observed by the licensee was that the linear channel unit
failed to uprange automatically with the unit in the reactor console in the
auto-ranging mode.  While testing the unit in the reactor console after
repair, the unit failed to uprange and exhibited spurious range-changing while
operating in the auto-ranging mode (Accession Number 9702120389).  During pre-
startup console testing following modification of the unit, it was 


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                                                            July 1, 1997
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found that actuation of the rod reverse demand would cause a range change with
the range control in manual mode.  During benchtop testing following repairs,
the unit exhibited
spurious range changes, including oscillatory behavior (changing up one level
when requested, but then changing back to the original level).  It was also
observed that the unit increased range to greater than the maximum range even
though this unit has by design a maximum range limit function.        

The unit used as a safety channel exhibited spurious down ranging during
reactor operation in the manual mode with the bistable set in the standard
mode.

Discussion

It is possible for a malfunction of these units to disable the scram function,
which, depending on the use of the unit, may be a high-power-level scram
required by the technical specifications (TS).  For the scram to be disabled,
the unit bistable would have to be in the smart bistable mode while a spurious
down ranging occurs, which places the unit on a range without scram protection
when such protection is needed.  Being on the wrong range disables the scram
function because the reactor could be operated beyond the scram setpoint power
level.  Operating the unit with the bistable in the standard mode with the
scram enabled on all ranges alleviates this problem.

If a spurious uprange malfunction occurs in which the range goes beyond the
maximum range limit, the scram function, if still present, will actuate with
the unit bistable in the standard mode.  This result occurs because the
invalid range setting forces the picoammeter to a maximum feedback state. 
This maximum feedback will cause the bistable input to go full scale for any
current above 10 picoamperes.  For this unit, 10 picoamperes represents either
7 or 8 decades below full power.    

The fact that the unit is on an incorrect scale can be readily detected by the
reactor operator by observation of the unit display.  The operator can then
take whatever action is necessary to respond to the specific situation.  The
impact of this type of problem is reduced by the TS requirement for multiple
high-power-level scrams using redundant channels.    

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 contact listed below.


                                       signed by S.H. Weiss for

                                    Marylee M. Slosson, Acting Director
                                    Division of Reactor Program Management     
                                    Office of Nuclear Reactor Regulation

Technical contact:  Alexander Adams, Jr., NRR
                    301-415-1127
                    E-mail:  axa@nrc.gov
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