Part 21 Report - 1997-090
ACCESSION #: 9702120384
North Carolina State University
Nuclear Reactor Program
Department of Nuclear Engineering
Box 7909
Raleigh, NC 27695-7909
(919) 515-2321
FAX (919) 515-5115 10 February 1997
U. S. Nuclear Regulatory Commission
Attn.: Mr. Alexander Adams, Jr.
Senior Project Manger,
Non-Power Reactors and
Decommissioning Project Directorate
Office of Nuclear Reactor Regulations
Washington, DC 20555
Subject: Nuclear Instrumentation information which may be useful to Non-
Power Reactor facilities
Dear Mr. Adams,
Executive Summary
The PULSTAR reactor at North Carolina State University has two Gamma
Metrics Wide Range Linear neutron flux channels. One channel, designated
the "Linear Channel" was operated in the auto-ranging mode and used for
power monitoring and control with a maximum range of 1 MW. The other
channel was designated as the "Safety Channel" and was operated in the
manual range-change mode on the 1 MW range.
In November 1995, the Linear Channel experienced a failure to
automatically up-range and the range control card was returned to Gamma
Metrics for repair. It was noted that both the linear and safety
channels had been operating with the neutron flux scram smart bi-stable
set so that it was enabled on the 1 MW range. The bi-stable mode was
changed at that time so that both units would produce a scram at 120% of
range on all ranges. In December 1995, the unit was successfully bench-
tested with the repaired card, but again failed to up-range and exhibited
spurious automatic range-changing when it was returned to service. The
unit was returned to Gamma Metrics at that time.
In January 1997, the Safety Channel spuriously down-ranged from the 1 MW
range to the 300 kW range when the reactor power was increasing through
150 kW. While operator manually scrammed the reactor, the scram function
remained operable at 360 kW due to the having the scram function enabled
on all ranges. This unit was also returned to Gamma Metrics for repair.
Facilities using the same type of equipment should be aware of these
range change problems in both automatic and manual range mode and the
fact that a spurious range
North Carolina State University is a land-grant university and a
constituent institution of The University of North Carolina
Mr. Alexander Adams, Jr. (2) 10 February 1997
U. S. Nuclear Regulatory Commission
change can defeat the scram function if the smart bi-stable is set so
that the scram is active on one range only.
Part 21 Assessment
The licensee has reviewed 10CFR Part 21 and concluded the Gamma Metrics
Wide Range Linear monitor ranging malfunctions experienced do not result
in disabling the protective function of the monitor by themselves and do
not create a substantial safety hazard. Nevertheless, the information
has been shared with the TRTR community, Gamma Metrics, and the NRC sine
our experiences and the findings may be useful to other licensees.
Discussion
NC State University Linear Channel
The first problem experienced with the Gamma Metrics Wide-Range Linear
Monitor occurred in November 1995 with the PULSTAR Reactor "Linear
Channel" that is ranged with reactor power, used for power control, and
provides redundant scrams for the PULSTAR Reactor "Safety Channel". This
event was previously reported to the NRC and was summarized in the
February 1996 TRTR Newsletter. The range control card was suspected and
returned to Gamma Metrics where several components were replaced. The
monitor was reassembled and successfully tested on the bench top and the
channel was returned to service. On December 20, 1995 the reactor was
restarted under testing authorization and multiple range changes were
deliberately introduced during a gradual power escalation. The channel
again failed to automatically up-range and the reactor was shutdown.
The entire monitor was returned to Gamma Metrics where modifications were
made in the internal grounding. During the subsequent receiving
inspection it was noted that several cable connectors were not seated
properly and that a pin on one of these connectors was burned. Gamma
Metrics provided a replacement connector and associated harness that was
installed by the reactor staff. During pre-start testing, it was found
that actuation of the rod reverse demand would cause the range to jump to
a different range when the range control was in the manual mode. A Gamma
Metrics field engineer spent one week at the site but was unable to
diagnose the problem. The monitor was then sent back to Gamma Metrics.
A PULSTAR facility design change was approved for re-installing the
linear channel picoammeter that had been used prior to the Gamma Metrics
Wide-Range Linear Channel.
In October 1996, the linear channel was returned to NC State University.
Changes that had been made included replacement of the printed circuit
board backplane. During the initial bench-top check-out of the returned
unit, new spurious range changes were observed including oscillatory
behavior. This included increasing the range to greater than 1 MW even
though this is inhibited by design in the NC State University version of
the monitor. The unit was again returned to Gamma Metrics where it
currently remains.
Mr. Alexander Adams, Jr. (3) 10 February 1997
U. S. Nuclear Regulatory Commission
NC State University Safety Channel
The same type of unit is operated on the 1 MW range in the manual range-
change mode. In January 1997 this unit spuriously down-ranged from the 1
MW range to the 300 kW range as the reactor power was increasing through
150 KW. At 150 KW, an internal trip circuit is enabled to protect
against loss of flow or the lower plenum flapper valve was not closed.
While the operator manually scrammed the reactor, the scram function
remained operable at 360 kW due to having the scram function enabled on
all ranges. This was verified during a post-shutdown check-out. This
unit has also been returned to Gamma Metrics for repairs.
Results
Our bench-top testing with the Linear Channel and consideration of the
circuit design indicated in 1995 that:
o In either the automatic or manual range change mode, spurious
ranging did not affect the protective features of the monitors
unless:
- There is a spurious down range and the smart bi-stable is set
so that the scram is active on one range only
- The unit is on the 1 MW range, a spurious uprange occurs, and
there is a concurrent or common mode failure of the maximum
range limit function
By setting the smart bi-stable so that it is operable on all ranges, the
protective function will be maintained for single-failure spurious range
changes. A spurious range change to a range greater than 1 MW requires
both a spurious range demand and failure of the maximum range limit
function.
It should be noted that the individual Gamma Metrics channels are not
single failure proof by design. Many other single failures could cause a
channel failure. Redundancy is provided through the use of two separate
channels for high neutron flux scram and low flow scram enable.
The first problem experienced with the Gamma Metrics Wide-Range Linear
Monitor was in November 1995. We performed a series of bench-top tests
at that time in an attempt to determine the root cause of the auto-
ranging problem.
The bench-top tests and review of Gamma Metrics documentation for the
unit demonstrated the following:
o Auto or Manual Mode ranging malfunctions are not likely to affect
the protective function(s) of the unit,
Mr. Alexander Adams, Jr. (4) 10 February 1997
U. S. Nuclear Regulatory Commission
o A spurious down-range can defeat the scram function IF the smart bi-
stable is set so that the scram is active on one range only,
o The smart bi-stable comparator should be set on the tenth rotary
position (dumb position) which causes the bi-stable to be operative
on ALL decades.
It should be noted that Gamma Metrics explicitly explains the smart bi-
stable options in their technical manual.
Conclusion
The Wide-Range Linear Monitor configured to provide protection only on
the full power range with a coincident spurious down-range would disable
the bi-stable and not cause a trip (over-range for the lower scale).
Such a scenario may lead to a Technical Specification violation for not
having a required protective action. This specific possibility is
eliminated by placing the rotary switch on the smart bi-stable card in
position "10" (dumb setting) on the NCSU units.
We have advised the TRTR community via the Internet recommending that
facilities with identical equipment should review the smart bi-stable
description and configuration.
Please feel free to contact us at (919) 515-2321 with any questions or
comments.
Sincerely yours,
Charles W. Mayo, Ph.D Pedro B. Perez
Director, Associated Director,
Nuclear Reactor Program Nuclear Reactor Program
cc: Mr. Craig Bassett, USNRC Region II
Senior Project Manager
Gamma-Metrics, Inc
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