Information Notice No. 93-78: Inoperable Safety Systems at a Non-Power Reactor
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
October 4, 1993
NRC INFORMATION NOTICE 93-78: INOPERABLE SAFETY SYSTEMS AT A NON-POWER
All holders of operating licenses or construction permits for test and
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to a problem which resulted in inoperable safety
systems at a non-power reactor. 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 28, 1993, after a number of spurious scrams had occurred, the
licensee for the University of Virginia Research Reactor shut down the reactor
and began troubleshooting activities to determine the cause of the problem.
The reactor had been experiencing spurious scrams for some time and the
licensee had concluded that the scrams were not caused by electrical
transients or line noise. The licensee suspected the reactor scram logic to
be the cause of the scrams. The reactor scram logic consists of two trains
that cause power to different magnets to be cut off on receipt of a scram
signal. This in turn releases control rods and shuts down the reactor. While
investigating the problem, the senior reactor operator (SRO) interchanged
components of the scram logic system between the two trains. Among these
components were solid-state relays and mixer-drivers (M/Ds) which act as a
28-channel "OR" gate in the scram logic. After consulting the facility safety
analysis report and visually inspecting the exterior of the components, the
SRO assumed that the interchanged components were identical. However, the
M/Ds had been internally modified in the 1970s to tie unused inputs together
and were no longer identical.
After the M/Ds had been exchanged and no spurious scram signals were received
for 30 minutes, the reactor administrator and the SRO started the reactor.
Neither the reactor administrator nor the SRO recognized that the
troubleshooting activities were actually a maintenance or modification
function and that testing to verify the operability of the reactor scram
system was required before the reactor was restarted.
October 4, 1993
Page 2 of 3
After operating for approximately 5� hours, the reactor was shut down by
driving the rods into the core. With three of the four control rods seated,
licensee personnel then introduced a scram signal into the reactor
electronics using the intermediate range channel switch. However, the
electronics did not generate a scram signal as expected. The licensee
investigated and found that the M/Ds were not internally identical as
described above. Further investigation showed that the following scram
signals were inoperable: the two power level scrams, intermediate range
period scram, primary coolant low flow scram, loss of power to the primary
pump scram, intermediate channel range switch scram, and key switch scram.
The licensee returned the M/Ds to their original positions and tested the
reactor to ensure that the reactor electronics had not been damaged.
To prevent a recurrence of this problem, the licensee made the following
changes: (1) revised the facility standard operating procedures (SOPs) to
clearly define maintenance and troubleshooting activities, (2) added a
checklist to the SOPs to specifically control maintenance activities, and
(3) added a checklist to the SOPs to verify the operability of the reactor
safety systems after an unplanned reactor scram. Other changes were also made
to the SOPs to ensure management control over maintenance.
The licensee checked the reactor electronics against the schematics and found
that changes had been made to console modules that were not reflected in the
schematics. The licensee found two instances where externally identical
components were internally different and therefore not interchangeable. After
completing this check, the licensee labeled the modules that had been modified
to clearly indicate that they were unique and not interchangeable.
The circumstances described above demonstrate the importance of recognizing
and controlling maintenance and modification actions. Clearly defining
maintenance activities in facility procedures and providing training to
personnel can be effective methods for controlling such activities. Also
important to reactor safety is updating schematics of reactor electronics to
reflect modifications to safety systems. Performing testing of the affected
safety systems after activities of the type described above may prevent the
occurrence of similar events.
October 4, 1993
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.
/s/'d by AEChaffee/for
Brian K. Grimes, Director
Division of Operating Reactor Support
Office of Nuclear Reactor Regulation
Technical contacts: A. Adams, Jr., NRR
C. Bassett, RII
List of Recently Issued NRC Information Notices
Page Last Reviewed/Updated Thursday, March 25, 2021