Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS
WASHINGTON, D.C. 20555
December 26, 1991
NRC INFORMATION NOTICE 91-84: PROBLEMS WITH CRITICALITY ALARM
COMPONENTS/SYSTEMS
Addressees
All Nuclear Regulatory Commission (NRC) fuel cycle licensees, interim spent
fuel storage licensees, and critical mass licensees.
Purpose
NRC is issuing this notice to remind licensees of the importance of adequate
reviews of plant modification, installation, maintenance, and response
actions, to ensure that required criticality alarm systems meet their
intended purpose. It is expected that recipients will review this
information for applicability to their facilities, distribute it to
responsible staff, and consider actions, as appropriate, to avoid similar
problems. However, suggestions contained in this information notice do not
constitute any new NRC requirements, and no written response is required.
Description of Circumstances
The following cases are recent events involving problems with licensee's
criticality alarm systems that have been reported to, or discovered by, NRC.
Case 1: During a routine test of the criticality alarm system, a licensee
discovered that several of the site's audible alarms ("howlers") did not
actuate. The licensee found that wiring to the alarms had been accidentally
broken while other electrical cables were being pulled through the cable run
that contained the criticality alarm wiring. The licensee's system provided
indication, prior to the next scheduled test, that some of the audible
alarms had been disabled.
Case 2: Engineering drawings describing modifications at a licensed
facility specified removal of "heat detectors (radiation)." The licensee's
review of the modification package did not recognize that it included
removal, rather than relocation, of criticality alarm system detectors.
When the specified detectors were removed no alarm was generated at the
system monitoring panel. Subsequent investigation disclosed that the alarm
panel was wired in such a way that, although a "failure" light was activated
at an intermediate panel (an unmanned location), a "loss of power/loss of
detector" signal was not generated at the monitoring panel in a normally
manned area.
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Case 3: A licensee experienced an activation of the plant's criticality
alarm system, but no criticality accident had actually occurred.
Investigation found that the alarm had been generated when the
uninterruptable power supply (UPS) circuit that powered the alarm system was
turned off by means of a switch in the facility's main computer room. The
switch had been backfitted to the system to allow for cutoff of all power to
the computer room in emergency situations and was not intended to affect
power to the criticality alarm system. The modification review associated
with the addition of the switch did not identify the fact that the planned
location was between the UPS source and the primary criticality alarm system
circuit.
Case 4: During an electrical storm, the criticality safety alarms at only
the Waste Treatment Facility (WTF) sounded because of a momentary power
interruption. Personnel in the WTF did not evacuate, but instead called
Security, who then notified Radiation Control and Electrical Shop personnel.
Electricians then entered the WTF to silence the alarms before obtaining
clearance from Radiation Control. An investigation of the incident
determined that the personnel in the WTF did not evacuate in accordance with
procedures required by 10 CFR 70.24, and that the electricians made an
uncontrolled entry into the WTF to silence the alarms prior to getting
clearance from Radiation Control.
Case 5: During a routine NRC inspection of a licensee's criticality
accident monitoring system, the inspector found that: (1) since 1970, no
evaluation had been performed to demonstrate that the system provided
adequate monitoring coverage for facility modifications or additions where
large quantities of special nuclear material were being stored or used; (2)
the licensee did not have a system to ensure that such technical evaluations
were performed and documented; and (3) no program existed to limit the
storage of intervening shielding materials between the neutron criticality
detectors and the monitored areas.
Case 6: During maintenance, a licensee discovered that under design basis
accident conditions (greater than 1 E+15 fissions) its criticality alarm
system might not function as intended. The alarm system used Geiger-Mueller
detectors that could become electronically saturated in a high radiation
field and not function. As a result, the licensee declared an Unusual Event
and discontinued transfers of fissile materials within the facility. The
licensee terminated the Unusual Event after connecting anti-saturation
circuitry to the criticality alarm system.
Discussion
All licensees are reminded of the importance of maintaining operable
criticality detection, monitoring, and annunciation capabilities, as well as
procedures and training for response to criticality alarms. As the
forementioned cases indicate, a lack of detailed knowledge of the system's
configuration and routing of detector or power circuits can result in
failure to recognize the possible impact of installation, modification, or
maintenance activities on the criticality alarm system. Physical and
electrical modifications have the
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IN 91-84
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clear potential to degrade or disable all or part of this important safety
system. Licensees should ensure that they have established a comprehensive
testing program, both on a routine basis and after maintenance and
modification activities, and a method of continuously monitoring the
integrity of criticality alarm system lines and components, through line
supervision, to preclude failure of criticality alarm system capabilities.
These activities should include clearly written implementing procedures, and
a training program to ensure appropriate implementation.
This information notice requires no specific action or written response. If
you have questions about the information in this notice, please contact one
of the technical contacts listed below or the appropriate regional office.
Richard E. Cunningham, Director
Division of Industrial and
Medical Nuclear Safety, NMSS
Technical contacts: Scott Pennington, NMSS
(301) 504-2693
Gerald Troup, RII
(404) 331-5566
Attachments:
1. List of Recently Issued NMSS Information Notices
2. List of Recently Issued NRC Information Notices
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