Information Notice No. 92-06: Reliability of ATWS Mitigation Systems and Other NRC-Required Equipment not Controlled by Plant Technical Specifications
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
January 15, 1992
NRC INFORMATION NOTICE 92-06: RELIABILITY OF ATWS MITIGATION SYSTEM AND
OTHER NRC REQUIRED EQUIPMENT NOT CONTROLLED
BY PLANT TECHNICAL SPECIFICATIONS
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 of the importance of maintaining the reliability
of equipment required by NRC regulations but not addressed in plant
technical specifications. It is expected that recipients will review this
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.
Background
In 1983, the Salem Nuclear Generating Station experienced an anticipated
transient without scram (ATWS) event. Following this event, efforts then in
progress to establish requirements to address ATWS events were completed,
and the NRC issued, on June 1, 1984, Section 50.62 of Title 10 of the
Code of Federal Regulations (10 CFR 50.62), "Requirements for reduction of
risk from anticipated transients without scram (ATWS) events for
light-water-cooled nuclear power plants." This regulation required that
each reactor have equipment, diverse from the reactor trip system, that
would automatically initiate actions to mitigate the consequences of an
ATWS. The regulation also required that the equipment for this system be
independent from the existing reactor trip system and be designed to perform
its function in a reliable manner. The NRC did not require licensees to
address the operability of this equipment in plant technical specifications
nor did the NRC require that this equipment be designated as safety-related.
Description of Circumstances
To meet the requirements of 10 CFR 50.62, the Houston Lighting and Power
Company, the licensee for the South Texas Project (STP), installed ATWS
mitigation system actuation circuitry (AMSAC). In May and June of 1991,
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the NRC inspected the implementation of this system at STP, Units 1 and 2,
and identified three situations in which the reliability of the ATWS
mitigation system had been compromised.
While inspecting Unit 2, the NRC found that the licensee had left open the
switches for the interface circuit between the AMSAC system and the
auxiliary feedwater (AFW) system. These open switches disabled the
automatic AFW flow initiation feature of the AMSAC, an important part of the
ATWS mitigation system. The NRC also found that the licensee had bypassed
the automatic initiation feature of the AMSAC system, causing the system to
be inoperable, at least 15 percent of the time that the system was designed
to be operable. While inspecting Unit 1, the NRC found that the licensee
had bypassed the automatic initiation feature of the AMSAC system 36 percent
of the time that the system was designed to be operable. The NRC issued the
licensee a Severity Level III violation and assessed a civil penalty.
The NRC also issued the licensee for Comanche Peak a Severity Level IV
violation because the ATWS mitigation system was bypassed for approximately
10 days while the reactor was operating at full power.
Discussion
In April 1985, the staff issued Generic Letter 85-06, "Quality Assurance
Guidance For ATWS Equipment That Is Not Safety-Related," to provide explicit
quality assurance guidance for the nonsafety-related equipment encompassed
by the ATWS rule. Although much of the equipment required by 10 CFR 50.62
is not designated safety-related, it does perform an important safety
function if the plant's primary reactor protection system fails. The
regulation was issued to reduce the risk posed by such an event. The NRC
considers the failure of licensees to comply with this regulation and to
ensure that the ATWS mitigation system and equipment function reliably to be
a significant regulatory concern.
The events described above indicate that licensees may not place an
appropriate level of priority on resolving problems with the ATWS mitigation
system because it is not a safety-related system and because the plant's
technical specifications do not govern its operability. However,
maintenance of this system is of high priority and appropriate because such
action assures the important safety function of the system and satisfies the
requirements of 10 CFR 50.62 for the system to function reliably. Actions
taken by licensees to ensure the reliability of the ATWS mitigation system
have included performing timely surveillance testing, completing repairs and
preventive maintenance promptly, implementing necessary modifications, and
updating procedures to reflect system changes.
Plant technical specifications only address the minimum personnel and
equipment requirements for the various modes of plant operations. However,
it is important that licensees maintain equipment and systems required by
NRC regulations, to ensure their reliability, even though they are not
addressed by a plant's technical specifications.
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IN 92-06
January 15, 1992
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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.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical contacts: R. Evans, RIV
(512) 972-2507
C. Paulk, RIV
(817) 860-8236
J. Mauck, NRR
(301) 504-3248
Attachment: List of Recently Issued NRC Information Notices
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