Page 35 - Nuclear Plant Journal - March April 2013
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Spent
Fuel Pool
Monitoring
By H.M. Hashemian, Jonathan Edwards,
Micah McFarland, and W.S. Johnson,
Analysis and Measurement Services
Corporation.
H.M. Hashemian
Dr. H.M. Hashemian is president of
Analysis and Measurement Services
Corporation, an
instrumentation
and control systems
testing company
headquartered
in Knoxville,
Tennessee, with a
worldwide list of
clients. He holds
three doctorates
in Electrical
Engineering,
Nuclear
Engineering,
and Computer
Engineering.
Dr. Hashemian
specializes in
process instrumentation, equipment
condition monitoring, on-line
diagnostics of anomalies in industrial
equipment and processes, automated
testing, and technical training. He is the
author of 2 books, 10 book chapters, 50
peer-reviewed journal articles, and more
than 250 conference papers. He holds 15
U.S. patents (9 awarded and 6 pending)
and is a Fellow of the International
Society of Automation (ISA), a Senior
Member of the Institute of Electrical and
Electronics Engineers (IEEE), and a
member of the American Nuclear Society
(ANS) and the European Nuclear Society
(ENS).
Summary
Following the events at Japan’s
Fukushima Dai-ichi Nuclear Power Plant
in March 2011, the Nuclear Regulatory
Commission (NRC) is emphasizing the
need for improved monitoring of spent
fuel pools. An NRC task force found
that, “The lack of information on the
conditions of the fuel in the Fukushima
spent fuel pools … contributed to a
poor understanding of possible radiation
releases and to confusion about the need
and priorities for support equipment.
The Task Force therefore concludes that
reliable information on the conditions
in the spent fuel pool is essential to any
effective response to
a prolonged station
blackout or other
similarlychallenging
accident.”
This
paper
presents the design
of a robust, cost-
effective solution for
enhanced spent fuel
pool
monitoring,
integrating
com-
mercially available
wide-range instru-
mentation (e.g., level
sensors, temperature
sensors, radiation-
hardened
video
equipment) with ex-
isting wireless data transmission devices.
Wireless data transmission will reduce in-
stallation costs, minimize the addition of
combustible material, and help overcome
issues associated with a potentially com-
promised cabling infrastructure caused
by an accident. This system can be used
to retrofit the existing fleet of nuclear
power plants to address post-Fukushima
requirements, as well as contribute to the
design of enhanced condition monitoring
equipment and techniques for instrument-
ing spent fuel pools in the next generation
of nuclear facilities.
Introduction
After spent nuclear fuel assemblies
are removed from a plant’s reactor, they
are maintained under water in a spent
fuel pool prior to long-term storage in
dry casks or a permanent storage facility.
The water in the pool serves to cool the
spent fuel assemblies, while preventing
radiation leakage to the surrounding
environment. It is well-known that the
Zircaloy cladding used in nuclear fuel
assemblies, when oxidized by steam,
produces combustible hydrogen. This
can occur if the assemblies are exposed
to air in either the reactor core or the
spent fuel pool. Such an event took place
at the Fukushima Dai-ichi nuclear power
plant in March 2011. After the explosions
in the reactor building of a previously
shutdown unit, plant operators initially
speculated that the water level in the
spent fuel pool had dropped, exposing
the spent fuel assemblies, leading to
hydrogen production as well as spent
fuel overheating and radiation leakage.
Though it was later determined that the
spent fuel was sufficiently immersed and
that the hydrogen was actually produced
in the reactor core of a conjoined unit, the
lack of pool condition information forced
emergency responders to divert significant
time and manpower to assessing a spent
fuel pool problem that ultimately did not
exist.
The event at Fukushima Dai-ichi has
served toheightenawareness in thenuclear
industry that existing instrumentation
cannot adequately inform plant operators
of the pool conditions outside normal
operations. As such, the NRC has issued
Order EA-12-051, “Order to Modify
Licenses with Regard to Reliable Spent
Fuel Pool Instrumentation”, which
requires all licensees to monitor the spent
fuel pool water level at three distinct
depths (as shown in Figure 1).
Current industry practice for spent
fuel monitoring often involves the use of
narrow-range instrumentation, as well
as limit switches, to detect when pool
parameters exceed acceptable limits.
This approach enables plant personnel to
detect the onset of a potential problem,
but it does not offer any indication of
the severity of a developing problem.
For example, if the water level in the
spent fuel pool begins to drop, the level
sensor will indicate the declining level.
However, if the water level drops below
the sensor’s narrow detection range, the
operators would be unaware of the exact
level.
One solution would be to simply
install additional narrow-range depth
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