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By Richard R. Schultz, Idaho National Engineering and Environmental Laboratory, Idaho Falls, ID; and David Schowalter, Fluent Inc.
View the pdf of this Supplement
Pathlines colored by temperature in the lower
plenum of a General Atomics GT-MHR reactor
show sufficient thermal mixing prior to the
turbine entrance (at left)
Courtesy of General Atomics
Nuclear system codes such as RELAP5-
3D® have proven extremely useful in
modeling transient two-phase phenomena
that are important for light water
nuclear reactor safety assessments. Years of
experience and validation have made these
codes fast, accurate, and robust for systems
in which the working fluid moves in welldefined
paths. Today, CFD is used for conventional
nuclear reactors as a supplement to
system codes when analyzing a process in
which the detailed three-dimensional flow or
heat transfer is important.
Some of the next generation reactor
designs now being explored are radically different
from traditional designs. Some use different
working fluids and many operate at
much higher temperatures. In all cases, a primary
goal is to rely on passive rather than
active safety systems whenever possible, and
to utilize fuel more thoroughly. Mixing and
natural convection play a dominant role in
many passive safety systems. These processes
are inherently two- or three-dimensional and
involve viscous, buoyant, and diffusive
processes that are difficult to model accurately
with traditional system codes.
For these reasons CFD has become an
essential ingredient in the suite of tools used
to analyze flow behavior in advanced reactors.
Following a critical review of commercial
CFD software conducted by INEEL in 2001,
Fluent was solicited to become a partner in
coupling FLUENT to RELAP5-3D [1]. FLUENT
was attractive to researchers because of its
versatility and large user base. Currently, a
number of efforts are underway to promote
the validation of both tools so that fluid
behavior calculations can be performed with
maximum confidence. Insights into the types
of validations required can be obtained
by reviewing the recent Nuclear Energy
Research Initiative Solicitation [2].
One validation of a passive safety system
that has been completed is the natural convection
cool-down of a pebble bed reactor
core. The fuel in a pebble bed reactor is
enclosed in many graphite-coated spheres in
a packed bed. Helium, the working fluid, is
heated by the pebbles and then used to drive
a turbine, generating power. One advantage
of this design is that in the event of a loss of
coolant pressure, the pebbles will continue to
be cooled by the natural convection of helium
through the core. Using the Eulerian granular
multiphase model to simulate the pebbles,
FLUENT has been successfully validated for
the prediction of temperatures during cool
down using data from a lab experiment [3].
Schematic of pebble bed validation
experiment and CFD model
Radial temperature profiles at three axial locations for
the pebble bed natural convection cool-down
problem are in excellent agreement with experiment
Another CFD application for advanced
reactors is the determination of mixing quality
downstream of a reactor core. The high
temperatures in gas-cooled reactors are a
challenge for turbine designers, and hot
spots at the turbine inlet must be avoided. In
one preliminary study, the lower plenum of a
General Atomics Gas Turbine Modular
Helium Reactor (GT-MHR) was simulated.
This reactor uses a graphite prism design in
place of the pebbles in the previous example.
The results show adequate mixing upstream
of the turbine, but also confirm that gas jets
entering the lower plenum near the entrance
to the hot duct are more likely to contribute
to potentially adverse temperature variations.
Concerns over global warming, fossil fuel
supplies, efficient hydrogen generation, and
growing energy needs, especially in Asia,
have led to a renewed interest in even safer
and more efficient nuclear power through
advanced reactors, and CFD will figure
prominently in their future.
References:
- R.R. Schultz, R.A. Riemke and C.B. Davis,
Proceedings of ICONE11, April 20-23, 2003.
- “Advanced Nuclear Research at Universities,” No.
DE-PS07-04ID014551 at http://e-center.doe.gov.
- A.A. Troshko and A.Y. Walavalkar, ICONE12-49089,
April 25-29, 2004.
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