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By Sarel Coetzee, Pebble Bed Modular Reactor, Pretoria, South Africa
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The Pebble Bed Modular Reactor (PBMR) is a small, inherently safe and
adaptable nuclear power plant. It is a helium-cooled, graphite-moderated
high-temperature reactor that uses a three-shaft recuperative Brayton
cycle for power conversion. The main power system consists of two primary
parts: the reactor, where thermal energy is generated by nuclear fission,
and the power conversion unit (PCU), where thermal energy is converted
to mechanical work and then electrical energy by means of a thermodynamic
cycle and a generator. The reactor uses 60 mm diameter fuel spheres consisting
of coated uranium-dioxide particles encased in graphite.
Temperature distribution in the pressure vessel, serving as
input to an FEM analysis
The PCU pressure vessel within the concrete enclosure, colored by temperature,
showing airflow pathlines surrounding it
Temperature profile on the outlet of the low-pressure
compressor and pathlines indicating the duct’s cooling flow
The PCU consists of three turbomachines: the high- and low-pressure
turbo units, and the power turbine (including the generator). Supporting
components include inlet volutes and outlet diffusers to the turbomachines
as well as various piping and interfaces. The high-efficiency recuperator
situated downstream of the power turbine uses high temperature helium
exiting from the power turbine to recover thermal energy. Lower energy
helium passes through pre- and inter-coolers and low- and high-pressure
compressors before it is returned to the reactor core through the recuperator.
The high pressure and temperature of the helium provides for the PBMR’s
comparatively high thermal efficiency. By comparison, the steam turbines
for Light Water Reactors (LWRs) operate at such low temperatures and pressures
that they are more costly to build and less efficient than the turbines
for a fossil-fuel-fired plant, where temperatures and pressures may be
several times higher. While a typical LWR has a thermal efficiency of
33%, an efficiency of about 40% is anticipated in the basic PBMR design.
Increases in fuel performance for higher temperature operation offer the
prospect of increasing the efficiency to almost 50%.
During the design of the PBMR, the determination of thermal loads on
various structures, systems, and components (SSC) is very important, and
FLUENT has been used extensively to study this in the PCU. A CFD analysis
of the PCU and its main components was recently conducted to verify design
parameters. The temperature distribution through the walls of the PCU
container (the pressure vessel) will be input to an FEM analysis to predict
stresses and displacements within the pressure vessel in order to verify
its structural design. The CFD model included most of the major components
within the PCU, the air cavity around it, as well as the concrete walls
surrounding it. The turbomachinery itself was excluded from the simulations,
with inlet and outlet boundaries in the appropriate locations accounting
for the flow into and out of the turbomachines. Inlet volutes and outlet
diffusers to the turbomachines were included, however. This resulted in
a model of approximately 3.8 million cells and 97 cell zones including
11 separate fluid flow paths, connected through fluid conduction, convection,
and conjugate heat transfer.
The CFD results have been used mainly for design verification and design
input, but also to understand the complex 3D flow fields within the various
components.CFD is recognized as an invaluable part of the design of the
PBMR.
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