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The
Pebble Bed Modular Reactor (PBMR) is a next generation
nuclear power plant with high thermal efficiency
and inherent safety characteristics. The extensive
use of CFD in the design of the PBMR allows the
engineers to tackle challenges during the design
phases that would have otherwise only been encountered
at high cost during the commissioning or operation
of the plant.
The PBMR utilizes a direct
cycle high temperature gas cooled Reactor Unit
(RU) and Power Conversion Unit (PCU). The plant
has a reactor of a pebble bed type and a three-shaft
helium Brayton Cycle (Fig 1). The helium gas is
heated by the reactor, passes through a high-pressure
Turbine, low-pressure Turbine and Power Turbine,
driving the generator. It passes through a Recuperator,
Pre-Cooler, low-pressure Compressor, Inter-Cooler
and high-pressure Compressor, back through the
Recuperator to the Reactor. Helium is chosen as
the working fluid due to the particular benefits
that it brings to closed cycle high temperature
reactors. Its advantages are that it is a chemically
inert gas and thus not affected by radiation, high
specific heat and its high sonic speed (three times
higher than air), allows higher circumferential
velocities on Turbomachinery blades. The disadvantages
are that some PCU components need to be either
specifically developed for helium or adapted from
existing components.
CFD
in the design process
CFD provides detailed information
to System Engineers from PBMR as well as external suppliers
and serves as input to FEM analyses when integrated
CFD-FEM results are required.
CFD provides component characteristics for complex geometries to Flownet,
a one-dimensional thermo-hydraulic network solver.
 Reactor
Unit, RU
The RU of PBMR consists of
a central column of graphite spheres surrounded by
an annular fuel pebble bed, enclosed by graphite blocks
on the inside of the core barrel. Between the core
barrel and the reactor pressure vessel is a gap filled
with helium. Between the reactor pressure vessel and
the concrete is an array of water pipes, protecting
the concrete against high temperatures (Fig 2 left).
CFD is employed to investigate local as well as global
thermal and fluidic effects.
These CFD results have led to several design changes to satisfy the PBMR
specifications, ranging from the design of the water pipes, support structure
of the vessel to the design of the helium inlet and outlet slots. It
is clear that the design of the RU has been greatly influenced by the
detailed CFD results.
Power
Conversion Unit, PCU
Cycle pressure losses and leakage
flows have a major effect on cycle efficiency. The
cycle pressure losses are primarily a function of the
individual component designs and layout. Leakage and
cooling flows are also a function of the component
design and component cooling strategy. These pressure
losses, leak flows and cooling flows must be determined
across interfaces between components from different
suppliers and through the components themselves. Some
of these interfaces have a high temperature and/or
pressure gradient. This calls for integrated CFD and
FEM analyses. Therefore, a complete CFD model of the
PCU was constructed, containing all the different components
and interfaces (Fig 3 and Fig 4).
All fluids and solids were solved simultaneously to obtain temperature
and pressure fields that were mapped onto a FEM mesh. The CFD results
were also used to calculate pressure drops across the different components.
The calculated loss coefficients are used by Flownet to improve the accuracy
of the cycle calculations. Detailed information could also be supplied
to the component designers regarding the thermal environment in which
their components will operate.
Spent
Fuel Storage Tanks
The spent fuel storage tanks
are used to store spent fuel from the power plant generated
during its production lifetime (40 years). Thereafter,
the tanks must store the spent fuel for another 40
years before being decommissioned. Detailed and accurate
temperature distributions throughout the complete Spent
Fuel Storage Area are needed, ensuring that the temperature
limits for the fuel, tank, supports and concrete are
not exceeded. CFD was used to simulate this complete
Spent Fuel Storage Area. This model included the fuel,
helium in the tank, the tank itself, the air surrounding
the tank and the concrete walls of the area (Fig 5).
From the results, temperature
distributions in all of the materials could be
obtained. The temperature distribution for the
tanks is shown in fig 6 and the temperature distribution
of the fuel is shown in Figure 7. The high heat
source from the "youngest" fuel can be
clearly seen. Note also the effect this has on
the tank temperatures. CFD supplied answers to
the tank designers, the HVAC designers, the building
designers and the nuclear physicists.
Conclusion
CFD has been a major
contributor to improving PBMR design. Optimizing
many design aspects before commissioning and
operating the plant has saved time and cost.
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Fig 1: Layout
of the PBMR
Fig 3: Meshed
volumes of the PCU
Fig 4: Close-up
of the PCU mesh
Fig 5: Geometry
of the spent fuel tanks
Fig 6: Temperature
contours of the
spent fuel tank walls 
Fig 7: Temperature contours of the
fuel pebbles
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