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| how the PBMR works |
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| The Pebble Bed Modular Reactor (PBMR) Power Plant is a helium cooled, graphite-moderated High Temperature reactor (HTR). |
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| The reactor has a vertical steel reactor pressure vessel which contains
and supports a metallic core barrel, which in turn supports the cylindrical
pebble fuel core. This cylindrical fuel core is surrounded on the
side by an outer graphite reflector and on top and bottom by graphite
structures which provide similar upper and lower neutron reflection
functions. Vertical borings in the side reflector are provided for
the reactivity control elements. Two diverse reactivity control systems
are provided for shutting the reactor down. |
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| The PBMR uses particles of enriched uranium dioxide coated with
silicon carbide and pyrolytic carbon. The particles are encased in
graphite to form a fuel sphere or pebble about the size of a billiard
ball. The core of the reactor contains approximately 360 000 of these
fuel spheres. |
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| Helium, which is used as the coolant, transfers the energy absorbed
in the core to a secondary loop through a special heat exchanger called
a steam generator. The helium in the primary circuit is circulated
by a blower. |
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| The secondary side of the steam generator contains water. The heat
absorbed, changes the water to steam which, in turn, is used to drive
a steam turbine connected to a generator to produce electricity in
the same way conventional power stations operate. In this configuration,
the reactor is an electricity producing plant. |
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| The secondary side of the steam generator may also be directly coupled
to a process plant to provide the energy as process heat. In this
configuration, the reactor is a pure process heat producing plant.
Another possibility is to configure the PBMR into a go-generation
plant, i.e. one that produces both electricity and process heat. |
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| The design is such that modules can be combined to suit the specific
energy requirements of the user. A noteworthy feature of the design
is that the reactor, steam generator and spent fuel storage will be
housed in a building called the nuclear island, while all the plant
connected to the secondary side of the steam generator will be housed
in a building (or buildings) called the conventional island. |
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| Online refueling is a key feature of the PBMR. Fresh fuel elements
are added to the top of the reactor while used fuel is removed at
the bottom while the reactor is at power. The aim is to operate uninterrupted
for six years before the reactor is shut down for scheduled maintenance.
For the first module, however, a number of interim shutdowns
will be required for planned evaluation of component and system performance. |
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| Shutdown will be done by inserting the control rods. Start-up is
effected by making the reactor critical, then using nuclear heat-up
of the core and circulating the coolant by motoring the turbo-generator
set. Heat is then removed by the pre-cooler and intercooler. At a
specified temperature, the cycle becomes self sustaining. |
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