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how the PBMR works
The Pebble Bed Modular Reactor (PBMR) Power Plant is a helium cooled, graphite-moderated High Temperature reactor (HTR).
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.
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.
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.
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.
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.
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.
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.
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.
Last Updated: 16 May 2017
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