Advanced Gas Cooled, Graphite Moderated Reactors and Pressurized Water Reactors: A Comparison

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Advanced Gas Cooled, Graphite Moderated Reactors
Introduction
In this type of reactor, elements of fuel among them fuel rods are enclosed within Magnox cans
and are then loaded into vertical channels inside a core that has been constructed using blocks of
graphite. Further, the vertical channels are composed of control rods which are string neutron
absorbers that can be dipped or withdrawn from the core to regulate the rate of the process of
fission and hence controlling the heat output (1). Carbon dioxide is blown past the fuel cans in
order to bring about a cooling effect of the whole assembly. Water is then converted to steam by
the hot gas inside a steam generator. In its early design, a steel pressure vessel was used that was
surrounded using thick concrete radiation shield but this was abandoned in the later design and a
dual-purpose concrete pressure vessel as well as a radiation shield were adopted.
How it works
Improving the cost effectiveness of this reactor called for achieving higher temperatures in order
to attain higher thermal efficiencies as well as higher densities of power that would minimize
capital costs. This involved increasing the pressure of the cooling gas as well as changing from
Magnox to stainless steel cladding. It also involved changing from uranium metal to uranium
dioxide fuel. In so doing, there was a rise in the need of increasing the proportion of U235 in the
fuel that was being used. This resulted into a new design called Advanced Gas-Cooled Reactor
as shown in the diagram below which still adopts graphite as the moderator (2).

The end products or purpose of this reactor type is production of electricity and plutonium.
Benefits of Advanced Gas Cooled, Graphite Moderated Reactors
 It is a simple fuel process
 There is no corrosion involved in the operation of the assembly system
 Makes use of graphite which is stable even at very high temperatures with higher thermal
efficiency (3)
 The possibilities of explosion are minimized through the use of carbon dioxide
 The use of uranium carbide and graphite are important in the resistance of high
temperature
Uranium carbide (UC2) is formed through the carbonization of uranium oxide using carbon in
which carbon monoxide and uranium carbide are formed as the only products
UO2+4C UC2+2CO
Uranium dioxide + carbon uranium carbide + carbon monoxide
In order to achieve the products, the process must be carried out under an inert gas since uranium
carbide is easily oxidized back to uranium oxide.

Advantages
• Not associated with corrosion
• It is a simple fuel process
• Uses graphite which is very stable even at very high temperatures
• The use of carbon dioxide eliminates the chances of explosion
• The use of uranium carbide and graphite help in resisting the high temperature in the
system
Disadvantages
• It is a complicated consolation
• Requires high quantity of fuel which is expensive
• It has very low power density
• It requires more power for the circulation of the coolant
• Due to the high critical mass, it requires very high initial fuel