Gas Cooled Reactors in Nuclear Plants

As the name itself suggests, a gas cooled reactor is cooled using a gas, and the heat extracted by the gas during the process of cooling the reactor is used either indirectly to generate steam which in turn is used for turbine propulsion, or this heated coolant could be used directly as the working fluid of the gas turbine thus eliminating the need for a separate steam circuit. Of course both these approaches have their own set of features and limitations. The moderator used in these types of reactors is Graphite which offers the advantages of being stable under conditions of high radiation as well as high temperatures.

There are several options available to choose for the coolant including gases but mainly carbon dioxide and helium are used as coolants apart from hydrogen in certain situations. Thermodynamically speaking Helium offers the best alternative since it has a high specific heat and low capture cross section for thermal neutrons but it is much expensive as compared to carbon-dioxide. The advantages that a gaseous coolant offers over light or heavy water are as follows

1. The gases are less prone to react chemically with the structural material of the reactor unlike water which has higher affinity for chemical reactions with these elements.
2. Gases are more flexible in terms of the temperatures and pressure ranges to which they can be subject to as compared to water. Of course certain practical considerations do limit these ranges but certainly they are more than those available for water.
3. Gas cooled reactors are more stable and safe because the reactivity of the reactor is not a function of the quantity of gas present in the core. Hence if a gas leak occurs accidentally the reactor would be much safer than a similar leak developed in water cooled reactor

As described above the direct system uses the same gas which is used as a coolant to act as the working fluid rotating the turbine blades from the enthalpy generated during heat absorption during the reactor cooling process. The main feature of such a system is that it gives the highest thermal efficiency of all types of nuclear reactors which are currently being used in the industry and typically gives efficiency of the order of nearly 42% within moderate operating temperatures.

And indirect circuit consists of cooling gas in the primary circuit while the secondary circuit consists of water as the working fluid which is converted into steam using a heat exchanger.

1. If CO2 is used as the cooling gas it eliminates the possibility of explosion which is always present in water cooled reactors
2. There is no need for cladding the metallic fuel which leads to simple fuel processing techniques as compared to other types of reactors where cladding is necessary
3. The main drawback of these plants is there low power density which requires large size of the reactor for relatively smaller power requirements
4. Although Helium is an excellent cooling medium from the thermodynamic point of view, its low neutron absorbing capacity makes it unsuitable for load control