Nuclear reactors using light water, or ordinary water, as reactor coolant are installed in about 80% of the commercial nuclear power plants today. Whether employing water at a high, though subcritical, pressure so that it does not boil in the reactor core and heat is transferred to raise steam in an adjoining cooling water circuit, or simply raising steam immediately above the core at a lower pressure, the steam is delivered to drive the steam turbine, which then turns the electrical generator to produce electricity.

For effective reactor cooling, the water needs to be kept well below its upper critical point (22 MPa and 374°C) but much lower figures are adopted in practice to accommodate the thermodynamic process of heat input and work output for power generation, that is, the steam cycle.

The resulting conditions for water are therefore typically at 15.5 MPa and 330°C above the core if boiling is not permitted there, and heat transfer to and steam raising at the adjoining circuit have led to steam entering the turbine at typically 7 MPa and 290°C, having a modest 130°C above boiling. The reactor design that permits boiling above the reactor core has similar steam conditions at the turbine inlet. In contrast, conventional power plants, for example those with coal, typically has substantially “superheated” subcritical steam at typically 15 MPa and 540°C entering the turbine.

Conventional wisdom in thermodynamics suggests that in a subcritical steam cycle, a higher steam temperature will yield a higher efficiency in turning heat input into work output. Indeed, light water reactor plants, even of the most advanced marketable designs, have a typical net efficiency of 34% while a mature conventional coal-fired plant working to a more advanced steam cycle will yield a few more percent in efficiency. A more modern coal-fired plant that operates above the upper critical point may achieve close to 50% efficiency.

The constraint of using water for reactor cooling upon plant efficiency can be avoided by using a different cooling fluid such as gas or liquid metal, and established gas and liquid metal reactor technologies have yielded a net efficiency well above 40%.

This article is contributed by Ir Richard Fung with the coordination of the Nuclear Division.