Advanced Gas-cooled Reactor

Once nuclear chain reactions were understood and harnessed for bombs and ships, nuclear engineers on both sides of the Iron Curtain turned their attention to the mundane but practical task of making electricity. Most nations adapted their submarine engines to the task, but the British found a different solution. 15 of the UK's 16 commercial nuclear power plants use high-temperature carbon dioxide rather than water to cool the reactor's core and heat its steam generator. You can read more about that here:
Advanced Gas-cooled Reactor

Mark's Standard Handbook for Mechanical Engineers has this to say about the thermodynamics of nuclear reactors:

...A nuclear reactor must be treated as a heat source which differs from a chemical heat source in that no oxygen is required and the heat does not have to be removed from gaseous combustion products which possess poor heat-transfer properties.

Most nuclear reactors in the United States, France, Japan, and the former Soviet Union (the largest producers of nuclear power) use water to channel the immense heat released when uranium atoms split to engines that convert some useful fraction of this heat into electricity. Liquid water is conductive, stable, nontoxic, and abundant where most (but not all) nuclear reactors operate, and its ability to absorb tremendous amounts of energy per unit volume lends compactness to any design using water as a coolant. Since most reactors used for commercial power share a heritage with ship and submarine propulsion systems, where space and power are precious things, much of the nuclear world gets its power from pressurized- and boiling-water reactors. The British decided to go a different route.

All else equal, the hotter a heat engine runs, the more efficient it is. Though water-cooled reactors are compact, they suffer a major problem here since water boils at a temperature well below the allowable temperatures of the structural metals that reactors are made of. Steam is far less dense than liquid water, and is impractical for use inside the reactor's core, so the boiling temperature of water imposes an upper limit on how hot water-cooled reactors can run. Increasing coolant pressure (standard practice in the pressurized-water reactors that make most American nuclear power) delays water's boiling to higher temperatures, but there are practical limits to this approach as well. The British technique of using carbon dioxide as coolant allows the reactor to run at much higher temperature and efficiency than can be achieved by the American, Russian, and French water-cooled designs. The AGRs are bulkier than their water-based cousins, but that poses little issue for powering the grid.

An interesting design feature of the AGR is that it outputs steam at exactly the same temperature, pressure, and power as previous coal-fired power plants built in the UK. This allows the generator-driving turbine to be agnostic to its power source. Heat from uranium is indistinguishable from heat from coal, but you'll breathe a little easier without all that soot in the air. If only someone would remind the the power that be in London of this...