|MadSci Network: Physics|
The efficiency you are reading about is almost certainly what is called the thermal efficiency of the plant. One of the more fundamental results of the second law of thermodynamics is that this efficiency is determined for all types of heat engines to depend solely on the temperatures between which it operates. Specifically the bigger the difference in temperature through which heat flow occurs the more efficient the engine will be. Since the temperature of the world we live in is pretty much fixed within a fairly narrow band determined by the seasons then the way to increase efficiency is to increase the temperature of the source from which heat flow occurs. Carbon, in whatever form, is not nearly as good a moderator for slowing fast neutrons as is hydrogen (heavy or light). However as long as you keep it away from chemicals with which it can react at high temperatures, like oxygen, you can raise the temperature of graphite much higher than you can that of water and without having to maintain the very high pressures needed to keep the water liquid as well. Itís not then that the graphite stores energy somehow as much as it simply acts as a moderator that will still let the reactor work at elevated temperatures. Pyrolytic graphite is used because it is mechanically stronger than ordinary graphite and its porosity can be varied over a fair range during manufacture. This aids in the design of a more robust fuel pebble and allows for better internal containment of those fission fragments that happen to be gaseous nuclides. The Wikipedia site has a very detailed and nicely written section on pebble bed reactors.
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