MadSci Network: Physics

Re: How does the pyrolytic Carbon in Pebble Bed Modular Reactor help efficiency

Date: Wed Aug 30 08:46:23 2006
Posted By: Calvin Cole, Faculty, Engineering Physics, Northeastern State University
Area of science: Physics
ID: 1156361233.Ph

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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