From: schillin@spock.usc.edu   
      
   "Paul F. Dietz"  writes:   
      
   >Dez Akin wrote:   
      
   >> Nitrogen for a coolant? We usually use helium for gas cooled reactors.   
   >> Neutron absorbsion cross section of zero and chemically nonexistant.   
   >> Why nitrogen? (Except its much cheaper)   
      
   >The speed of sound in helium is too high to conveniently expand it   
   >in a turbine, I think.   
      
   You don't need supersonic, or even compressible, flow to run a perfectly   
   good turbine.  The obvious example being hydraulic turbines as used in,   
   e.g., hydroelectric power plants.   
      
   However, if you want to use the existing gas turbine technology base   
   without extensive and perhaps unnecessary development work, there is an   
   argument to be made for using a working fluid at least roughly similar   
   to air, such as nitrogen...   
      
      
   >This is why the proposed solar dynamic power system for the station (long   
   >since shelved) would have used a He/Xe mixture (He for high thermal   
   >conductivity, Xe to keep the average molecular weight high.)   
      
   ..or a proper He/Xe mix, though that seems needlessly contrived compared   
   to straight N2.  Argon, anyone?   
      
      
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