From: rcp27g@gmail.com   
      
   On 2014-04-14 14:41:59 +0000, hancock4@bbs.cpcn.com said:   
      
   > On Monday, April 14, 2014 7:07:47 AM UTC-4, rcp...@gmail.com wrote:   
   >   
   >> The problem is nuclear reactors are very slow to change their output.   
   >> The rate at which demand changes is far more rapid than the rate at   
   >> which power station type nuclear reactors can respond.   
   > Slower than coal?   
      
   Yes, significantly.   
      
   > A large coal plant takes a long time to ramp up or ramp down.  Usually   
   > demand, averaged out over the grid by many consumers, changes gradually   
   > and predictively.  Utilities watch consumption and various factors,   
   > such as weather, external events, etc., that might impact demand.   
      
   The issue with nuclear plant is that the rate of heat output from the   
   reactor takes a long time to vary.  With thermal plant, the heat from   
   combustion can vary as fast as it takes to increase or decrease the   
   fuel and air supply rates.  In reality the rate of change has to be   
   moderated because the structural parts of the plant exposed to high   
   temperatures are limited in the thermal transients they can sustain   
   without mechanical failure.   
      
   For nuclear plant, it takes a long time for the rate of heat output   
   from the reactor to respond to changes in control parameters.  While it   
   varies depending on reactor design, the kinds of timescales involved in   
   ramping up and down power are measured in tens of hours.  This is far   
   longer than the timescales of demand change from the power grid.   
      
   In a nuclear (or big steam) plant, the actual power output can be   
   reduced by throttling steam and dumping it straight to the condenser   
   rather than sending it through the turbine, but this is not really an   
   ideal way of running things.  In an ideal world, you would have a   
   system like Switzerland, where there is nuclear power to cover the   
   baseload and hydro/pumped storage to manage the peaks.  That, however,   
   requires the kind of geography of Switzerland, that few other countries   
   can manage.   
      
   Robin   
      
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