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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