Re: [Phys-l] Nuclear Power and the Grid

[John Denker <jsd@av8n.com>]

On 09/10/2011 07:05 AM, Michael Edmiston wrote:

> [4] If a 3000 MW reactor system has the capability of getting rid of the 
> 2000 MW of power that is not going to the grid, then it certainly has the 
> ability to get rid of 1500 MW of thermal energy (without using the grid) if 
> the reactor were shut down to 50% (or less)

Consider the contrast:
 ++ From the point of view of basic physics, we agree that there
  are no deep principles that prevent the plant from dissipating
  1500 MW.
 -- OTOH there is more to the story than basic physics principles.
  From the point of view of engineering, in practice there is a
  very big difference between 2000 MW here and 1500 MW over there.
 -- Also there is a lot of residual decay heat, so it takes a while
  to get down to 1500 MW, so in the short term we are really talking 
  about 2000 MW here versus 3000 MW over there.

Specifically:  In normal operation there is 2000 MW dissipated by
heat exchange in the condensers downstream of the turbines.  There
is also electrical work being done at the rate of 1000 MW.  When
the turbines aren't spinning, those condensers don't contribute,
and the electrical work isn't being done ... so you need a somewhat
different system to take 3000 MW out of reactor.

We agree there is no deep reason why this can't happen.  OTOH it 
isn't "certain" to happen.  It doesn't happen automatically.  Maybe
the second system has lots of parts in common with the normal system,
or maybe not.

You might hope to get by with just a huge valve to allow steam 
to bypass the turbine, but in practice I doubt it is that simple.  
In this business, details matter.

Existing plants are nowhere near having "passive safety".
  a) You can't just walk away from the plant while the reactor
   is running and expect things to be OK.
  b) What's far worse is you can't even scram the reactor and
   walk away.  There is a lot of residual decay heat, and if 
   you don't take care of that, Bad Things are going to happen.
   Everything the industry has said about multiple layers of
   safety is hogwash.  The fuel pins *will* be breached.  The
   reactor pressure vessel *will* be breached.  The primary
   containment *will* be breached.  The secondary containment
   *will* be breached.  The countryside for miles around will
   become uninhabitable for thousands of years.  We know this
   because the experiment has been done.
     http://en.wikipedia.org/wiki/Fukushima_Daiichi_nuclear_disaster
   This should not even have been a surprise.  It was known in
   advance (albeit not widely known) that prolonged station
   blackout is a single-mode failure that leads to disaster.

Also: Spent-fuel storage is a problem.  One of the four explosions
at Fukushima occurred in a reactor building where the reactor
pressure vessel was empty.  The failure to deal with the back
end of the fuel cycle is another of those issues that leads to
all-too-predictable disasters.  People in Nevada objected to
Yucca Mountain, saying it wasn't safe.  The alternative is de
facto long-term storage at each reactor site, which is orders
of magnitude less safe.  This is nuts.  (FWIW carbon fuels have 
problems at the back end, too.)

================

There is one fairly fundamental piece of physics that is relevant
to the problem we are discussing, namely the physics of radioactive 
half-lives.  This dictates that you cannot modulate the power output 
of the reactor on any short timescale.

As a corollary, this means it is going to be difficult to pick up 
the load after a blackout, because that involves large step-function 
changes in the load, plus other weird transients.

Life would be easy if you had a 1000 MW dummy load, and a magic
switch that could transfer just the right amount of power from 
the dummy load to the real load, but you don't have either of 
those things.

===============================

> [3] Depending on where the reactor was in its fuel cycle, if a SCRAM 
> occurred, wouldn't there be enough residual heat to run a 100 MW or smaller 
> turbine/generator? 

There is.

>  If this is true, why aren't power plants designed this way?

Some of them are.  As previously mentioned, there was such a thing at
Fukushima, but it failed.  The operators didn't and/or couldn't set 
the valves correctly to spin up the mini-turbine.