[LMB] reactor design

[David McMillan]

James Nicoll wrote:
> On Sat, 5 Jan 2008, David McMillan wrote:
> 
>> Stewart Dean wrote:
>>> Found the NYTimes article I mentioned...from 1988...more than a few :( years
>>> http://query.nytimes.com/gst/fullpage.html?res=940DE3DA1039F936A25752C1A96E948260
>>> It /is/ uranium fueled but with the uranium is in high temperature
>>> encased pellets instead of fuel rods.  Thorium sounds better tho.
>>>
>>> Re the graphite rods for safety shutdown...using these to immediately
>> 	Graphite?!?  Are you certain?  B/c I could have sworn that graphite was
>> a reaction *enhancer* -- IIRC, it slows down neutrons that would
>> otherwise have been going to fast to stay inside the core and trigger
>> more fusion reactions.  I thought the control rods were made out of
> 
>  	Fission

	<facepalm>  I knew that.  Really, I did.  Nuclear fusion is still 30 
years away <rimshot>.

>> cadmium or some similar neutron-capturing material.
> 
>  	As I recall, the graphite was used as a tip on the control rods 
> because it made a good lubricant.

	I think I mentioned that.  Problem is, if I'm recalling the NRC report 
on Chernobyl correctly, Graphite *also* has a *positive* effect on 
fission (hah!  got it right this time!) reactions, and when inserted the 
rods actually *caused* the reactor to go prompt critical.  I don't 
recall if the rods were never intended to be completely withdrawn, or if 
the design counted on being able to move the graphite tips through the 
critical zone quickly enough to avoid what actually happened.
	Still, even though the Chernobyl reactor design is generally regarded 
as being pretty poor, it *still* took an unprecedented degree of 
incompetence to cause the meltdown -- and even then, a properly-designed 
containment vessel (for cost reasons, the Chernobyl reactor had 
essentially *no* containment vessel) could probably have contained the 
resulting fire and contaminants.  It's almost enough to give one a 
positive feeling about nuclear power.