Re: Why would weapons-grade plutonium cause a meltdown in a nuclear submarine?

Using weapons grade plutonium in a nuclear reactor will not cause it to 
melt down, as suggested by the recent Bond flick.  

A reactor core requires a balance of what nuclear engineers call 
reactivitiy.  A neutron that does not cause a fission in a reactor is 
considered "lost".  For example, a neutron that is absorbed by the 
structural steel or by a control rod is lost.  If something causes more 
neutrons to be produced or fewer neutrons to be lost, it is said to insert 
positive reactivity.  If it has the opposite effect, it inserts negative 
reactivity. 

The fission of Plutonium-239, the main isotope in weapons grade plutonium, 
produces a higher average number of neutrons than Uranium-235.  Therefore, 
the addition of plutonium to a nuclear reactor would insert positive 
reactivity.  But, this would not occur in an uncontrollable fashion. By 
adjusting control rod positions the nuclear reactor could be operated 
safely with fuel made from weapons grade plutonium.  A large positive 
reactivity insertion without a corresponding correction by other means, 
such as insertion of control rods, could cause a reactor to meltdown.  
Reactors are designed with specific automatic safety systems to prevent 
such a thing from happening.

Matter of fact, about 30% of the energy produced in a nuclear reactor comes 
from the fission of plutonium that is produced inside the reactor.  
Uranium-238 absorbs neutrons (another way to lose neutrons) but then decays 
to Pu-239, which can subsequently fission. So, plutonium is continuously 
burned in all nuclear reactors.

The USA, as well as Russia, is pursuing an effort to dispose of weapons 
grade plutonium by "burning" it in nuclear reactors.  The weapons grade 
plutonium will be used to make mixed oxide (MOX) fuel that will be inserted 
into reactor cores to produce electricity at the same time that it reduces 
the world's inventory of weapons grade plutonium.  

For more information on the use of MOX fuel in nuclear reactors see one of 
the following web sites:
 http://www.moxfuel.com:8
084/Mox/MoxFuel.nsf

Try the following links for two excellent web sites with further 
information on how nuclear reactors work and other related information.
 http://www.cannon.n
et/~gonyeau/nuclear/index.htm
 http://www.uic.com.au/