[ RadSafe ] Used Fuel

SFisher373 at aol.com SFisher373 at aol.com
Fri Apr 29 14:36:43 CDT 2011

Franz, I beg to differ with what you say regarding used fuel.  Within  Used 
Fuel a number of things are happening.  For one, the U-238 is being  
changed to Pu-239.  As well, there is Pu-241 being formed.  And Pu-239  and Pu-241 
are more fissile than the U-235.  On the down side, you have the  poisons 
building up in the fuel.  
The main reason that we can not run our fuel in Candu reactors too long is  
not that they are too depleted, but that the increased Pu-239 causes a 
lower,  delayed neutron fraction, beta.  The lower beta causes the reactor to 
react  faster, and to increase the chances of prompt criticality.  The used 
fuel  causes the reactor to react faster than our shutdown systems could 
handle.   So we change the fuel, to ensure that we are within our safety  
At PACS in Hungary, there was a prompt criticality with used fuel as  
detailed in the WANO SOER 2007-1(I think) on reactivity management.   They were 
washing the used fuel to remove deposits on it, when the pressure was  
released too quickly, and the fuel bundles cracked, and the pellets ended up in  a 
pile. So prompt criticality is a possibility with used fuel.  Once the  
neutrons went up, the safety injectors injected Boric Acid and stopped the  
Spencer M Fisher
Reactor Physics Instructor
Ontario Power Generation
In a message dated 29/04/2011 12:43:17 P.M. Eastern Daylight Time,  
radsafe-request at health.phys.iit.edu writes:

Even  more this is true
for used fuel rods with considerably lower U-235  concentration - otherwise
they would be still used and their potential  would not be disregarded!
People who think of a nuclear reaction from used  fuel have obviously read 
much about criticallity accidents with highly  enriched or bomb-grade 

Therefore I believe that these news are  the same bogus as 90% of the
distributed news about the Fukushima accident.  It is in the interest of 
media, "environmentalist" groups and  especially groups who employ the 
stories commercially (Greenpeace).  

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