[ RadSafe ] Re: Spent Fuel and Decay

[JGinniver at aol.com]

In a message dated 30/07/2007 18:38:58 GMT Standard Time,  
Mike.Brennan at DOH.WA.GOV writes:

I took a quick look through the report, and I wasn't able to find  the
passages you quote, but I accept without reservation that they are  there
and you presented them in context.
 
Hi Mike, I had another look at the report and the section I quoted from  is 
Appendix 3 - nature of the hazard starting on p. 341

I  have a suspicion, however, that
someone who had a hand in the report was  sliding something by.  Two of
the three passages specifically state  "Natural Uranium ore" or "the
equivalent uranium ore", and the other is  ambiguous as to what is being
compared (used fuel to "Natural  Uranium").  A couple numbers are given,
from 130,000 to a million  years, with the latter mentioned in each
quote.  I note, however, two  points that cause me some concern.

Having re-read the section I had quoted from I must admit that my  impression 
is that the report is simply using the terminology used in the source  
material.  Unfortunately without reading the source material I can't  confirm this.  
Perhaps someone else on the list is familiar with the source  material (all 
listed at the end of my last e-mail and might be able to clarify  this)
 
 


The  first is the use of the word "ore".  "Ore" generally means  the
material in question (in this case uranium) mixed with the rock it  is
found in.  While ore is interesting, I don't think it is in this  case
particularly germane.  While I am not familiar with CANDU fuel, I  am
reasonably certain that the ore is processed to remove the uranium,  and
the uranium is then mixed with other materials and fabricated into  fuel
elements.  I suspect that the fuel elements have a higher  percentage of
uranium than ore does (though quite possibly a lower total  activity, as
the non-uranium isotopes in the decay chain would not be  present in the
fuel).  Thus, even if the fuel was never used it would  have higher
activity than ore, particularly if one was only considering the  uranium
in the ore, and all radioactive isotopes in the fuel.    

The same appendix in the report gives a brief description of CANDU  fuel.  
What I found interesting is that although it is natural uranium  (i.e. approx. 
99.3% U-238, and 0.7% U-235) it is in a ceramic UO2  matrix.  Whereas I had 
previously assumed that is natural uranium metal  rods like UK Magnox fuel.  
 


The  "one million years" figure I find interesting.  When one gets past
the  time where the total activity of spent fuel is dominated by  fission
fragments and activation products, say three or four hundred years,  you
settle into for the long haul, with the total activity driven by  U-235,
U-238, and Pu-239.  The fresh fuel stated with the natural  ratio of
U-235 to U-238, in which the overwhelming majority of the number  of
atoms are U-238, but with U-235 providing a fair amount of the  activity,
due to its higher specific activity.  While the burn history  of fuel may
vary, the goal is to consume as much of the uranium as can  profitably be
converted into energy.  In fuel I am familiar with, the  limiting factor
usually is the build up of neutron poisons, but most of the  U-235 is
consumed, with some of the U238 fissioned directly, and some  converted
into Pu-239 (mostly).  Of the Pu-239, some fissions and some  remains.  

The report states that 'used' CANDU fuel contains approx 0.23% U-235  and 
0.25% Pu-239, this compares with approx 0.84% U-235 and 0.94% Pu (estimated,  
based on 4.32Kg of Pu per PWR fuel assembly with an initial mass of 461.4 Kg  
Uranium) at Discharge, unfortunately the Nuclear Fact Book doesn't quote  the % 
of Pu present and doesn't give individual isotopes/
 


The  question of how radioactive the spent fuel is after the short
half-life  (let's say 100 years) stuff decays seems to be mostly driven
by how much of  the original U-238 is left, and how much Pu-239 was
created.  The  Pu-239 has a much higher specific activity than U-238, as
its half life is  only 24,000 years, as opposed to 4.5 billion years for
U-238.  As the  Pu-239 decays into U-235 the total activity of the spent
fuel decrease,  eventually reaching the same total activity of the
original fuel (the  activity from the additional U-235 remains small).

This is where the  one million years seems problematic to me.  By the
time we get out to  a million years, we have about 4.5 E-11 of the
original Pu-239.  Even  if you started with a lot, that's not much left.
Given the loss of some of  the original uranium from the fuel, I find it
difficult to believe that  this many half lives of Pu-239 have to occur
for total activity to get down  to that of the fresh fuel.  If someone
sees a problem with my math, I  would be indebted if you would point it
out.

Unfortunately the only way to work out how the values of 500,000 or  
1,000,000 years have been derived would be too look at the original reports from  the 
IAA and EN and the NEMO report doesn't provide any detail.
 


If  the total activity is not being compared to the fresh fuel, but
rather to  the ore that provided the uranium that went into the fuel, I
see several  problems, the biggest of which is that of concentration.   If
concentration is the sin, then dilution is the solution.  In  other
words, if those managing the spent fuel are being held responsible  for
the increase in activity per kilo that occurred when the ore was  turned
into fuel, it only seems fair that they be allowed to dilute the  spent
fuel to get back to the original total mass.  This lets you  compare on a
same-weight basis.  Of course, once you've started  diluting, there is no
real reason to stop at the original ore weight.   You can continue adding
filler until the concentrations are  negligible.


Unfortunately the modern regulators mantra is 'concentrate and  contain.'  
Dilute and disperse in no longer considered  acceptable.


If I  made a mistake in interpreting the words out of the report,  I
apologize.  I have no desire to misrepresent anyone's work.  It  is just
that I have on far too many occasions seen individual with a  particular
agenda choose inappropriate comparisons in order to make the  "other
side" look worse than they really are.    


I don't see that you made any mistakes at all.  It would have been  better if 
I had been able to include more of the original report, but I could  cut and 
paste from the pdf file and the time I had available meant that I was  only 
able to cribs what I thought would be the relevant sections.
 
Regards,
Julian (not Julia lol)