[ RadSafe ] Re: Spent Fuel and Decay

Mon Jul 30 12:34:47 CDT 2007

Hi, Julia.  Thank you for the reference.

I hadn't given much thought to CANDU fuel, as I am a light water kinda
guy (I'll even admit a preference for pressurized over boiling).  As
CANDU fuel does not have to be enriched, the uranium mix within the fuel
starts out with a lower specific activity, though I don't know how the
total activity per fuel rod compares, and I am not sure that is even a
useful comparison.

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.  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.

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 "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 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.

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.

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.   

-----Original Message-----
From: radsafe-bounces at radlab.nl [mailto:radsafe-bounces at radlab.nl] On
Behalf Of JGinniver at aol.com
Sent: Saturday, July 28, 2007 9:50 AM
To: llowe at senes.ca; radsafe at radlab.nl
Subject: Re: [ RadSafe ] Re: Spent Fuel and Decay

A quick look on google came up witha report from the Nuclear Waste
Management Organisation (of Canada)

_http://www.nwmo.ca/adx/asp/adxGetMedia.asp?DocID=1487,20,1,Documents&Me
diaID=
2703&Filename=NWMO_Final_Study_Nov_2005_E.pdf_
(http://www.nwmo.ca/adx/asp/adxGetMedia.asp?DocID=1487,20,1,Documents&Me
diaID=2703&Filename=NWMO_Final_Study_
Nov_2005_E.pdf) 

This states that:

SNIP>The radioactivity of used [CANDU] fuel decreases to about 1% of  
SNIP>it's
initial value after 1 year, 0.1% after 10 years and 0.01% after 100
years  (AECL 1994).  After approximately one million years, the
radioactivity in  used fuel approaches that of Natural Uranium (AECL
1994; NWMO 2003; Mc Murray et  al 2003)<SNIP

and that

SNIP> Radiotoxicityt must also be considered.  Note that the  
SNIP> radiotoxicity
of used fuel also becomes comparable to that of Natural Uranium  ore on
a one million year time frame<SNIP

and also

SNIP> Similar analysis for used PWR fuel with enriched U-235 suggests  
SNIP> that
teh radiotoxicity of used [PWR] fuel becomes equal to the equivalent
uranium ore after about 130,000 years (IAEA 2004).  Other analysis
sugests  the time period is between 500,000 and one million years (OECD
2004).

The refrences were given as :
IAEA 2004, Implications of partitioning and transmutation in Radioactive
Waste Management IAEA Technical Report Series No. 435

OECD 2004, The handling of timescales in Assessing Post Closure  Safety.
NEA Report NEA No. 4435, ISBN 92-64-02161-2

Mc Murray J, D.A. Dixon, J. D. Garromi, B.M. Ikeda, S. Stroes-Gascoyne,
P.  
Baumgarter & T.W. Melnyk 2003.  Evolution of a Canadian deep Geologic
repositry - Base Scenario, AECL, OPG Nuclear Waste Management Division
rep 06819-REP-01200-10092-R00

Regards,
        Julia

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