[ RadSafe ] Global Warming
George Stanford
gstanford at aya.yale.edu
Wed Dec 9 22:32:22 CST 2009
Mike & Peter:
Just to be clear, the fission-yield percents
below do not represent the fraction of the fission
products. The fission "yield" is defined as the
fraction of fissions that yield the isotope in
question. Because there are two fission
fragments per fission, 93Zr (for example), with
a fission yield of 6.30%, comprises 3.15% of
the fission products. Not that it makes much
difference in the current context.
Here's Peter's table with the half-lives added:
99Tc: 0,2M yr 6.05% thermal 235U fission yield
135Cs: 2.3M yr 6.33%
129I: 15.7M yr 0.66% only
93Zr: 1.5M yr 6.30%
Because the specific activity of the long-lived
fission products is so low, there's a number of
relatively easy ways to manage them safely. One
possibility would be to match each nuclear plant
with a coal plant of equal capacity, and mix the
long-lived fission products uniformly with the
coal ash before shipping it off to the land fill.
Here's a ball-park calculation:
A 1-GWe coal plant produces about 2 million
tons of ash per year, with a uranium content (at
~170 ppm) of ~340 tons. A 1-GWe nuclear plant
produces about 1 ton of fission products per year,
including ~200 pounds total of the four isotopes
listed.
For the first half-million years or so, the fission-
product activity is dominated by the 99Tc. In curies,
if my figgerin' is correct, the 238U activity in the
coal ash is some 13 times the 99Tc activity.
In other words, the radioactivity of those long-
lived nuclides would not increase the activity of the
coal ash by even 10% -- and that neglects the
tons of thorium and whatever else might be in
the ash.
Corollary: At the <1% resource utilization
of today's thermal reactors, that 340 tons of
uranium would yield ~3.4 GWe-years of nuclear
energy -- i.e., 3.4 times as much energy as came
from burning the coal. With fast reactors, at 99%
utilization, that uranium would yield 340 times as
much energy as the coal did.
Maybe someone would like to check my
numbers -- I've been know to slip a decimal place
now & then.
Cheers,
-- George
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
At 04:37 PM 12/9/2009, Brennan, Mike (DOH) wrote:
Hi, Peter.
First, thank you for presenting me with a reason to look at this subject
again. It has been a while since I paid more than passing attention to
fission fragments, and it is an interesting topic, core to some of the
worst (from a technical aspect, anyway) of the arguments surrounding
nuclear power.
>small ?
>99Tc: 6.05% thermal 235U fission yield
>135Cs: 6.33%
>129I: 0.66% only
>93Zr: 6.30%
OK, so about 1/5th. Depends, I guess, on your definition of "small".
On the other hand, if you look at activity when the fuel comes out of
the reactor, the long lived fission products (LLFP) are by anyone's
definition a small fraction of the total. As an example, 135Cs and
137Cs both account for about 6.3% of the atoms in the fission fragment
inventory. Because of the difference in their half lives, however, in
fresh spent fuel 137Cs produces about 76,000 times as much radiation as
the 135Cs in the fuel. The 137Cs decay is also about 4 times as
energetic.
I might concede "small" and go up to "modest" on the % of atoms in fresh
spent fuel that are LLFP. On the other hand, I would revise the % of
activity from "small" to "vanishingly small" or "trivial" in comparison
the short half life fission products.
I am now uncertain when the total activity of the spent fuel becomes
less than the total activity of the fuel before going into the reactor,
but I am pretty confident of two things: (1) It is a long time; long
enough that I won't be personally concerned, and (2) in a much shorter
time frame, a couple of hundred years or so, the activity of the fuel
will have decreased to the point where the greatest danger is from one
of our less sophisticated descendants taking a length of fuel rod and
beating someone over the head with it.
As for transmutation of LLFP, I've seen some articles speculating on it,
but I frankly don't see the need. If it happens when spent fuel is put
in a fast flux reactor to burn more of the fissile material, cool, but I
just don't see that in and of itself transmutation (with any foreseeable
tech) is worth the effort.
Thanks again for pointing my attention this way.
-----Original Message-----
From: Peter Bossew [mailto:Peter.Bossew at reflex.at]
Sent: Wednesday, December 09, 2009 12:04 PM
To: Brennan, Mike (DOH)
Cc: radsafe at radlab.nl
Subject: Re: [ RadSafe ] Global Warming
"Brennan, Mike (DOH)" <Mike.Brennan at DOH.WA.GOV> writes:
>There aren't actually a lot more isotopes after the etc.
this is true.
>, but yes, there
>are some fission fragments that have long half lives. However, they
>represent a small fraction of the inventory of fission products in
>"fresh" spent fuel, either on an atom basis or an activity basis. They
>also generally represent a small fraction, again on an activity or atom
>basis, of the fissile material that was consumed by the reaction (the
>exact ratio is dependent on many factors).
small ?
99Tc: 6.05% thermal 235U fission yield
135Cs: 6.33%
129I: 0.66% only
93Zr: 6.30%
while 99Tc and 129I can be subjected to transmutation (in principle; if
someone is willing to pay for this), 135Cs and 93Zr cannot, to my
knowledge.
Has anybody calculated what a system - certainly scientifically
beautiful,
on paper - would cost (e.g. in Euro / MWh) which includes advanced
reactors, reprocessing of high burn-up fuel, actinide burning,
transmutation of fission products, and including probably decades of
development until it works large scale ? Any figures available ?
Peter Bossew
> In most cases, even with the
>long lived fission fragments mentioned, somewhere in the 300 to 500
year
>range the spent fuel will probably become less radioactive than the
>fresh fuel was, even including the unused fuel and fissile
transuranics.
>It gets even better with reprocessing.
>
>So, if the assumption is that radioactive material is bad, and that we
>need to be willing to sacrifice now in order to protect less
>technologically sophisticated later generations, it is clear that the
>ethically sound choice is to use as much uranium as possible now to
make
>electricity, so that there will be less radioactive material in the
>world later.
>
>-----Original Message-----
>From: radsafe-bounces at radlab.nl [mailto:radsafe-bounces at radlab.nl] On
>Behalf Of Peter Bossew
>Sent: Wednesday, December 09, 2009 3:47 AM
>To: gstanford at aya.yale.edu
>Cc: radsafe at radlab.nl
>Subject: Re: [ RadSafe ] Global Warming
>
>George Stanford <gstanford at aya.yale.edu> writes:
>
>(...)
>>
>>
>> Then the only waste would consist of fission
>>products, which can be easily isolated in various
>>ways for 300 - 500 years, by which time their
>>radioactivity has decayed below any reasonable level of concern.
>
>(...)
>
>129I: half life 1.57e7 a
>99Tc: 2.11e5 a
>135Cs: 2.3e6 a
>etc.
>
>
>Peter Bossew
>
>_
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