[ RadSafe ] Natural Uranium - Isotope Abundance

[Dan W McCarn]

>>> "Redmond, Randy (RXQ)" <redmondrr at y12.doe.gov> 10/24/07 12:45 PM >>>

That's exactly what I'm looking for - the natural variation in relative

abundance of uranium isotopes.

 

Hi-

 

Oh! Randy, please be very careful what you ask for because it may give you
the wrong answer!

 

Are you asking, 

 

1)     "What is the natural, crustal abundance of uranium isotopes within
mineral grains > 2 my in aggregate?" or

2)     "What is the natural distribution of the abundance of uranium
isotopes exposed to varying degrees of weathering and reconcentration under
meteoric conditions?"

 

Because these two questions give significantly different results!  This is
not something that you can just "pull" from an isotope abundance value in
someone's nuclear data compilation.

 

Just a point from the perspective of the mineralogy of natural uranium -
usually coffinite or uraninite.   Since so many RSOs are only exposed to
highly purified product, the wild world works a little differently!

1) When U-238 decays to Th-234, the alpha recoil (~5MeV) dislodges the atom
from its "normal" position in the mineral lattice, and as that decays to
Pa-234 and then to U-234, it leaves the atom dislodged.  If
"weakly-oxidizing" solutions pass over the mineral grain, the U-234 is
selectively leached over the U-238 and U-235.  This allows a slightly higher
U-234 ratio than occurs in the original mineral grain.  Thus, this leads to
an apparent "over-abundance" of U-234 than occurs naturally and a slightly
lower value for U-238 and U-235 because they have not been dislodged from
the mineral lattice.  Also, it allows for the selective movement of U-234
within a deposit.  Overabundance of U-234 will occur down the hydrologic
gradient from where uranium minerals are being weakly leached by natural
processes.  Secondary concentrations onto soils or other media will reflect
this.   As a sidebar, production of uranium deposits, especially with ISL
mining, can have an issue with an increased U-234 to U-238 ratio because of
this issue.  The problem occurs during enrichment because the U-234 is
concentrated with U-235, and potentially causes the activity of the enriched
fuel to be appreciably higher. There was a Cogema paper given at an IAEA
conference some time back that addressed this problem.  The paper (G.
Campus, 2004) was presented at the IAEA Beijing Conference in 2002 published
as a IAEA-TECDOC-1396  in 2004.  (Tooting my own horn, I have two papers in
the proceedings!) The abstract is below.

 

Quoting Campus from TECDOC-1396, pp.213-220

"In 1959, Rosholt was the first to publish data and analysis of uranium ore
samples showing significant discrepancies with a reference isotopic content,
including uranium isotopes and decay progeny.  In 1964, [2] Rosholt et al.
have shown that the so-called uranium roll type deposits in Wyoming are
functioning in a way that could led to uranium "isotopic fractionation".
This fractionation was observed only for the 234U content, and the
explanation was found within the decay chain of 238U, leading to 234U
through short-lived compounds having a very different chemical behavior than
U, and also by a possible selective leaching of 234U due to its special
position in the crystal lattice or oxidation state, after 238U decays."

 

2) Full ingrowth of U-234 takes about 1-2 million years in nature and U
deposits are frequently "out of secular equilibrium" not only from young
age, but from one place to another within the deposit, especially in
roll-front sandstone deposits that dynamically move as a response to mildly
oxidizing groundwater flow.  Deposits that are "frozen" in time e.g.
Proterozoic Unconformity type deposits (Athabasca Basin in Canada) tend to
be in secular equilibrium.

 

3) In natural, meteroic conditions, uranium minerals are frequently exposed
to "weakly leaching" conditions, so I am not at all surprised that many
samples may have more (or less) U-234 in any given sample, and somewhat less
U-235 than the aggregate uranium in a host rock would suggest.

 

4) Remember that most uranium ore that was mined was exposed "in aggregate"
to strongly leaching conditions e.g. Conc. H2SO4, etc., so the "purified"
sample of natural uranium will probably reflect this aggregate isotopic
composition.  The exception to the rule is Oklo, Gabon.  

 

These mechanisms might account for an overabundance of U-234 in natural
uranium.

 

5) During enrichment, both the U-234 and U-235 are selectively aggregated.
Depleted Uranium will also be quite depleted in U-234.  Because ingrowth
requires 1-2 million years (See Bateman, 1910) for secular equilibrium:
n1λ1 = n2λ3 - n3λ3.  and so on. the source appears to be natural.  Any
sample that shows an elevated concentration of U-234 is most likely to have
a "natural" source.  My expectation of a source of "depleted" uranium would
also include a quite low U-234 activity.

 

Assumption: Sample with reconcentrated uranium in soils.

 

Measured Data:

Uranium-238  = 0.331

Uranium-235 = 0.015

Uranium-234 = 0.353

 

U-234 / U-238 activity ratio = 0.353/0.331 =1.066, or slightly enriched in
U-234.

 

Calculated Equilibrium Conditions (pCi/1 ug) (Activity of U-238 = U-234)

Uranium-238eq = 0.3337 

Uranium-235eq = 0.0154

Uranium-234eq = 0.3337

 

Ratios of Measured Data / Equilibrium Conditions (What is the measurement
accuracy?)

U-238/U-238 eq                       0.9919         Slightly Low

U-235/U-235 eq                       0.9758         Slightly Low or likely =
1

U-234/U-234 eq                       1.0578         Slightly High

 

Conclusion: NATURAL URANIUM, PROBABLY LEACHED FROM NEARBY SOURCE MINERALS AT
EQUILIBRIUM, AND RECONCENTRATED.

 

Based on the U-234 / U-238 activity > 1  (0.353/0.331 =1.066), but near
unity, I assume the source is close to equilibrium.  Also, because both
U-238 and U-235 show similar drop below equilibrium, that is they are both
slightly below 1, it appears that the sample has been leached from uranium
minerals in equilibrium conditions (probably > 2 million years old).  That
the sample shows a slight increase in U-234 is to be expected if the sample
was collected some distance down the hydrologic gradient and the source
minerals are being weakly leached.

 

Also, to emphasize, depleted uranium is significantly depleted in U-234 and
U-235, NOT ENRICHED!

 

Recommendations:  Sample both rock and subsurface and examine
petrographically for uranium minerals, or minerals that tend to concentrate
uranium.  Determine the spatial extent of the U distribution as well as
isotope ratio.  Develop a working model of leaching / reconcentration.

 

Hope that helps!

 

Dan ii

 

Dan W McCarn, Geologist

 

 

Downstream constraints on product specification and ISL mining methods

G. Capus

COGEMA, Velizy, Cedex, France

Abstract. As uranium is more and more considered as a commodity, miners are
producing natural uranium "as it comes out of the ground", forgetting
sometimes that its isotopic composition can vary. Focusing on the 234U
content of commercial concentrates coming from mining operations, it is
reported that some batches are presenting difficulties. For these batches,
ASTM C996 Standard specification requirements are not always met. It seems
that this is only the case for certain uranium batches recovered through
solution mining. Out of specification batches reaching the market now are
more likely to cause problems than in the past. The main reason is that
downstream from the front-end of the fuel cycle, the average U235 assay
tends to increase significantly with the fuel burnups. And the economic
ratio between uranium and SWUs prices is leading towards high tails assay.
As ISL is increasing its output, it is important to understand the
phenomenon at its roots and analyze the consequences of the problem. This
paper is aimed at recalling the major facts explaining the U234 content
variations and their potential consequences. To conclude, possible
mitigation measures are discussed.