Vandergraaf, Chuck (vandergraaft@aecl.ca)
Tue, 24 Feb 1998 15:44:31 -0500

I've managed to find some time to write a brief description of the Oklo

Hope this will be of interest and that the superscripts can be
transmitted as typed. I look forward to your comments, especially from


T.T. (Chuck) Vandergraaf
Pinawa, MB

The Oklo Phenomenon

The story of the Oklo phenomenon goes back to June of 1972 when a group
of analysts at the French CEA in Pierrelatte found an anomaly in the
235/238 U ratio in some uranium feed stock. The natural abundance of
235U is 0.7202 +/- 0.001 atom percent. These analysts obtained a value
of 0.7171, well outside the expected range. To their credit, they
checked their results, checked the calibration of their mass
spectrometer and came to the conclusion that their value of 0.7171 was

A value lower than 0.7202 usually means that the sample has been
contaminated with depleted uranium. Depleted uranium is the stuff that
is left over after the enrichment process and, because of its high
specific gravity, is used as ballast in some (expensive) sailboats and,
because of its propensity to react with oxygen, in armor-piercing
bullets, I believe. It has an 235U atom percent of much less than

The reason why the 235/238 U ratio has to be increased is that this
ration is too low to sustain a chain fission reaction in light-water
nuclear power reactors (except for nuclear reactors that use heavy
water, D2O, as moderator such as the Canadian CANDU reactor, but that's
another story). So, to make a PWR or BLW work, UO2 with a 235/238 U
ratio of about 3 atom percent is needed.

Anyway, after much checking, the people at Pierrelatte ruled out any
contamination, and found that the ore from which the uranium samples had
been refined, came from Oklo, in Gabon, Africa. Further investigations
into this uranium deposit discovered uranium ore with a 235/238 ratio as
low as 0.440 atom percent. In addition, neodynium and other elements
were found with isotopic compositions different from natural. For
example, natural Nd contains 27 atom percent 142Nd; the Nd at Oklo
contained less than 6 atom percent but contained more 143Nd than natural
Nd. Assuming that the Nd in the Oklo ore was a mixture of natural and
"other" Nd, and subtracting the natural Nd from the Oklo-Nd, the
isotopic composition of the "other" Nd matched that produced by the
fissioning of 235U.

Similar investigations into the isotopic composition of ruthenium at
Oklo found a much higher 99Ru atom percent than in natural Ru (27-30
atom percent vs. 12.7 for natural Ru). This anomaly could be explained
by the decay of 99Tc to 99Ru. Other investigations led to the same
conclusion: sometime in the past, the Oklo deposit had been operating as
a natural fission reactor, complete with the generation of fission

Now remember that no chain reaction can be sustained in uranium with a
235U atom percent of 0.7202, no matter how concentrated the uranium is.
In nuclear reactors, U is present as near-stoichiometric and very pure
UO2, with low concentration of impurities. And yet, at Oklo, a nuclear
reaction apparently took place in a non-engineered setting, complete
with dirt, sand, water and who-knows-what else. The only plausible way
to explain how this chain reaction could have taken place, is to look at
the half lives of 235U and 238U. These are 7.13 x 10^8 and 4.51 x 10^9
years, respectively.. Because the half life of 235U is shorter than
that of 238U, the 235/238 U atom ratio is decreasing with time, but must
have been higher in the past. Sometime in the past, the 235/238 U ratio
must therefore have been high enough to allow a fission process to be
sustained. Water surrounding the deposit is thought to have been the
moderator to slow the fast neutrons down to a low enough energy to
fission other 235U atoms and the deposit apparently only had low
concentrations of neutron poisons or neutron absorbers. The deposit was
likely formed 1.7 billion years ago when the 235/238 U ratio was high
enough to sustain a chain reaction. The reactor(s) operated off and on
for a period of 600 000 to 1.5 million years. Presumably, when the
temperature became too high, water was driven away from the deposit and
the reaction stopped due to loss of moderator. When the deposit cooled
down, water reentered the deposit and the reaction started again.

Note the reference to 99Tc and its decay into stable 99Ru. Tc-99 has a
half life of 2.13 x10^5 years. The observation that all 99Tc has
decayed into stable 99Ru confirms that the formation of 99Tc ceased at
least 10^6 years ago (five half lives as a rough guess).

Interestingly, a very high grade uranium deposit has been found in
northern Saskatchewan, the Cigar Lake deposit. It is extremely high
grade ore, with up to 55% U in places. Yet, no evidence has been found
that this deposit ever operated as a nuclear reactor, most likely
because it was formed too recently (1.3 billion years ago) when the
235/238 U ratio was already to low to sustain a chain reaction. Some
239Pu, 99Tc and 129I have been detected but they appear to be in secular
equilibrium, i.e. they are decaying as fast as they are being produced
by a natural fission process.

This is just a thumb nail sketch of the Oklo phenomenon. I could go
into much more detail such as the corrections that had to be made to
distinguish the fission products of 235U and 239Pu, but these
calculations are not germane to the general conclusion that all
observations point to a very old deposit and that the nuclear reactor
operated long enough ago for the fission products to have decayed to
stable daughters.

My question then is, how do YEC scientists reconcile this with a young
earth? It seems to me a open-and-shut case, but I am willing to keep an
open mind on this.


P.K. Kuroda. 1982. The origin of the elements and the Oklo phenomenon.
Springer-Verlag, Heidelberg, New York

J.J. Cramer. 1995. The Cigar Lake uranium deposit: analog information
for Canada's nuclear fuel waste management concept. AECL Report 11204.