The Society for the Preservation of Canada’s Nuclear Heritage has among its artifacts pieces of graphite from the first nuclear reactor, CP1 (Chicago Pile 1), as well as large graphite blocks from the first nuclear reactor outside of the USA, Canada’s very own ZEEP (Zero Energy Experimental Pile) reactor. We have been telling visitors to the museum about these “firsts” but really must be careful with our descriptions. We should always be saying that these were the first “human-made” nuclear reactors.
The reason we need to be more precise is that just over 50 years ago a French mining company came up with an unusual result when they assayed a uranium sample from an ore body near Oklo in Gabon. Natural uranium contains three isotopes with uranium-235 (U-235), the isotope of interest for nuclear reactors, existing at only 0.72%. To the surprise of the mining company lab, their sample was “depleted’, that is to say, they only found about 0.6% of the uranium to be U-235. Other samples in the ore body showed a percentage as low as 0.44%. How could that have happened?
Further analysis of the ore body showed the existence of elements and isotopes normally associated with fission products. This additional information, a bit of speculation and a few calculations then led to the theory, since confirmed through further experiments, that a “natural” nuclear reactor had existed in Gabon in the distant past.
Uranium-235 has a half-life that is factor of six shorter than the dominant (99.2%) isotope uranium-238 (U-238) and hence decays more quickly. If we turn back the geological clock 1.7 billion years, the fraction of U-235 in an ore sample would be significantly higher, at about 3.1%. That number happens to be in the range where one does not require heavy water for a reactor to operate but can use ordinary water as is done in many (human) non-CANDU reactors.
The conclusions of the various groups who have studied the data carefully is that when rainwater or other water sources penetrated to the ore body at Oklo a “natural” nuclear reactor reached critically and would operate for some time generating fission heat. The heat in turn would eventually boil off the water and dampen the reaction; the cycle (estimated as lasting about 3 hours) would repeat, probably again and again for thousand of years. Estimates of the power of several reactors that operated in the ore body are that up 100 kW of thermal energy was generated.
During the process above, plutonium, an element normally regarded as one of the first “human-made” elements was undoubtably generated. Another “first” that needs to claimed with care.
While the artifacts in the museum at 51 Poplar St do not contain a sample of Oklo ore, the Society does have a wide range of artifacts to show its visitors, even though we might be regarded as “johnny-come latelies” in the nuclear reactor field. Tours can be arranged by contacting any Board member or sending an email to www.nuclearheritage.com.
Many of the details above were extracted from Wikipedia.
