So this week I was sitting in my reactor physics class, and my professor, Dr. King, began talking about the natural percentage of uranium-235 in uranium ore. Just like in metals such as copper, isotope percentages are constant throughout the world in a pure substance. This hold true for uranium in every part of the world except for in one place: at Oklo in Gabon, Africa. In 1972, a French processing plant received a shipment of ore from this site, and upon doing tests to check in the level of U-235 in the ore, discovered it was lower than expected. This was very alarming as U-235 is a highly controlled material due to its need in weapons. The lower levels of U-235 meant that someone was extracting some of the material before sending the ore to be processed. Either that, or they were sending already used fuel to them.
As it turns out, such was not the case. To get a good summary of the events, I suggest checking out the Wikipedia page, as it provides the best quick summary of what happened at Oklo. When further looking into the site, something never expected was discovered. Tests on the ore at the site revealed fission fragments in the fresh ore. But it could not have already been used in a modern power plant because it just came out of the ground! So what does this mean?
Well, it means that this was the first nuclear reactor on Earth, running roughly 1.5 billion years ago! It seems that following the decay chain of uranium, the percentage of U-235 in the soil at Okla would have peaked at about 3% around this time. This is the lower limit of what modern commercial reactors are enriched to for power generation. This high level of U-235 covered such a large area, and the fact that the rock in the area was porous due to volcanic activity meaning that water was running able to run through the rock at Oklo created what is the basis of how our light water reactors are designed today. The large area allowed for the neutrons to be kept within reach of fissile material long enough to likely fission. The presence of the water acted as a moderator, which means that the water slowed down the neutrons to appropriate speeds so that they were likely to fission. I can talk more about the physics of fission later, but the fact that this naturally occurred is just mind blowing!
The conditions in the soil outside of Oklo enabled the uranium ore to go critical, meaning that it caused a stable nuclear reaction as long as the water was present. Could you imagine walking along and wondering why the ground is hot? It is not geothermal energy. It is not fire in the ground. It is a release of nuclear energy done by the hand of nature! You can still today see the evidence of the reaction.
Once upon a time, this natural nuclear reactor operated at about 100 kW, which in today's terms of commercial reactors is very small. These reactors started up in several different locations in the area, but none of them are active today. Though these are not viable sources for power production, they are interesting for other reasons. Largely, this allows us to study the storage and disposal of nuclear waste over billions of years. The studies that we do in labs give us a lot of information, but they do not allow us to place in the time factor. I don't want to go into discussing nuclear waste here as this is the subject of my upcoming posts! I love it when things just lead in so nicely to where you are going.
As to my original point, does anyone else see this as a bit of a miracle? I mean, nature didn't only supply us with the material to build a nuclear reactor, it also took it upon itself to show us that it can be done. It is not a power source that is foreign to nature, but one that occurs on Earth naturally. It is almost as though God is trying to give us a hint!
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