Answer to Question #4106 Submitted to "Ask the Experts"

Why does the uranium in the earth's core not undergo a chain reaction? Are there moderator materials involved or does ²³⁸U not fission, even under extreme pressures?

There are many reasons for this, involving both the physics of nuclear reactions as well as the geochemistry and cosmic chemistry of uranium (U). Let me start at the chronological beginning and we'll go from there. 

All of the uranium on earth was formed during a supernova explosion about five to six billion years ago. When a star explodes, the extraordinarily high concentration of energy, atoms, and other particles leads to the formation of all of the elements in the periodic table that are heavier than iron. The explosion then throws this into space in what is called a supernova remnant—the Crab Nebula is an excellent example of a relatively young supernova remnant. There is a LOT of material formed—estimates are that the mass of nickel formed in a supernova is as much as the mass of our sun, and that is only a part of all the material blown into space.
 
Over time, the expanding shock wave from the supernova will plow into a cloud of gas and dust that's just sitting there. This shock wave will form instabilities in the gas cloud, causing it to start to collapse.  During the course of the collapse, the supernova debris will be well mixed in with the hydrogen, helium, carbon, and other elements in the cloud. As the cloud collapses, it will form a star (or multiple stars) and planets as well as comets and other bodies, and all of these will have a composition that is fairly similar to the original nebula. This is how our solar system got started.
 
So early earth was probably a big ball of fairly well-mixed rock—the core probably had a composition very similar to the crust. Uranium would have been fairly evenly distributed through the earth, too. The earth we see today is very heterogeneous—we can clearly see that the basaltic sands in Hawaii are very different from the quartz sands in Florida, and we know that earth's core, mantel, and crust are greatly different from each other. And, in particular, there is a lot more uranium in the crust than in the core.
 
Uranium and thorium (Th) and potassium (K) are large ions. This makes them "prefer" to remain in liquid rock rather than in solid crystals. So, when a magma chamber forms in the mantel, the liquid in the chamber will have more U, Th, and K than the surrounding rock. As soon as a magma chamber forms, the magma starts to crystallize, and these crystals (which remain underground) have LOWER amounts of U, Th, and K than the remaining melt. As the magma continues to crystallize, the residual liquid becomes increasingly enriched in U, Th, and K. When it finally rises and erupts at the surface (or forms granite just below the surface), the rock that is formed has most of the U, Th, and K of the original magma. Doing this repeatedly over billions of years has caused most of the U, Th, and K in the earth to become partitioned into the earth's crust. We can measure this—when we find rocks that formed in the mantel, we see that they have far less U, Th, and K than rocks formed in the crust. We can also see that, over time, the amount of U, Th, and K in the crust has increased steadily—just what we'd expect from the explanation given above. So the first answer to your question is that there is no chain reaction in earth's core because there probably isn't enough uranium there to initiate a chain reaction—and this is because of the cosmic and geochemistry of uranium.
 
The rest of the answer lies with the physics of a nuclear reactor. Today, natural uranium consists of 99.2 percent ²³⁸U and 0.7 percent ²³⁵U (the isotope that fissions most readily). Today, natural uranium is not physically capable of sustaining a nuclear chain reaction without very careful engineering and the use of heavy water or graphite as a moderator. In earth's core, we do not have either of these, so any uranium that is there will just be sitting there. Even in the distant past, when ²³⁵U constituted up to 16 percent of all uranium, it would have been difficult to sustain a chain reaction because uranium fissions primarily with slow neutrons, but there would be no way to slow down the fast neutrons emitted by fissioning uranium atoms. So, there is not enough ²³⁵U in uranium on earth to sustain a nuclear chain reaction, and even if there was, the "construction" of the earth's core won't support a chain reaction either because there's no way to slow down (moderate) the fast neutrons.
 
There was a natural nuclear reactor in Oklo (in the nation of Gabon, in west Africa) a few billion years ago. However, this formed at a time when ²³⁵U concentrations were higher than today. It formed near earth's surface, where there were several mechanisms to concentrate the uranium into a fissionable mass, and there was plenty of water to moderate the reaction. None of these holds true in earth's core. You can find more about the Oklo reactor at the Idaho State University Department of Physics Web site.

P. Andrew Karam, PhD, CHP