Rare Earth Revelry #5: Fission and neutron capture products

With interest in the Fukishima reactor disaster decaying at an exponential rate, I thought it would be an opportune time to take the long view of this phenomenon, and consider what sort of isotopic anomalies are left behind billions of years after an uncontained uranium fission reactor. And the best place to find these, of course, is Gabon.

Gabon is not known for its nuclear ambitions or industrial fiascos. It is a sparsely populated sub-saharan country known mostly for tropical rainforest and oil reserves. But crucially, it is also the host of the Oklo natural reactors. These are uranium deposits which naturally achieved criticality about 2050 million years ago. At that time, natural uranium had a much higher proportion of the fissionable ²³⁵U than modern uranium does, because the ²³⁵U had not yet radioactively decayed. So at this time, all uranium had a similar isotopic ratio to the isotopically enriched uranium used to commercial reactors (~3%). In the Oklo reactors, this allowed parts of these uranium deposits to achieve criticality, and they bubbled away as natural boiling water reactors for tens to hundreds of thousands of years.

Because there was no containment system- it was just a rock- these fossilized reactors are great for tracking the movement of radioactive materials and their daughter products. These have been extensively studied, and the study we’ll focus on today is Hidaka and Kikuchi (2010).

There are two main types of tell-tale isotopes produced in nuclear reactions.

The first are the fission decay products. When a uranium atom splits, these are the fragments left over. Must fission decay products are neutron rich and highly radioactive, and they to quickly decay by beta decay to form stable isotopes of the same nominal mass. The mass of fission decay products can be quite variable, but the heavier ones, with a mass greater than 139 amu, end up decaying to form light rare earth elements such as cerium or neodymium. The ratio of isotopes produced by fission is different to the ratio of isotopes produced by stellar nucleosynthesis, so fissionogenic Nd or Ce be distinguished from naturally occurring Nd or Ce by its isotopic ratio.

Nuclear fission of uranium does not just produce fission products. It also produces free neutrons. The neutrons are what allow the process to continue. A ²³⁵U nucleus which absorbs a neutron will often fission, releasing additional neutrons, so when the ²³⁵U is concentrated enough for one neutron from each fission to cause another fission, the reaction is considered critical, and will be self-sustaining.

Of course, ²³⁵U is not the only nucleus which can absorb a neutron. A variety of nuclear reactions can absorb neutrons. One of the most common is neutron capture, where a neucleas simply captures the neutron, increasing its atomic mass by one and releasing the excess binding energy as a gamma ray. The ability of nuclei to capture neutrons is expressed as their neutron capture cross section, and the cross sections of various nucleii vary by many orders of magnitude.
Three of the nuclei with the largest cross sections are rare earth elements, ¹⁴⁹Sm, ¹⁵⁵Gd, and ¹⁵⁷Gd. These react with neutrons to form ¹⁵⁰Sm, ¹⁵⁶Gd, and ¹⁵⁸Gd, respectively, and these reaction products have much lower cross sections, so are unlikely to absorb further neutrons. As a result, the Sm and Gd isotopic ratio of pre-existing Gd and Sm in the reactor zone will change. Table two shows that in the reactor zone, the ¹⁴⁹/¹⁴⁷ ratio can be two orders of magnitude lower than the non-radiogenic value.

What the Hidaka and Kikuchi study does is to look at the isotopic ratios of Ce, Nd, and Sm (along with other non-REE isotopes) in individual mineral grains from various Oklo rocks using the SHRIMP. This allows them to determine where the isotopically distinct REE from the reactor zone have traveled, and which mineral phases and reactions have caused them to be immobilized.

What the study shows is that the isotopcially anomalous REE dispersed quite easily through the underlying sandstone, but were effectively adsorbed by the clay overlying the deposit, so that the black shales above the clay layer show lower degrees of REE migration from the reactor core. The study also shows that this migration most likely occurred 1200 million years after the reactor went critical, in response to heating from a nearby mafic dyke intrusion.

Hidaka, H., & Kikuchi, M. (2010). SHRIMP in situ isotopic analyses of REE, Pb and U in micro-minerals bearing fission products in the Oklo and Bangombé natural reactors: A review of a natural analogue study for the migration of fission products Precambrian Research, 183 (1), 158-165 DOI: 10.1016/j.precamres.2010.07.012