Outdone only by nuclear fusion, the process of nuclear fission releases enormous amounts of energy. The ‘spicy rocks’ that are at the core of both natural and artificial fission reactors are generally composed of uranium-235 (U-235) along with other isotopes that may or may not play a role in the fission process. A very long time ago when the Earth was still very young, the ratio of fissile U-235 to fertile U-238 was sufficiently high that nuclear fission would spontaneously commence, as happened at what is now the Oklo region of Gabon.

Although natural decay of U-235 means that this is unlikely to happen again, we humans have learned to take uranium ore and start a controlled fission process in reactors, beginning in the 1940s. This can be done using natural uranium ore, or with enriched (i.e. higher U-235 levels) uranium. In a standard light-water reactor (LWR) a few percent of U-235 is used up this way, after which fission products, mostly minor actinides, begin to inhibit the fission process, and fresh fuel is inserted.

This spent fuel can then have these contaminants remove to create fresh fuel through reprocessing, but this is only one of the ways we have to extract most of the energy from uranium, thorium, and other actinides like plutonium. Although actinides like uranium and thorium are among the most abundant elements in the Earth’s crust and oceans, there are good reasons to not simply dig up fresh ore to refuel reactors with.

All About The Neutrons

A photo of yellow cake uranium, a solid form of uranium oxide produced from uranium ore. Yellow cake must be processed further before it is made into nuclear fuel. Courtesy of Energy Fuels Inc.
Forming nuclei as heavy as uranium requires something more than the standard nuclei-forming process (s-process) in the average star. An integral part of nuclear astrophysics, the s-process stands for ‘slow’ and refers to the rate of neutron capture of nuclei. Essentially it refers to the number of neutron captures that happen before nuclear decay can occur. The s-process is sufficient to create many of the elements we know from the periodic table heavier than iron (Fe) via various decay chains, with the remaining elements requiring the much higher neutron-density flux of the rapid, or r-process.

The difference between the s- and r-process is quite severe, with the s-process requiring seed nuclei from the proton, or p-process, while the r-process with many orders of magnitude higher neutron capture events can create its own own nuclei and from them the heavy elements such as the actinide series that include americium, plutonium, as well as a range of synthetic elements commonly referred to as the transuranium elements: the transuranics. Within an astrophysical context, however, neutron stars are probably the biggest source of these heavier elements.

Once this proverbial stardust has gone through planet formation, the first multicellular life can evolve into intelligent life over the course of a few million years. After this, said intelligent life can then proceed to dig up uranium ore for use in a nuclear fission reactor.

During the millions of years that humanity took to evolve to this point, however, the fissile U-235 has largely undergone decay already, while the fertile U-238 isotope, whcih can become fissile upon neutron capture, now makes up most of the uranium ore recovered today. This is why for certain types of fission reactors that use slow neutrons the uranium fuel is ‘enriched’, meaning that the amount of U-235 in it is increased from the approximate natural 0.7% to 3 – 5% for use in LWRs.

After the optional enrichment step, fuel fabrication can commence. The typically ceramic fuel pellets are then inserted into a fission reactor and the U-235 is exposed to a neutron source that then kickstarts a nuclear chain reaction.

Reprocessing And Pyroprocessing

Schematic view of PWR fuel assembly (Credit: Mitsubishi Nuclear Fuel)
The exposure of fissile isotopes to neutrons results in rapid nuclear decay, along well-known decay chains into a range of different isotopes. Some of these are helpful – like fissile Pu-239 – but minor actinides, Pu-240 as well as other isotopes that are formed inside the ceramic LWR fuel pellets will interfere with the nuclear chain reaction, reducing its efficiency. After replacing such spent fuel with fresh fuel, the spent LWR fuel can then be processed in a number of ways to use up the remaining fissile isotopes, the primary ones being reprocessing and pyroprocessing.

Effectively, the spent LWR fuel isn’t so different from the uranium ore, with the isotopes being separated from the ceramic material rather than the minerals in uranium ore. This process can be performed in a few ways:

• hydrometallurgy – dissolving into an aqueous solution, e.g. PUREX.
• electrometallurg – using electric current.
• pyrometallurgy – smelting the pellets to separate the metal from the mineral.

Of these hydrometallurgy is the oldest method, as well as the one most commonly used. France’s La Hague reprocessing plant processes about 1700 tons of spent fuel per year using the PUREX (plutonium-uranium-extraction) method which uses concentrated nitric acid to assist in separating the uranium and plutonium via solvent extraction steps along which various other isotopes (e.g. neptunium for Pu-238 production) can be separately recovered.

The remaining liquid after PUREX contains about 3% of the original used fuel material, which is generally disposed of as high-level waste with this reprocessing process. The recovered uranium and plutonium is then used together with fresh uranium to create a blend (mixed oxide, or MOX) fuel that can be used in LWRs again. There are a few variations on the basic PUREX process, but they all come with various trade-offs and the necessity for a long and tedious process.

This is where pressurized heavy water reactors (PHWRs) and fast neutron reactors (FNRs) can provide the missing link to fully close the uranium fuel cycle.

Fast Neutrons

Overview of the thermal energy transfer in the Natrium reactor design. (Source: TerraPower)
The PHWR reactor uses so-called ‘heavy water’ (deuterium), which unlike its lighter sibling does not moderate neutrons, thus allowing for a PHWR to also work with fast neutrons. Unlike the slower, thermal neutrons in water-moderated LWRs, this means that these reactors can also use much more of the fertile isotopes in the fuel. An interesting approach here is the direct use of spent LWR fuel in Canadian-designed CANDU PHWRs, called DUPIC. Normally CANDU reactors use either natural or only slightly enriched uranium fuel, using which they can achieve very high burn-up rates.

This DUPIC method does not require any reprocessing, but takes the ceramic fuel and merely puts it into fuel assemblies that work in the CANDU reactor. This and similar approaches are being trialed by South Korea, and China.

Effectively the use of PHWRs is similar to FNRs, which are a popular choice for closing the uranium fuel cycle, including the currently under construction Natrium reactor by TerraPower. By increasing the selection of available neutrons (thermal and fast), FNRs can effectively function as a breeder (turning fertile isotopes into fissile ones) while burning up all fissile isotopes. As this includes minor actinides and transuranics, this means that in an FNR theoretically every single last bit of radioactive fuel can be used, leaving no radioactive waste to handle. Through a constant process of neutron capture within the reactor, the isotopes will rush through their decay chains until finally reaching a state where their nuclear cross-spectrum no longer makes them viable nuclear fuel, or a radiological hazard.

So why is that we don’t fully burn up uranium fuel fully today, but instead usually use a once-through fuel cycle?

It’s The Economy, Silly

Schematic overview of dry cask storage of spent LWR fuel. (Credit: NRC)
One defining characteristic of nuclear fission plants is that the fuel costs end up being practically a rounding error over their operating life. This is defined both by the abundance of uranium ore and the relatively small amounts of it needed by an LWR’s roughly two-yearly refuel cycle. Although countries like France reprocess virtually all of their LWR fuel, this is more a part of their energy independence strategy, even if it has the benefit of minimizing the amount of nuclear waste. While the PUREX process results in high-level waste, this type of waste also decays very rapidly, reaching background levels within a matter of decades.

With the current resurgence in new nuclear construction, uranium commodity prices have also gone up, along with newly (re)opened uranium mines getting a lot of investor interest. Although uranium is incredibly abundant in the soil (with much more dissolved in the oceans), economics says that with more economical ways to close the uranium fuel cycle, reprocessing, reusing and burning up uranium fuel becomes the logical approach.

Before even tapping into fertile isotopes like thorium 232, the uranium we can extract today for energy production should be enough to last us many thousands of years. All because of the neutron flux in stars capturing all this energy, which is a process that continues to this day throughout the Universe.

Featured image: Solar flare on the Sun’s surface. (Credit: NASA)

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