4.6.5: Generation IV Reactors and Thorium Fuel

Generation IV reactors are not a reality – not yet – they exist mostly as concepts, some of them in advanced phase of planning, and only in a few cases there exist functioning prototypes. In view of that, it does not make sense to discuss details of those new concepts in the present e-text. Rather, it seems to be a better idea to provide links to Web sites where much information about those innovative ideas can be found.

There exists an international organization, the Generation IV Interna-tional Forum (GIF). In its Web page, it declares: GIF is a co-operative international endeavour which was set up to carry out the research and de- velopment needed to establish the feasibility and performance capabilities of the next generation nuclear energy systems.

GIF has selected six reactor technologies for further research and development. They are listed in this Web page. By clicking on each item one can get much more information about the given concept.

It should be noted that out of those six goals, four are fast-neutron reactors. We haven’t talked yet about fast-neutron reactor technologies in this Chapter. Until now, we discussed only the principles of operation of reactors using natural or low-enrichment (3-5%) uranium as a fuel. Such reactors do need moderators. However, a chain reaction may also be supported by fast neutrons: this is the case in nuclear weapon. Atomic bombs are made of highly enriched uranium (80% or more of U-235) or Plutonium-239¹ which is 100% a fissile material. Well, but atomic bombs are expected to explode in a most violent way. This is achieved by making the mass of U-235 or Pu-239 critical (e.g., by combining two or more blocks of sub-critical mass into a single block). In a fast-neutron reactor the mass of nuclear fuel must reach the critical level, too, to start the chain reaction – but the goal is to keep the rate at a steady controlled level.

Out of the four fast-neutron reactor types on the GIF goal list, only one type has underwent extensive testing in several large-scale prototypes – namely, the liquid sodium cooled type. Two large-scale French devices, Phoenix and Super Phoenix were tested for about 25 years before the year 2009, demonstrating that there were capable of generating power at the level of hundreds of Mega Watts for prolonged periods. But the unquestionable leader in research on such reactors is Russia, where the testing program was crowned by commissioning the first-in-the-world regular power plant BN-800 which has started supplying the grid with 880 MW in November, 2016 – with an even larger reactor, BN-1200, under construction and expected to be commissioned in the near future.

The fast-neutron reactors are especially interesting due to their “fuel breeding capability”. Namely, when operating, they release more fission neu- trons than are needed to maintain the chain reaction. The “extra” neutrons may be absorbed by U-238 nuclei – either those in the fuel, or those in a spe- cial “blanket” surrounding the reactor core – to create Plutonium-239, which can be later used as the fuel for the same reactor. It turns out that amount of Pu-239 produced may be even higher than the amount of fissile material spent for maintaining the chain reaction. In other words, the reactor may “self-supply” itself with nuclear fuel. In practice, it means that the fertile⁸ U-238 can be converted to valuable fuel, so that not only 0.7% of the nuclei present in natural uranium, but 100% of them can be used for generating power in future nuclear power plants.

Thorium Fuel

One more fascinating opportunity is that fast-neutron reactors can use not only U-238, but also Thorium-232 for “breeding”. Thorium-232 is a non- fissionable element, but irradiated with neutrons, it changes to Th-233, which decays with a half-life of 22.3 minutes to protactinium Pa-233, which in turn decays with a half-life of 27 days to Uranium-233:

\[
{ }_{90}^{232} \mathrm{Th}+{ }_{0}^{1} \mathrm{n} \longrightarrow{ }_{90}^{233} \mathrm{Th} \underset{22.3 \mathrm{~min}}{\stackrel{\beta^{-}}{\longrightarrow}}{ }_{91}^{233} \mathrm{~Pa} \underset{27 \text { days }}{\stackrel{\beta^{-}}{\longrightarrow}}{ }_{92}^{233} \mathrm{U}
\]

Uranium-233, like U-235, is a fissile isotope. There is one problem with such "breeding" reaction because the Protactinium-233 has a high cross section for neutron absorption. If it captures a neutron before it decays to U-233, it leads to the production of a lighter Protactinium-232 isotope in an exotic reaction:

\[ { }_{91}^{233} \mathrm{~Pa}+{ }_{0}^{1} \mathrm{n} \longrightarrow{ }_{91}^{232} \mathrm{~Pa}+2{ }_{0}^{1} \mathrm{n} \]

which decays to U-232, which is an unwanted contamination. Extra means are needed to deal with it; nonetheless, using Thorium as a nuclear fuel is definitely feasible. Because thorium is three to four times more abundant than uranium, thorium fuel may become an important player in the future system of satisfying the growing appetite for power on the planet of Earth.

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1Plutonium ²³⁹Pu is a “man-made” element which is created during a normal operation of an uranium reactor by absorption of neutrons by U-238 nuclei. The resulting ²³⁹U undergoes two β decays and becomes ²³⁹Pu, the half-lifetime of which is over 20 000 years. Plutonium is chemically separated from spent fuel rods. U-238 is not fissile, but if it captures a neutron, then a decay chain starts, in which after a short time a fissile nucleus emerges – namely, Plutonium-239. Therefore, U-238 is called “fertile” – as is another non-fissile material, Thorium-232, which can also be transmuted to a fissile Uranium-233 through irradiation with neutrons.