Fusion Disposal of Nuclear Waste
Since Hans Bethe, once referred to as the “supreme problem solver of the 20th century,” published “The Fusion Hybrid,” his seminal paper in 1979, the fission-fusion hybrid reactor has taken its place in the nuclear science community as an incredibly novel idea, but one that simply would not be practical enough to implement. Bethe explained the advantages of the hybrid reactor beautifully: The fusion reaction (for example, between deuterium and tritium) will produce excess neutrons. Each extra neutron will collide with a uranium or thorium nucleus, and will either yield two more neutrons in the resulting neutron capture, or induce a fission reaction and yield four neutrons. “Because the abundant isotopes U-238 and Th-232 are used, all of the uranium and thorium will be available for power production, not just a fraction of a percent as at present in light-water reactors [the most common type of nuclear reactor]. Much lower grade uranium and thorium ores can therefore be used,” said Bethe. But due to fusion’s slow development, the fission-fusion hybrid that Bethe proposed has not become a significant reality. Nevertheless, as the challenges we face today grow in magnitude exponentially, such as the issue of nuclear waste, the fission-fusion hybrid is getting a second look.
In the last several years, as the problem of nuclear waste continues to grow larger, scientists have tried to address this concern by leveraging transmutation—the conversion of one chemical element to another via a nuclear reaction—on a massive scale. In particular, many scientists have suggested using fission-fusion hybrids to deal with this issue through what's known as a Fission-Fusion Transmutation System (FFTS). This work builds off of the previously established knowledge of transmutation with fusion neutrons. When dealing with nuclear energy, the problem often is dealing with isotopes that are either very unstable (i.e., they emit high levels of radiation and decay almost immediately to other isotopes) or have very long half-lives, as is often the case with minor actinides that form when faster neutrons collide with plutonium (Pu) nuclei. Imagine for example, that we start with 1,000 kg of nuclear fuel, consisting of a safe (i.e., not weapons-grade) mixture of U-233 and U-238. Here, the Pu nucleus, which is formed from U-238, does not always undergo fission, and can instead undergo transmutation to a higher actinide. The end result (after all of the U-233 and Pu-239 is burned) is about 700 kg of radioisotopes, 50 kg of which have very long half-lives, which range from 30 years to upwards of 2 million years. In this case, fusion neutrons are used to transmute these radioactive fission products, such as technetium-99 (Tc) and Cesium-137. In the case of Tc-99 (an isotope with a half-life of 200,000 years), a slow neutron will be absorbed by the nucleus, thus forming Tc-100. This quickly decays into Ruthenium-100, which is stable.
Knowing this, the proposed Fission-Fusion Transmutation System takes the previously described process one step further. The key in the FFTS is the use of a high-density compact fusion neutron source (CFNS) that produces fast neutrons, which augment the rate of nuclear reactions. The CFNS will be encapsulated by a blanket of fissile material that is subcritical, meaning it cannot reach critical mass and sustain a chain reaction on its own, and therefore must rely on the CFNS for supplemental neutrons. As the reaction takes place and the fissile blanket becomes more depleted, more transuranic elements become part of this subcritical blanket. These transuranic elements will then undergo transmutation, either transmuting into fissile material or stable isotopes. This is a significant improvement over light-water reactors, in which these long-lived, biologically hazardous isotopes do not have the nuclear reactivity to undergo fission. Moreover, traditional ways of dealing with hazardous waste, such as fast-spectrum reactors, still do not deal with this biohazardous “sludge.”
The key to the success of this transmutation system is the CFNS, which has to be compact, but also capable of producing a high-density stream of fast neutrons. If a CFNS is designed to be small enough to fit inside a blanket of fissile material, but powerful enough to address the need for transmutation, then the fission-fusion hybrid will be able to significantly improve nuclear waste management.
Cover image "Fuel Assembly Removal Process (02813602)" by Greg Webb / IAEA is licensed under CC BY-SA 4.0.