The hybrid thorium reactor (HTR) uses a fusion reaction to generate neutrons that transmute Thorium-232 into Uranium- 233, and which then cause the U233 to fission, producing energy.
Fusion Stage of the HTR Reaction – The fusion reaction is created by bombarding deuterium, an isotope of hydrogen, with microwaves. This generates and heats a plasma that can be contained in a magnetic field and raised to an extremely high temperature. The plasma is so hot that the deuterium nuclei smash into each other at such high speeds that they fuse together. This fusion reaction produces neutrons. The fusion reaction chamber is surrounded by a shell or “blanket” of thorium, so that the neutrons ejected in every direction by the fusion reaction end up penetrating the thorium blanket.
Fission Stage of the HTR Reaction – Bombardment of the thorium blanket with neutrons causes a transmutation reaction in which the thorium is converted into the isotope Uranium-233. The U233 also is bombarded by the neutrons, which cause it to fission into lighter elements, releasing a tremendous amount of energy. The U233 fission itself produces some additional neutrons, so the fission reaction is partially self-sustaining, making the entire process more efficient. However, the fission reaction alone does not produce sufficient neutrons to sustain the reaction, so if the fusion reaction ceases, cutting off the external neutron source, the U233 fission will very quickly come to a halt. This is fundamentally different than how reactions are sustained in conventional nuclear reactors, where fission of enriched Uranium-235 or Plutonium results in a self-sustaining chain reaction. The chain reaction in conventional fission reactors produces so many neutrons that they will grow at an ever-increasing rate unless mitigated by control rods or other active processes. If the control system fails, as it did recently at Fukushima, a runaway reaction can take place, resulting in a meltdown.
Key Technical Challenges – HTR entails two major technical challenges: (1) Development of a containment mechanism capable of sustaining the high temperatures created by the fusion plasma, and (2) Development of a means of converting the large amounts of heat generated by the fission reaction into electricity.
To address the first of these challenges, the University of Michigan (UM) has developed a Gasdynamic Mirror (GDM) technology, which has been demonstrated in small-scale prototype systems in both the U.S. and Russia, such as the one shown here. Use of fusion alone for cost-effective power generation is not practical because fusion must achieve ten times energy breakeven to be economical on its own. By contrast, when used solely to produce sufficient neutron flux to sustain the thorium cycle of reactions, the fusion reaction needs to achieve only one-tenth or less of energy breakeven. This is within present-day capabilities. The prototype GDM pictured here was used to successfully generate a plasma at NASA’s Marshall Space Flight Center in the late 1990s, and Russian physicists have successfully operated a GDM as a neutron source. Although further research is needed, confidence is high that the GDM will meet the need.
The second key challenge relates to effective utilization of the tremendous amounts of energy produced by the fission stage of the HTR reaction. The feasibility of producing energy via the thorium cycle has been established for decades, and working thorium reactors have been built. The research needed to optimize design of the thorium shell is straightforward. The key remaining challenge is to address the energy conversion problem. TransPower and UM are working together, along with other major industry partners, to investigate this problem and to develop highly effective means of heat transfer. This is a key challenge because the net energy production of a HTR plant will be limited by the rate at which heat can be removed from the reactor and converted to electricity. UM calculations suggest that a HTR reactor can theoretically produce a hundred times as much energy as a conventional reactor, but managing this level of energy output may be beyond the capabilities of today’s heat transfer technologies. Nonetheless, achievement of just five or ten times the net energy output of conventional nuclear power plants would be a game-changing development – and energy production at these levels may be well within our grasp.
Success in meeting these engineering challenges will make HTR generating plants commercially attractive and profitable for a very broad range of applications, from small distributed power plants to extremely large plants that can match or exceed the output of the largest power generating facilities operating in the world today.
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