|Fig. 1: Fusion Reaction. (Courtesy of European Joint Undertaking for ITER and the Development of Fusion Energy - Fusion for Energy)|
The world's current fleet of 345 nuclear reactors all use fission technology, requiring continual production of enriched uranium and leaving countries around the world with waste management and proliferation challenges. Although developing new commercial scale nuclear technologies could cost billions, venture capitalists such as Bill Gates and Jeff Bezos, the founders of Microsoft and Amazon.com, have invested in alternative nuclear power plant technologies that have the potential to address some of these problems. Bill Gates has backed TerraPower, a Seattle-based company pursuing a traveling wave reactor technology. Meanwhile, Canada-based General Fusion, supported by an investment from Jeff Bezos, is developing a magnetic-target fusion technology. If successfully developed, the two technologies described below have the potential to offer the world safer, more environmental nuclear power options. The ideas behind these two technologies have been around for decades. With the rapid increases in computing power, it may be possible to use sophisticated modeling and controls to make these technologies a reality.
The initial idea of a travelling wave reactor (TWR) dates back to 1958 when Saveli Feinberg proposed a "breed-burn" reactor, where fissile material is produced inside the reactor core.  The basic concept behind the TWR is that the reactor is seeded with a small amount of U-235 and the rest of the fuel for the fission reaction can be U-238 sourced from existing depleted uranium waste. After ignition of the initial U-235, the neutrons that escape can transform the U-238 into Pu-239, breeding fissile material inside the reactor. Over its lifetime, by moving the fuel rods around in a process called "shuffling," the reaction spreads through the fuel in a "travelling wave." The reactor is cooled using liquid sodium, which then fuels a steam turbine to produce electric power, in a similar way to existing fast reactors. The major innovation for the TWR is in the core.  Since the reactor breeds its own fuel and the fuel shuffling occurs mechanically using robotic elements within the core, the reactor vessel can theoretically be sealed and continue to operate for decades. The challenge is to find a cladding material (used to contain the fuel) that can withstand radiation for prolonged time periods. 
Since the core is sealed throughout the lifetime, the TWR faces lower proliferation risk than other types of reprocessing of spent fuel. In addition, the TWR uses uranium more efficiently over the fuel cycle. TerraPower estimates that the TWR will produce much less enriched uranium over the lifetime of the reactor compared to a light water reactor (LWR), leading to less waste production and lower costs over the lifetime. Since the sodium coolant operates at a higher temperature than a light water cooled reactor, the greater thermodynamic efficiency can also lead to efficiency gains in power production. For a 1GW power plant, the company projects that these savings could count in the hundreds of millions of dollars. 
Fusion is based on the following simple reaction, deuterium and tritium, two isotopes of hydrogen combine to form a charged helium nucleus, a free neutron and energy. (Fig. 1) The major challenge in fusion is to create this reaction in a controlled way so the energy can be harvested. General Fusion's magnetic-target fusion reactor approach combines the two primary fusion technology pathways for fusion energy, inertial confinement magnetic fusion.
In inertial confinement fusion, high-energy laser beams are focused on a deuterium-tritium fuel target, either directly or indirectly heating a shell, which releases x-rays inward on the deuterium-tritium fuel target. As the fuel target compresses to very high temperatures and density, the ignition starts in a central hotspot. The fusion burn propagates outward from the hotspot through the fuel target, moving more quickly than the fuel capsule can expand given its mass inertia. The neutrons that escape the reaction heat the surrounding lithium coolant. Through heat exchange the heated lithium could power a steam turbine and at the same time, the neutrons and lithium would react to form new tritium for the targets. 
In magnetic fusion energy, deuterium-tritium plasma is heated to extreme temperatures of around 100 million degrees Celsius, the point at which fusion occurs. The plasma is contained inside a torus (donut) shaped vessel, using magnetic fields. Once the fusion reaction begins, the charge in the helium nuclei will cause them remain in the magnetic field. The neutrons escape from the magnetic field at high velocity since they have no charge, and are absorbed by the surrounding confinement chamber.  The heat created in the chamber walls could theoretically be used to produce energy via a steam turbine.
Combining these two concepts, General Fusion's approach builds on a concept first developed by the U.S. Naval Research Laboratory in the 1970s.  General Fusion's idea first uses electrical capacitors to create a heated and magnetized "puff" of deuterium-tritium plasma shaped like a torus, in an approach similar to magnetic fusion energy. Two puffs are released simultaneously from either end of a spherical chamber, combining in the center to form one plasma bubble. The chamber is filled with liquid lead-lithium alloy, which is injected into the chamber via pumps so that it creates a vertical vortex through which the plasma can pass. Then, steam-powered pistons impact the chamber from all sides, creating an acoustic shock wave that compresses the already heated plasma to the point of nuclear fusion. The neutrons released from the reaction heat the lead-lithium mixture, which is then pumped out to a heat exchanger to power a steam turbine. Like in inertial fusion, the neutrons combine with the lithium to breed more tritium. 
Magnetic target fusion offers the potential for a simpler, more cost-effective pathway to fusion energy since the energy. Since the fusion reaction occurs much more quickly than in traditional magnetic fusion energy, it reduces the complexity of controlling the heated plasma. Additionally, it requires less energy to create compression in the already heated plasma compared to inertial confinement fusion. Fusion has low environmental impacts and proliferation risks. Deuterium and lithium are both readily available elements. Aside from the initial tritium required, additional tritium can theoretically be produced within the reactor.  And unlike fission, the only waste product produced is helium. Furthermore, with fusion there is no risk of a runaway chain reaction or meltdown.
For now, these two these two technologies remain to be proven. Time will tell whether they can deliver on the promise of cheap, clean, safe and reliable energy.
© Becca Levin. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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