|Fig. 1:Schematic diagram of a tokamak fusion reactor.  (Courtesy of the ARIES Program)|
Like it or not, our civilization is addicted to energy from fossil fuel, which cannot last forever. Simple calculations show that at current rate of depletion, world oil reserve will run out in less than 100 years  while the earth's coal is likely to finish in roughly 300 years.  With this in mind, the energy sector races to discover and invest in alternative energy resources, like nuclear energy. Although nuclear fission using uranium fulfills a considerable and increasing amount of the world's current energy demand, it will not save us when fossil fuel is gone: mined at the current rate, the world's uranium supply will also end in less than 300 years. 
Many nuclear scientists hence cast their sight on fusion, which combines lighter nuclei like hydrogen isotopes, deuterium (D) and tritium (T), to form heavier nuclei like helium and release energy. Since helium is non-radioactive while hydrogen isotopes are almost limitless, nuclear fusion energy can be green and renewable. Deuterium exists naturally in abundance in water and tritium can be bred from equally abundant lithium metal. In facilities such as the Joint European Torus (JET), Laboratory for Laser Energetics (LLE) and Air Force Research Laboratory (AFRL), there are three types of processes under study: magnetic confinement, inertial confinement, and magnetized target fusion. Among these, magnetic confinement fusion is the most researched and best understood; therefore, we will devote our attention to this process and its energy applications due to space limitations.
|Table 1: Nuclear reactions of greatest relevance to magnetic fusion, and their total equivalent energy .|
One challenge for nuclear fusion is that the lighter nuclei do not spontaneously fuse. The density of fuel ions must be sufficiently large, as the number of fusion reactions per volume is roughly proportional to the square of this density, which JET maintains at 1 - 2 × 1020 particles/m3. In addition, fusion reactions occurring at a sufficient rate need high temperatures for the charged ions to form plasma and overcome their natural repulsive tendencies.  Therefore, the first reaction in Table 1 is the most attractive, since the reaction becomes self-sustaining at a minimum temperature of 58 million Celsius (104 million Fahrenheit) and its optimal reaction temperature is just three times this minimum. The other reactions need roughly 4 to 6 times of that minimum temperature to occur at sufficient rate.
On top of density and temperature, nuclear fusion also needs to have its energy confined for 4 to 6 seconds to take place. As its name suggests, magnetic fusion uses magnetic fields to separate the hot plasma from its relatively cold vessel.  In a typical tokamak reactor (Fig. 1), like JET, a moderator of the neutrons (blanket) and a shield that absorbs the neutrons and as well as gamma rays protect the magnetic coils from nuclear radiation. In the case of deuterium-tritium (D-T) plasma, the blanket contains lithium to breed tritium as the reaction's fuel. Meanwhile, the outer magnetic flux diverts onto targets, which absorb the heat, thus isolating most of the chamber wall from direct contact with the plasma. The first wall and the blanket form a vacuum chamber. A coolant then removes the heat from the outer elements, and transports it to heat exchangers and generators to produce electricity. 
|Fig. 2: Internal view of stripped-out JET vacuum vessel as of 1994. (Courtesy of EFDA JET)|
Despite the prospect and advancement in pure fusion generators, poor power output is a major drawback. The ratio between energy released by fusion and the kinetic energy injected into the plasma, Q, is a good measure of an ideal fusion generator. In practice, we also need to consider the energy loss while converting electricity into plasma kinetic energy, and converting heat into electricity. Therefore, a fusion generator's practical efficiency becomes roughly 
This means Q needs to be at least 3 for a pure fusion generator to break even, but obtaining a high Q is extremely difficult. One of the most efficient fusion reactors, JET, can only achieve Q of 0.65 by 1997, yet a typical fission reactor can easily have Q in the hundreds.Other fusion experiments underway are IFMIF and ITER (under construction). Once the engineering systems have been tested, there are also plans to build a demonstration fusion power plant called DEMO.
To tackle the problem of obtaining high Q, scientists have proposed to combine fusion and fission into hybrid generators. As briefly discussed before, one of the major concerns with fission-power is uranium depletion. In addition, only rare isotopes of uranium like 233U and 235U can serve as fissile fuel, and this exacerbates the situation. However, when the much more abundant non-fissile isotopes capture neutrons, they can generate the scare fissile isotopes. Recall from Table 1 that the popular deuterium-tritium fusion reactions generate neutrons as by-products. We can then combine the two reactions, so the fusion can produce neutrons and help the fission to be more fuel-efficient. 
Nuclear waste disposal is another application of the fusion-fission hybrids. By bombarding fission generators' waste material with high-energy neutrons created through fusion, we can reduce or even eliminate the waste's radioactivity, making it easier to manage.
Safety is also an often-quoted advantage of hybrid reactors. To maintain the pure fission reaction, traditional nuclear generators need to have enough neutrons from within the fissile material. Operating at this critical level can make the reaction difficult to constraint. In the case of fusion-hybrid, the required neutrons come externally from an easily controlled reaction. Criticality accidents are thus physically impossible in hybrid reactors. Therefore, they are worthy of extensive studying to come to commercial stage. Supply of the heat load, neutron wall loading, refuelling, and impurity removal from the plasma are some of the major issues. Meanwhile, materials, remote handling, reliability and maintainability of the hybrid reactors also need research. 
Over all, nuclear fusion provides a clean and renewable energy source, yet its high investment and low power out-put is a major hurdle to overcome. Experts estimate that it will take roughly 35 to 50 years for pure fusion to be commercially viable.  At the meantime, fusion-fission hybrid generators can serve as the transition. Because of the economy and readily available fissile fuel, hybrid technology may mainly serve to handle radioactive waste from traditional nuclear power plants in short-terms. In the long run, it can step up as electricity generator and pave the way for fusion as the truly green while renewable nuclear energy.
© Rebecca Nie. 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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