Fusion reactors: Not what they’re cracked up to be
Fusion reactors have long been touted as the “perfect” energy source. Proponents claim that when useful commercial fusion reactors are developed, they would produce vast amounts of energy with little radioactive waste, forming little or no plutonium byproducts that could be used for nuclear weapons. These pro-fusion advocates also say that fusion reactors would be incapable of generating the dangerous runaway chain reactions that lead to a meltdown—all drawbacks to the current fission schemes in nuclear power plants.
And, like fission, a fusion-powered nuclear reactor would have the enormous benefit of producing energy without emitting any carbon to warm up our planet’s atmosphere.
But there is a hitch: While it is, relatively speaking, rather straightforward to split an atom to produce energy (which is what happens in fission), it is a “grand scientific challenge” to fuse two hydrogen nuclei together to create helium isotopes (as occurs in fusion). Our sun constantly does fusion reactions all the time, burning ordinary hydrogen at enormous densities and temperatures. But to replicate that process of fusion here on Earth—where we don’t have the intense pressure created by the gravity of the sun’s core—we would need a temperature of at least 100 million degrees Celsius, or about six times hotter than the sun. In experiments to date the energy input required to produce the temperatures and pressures that enable significant fusion reactions in hydrogen isotopes has far exceeded the fusion energy generated.
But through the use of promising fusion technologies such as magnetic confinement and laser-based inertial confinement, humanity is moving much closer to getting around that problem and achieving that breakthrough moment when the amount of energy coming out of a fusion reactor will sustainably exceed the amount going in, producing net energy. Collaborative, multinational physics project in this area include the International Thermonuclear Experimental Reactor (ITER) joint fusion experiment in France which broke ground for its first support structures in 2010, with the first experiments on its fusion machine, or tokamak, expected to begin in 2025.
As we move closer to our goal, however, it is time to ask: Is fusion really a “perfect” energy source? After having worked on nuclear fusion experiments for 25 years at the Princeton Plasma Physics Lab, I began to look at the fusion enterprise more dispassionately in my retirement. I concluded that a fusion reactor would be far from perfect, and in some ways close to the opposite.
Scaling down the sun. As noted above, fusion reactions in the sun burn ordinary hydrogen at enormous density and temperature sustained by an effectively infinite confinement time, and the reaction products are benign helium isotopes. Artificial (terrestrial) fusion schemes, on the other hand, are restricted to much lower particle densities and much more fleeting energy confinement, and are therefore compelled to use the heavier neutron-rich isotopes of hydrogen known as deuterium and tritium—which are 24 orders of magnitude more reactive than ordinary hydrogen. (Think of the numeral one with 24 zeroes after it.) This gargantuan advantage in fusion reactivity allows human-made fusion assemblies to be workable with a billion times lower particle density and a trillion times poorer energy confinement than the levels that the sun enjoys. The proponents of fusion reactors claim that when they are developed, fusion reactors will constitute a “perfect” energy source that will share none of the significant drawbacks of the much-maligned fission reactors.
But unlike what happens in solar fusion—which uses ordinary hydrogen—Earth-bound fusion reactors that burn neutron-rich isotopes have byproducts that are anything but harmless: Energetic neutron streams comprise 80 percent of the fusion energy output of deuterium-tritium reactions and 35 percent of deuterium-deuterium reactions.
Now, an energy source consisting of 80 percent energetic neutron streams may be the perfect neutron source, but it’s truly bizarre that it would ever be hailed as the ideal electrical energy source. In fact,
these neutron streams lead directly to four regrettable problems with nuclear energy: radiation damage to structures; radioactive waste; the need for biological shielding; and the potential for the production of weapons-grade plutonium 239—thus adding to the threat of nuclear weapons proliferation, not lessening it, as fusion proponents would have it.
In addition, if fusion reactors are indeed feasible—as assumed here—they would
share some of the other serious problems that plague fission reactors, including tritium release, daunting coolant demands, and high operating costs. There will also be additional drawbacks that are unique to fusion devices:
the use of fuel (tritium) that is not found in nature and must be replenished by the reactor itself; and unavoidable on-site power drains that drastically reduce the electric power available for sale.
All of these problems are endemic to any type of magnetic confinement fusion or inertial confinement fusion reactor that is fueled with deuterium-tritium or deuterium alone. (As the name suggests, in magnetic confinement fusion, magnetic and electrical fields are used to control the hot fusion fuel—a material that takes an unwieldy and difficult-to-handle form, known as a plasma. In inertial confinement, laser beams or ion beams are used to squeeze and heat the plasma.) The most well-known example of magnetic confinement fusion is the doughnut-shaped tokamak under construction at the ITER site; inertial confinement fusion is exemplified by the laser-induced microexplosions taking place at the US-based National Ignition Facility.
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Originally Posted by Zippyjuan
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