Do We Really Need Nuclear Fusion for Power?
February 16, 2014
Ingold and Thorner: Do We Really Need Nuclear Fusion for Power?
I remember my father-in-law saying there is enough uranium above ground, much of it stored in Oak Ridge, TN, where he worked, to power 1000 reactors of 1000 MW each. To put that in perspective, each of those reactors would have twice the output of all the windmills in the US. So-called “fast” reactors refer to the harnessing of high energy (fast) neutrons to “burn” naturally occurring uranium 238. Unfortunately the Fast Breeder Reactor Project at Oak Ridge National Laboratories (ORNL) was halted by the renowned “nucular [sic] engineer,” President Jimmy Carter, and the scientists involved were re-tasked to harnessing the limitless power of coal. The other “nuclear” problem, spent fuel disposal, can be credited to another famous Navy veteran, President Richard Nixon, who halted development of fuel reprocessing. We aren’t burying nuclear ashes. To the contrary, only about 5% of nuclear fuel is consumed before fission products accumulate, absorbing neutrons, until the fission reaction cannot be sustained.
Fusion reactors don’t burn “limitless” fuel, vis-à-vis hydrogen. They burn relatively rare isotopes of hydrogen – deuterium and tritium. Deuterium constitutes only 0016% of naturally occurring hydrogen, as found in water. The separation process consumes huge amounts of electricity and a vast supply of water. A 200 MW power station, dedicated to producing deuterium, would yield about twelve liters of “heavy water” (D2O) a year. Tritium does not occur naturally (12 year half-life), but is made in fission reactors. As the good professor points out in the attached article, you can make tritium on the fly by irradiating lithium with fast neutrons. Incidentally, that’s how it works in a hydrogen bomb, packed with (among other things) solid lithium deuteride. One downside is that 99% of the world’s lithium is found in the mountains of Peru and China, and most of what we import goes into batteries.
There also some questions about the “limitless” energy available from fusion reactions. The project hailed in the Forbes article uses a D+T reaction, which yields helium and a fast neutron. About 80% of the energy of this reaction is imparted to the neutron. The tritium (T) comes from neutron bombardment of lithium, which is endothermic (consumes energy). The net result is 99% of the energy is in the form of fast neutrons. Since neutrons don’t interact well with materials, only about 30% of this energy can be converted into heat for turbines, and replacing the heat needed to sustain the fusion reaction. The by-products of the fusion reactions are not radioactive (other than tritium, which is difficult to contain), but the neutrons render everything they contact radioactive. In short, instead of burying spent fuel, you bury the reactor, once the materials of its construction are transmuted until they are not structurally sound.
It’s also puzzling why it’s claimed that this experiment produced more energy than it consumed. The brief (7 billionths of a second) reaction released about 9400 joules of energy due to the fusion reaction, above that due to the high temperature (heat content) of the reactants. To achieve this, approximately 1.8 trillion joules of energy was imparted by a bank of X-Ray lasers, which occupy a 10 story building with a footprint of over an acre. An inveterate gambler is happy to go home with $500 of winnings, after laying down $5000 on the ponies during the season, sort of like Congressional Economics 101.
There’s nothing wrong with the science, and it’s important to continue. For the foreseeable future, we should recognize that the most important gains are in the form of knowledge and technology, rather than a viable source of electricity. How few men stepped on the moon, but who doesn’t benefit from the technology which came out of the Apollo project. Who hasn’t worn or used something with Teflon, used a computer, watched a program broadcast by satellites, or handled a cell phone? Some day, there will be a Scottie who knows just what to do with a dilithium crystal or two.
Saturday, February 15, 2014 at 10:00 AM | Permalink