MIT proposes New Compact Fusion Reactor Design Based on REBCO Superconducting Magnets

A group of MIT scientists have recently published a design proposal for a compact Fusion Reactor based on rare earth barium copper oxide (REBCO) superconducting magnets. REBCO magnets can produce higher magnetic fields than any previous generation of magnets. The magnetic field is used to confine the super hot plasma of fusing atomic nuclei in a so called magnetic bottle. The superior confinement brought about by the REBCO magnets would allow a smaller cheaper reactor design than the ITER tokomak fusion reactor being built in France. The MIT design is also a tokomak (Tokomak is a Russian acronym for a toroidal magnetic plasma confinement technology first developed in the Soviet Union in the 1950s) design, but the MIT scientists estimate that REBCO magnets would allow total costs to be cut by a factor of four.

A working 300kW reactor would probably still have a price tag of 10 billion dollars which is still a long way from commercial practicality.
In press coverage of the MIT announcement about this design, the story talks about the possibility that fusion reactors capable of providing ‘virtually limitless energy’ could be available within a decade. In my view these statements are hyperbolic. Conceivable a working research reactor could be operating within a decade’s time, but the availability of a low cost, high reliability design ready to roll of the assembly line within such a short time period highly unlikely in my view. Fusion energy is often viewed as being safer and cleaner than fission based nuclear energy since long term releases of radiation from a core meltdown or from long lived daughter nuclei are not possible in a fusion reactor. Nevertheless a fusion reactor will be an enormously complex piece of technology, so that the achievement of low operating cost and long term reliability would not necessarily quickly follow a research demonstration of successful power production from such an energy source.

Aside from the issue of the capital cost and the complexity of operation a fusion reactor it is worthwhile to take a moment to examine the claim of a ‘virtually limitless’ fuel supply. The fuel is deuterium and tritium. Deuterium is the nucleus of a heavy hydrogen atoms which contains a neutron in addition to a proton. Tritium is the nucleus of a super heavy hydrogen atom which contains two neutrons in addition to the proton. The deuterium/tritium combination is an enormously energy dense fuel. One deuterium nucleus and one tritium nucleus combine to form a helium nucleus and one free neutron. In the process energy is liberated. If one half of this energy is covered to electrical energy then then the fusion of 2kg of deuterium with 3kg of tritium would produce 24 million kwh of electrical energy. By contrast more than seven thousand metric tons of coal would be required to carry out the same task assuming the same 50% conversion rate to electrical energy.

Deuterium is a stable isotope of hydrogen and is present in the hydrogen of ordinary sea water at a level of 0.014%. This percentage may sound low, but heavy hydrogen is fairly easy to separate from ordinary hydrogen and the earth’s oceans contain 1 billion 338 million cubic kilometers of water. An adequate supply of deuterium will be available for quite a long time.

The nucleus of tritium, on the other hand, is radioactive with a half life of 12.5 years. Any tritium nuclei formed in the in early stages of the universe has long since decayed away. Tritium can be manufactured from lithium by radiating it with neutrons. A lithium nucleus can can capture a neutron and then decay into a helium nucleus and a tritium nucleus. The fusion of tritium and deuterium naturally produces neutrons. Most fusion reactor designs propose including lithium in the liquid cooling blanket which will surround the hot plasma core of the reactor. This cooling blanket will transfer heat from the fusing core to the working substance of the external combustion engine that will actually generate electricity. The neutron flux will convert the lithium to helium and tritium so that the reactor will generate its own tritium fuel.

The supply of tritium will then depend on the supply of lithium. Let us suppose that we wish to supply nine billion people with 24,000kwh of electrical energy per year (Current US consumption is 12,000kwh per year. However, in a fossil fuel free future, transportation fuel, fuel for space and water heating and various other important economic processes will have to be supplied by alternative forms of energy). How much lithium is required to prove this amount of electrical energy? The fusion of one deuterium nucleus with one tritium nucleus produces 17.6 Mev. Mev stands for a million electron volts. This energy unit is convenient for use in particle physics but not much used in the economics of energy production. However, if we convert MEV to kwh and assume a 50% conversion rate of the released energy to electrical energy we find that the required yearly consumption of lithium is 1370 metrics tons. According to this USGS data sheet the global production of lithium metal in 2012 was 37,000 metric tons. Therefore 1370 metric tons constitutes 3.7% of global lithium production in 2012. These numbers imply that lithium supplies would not limit energy production from fusion for a considerable period of time into the future. Whether or not fusion represents ‘virtually unlimited’ energy is more doubtful.

Frankly if reasonable cost, high reliability fusion reactors are rolling off the assembly lines by 2035 I will be very surprised. But for the moment let’s be optimistic and suppose that this scenario is correct. Would this development imply that the human race is saved from any responsibility in limiting its consumption or embracing ecological intelligence in its relations with the biosphere? All we have to do is embrace clean energy and we can race to exponentially increase the flow of dollars forever?

I think that the answer to both of these questions is ‘No’. In the long run we clearly need an alternative to fossil fuels for our energy supply, but we also need structural intelligence in our system of economic production and distribution. No technological miracle of ‘clean energy’ is going free us of this necessity.