A Picture Worth a Thousand Words - by Rolf A. F. Witzsche

 

The Great Nuclear Fusion Delusion


(c) Corel Corp.

 

A jug of ocean water contains enough energy to supply a the energy needs of a family for a year.

Oh, really?

The reality is different

Nuclear fusion fuel is made up of two parts: 40% deuterium and 60% tritium. (both are 'heavy' isotopes of hydrogen)

Deuterium is plentiful in the oceans, but is highly diluted. It takes the processing of 600,000 tons of water to extract enough deuterium for one ton of fusion fuel that will power a gigawatt rector for a year. This part is 'easy.' It is typically accomplished with the separation of 'heavy' water and the distillation of the 'heavy' isotope from it.  The process only takes a lot of energy, for which the 'excess' heat from nuclear reactors is typically used.

Tritium is rare. Tritium is mildly radioactive with a half life of 12 years. For this reason, virtually no natural resources exist. However, it can be manufactured by irradiating lithium-6 in nuclear fission reactors. 

It is hoped that with this process of fissioning lithium, tritium can also be produced in nuclear fusion reactors. The lithium-fission process would then use the neutrons released in the fusion-power process to breed more tritium, in order to keep the reactor supplied with its own fuel. However, here the numbers don't add up. It takes 1 tritium atom to produce 1 neutron in fusion, and it takes 1 neutron irradiating lithium-6 to produce 1 tritium atom for new fuel. (lithium-6 is also quite rare, a mere  7.5% of natural lithium is lithium-6). However, if not a single neutron was lost, the process of breeding tritium would be self-sustaining. In practice the 100% rate of efficiency cannot be achieved, or anything close to it. To overcome this problems, it is proposed that lithium-7 (which has an extra neutron and is the most abundant form of  lithium) be used from breeding of tritium in fusion reactors. This, however, requires high-energy neutrons for activation. Unfortunately, most of the high-energy neutrons in fusion reactors will have been moderated in the heat energy production that occurs, before the neutrons reach the coolant of the reactor for which liquid lithium could be used. The few neutrons that would get through the rector lining to the lithium-7 coolant, would react with it and produce a tritium atom. This fission process also gives off a low-speed neutron, which then could be used to activate lithium-6, (the 7.5% portion of natural lithium). Liquid lithium can be used as a coolant. It melts at 180 decrees Celsius with a boiling point above 1,300 degrees, and a specific heat carrying capacity of slightly less than water.

Some tritium might be produced in this manner, but likely not enough for a full break-even fuel cycle. The inefficiency in the tritium breeding cycle will no doubt be the make or brake factor in nuclear-fusion process as a practical energy-producing system. The huge international ITER project, the largest technology experiment in history, is designed to explore, among other things, this critical fuel-cycle efficiency factor. This efficiency question is one of the key questions that ITER is designed to answer over its 30-year project-cycle, if the project is actually carried out. The theoretical outlook is far from encouraging.

The other questionable factor in the fuels cycle is the lithium supply itself. About 10 to 20 millions tons are available on the planet, bound up in mineral deposits. The use of this resource must compete with other commercial requirements.  Most of the world's lithium exists dissolved in the oceans, some 230 billion tons of lithium, though at a low concentration of 0.1 to 0.2 parts per million, which makes the recovery impractical.

Considering the high cost of the fuel cycle, and the huge cost of the fusion facilities themselves, if indeed they can be made to work at all, makes D-T fusion impractical, especially in considering the alternative, the high efficiency of thorium fission in the Liquid Fluoride Thorium Reactor (LFTR). For this this type of nuclear energy production 2 million tons of fuel are readily available, which carries the exact same recoverable energy content as the D-T fusion fuel. Also, few commercial requirements compete for the thorium-fuel resource. 

On a factor by factor comparison with thorium fission power, D-T fusion power doesn't have a hope to ever make the grade to become a practical energy resource. Nor would it be a clean resource, because of the neutron-induced radiation of the equipment. In comparison the LFTR has the capacity to 'burn up' a lot of the high-level radioactive 'waste,' so-called, that poses many of the nuclear-power problems today. Choosing the LFTR option thereby also extends the uranium fuel cycle, and accomplishes the burn-up of the nasty plutonium that has become weapons-grade plutonium, while the LFTR's own waste product has a short life span and has no weapons potential. In comparison with that, the D-T fusion fuel is a key element for nuclear weapons, especially the tritium that is currently strictly controlled for inhibiting weapons proliferation.


 

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Published by Cygni Communications Ltd. North Vancouver, BC, Canada -  2010  Rolf A. F. Witzsche

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