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