Page 242 - Managing Global Warming

Nuclear fusion: What of the future? 201

The second reaction is endothermic and so most fusion concepts assume the tritium
6
breeder is highly enriched with 3 Li to maximize energy output. In addition, a neutron
multiplier such as lead or beryllium must be mixed with the lithium to ensure there are
sufficient neutrons available to breed more tritium than is consumed by the reaction.
3
D- 2 He and p-B fusion reactions are aneutronic and do not require the breeding of tri-
tium, so reactor concepts can be simpler (see Technology Challenges, Section 5.3) but
require much higher temperatures to produce significant power. At these high temper-
atures, energy losses from the plasma through Bremsstrahlung radiation can cool the
plasma more powerfully than the fusion products heat it, making sustained power
production difficult.
The old joke is that nuclear fusion is 30years in the future, and always has been. It is
the case that research into fusion for energy has been a slower-than-expected process,
but it has also been more difficult to realize than anticipated. Much of the work to date
has revolved around controlling and heating plasmas to fusion-relevant regimes,
discovering a wide range of instabilities and interesting behaviors on the way [1].
Recently, consideration has turned to the technological problems of actually building
a fusion power plant, which has brought to light a new set of challenges, some of
which are reviewed later. If constructed, a fusion power plant would be one of the most
complex machines ever built, with environments requiring semiautonomous robotic
maintenance. Engineering such a system to meet power-supply requirements of
reliability, availability, maintainability, and inspectability is a considerable challenge.
The presentation of fusion in this chapter focuses mainly on D-T fusion in toka-
maks (see Section 5.2), but most of the challenges and conclusions also apply to other
concepts.

5.1.1 Fusion resources
As discussed earlier, the main fusion fuels are deuterium and lithium. Approximately
one in 7000 hydrogen nuclei in the universe is deuterium, so 1 L of water contains
0.033g of deuterium, which, when combined with tritium, releases the same energy
as burning 280L of oil. As there is no shortage of water, there is enough deuterium for
billions of years of global energy supply. However, as discussed, tritium must be bred
from lithium. Since the fusion reaction releases a lot of energy, it does not take much
lithium to supply the needs of a single individual; a few grams of lithium, as found in a
single laptop battery, can provide enough tritium to release 250,000kWh, a typical
European lifetime consumption of energy. Known land-based lithium resources pro-
vide enough lithium for around a thousand years of global energy demand if it is all
used for fusion (see, for example, [2]). There is competition, though, from energy stor-
age demands such as laptop batteries. However, although commercial methods do not
yet exist, lithium is also extractable from seawater and in total the potential reserves
from this source are orders of magnitude higher [3]. The lithium must, as previously
6
stated, be enriched to 90% Li. A number of methods to accomplish this exist at lab
scale, but will require scaling up to commercial levels if fusion is to be successful as an
energy technology.