As already mentioned, the fusion reaction involving deuterium and tritium is the most accessible and, obviously, is the reaction on which the most credible concepts are based. Tritium is a radioactive element with a relatively short half-life (12.3 years) that will be produced on-site by neutron capture in lithium.
Therefore, the primary fusion fuels are deuterium and lithium (Fig. 1), two nonradioactive elements.
Deuterium is a stable isotope of hydrogen. It is very abundant and may be extracted from seawater (33 g of deuterium per m3 seawater) by conventional industrial processes (e. g., distillation, electrolysis, isotopic exchange). The estimated resource in oceans is on the order of 4.5 x 1013 metric tons, corresponding to an energy potential of 5 x 1011 TW per year. Bearing in mind that current global energy consumption is on the order of approximately 12 TW per year (year 1990), deuterium energy resources would exceed the lifetime of the sun (~ 5 billion years).
The lithium content in Earth’s crust is approximately 50 ppm (0.05 g/kg). It is more abundant than tin or lead and is even 10 times more abundant than uranium (3-4 ppm). Lithium occurs naturally in a mixture of two isotopes: lithium-6 (7.5% of naturally occurring lithium) and lithium-7 (92.5% of naturally occurring lithium). Although it would be possible to use naturally occurring lithium in a fusion reactor, mixture enriched with lithium-6 (40-90% according to the design) would be preferable because, in this case, reactions are exothermic and feature higher cross sections. The enrichment processes are well known and have been validated (lithium-6 is used to produce tritium for nuclear weapons). Lithium may also be extracted from seawater that contains
0. 17g/m3, constituting a potential reserve of 230,000 million metric tons. Proposed extraction methods are based on conventional ionic exchange, solvent extraction, or coprecipitation-type chemical processes. In a fusion reactor generating 1 GWe per year,
between 0.5 and 3.5 metric tons of naturally occurring lithium would be consumed depending on the enrichment implemented. Therefore, the use of telluric lithium would ensure reserves for several thousand years, and use of lithium extracted from seawater would push this limit to several million years.
Use of ‘‘advanced’’ fuels (D-D or even D-3He reactions) features the dual advantage of avoiding use of tritium and production of high-energy neutrons. However, conditions regarding temperature, density, and confinement time necessary to compensate the small cross sections render their implementation extremely hypothetical and, in any case, impossible in the near future. It should also be emphasized that in a reactor based on these reactions, nearly all fusion energy produced is transferred to the first wall elements, accentuating the constraints on these components that were already very high (in the case of D-T reactions, 80% of fusion energy is carried by neutrons that give up their energy in the tritium breeding blanket).