Thermonuclear Fusion : Basic Principles, Achievements and Prospects

All long-term economic development scenarios predict a doubling, at least, of global energy consumption towards the end of this century. Current consumption is of the order of 10 ²⁰ joules per year and, given the impact of fossil fuel use on our environment, thermonuclear fusion is the only development path that can cope with this doubling of energy demand, while offering a very long-term perspective (> 10 ³ years), free from the problems of proliferation, runaway and high-level waste.

In terms of specific energy density (J/kg), thermonuclear energy is a million times more dense than chemical energy. An energy conversion system based on the thermonuclear combustion of deuterium (D) and tritium (T), the isotopes of hydrogen, according to the exothermic reaction: D + T → α (alpha) + n (neutron), generates low-level radioactive waste and presents no risk of runaway.

Deuterium is found in abundant quantities in water, in a proportion of 1/6,700 to hydrogen; the mass of the oceans being of the order of 10 ²¹ kg, the potential energy reserves of terrestrial deuterium are therefore of the order of 10 ¹¹ years based on current energy consumption.

This optimistic estimate needs to be revised, as tritium is unstable and has a half-life of 12.3 years; it therefore does not occur naturally and must be produced in the reactor blanket using the neutron flux from fusion reactions D + T → α + n. Lithium (Li) exhibits two neutron reactions enabling tritium regeneration. With the light isotope ⁶ Li, the n + ⁶ Li → α + T reaction is exothermic and exhibits significant reactivity with thermal neutrons. Lithium is found in abundant quantities in the earth's crust; the natural abundances of the light and heavy isotopes are 7.4% ⁶ Li for 92.6% ⁷ Li respectively. Theoretical neutron studies have estimated that 95% of the neutron energy can be deposited in a metre-thick tritiated blanket. For the natural abundance of lithium, potential energy reserves range between 10 ⁴ and 10 ⁷ years, the low estimate being restricted to continental resources and the high estimate taking into account the exploitation of ocean reserves.

For a fusion power reactor with an electrical output of 1 GW, typical of today's fission reactors, the annual requirement is: 123 kg deuterium + 184 kg tritium, which...