Magnetic fusion : fusion resource

Fusion: a practically unlimited source of energy

Energy resources, global consumption

Fusion is often presented as being a source of energy for which fuel is abundant and even unlimited. It obviously compares favourably with other energy sources (coal, oil, gas and fission). For the latter it must be noted that current reserves are around 100 years for oil, gas and conventional fission, around several hundred years for coal and around several thousand years for breeding fission. How is fusion situated in all this ?

Fusion reactions and fuel to be taken into account

The Deutérium-Tritium reaction: D + T to ₄He (3.5 MeV) + n (14.1MeV). This is the most easily attainable reaction (maximum effective cross section at an energy of 100 keV). Tritium is an element with a relatively short period of radioactivity (12. 3 years). It will be produced on site from lithium via the reaction Li + n --> 4He + T. The final result is written opposite.

The Deutérium- Deutérium reaction : D + D to ₃He (0.8 MeV) + n (2.5 MeV), D + D to T (1 MeV) + H (3 MeV) This reaction is worthwhile since it eliminates the necessity of generating the tritium in-situ. Besides, the neutrons produced are less energetic and therefore less harmful to the reactor structures. On the other hand, between 1 and 100 keV its effective cross-section is around 100 times less than that of the D-T reaction, which explains why controlled fusion research focuses on this latter reaction process.

The fuels to be considered are therefore : deuterium and lithium.

Resources in deuterium

Deuterium is a stable hydrogen isotope. It is very abundant and may be cheaply extracted from seawater (33 of D per cubic metre of seawater). The estimated resource in the oceans is 4.6 10¹³ metric tons or around 5,10¹¹ TW/yr. Bearing in mind that current world energy consumption is around 14 TW/yr, the energy resources in deuterium represent more than 10 billion years of average annual world consumption (reference year 2000).

Resources in lithium

The average quantity of lithium in the Earth’s crust is around 50 ppm. It is more abundant than tin or lead and even ten times more abundant than uranium (3 to 4 ppm). The price of a kilo of lithium is under 5$ and reserves are estimated at 12 million metric tons (for comparison, reserves in natural uranium are estimated at less than 4 million metric tons and the price of a kilo of uranium is close to 100 $).

Lithium may also be taken from seawater (0.17g/m3), which makes a further potential 230,000 million metric tonnes.

In a fusion reactor based on the D-T reaction, the component charged with producing the tritium in-situ is called the tritium-breeding blanket. It contains components with a lithium base, in liquid or solid form and with enrichments in lithium-6 variable according to the concepts. Typically, we may note that a 1000 MWe reactor based on the D-T reaction requires 100 kgs of deuterium in a year, around 150 kgs of tritium (produced from lithium) and around 300 kgs of lithium-6. If we were to put forward the (unrealistic) hypothesis that the whole world nuclear production were provided by fusion reactors, then the mining seams of lithium would be used up in 5000 years. The use of lithium in seawater pushes back this limit to several million years. We can conclude by indicating that the cost of energy produced by a fusion reactor is determined by the extent of initial investment (around 2/3 of the cost), to which must be added the cost of regularly replacing used components (around 1/3 of the cost): the cost of fuel is therefore marginal.

Conclusions

The production of energy through thermonuclear fusion using the Deuterium-Tritium reaction involves on-site production of tritium from lithium. The fuel resources, limited by lithium, are estimated at several thousand years if the lithium is of telluric origin and at several million years if the lithium is taken from seawater.

For further information :

U.S. Geological Survey, http://minerals.usgs.gov/,