CONTROLLED FUSION :
THE ENERGY OPTION FOR THE 21st CENTURY

World Population and Energy Needs

Reasonable estimates indicate that the world population will be around 10 billion in the middle of the 21st century.

In 1990, annual per capita primary energy consumption was 2.2 x 1011 Joules or 5.1 toe (tonnes of oil equivalent) in the industrialized countries - ten times more than in developing countries. In 2050, world primary energy consumption could be anything from two to three times its current level, depending on which energy demand scenario is chosen (see the table below). Most of this demand will have to be met by :

  • fossil fuels (mainly coal, because crude oil and natural gas reserves will be greatly depleted),
  • nuclear energy : fission and fusion,
  • renewable energy resources : hydroelectric, solar, wind, wave, tidal, geothermal, biomass, etc.

Even though renewable resources will probably be able to meet a greater proportion of the World's energy requirements than they do at present, experts agree that they will not be able to satisfy the total demand. New energy options must therefore be developed - systems which are optimally safe, environment-friendly and economical. Controlled thermonuclear fusion is one of these rare options.

Fusion is the process by which the nuclei of light atoms (such as hydrogen) combine, or fuse, to form heavier elements. Temperatures of around 100 000 000 degrees C are needed to overcome the electrostatic repulsion between positively-charged nuclei and thus bring them close enough together for fusion reactions to occur at a sufficient rate. At these temperatures, the gas becomes a plasma (the ions and electrons form a macroscopically neutral fluid) and obviously cannot be in contact with material walls. The fusion power plant design based on a tokamak (the most performing device) uses magnetic fields to keep the plasma thermally insulated from the reactor walls. The volume of plasma, in such device, would be about 1000 m3 and the fusion energy would amount to several gigawatts.

 

 

Thermonuclear Reactions in the Stars

Fusion is the energy process which fuels the sun and other stars. Under the effect of gravity, the matter in a star's core eventually reaches densities and temperatures sufficient to trigger thermonuclear reactions. The latter release energy which dynamically balances the gravitational forces - and the star begins to shine.

 
Group of countries

Per capita (toe/year)

Global demand (109toe/year)

Table showing the estimated growth in world energy demand (according to two types of scenario : standard demand or low demand).

1988

2050

1988

2050

standard

low

standard

low

OECD

5.2

5.2

2.6

4.0

4.6

2.3

Central and Eastern European Countries

4.4

4.4

2.2

1.9

2.1

1.1

Developing countries

0.5

1.5

1.0

2.0

13.8

9.2

World (total)

1.5

2.0

1.2

7.9

20.5

12.6

(toe (Tonne of oil equivalent) : 4.4 x 1010 Joules = 12000 kWh)

Initially, the main type of reaction is the fusion of four hydrogen nuclei to form two helium nuclei, with photons, neutrinos, electrons and positrons being mitted. To a lesser extent, the helium also starts to fuse, forming heavier elements (beryllium, boron) and releasing more energy.

The sun, a natural reactor with a power output of 4x1026 watts, is not particularly calm: turbulence and instabilities frequently occur, producing gigantic "flares" on its surface.

 

In massive stars, after the hydrogen has been completely depleted, gravity further compresses the core of the star : helium fusion reactions thus become possible, producing heavier elements. In a succession of contractions and new types of thermonuclear reaction, new nuclear species are produced and, in their turn, burned in the stellar crucible. When thermonuclear reactions can no longer occur in a star - once it has a core of ron, for example - its life cycle is over. Depending on its mass, the star may then explode in spectacular fashion as a nova or may die slowly as a white dwarf (among other scenarios).

The challenge facing fusion research is to reproduce, on Earth, the kinds of fusion reactions which occur within the sun, and to use them for the benefit humanity.

Controlled Thermonuclear Fusion

On Earth, the fusion reaction of most immediate interest is that between the nuclei of the two heavy forms (isotopes) of hydrogen: deuterium (D) and tritium (T)

D + T -> He + n + 17.6 MeV

Deuterium is abundant in sea water (30 g/m3) but tritium is radioactive, with a half-life of 12.36 years and therefore does not occur naturally. It has to be manufactured.

In a fusion reactor, neutrons (n) - which account for 80% of the energy produced - will be absorbed in a ´blanketĒ surrounding the reactor core. This blanket contains lithium (Li) which is transformed into tritium and helium :

6Li + n -> 4He + T + 4.86 MeV

7Li + n -> 4He + T + n - 2.5 MeV

Natural lithium, consisting of 7Li (92.5%) and 6Li (7.5%), is found in large quantities (30 ppm) in the Earth's crust and in weaker concentrations in the oceans.

The blanket must be sufficiently thick (roughly one metre) to slow down the 14 MeV neutrons. Slowing the neutrons heats the blanket and a coolant flowing through it transfers heataway from the power plant to generate steam and finally, by conventional methods, electricity.

Thermonuclear fusion, as a major new source of energy, would have certain intrinsic advantages :

  • the basic fuels (D, Li) are non-radioactive, plentifully available and fairly evenly distributed throughout the Earth's crust ;
  • a runaway fusion reaction is intrinsically impossible. Furthermore once its supply of fresh fuel is cut-off, the reactor can continue operating for only a few tens of seconds ;
  • there are few radioactive waste problems : fusion generates no radioactive ash, and the unburnt gases are treated on site. Structural components of the reactor which have become radioactive through exposure to the neutrons will have to placed in storage - but, provided they are made of carefully-selected materials, the storage time could be less than one hundred years.
In the reactor core, fusion reactions take place at temperatures exceeding 100 million degrees Celsius. The confined helium heats the fresh fuel and then, once it has cooled, is extracted from the core. Neutrons pass through the wall and interact with the lithium in the reactor blanket, thus breeding more tritium in situ. A coolant flowing through the blanket removes the heat so as to generate electricity.

It is too early to give a precise assessment of the economic impact and viability of fusion energy. The investment costs will certainly be higher than for coal-fired or nuclear fission power stations, but fuel costs will be very low.

Fuels other than (D-T) could be used by a second generation of fusion power plants. Reactions involving these ´advancedĒ fuels, which might be possible in the long term, produce fewer high-energy neutrons (D-D) or no neutrons at all (D-3He). They require no tritium-breeding blanket and induce a lower level of radioactivity in mechanical structures. However, much higher temperatures than for the (D-T) cycle would be needed to ´burnĒ these fuels. Moreover, although deuterium is very abundant on Earth, 3He is found only as a trace element and would have to be extracted from the surface of the moon.

Lake Geneva alone contains enough deuterium to supply all the primary energy needed by our planet for several thousand years.

 

Non-Thermonuclear Fusion

Fusion is also possible at ambient temperature if the electrons in the deuterium and tritium molecules are replaced by much heavier negative particles. One such particle is the negative muon, an unstable particle with a mass equal to 207 times that of the electron and a life time of 2.2 ms. The physics of fusion reactions catalysed by muons is well established, but results so far indicate that a positive energy balance cannot be expected, for the following reasons :

- muons must be produced by particle accelerators which consume a great deal of energy ;

- the muon becomes attached to the helium nucleus (produced by D - T fusion) before it has had time to catalyse enough fusion reactions to make the process worth while.

In recent years, spectacular claims have been made for another approach, known as ´cold fusionĒ. This involves the electrolysis of heavy water using palladium electrodes in which deuterium nuclei are said to concentrate at very high densities. Many experiments have been performed to verify whether such unexplained electrochemical phenomena can actually be produced, but none have conclusively proved that energy is generated or that fusion reactions occur.


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