Faster fusion reactions are possible with a range of mixtures involving the iso­topes of hydrogen, helium, and lithium. These include:

2D + 2D ^2He + n + 0.96×10-B J 2D + 2D ^2T + ’H + l.19xi0-15 J 2D + 5D ^ ‘He +n + 5.2x 10_11 J

Most research effort is being directed at the last of the reactions because it is the least difficult reaction to achieve (Figure 9.1 ).

Most (80%) of the energy released is in the form ofkinetic energy of the neu­tron. Note that though the energy released per fusion reaction is typically 10 times less than for a single fission reaction, the neutrons are released with per­haps 5 times as much energy.

Deuterium, as we saw in Chapter 3, occurs in ordinary water at a concentra­tion of 0.016%) and can be readily separated by chemical processes. Tritium

does not occur naturally but can be produced from lithium by bombardment with a neutron. Thus:

6 Li +n ^ *He + ‘T + 7.7 x lO’13 J
’Li + n ^ ‘He + T + n — 4.0 x 10~u J

As we shall see later, it is possible to arrange the system so that the neutrons from the fusion reaction are used to breed more tritium from these reactions. This is done in a blanket in a manner similar to that used in a fast fission reactor. The power generated in such a fuel cycle for each gram of lithium is 36 million joules (10,000 kWh).

It is interesting to compare the energy available from these isotopes with, say, the figures given in Table 1.2. There, we saw that the world’s readily avail­able uranium resources, utilized in fast reactors, could release around 1023 J and using the uranium in the ocean could increase this figure to around 1026 J. These figures compare with the current world electrical consumption of 1.8 x 1 0iy J/year. In fusion reactions the deuterium in the oceans could release around 3 x 1031 J, while the land-based lithium reserves could yield around 1028 J, and including the lithium in the oceans would raise the figure to 2 x 102H J. Thus, the fuel resource with fusion reactions can be considered limitless, cer­tainly beyond a million years.

Let us therefore turn our attention to how we might tap into this immense source of energy. The fusion reaction is difficult to achieve because the deuterium and tritium nuclei are each positively charged electrically. Like charges repel each other and this force can be overcome only if the nuclei approach each other with sufficient velocity—millions of kilometers an hour—to overcome the mutual re­pulsion. That means heating up the gaseous deuterium-tritium mixture to a tem­perature around 100 million degrees or more. At a temperature of a few thousand degrees the gas becomes ionized; that is, the electrons separate from the atoms and the separate electrons and nuclei move randomly (Figure 9.2). Such a mater-

ial is known as a plasma Plasmas exist in the sun and stars and also in such everyday items as neon signs and electric arcs.

It is not enough to heat the plasma to the required temperature. It is also nec­essary to hold the plasma at that temperature for sufficient time for the reaction to take place. Clearly, the length of time will depend on the number of nuclei in a given volume of plasma. The required conditions have been identified in the Lawson criterion, which states that the product of the time for which the plasma is confined ‘tr and the density of the plasma (n) must be greater than 102° s/m3

n x X > 1020 s/m2

Thus, if the density of the plasma is 102° nuclei per cubic meter, the plasma must be held at 100-200 million degrees for 1 s.

9.2 CONFINEMENT

How are we to “confine” the plasma long enough so that it does not touch (and melt) the walls of the vessel in which the reaction is to take place? In the Sun and stars the fusion plasma is held together by large gravitational forces. On Earth we obviously cannot use such forces to contain a plasma in any convenient-sized ap­paratus. Two ways have been tried to provide this confinement of the plasma.

1. Magnetic confinement. Since plasmas are excellent conductors of electricity, they can be acted on by magnetic fields (Figure 9.3). Thus, magnetic fields can be used to shape and confine the plasma in such a manner that it does not touch the walls of the vessel in which the gaseous mixture is held. If the plasma did come into contact with the vessel walls, it would quench, losing its energy and high temperature very rapidly.

2.

Inertial confinement. The alternative to magnetic confinement is to contain the isotopic mixture frozen solid at about 15 K as a small spherical pellet or

bead (Figure 9.4). This spherical pellet is then bombarded from eveiy direc­tion by beams of high-powered lasers, which compress and heat the mixture to fusion temperatures. Inertia holds it together long enough—perhaps a nanosecond (10-9s) for the fusion reaction to take place. This time can be so short because of the very high densities achieved.

Considerable research is being done on the inertial confinement process at the Lawrence Livermore Laboratoiy at the University of Rochester, New York and at the Los Alamos Laboratory, New Mexico. There are, however, funda­mental difficulties with this route to a practical system. These are the low effi­ciency of the laser (1-2%), the low fraction of fusion energy released to date (-0.01%), and the difficulties of engineering a device to produce a continuous power output involving ignition of a stream of frozen pellets at a high rate.

Most effort is therefore being devoted to trying to achieve a fusion reaction using magnetic confinement. The sticture of magnetic fields is often indicated by lines of force or field lines’, the stronger the field, the greater the density of the lines. Within a magnetic field, charged nuclei take a spiral path in the direction of the field lines as illustrated in Figure 9.5a. A magnetic field line causes a charged nucleus to spiral around it (Figure 9.5a). If the field is ananged so as to close on itself in a circle within a circular chamber (Figure 9.5b), the particles will spiral around the field and remain trapped within the circular chamber, or toms. Unfortunately, this does not always happen in practice due to instabilities that occur in the plasma. Nevertheless,

Г^І The sudden inwards movement compoeases and heats the D & T mixture up to conditions for fusion to take place

Figure 9.5: (a) Particle spiraling around a magnetic field line. ( {J) A closed toroidal system. ( cl A magnetic mirror or bottle.

most ofthe experiments that have tried to achieve controlled ft sion reactions make use of this closed doughnut-shape configuration. Another possibility is to constrict the magnetic field lines at each end of a tube. Particles trying to escape by spiraling along the field lines are reflected back into the central region. This arrangement is called a magnetic mirror or bottle (Figure 9.5c).