Prospect for the Future

9.1 INTRODUCTION

In the preceding chapters we have seen how uranium, mined from the earth’s crust, is utilized in a nuclear reactor to create energy and how the resulting waste products can be dealt with safely. We have concentrated on the thermal or heat-generating aspects of the materials at the various stages of the cycle. We have seen that the energy that can be recovered from nuclear fission of 1 ton of uranium can be increased 60-fold by the use of fast reactors and that this can extend our use of fission power from a few tens to many hundreds of years. Nevertheless, the world’s uranium resources are finite, and energy resources will increasingly be required by the developing world. Scientists have therefore turned to alternative ways to release nuclear energy. What more natural place to look than to the ultimate source of the earth’s energy—the sun. The energy generated by the sun is not the result of splitting up nuclei of heavy elements but of the joining together—fusion—of nuclei of light elements such as the iso­topes of hydrogen or lithium. These elements are abundant and easily available on the earth, so what is the problem of releasing fusion energy for our use?

The problem is that to release the energy of fusion in a controlled manner re­quires heating the reacting nuclei to temperatures of tens to hundreds of mil­lions of degrees and holding them in sufficient quantities at these temperatures long enough for the reaction to take place. A device capable of creating such a reaction is called a thermonuclear reactor.

The energy release in the sun results from the conversion of hydrogen into helium. Effectively four protons fuse together to form one helium nucleus with

an energy release of 7.7 x 10 ~’.J joules. Thus the conversion of 1 gram of hy­drogen to helium produces 0.71 x 1012 joules. The energy released by the sun is almost incomprehensibly large: 0.39 million million gigawatts (3.9 x 1026 watts). This requires the consumption of 5.5 x 101’* grams/s. (or alternatively, 550 million tons per second). Even so, the sun has an expected lifetime of 10,000 million years!

On Earth it is not possible to reproduce the solar conditions. The specific ther­monuclear reaction is too slow to produce a practical size of reactor. Fortunately there are other fusion reactions that might form the basis of a practical reactor.