Expert: Osman Kemal Kadiroglu - 10/9/2000

Question
Could you please explain the proccess of nuclear fusion? This would be very helpful. If it is not to much to ask could you explain what waste matter is left over.
Thank you,
Josh.  

Answer
Nuclear fusion is just the opposite nuclear reaction of nucear fission, which is the source of nuclear power at present day. Some atomic nuclei are not stable and they tend to change to other nuclear configurations. Heavy nuclei tend to reduce their masses and become small nuclei whereas light nuclei tend to increase their masses and become heavier nuclei. Nuclei like cobalt, iron etc. are the most stable nuclei and uranium tend to fission, and hydrogen tend to fusion to reach nuclei of higher stability.

If one could join two hydrogen nuclei to form a helium nuclei one can achieve nuclear fusion. The total mass of the particles getting into reaction is larger than the total mass of the reactants. This mass difference is the source of fusion energy. Such a reaction is very easy to explain than realize. Hydrogen nucleus (actually deterium, heavy hydrogen) are charged particles and it is very difficult to bring then close enough to have a nuclear reaction. Just like two magnets N poles repulse each other two deteruim nuclei repell each other with a huge force. To overcome this force one must accelerate the deterium nuclei to very high velocities. This is done at very high temperatures, like million degrees Kelvin. This could be done in magnetic fields (inertial confinement) or with lasers.

The enregy generated from a fusion reaction per mass of fuel is much larger than the enrgy relased by fission. A bad and unethical example for fusion reaction is the H bomb and for fission reaction is the A bomb.

Presently there are some international machines trying to harness the fusion power. ITER is one of them. Research in this field is very expensive and many nations join the program to minimize the cost. Similar research is pursued in laser fusion but these techologies are restricted technologies. In my opinion fusion power that would generate economical electric power will be available within a century. In 50's it has been promoted as "too cheap to measure" kind of new energy source. In 60's and 70's there was always 20 years for the demostration plant. presently same argument could be heard in the nuclear industry.

The most efficent fuel for fusion is deterium but it is much easier to us deterium and tritium as the fuel. In both cases the waste of the reaction will be helium atoms. There will be lots of very high energy neutrons coming out of the reaction. These neutrons will evidently interact with the surrounding structural and magnetic material thus producing radiactive waste much similar to fission reactors waste. Most probably the first fusion reactors would be what is called 'hybrid reactors' that utilize these high energy neutrons. Fusion reaction will generate heat and neutrons at the center of a hybrid reactor and high energy neutrons will interact with uranuim, plutonium or thorium rods surrounding the fusion reactor, thus producing more power. Waste of such a reactor would be very similar to the waste of a fission reactor.

More introductory information is available in chapter 14 of

Krane, Kenneth S., Introductory Nuclear Physics, John Wiley & Sons, 1988 (QC777.K73 1987)