The most common type of fusion reaction discussed for fusion energy in the near future is the fusion of two hydrogen isotopes: deuterium (2H) and tritium (3H). It is the easiest fusion reaction to achieve on Earth, and will most likely be the type of reaction found in first generation fusion reactors. The actual reaction involves a deutrerium nucleus fusing with a tritium nucleus to form an alpha particle (4He nucleus) and a neutron. The products contain around 17.6 million electron volts (MeV) of released kinetic energy through the loss of mass in the fusion process.

The D-T reaction is the easiest because the extra neutrons on the nuclei of the deuterium and tritium increase their size and thus the probability of a fusion reaction. They also each have the smallest possible positive charge (since hydrogen has only one proton), making it relatively easy to have the two nuclei overcome their repulsion and fuse together.

A Few Problems with DT Reactions - And Solutions

One problem in using this reaction is the release of the neutron. This poses a problem because neutrons often "stick" to other nuclei, usually causing the nuclei to become radioactive or to initiate new reactions. For example, some neutrons can be absorbed by the walls of a reactor, creating a radioactive waste for disposal later on. This is one problem scientists face in the construction of possible fusion reactors.

Another problem is acquiring some of the fuel. Deuterium can be found on Earth, although in a very small quantity (.015% of natural hydrogen is deuterium). However, this small amount is more than enough to suppy energy for thousands of years at our current energy demands. One gallon of sea water has the energy content of 300 gallons of gasoline. Tritium, on the other hand, is radioactive, with a half-life of 12.3 years; therefore tritium does not last long enough to acquire in significant amounts naturally. Fortunately, both problems are solved by using the neutron in another reaction like this:

6Li + n 4He + T

The lithium absorbs the neutron and generates a tritium while releasing a bit more energy in the process. There is plenty of lithium available in nature.

However, the neutron problem is not totally eliminated through the above solution. Not all neutrons will fuse with the lithium, and instead fuse with other parts of the reactor, possibly inducing radioactivity. Neutron multipliers may be used in a reactor to compensate for this neutron loss, or reactions that yield more neutrons might be implimented, such as 7Li + n 4He + T + n. As for limiting the amount of high-level nuclear waste, careful selection of the materials used are expected to minimize the handling and disposal of such radioactive material. For example, the development of advanced, low-activation materials (like vanadium-based materials), or through the use of neutron-free reactions, could be implimented in future reactors.

Other Possible Reactions

There are other reactions besides the D-T that would work, incuding D+D, T+T, and D+3He reactions. The D+3He reaction in particular is a promising reaction, in that this reaction is the easiest "aneutronic" reaction, producing 4He and a proton. Aneutronic means it does not produce a neutron. This is good because radioactive waste caused by neutron absorption is eliminated. This is considered a more advanced fuel, however, and will most likely not be used in the first generation of commercial power plants.

Let's Review:

Describe the deutrium-tritium reaction.

Explain why the D-T reaction is the easiest to achieve.

What problem do the neutrons released pose?