Why Technical English

Fuel cycle in fusion reactors | May 25, 2011

Composed by Galina Vitkova

Common notes

The basic concept behind any fusion reaction is to bring two or more nuclei close enough together, so that the nuclear force in nuclei will pull them together into one larger nucleus. If two light nuclei fuse, they will generally form a single nucleus with a slightly smaller mass than the sum of their original masses (though this is not always the case). The difference in mass is released as energy according to Albert Einstein’s mass-energy equivalence formula E = mc2. If the input nuclei are sufficiently massive, the resulting fusion product will be heavier than the sum of the reactants’ original masses. Due to it the reaction requires an external source of energy. The dividing line between “light” and “heavy” nuclei is iron-56. Above this atomic mass, energy will generally be released by nuclear fission reactions; below it, by fusion.

Fusion between the nuclei is opposed by their shared electrical charge, specifically the net positive charge of the protons in the nucleus. In response to it some external sources of energy must be supplied to overcome this electrostatic force. The easiest way to achieve this is to heat the atoms, which has the side effect of stripping the electrons from the atoms and leaving them as nuclei. In most experiments the nuclei and electrons are left in a fluid known as a plasma. The temperatures required to provide the nuclei with enough energy to overcome their repulsion is a function of the total charge. Thus hydrogen, which has the smallest nuclear charge, reacts at the lowest temperature. Helium has an extremely low mass per nucleon and therefore is energetically favoured as a fusion product. As a consequence, most fusion reactions combine isotopes of hydrogen (“protium“, deuterium, or tritium) to form isotopes of helium.

In both magnetic confinement and inertial confinement fusion reactor designs tritium is used as a fuel. The experimental fusion reactor ITER (see also The Project ITER – past and present) and the National Ignition Facility (NIF) will use deuterium-tritium fuel. The deuterium-tritium reaction is favorable since it has the largest fusion cross-section, which leads to the greater probability of a fusion reaction occurrence.

Deuterium-tritium (D-T) fuel cycle

D-T fusion

Deuterium-tritium (D-T) fusion


The easiest and most immediately promising nuclear reaction to be used for fusion power is deuterium-tritium Fuel cycle. Hydrogen-2 (Deuterium) is a naturally occurring isotope of hydrogen and as such is universally available. Hydrogen-3 (Tritium) is also an isotope of hydrogen, but it occurs naturally in only negligible amounts as a result of its radioactive half-life of 12.32 years. Consequently, the deuterium-tritium fuel cycle requires the breeding of tritium from lithium. Most reactor designs use the naturally occurring mix of lithium isotopes.

Several drawbacks are commonly attributed to the D-T fuel cycle of the fusion power:

  1. It produces substantial amounts of neutrons that result in induced radioactivity within the reactor structure.
  2. The use of D-T fusion power depends on lithium resources, which are less abundant than deuterium resources.
  3. It requires the handling of the radioisotope tritium. Similar to hydrogen, tritium is difficult to contain and may leak from reactors in certain quantity. Hence, some estimates suggest that this would represent a fairly large environmental release of radioactivity.

Problems with material design

The huge neutron flux expected in a commercial D-T fusion reactor poses problems for material design. Design of suitable materials is under way but their actual use in a reactor is not proposed until the generation later ITER (see also The Project ITER – past and present). After a single series of D-T tests at JET (Joint European Torus, the largest magnetic confinement experiment currently in operation), the vacuum vessel of the fusion reactor, which used this fuel, became sufficiently radioactive. So, remote handling needed to be used for the year following the tests.

In a production setting, the neutrons react with lithium in order to create more tritium. This deposits the energy of the neutrons in the lithium, for this reason it should be cooled to remove this energy. This reaction protects the outer portions of the reactor from the neutron flux. Newer designs, the advanced tokamak in particular, also use lithium inside the reactor core as a key element of the design.

PS: I strongly recommend to read the article FUSION(A Limitless Source Of Energy). It is a competent technical text for studying Technical English. Consequently it offers absorbing information about the topic.



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