generalfusion

How a nuclear fusion reaction works

Nuclear fusion involves fusing small atoms together and occurs when two light nuclei collide to form a heavier nucleus. Like nuclear fission, the reaction is accompanied by a significant energy release. Unlike nuclear fission, there is no potential for meltdown, there are no radioactive by-products, and fuel is virtually unlimited. Nuclear fusion is the mechanism that fuels the sun, which is the solar system’s largest fusion energy source.

A variety of atoms can be used as possible fusion fuels, but the most practical involve isotopes of hydrogen. Hydrogen normally consists of one proton and one electron with no neutrons (denoted as 1H). Two other isotopes of hydrogen include one and two neutrons respectively. These are denoted as 2H and 3H, but also have the specific names deuterium and tritium respectively, sometimes denoted as D and T.

The easiest of all fusion reactions combine deuterium with tritium, which form helium and a free neutron with an output energy of 17.5 MeV.

The fusion mechanism is such that it only acts on a small amount of material and can only occur if suitable conditions can be created and maintained for a sufficient time. If any part of the

process does not work perfectly, fusion does not occur. It is for this reason that a nuclear fusion meltdown is not possible: in fusion, a small amount of fuel is added to a device and conditions are created to enable fusion to occur; in fission, fuel is added in bulk, and the reactor controls the rate at which the chain reaction occurs.

The final by-product of the fusion reaction is helium, which is a safe, stable and environmentally friendly gas. As a result, fusion does not have any of the long-lived radioactive waste problems associated with nuclear fission.

Sources of deuterium and tritium

Deuterium is a stable isotope of hydrogen and occurs in nature at a frequency of one atom of deuterium for every 6,000 atoms of normal hydrogen. Deuterium can be separated from regular hydrogen (of which seawater is an abundant source) and collected for use in a fusion power plant.

Tritium only forms in nature under unusual circumstances, such as high in the atmosphere, and is unstable, decaying radioactively into helium with a half-life of 12.2 years. Thus, tritium cannot be collected from nature and once created synthetically, it does not persist.

Tritium can be produced as a by-product of the fusion reaction by allowing the free neutron to react with lithium (installed in a blanket around the fusion core), which then breaks into tritium and helium. The tritium can be rapidly extracted from the blanket and sent back into the fusion reaction, thereby establishing a self-generating tritium fuel supply that uses deuterium and lithium as input fuels, both of which are consumed in the process.

Lithium is an abundant, inexpensive metal that occurs naturally in the earth’s crust. At a rate equivalent to today’s total global energy consumption there is enough lithium for 23,000 years of fusion energy. Lithium can also be extracted from seawater, which could fuel fusion for an additional 207 million years.

When the deuterium found in 1 L of seawater is fused with the tritium produced from 0.5 g of lithium, the energy released is equivalent to that of burning 1,000 L of gasoline.

Creating the conditions for fusion

The difficulty with nuclear fusion lies with creating the conditions that allow fusion reactions to take place. Atomic nuclei are positively charged and therefore repel each other. In order to fuse, nuclei must be brought close enough together for the nuclear force to overcome the electrostatic force, which in practical terms means they must be hurled at each other at very high velocity. The velocity of a particle corresponds to its temperature, which for successful deuterium-tritium fusion amounts to 150 million °C.

Significantly, at temperatures above even a couple of thousand degrees, the collisions between the atoms are so violent that the electrons are knocked off their nuclei, creating a soup of free electrons and free nuclei called plasma. Since the electrons move freely, plasmas are conductive and have magnetic properties, which provide an important means of manipulating and containing the hot substance.

Once the nuclei have suitable velocity, the chance of their collision and therefore fusion is probabilistic in nature – and chances are low since nuclei are very small. Dense plasmas and long dwell times increase the chance of fusion and the resulting energy output. Creating and maintaining these conditions requires a considerable amount of energy, which must be drawn from the fusion reaction before any net energy can be extracted.

The conditions under which enough fusion reactions occur to produce more energy than initially invested to heat the gas is called “break-even”. Any energy gained through fusion that is greater than the amount of energy put into it is called “net gain”.