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Nuclear fusion

Hydrogen isotopes

Nuclear fusion, often hailed as the energy source of the future, is based on the fusion reaction between two heavy hydrogen isotopes, deuterium and tritium. If a common hydrogen atom (symbol H) has a mass of 1 unit, then a deuterium atom (symbol D or H-2) has mass 2 and tritium (symbol T or H-3) mass 3, hence the names: 'deuterium' means the second and 'tritium' the third. Tritium is radioactive and comes into being in nature by reaction of cosmic radiation with molecules high in the atmosphere. The naturally occurring tritium on Earth amounts to less than 9 kilograms. Deuterium is not radioactive and occurs in water molecules, one of every 6700 hydrogen atoms in nature is a deuterium atom. Heavy water consists of molecules with deuterium atoms instead of common hydrogen atoms.

Fusion principle

At temperatures of around 100 millions of degrees celsius deuterium nuclei can fuse with tritium nuclei, generating helium-4 nuclei, neutrons and heat. This thermonuclear reaction is applied in the explosion of a hydrogen bomb, in which the required high temperatures and pressures needed to ignite the fusion are generated by the explosion of a fission bomb.

The tritium for nuclear weapons is generated in special nuclear fission reactors. Since the start of the atomic age the quantity of man-made tritium worldwide amounts to a few hundreds kilograms, intended for use in nuclear weapons, most of which has decayed during the past decades. Tritium has a half-life of 12.5 years, so after a storage time of 12.5 years, half of each kilogram tritium has been decayed into helium-3, after 25 years 0.25 kg tritium remains. From each kilogram tritium produced in 1960, 62.5 grams still exist today. Consequently the man-made tritium inventory has to be replenished constantly. At present the world tritium inventory is estimated at some 20 kg.

Controlled fusion: a moving target

Understandably the notion of the potentially huge amounts of energy released by the fusion reaction of a tiny amount of matter, as demonstrated by the H-bomb, sparked the research towards a controlled fusion process, to construct a reactor which would make available a nearly unlimited energy supply in a controlled and safe way.

During the 1960s the nuclear world claimed that first fusion power station would come online within ten years. During the 1970s the first commercial fusion station would come online during the 1980s. During the 1980s the industry assured the first fusion power station would be ready by the year 2000. After the year 2000 the date of the first operating fusion power station moved back to 2050, or later.

Challenges

Before the first fusion reactor can come online, several technical challenges are to be overcome:

• Stable and continuous operation of the fusion process during tens of years. At present a period of only a number of seconds seems to be within reach.

• Provisions making a run-away fusion reaction and meltdown impossible.

• Supply of tritium.

• Materials able to withstand very high temperatures, pressures and neutron radiation under highly corrosive conditions.

• Transfer of the fusion heat generated in the reactor to a medium outside of the reactor for conversion into useful energy (electricity).

• Safe handling of unprecedentedly large amounts of tritium, biomedically a dangerous radionuclide.

Tritium supply

A fusion reactor with an output of 1 GWe (gigawatt electric) needs some 200 kg of tritium to start up, ten times as much as is available world wide at present. Tritium can be generated in nuclear fission reactors and has to be separated from spent fuel, cooling water and/or special target elements for neutron irradiation.

After startup the fusion reactor has to generate on its own the tritium it consumes, tens of kilograms a year. This breeding process would be based on neutron irradiation of lithium-6, one of the natural lithium isotopes, and should occur in the wall of the reactor. After irradiation the contaminants of the mixture have to be removed and the remaining lithium and the newly formed tritium have to be separated and purified.

In addition the contents of the reactor have to be purified constantly, to remove the newly formed helium and the contaminants spallating from the reactor walls during operation. The purified mixture of deuterium and tritium can be fed back into the reactor, with fresh deuterium and tritium to make up the consumed quantities plus the unavoidable losses.

Materials

Deuterium is extracted from purified water. This is a technically mature process and very energy-intensive, the more so if the deuterium has to be prepared from seawater.

The reactor vessel and magnets needed for confinement of the superhot plasma are made of exceedingly high-grade materials, containing exotic and scarce metals, such as columbium (niobium). The life span of a fusion reactor is limited due to the high thermal and mechanical stresses, combined with an intense irradiation of fast neutrons. Likely it will be necessary to replace the reactor vessel a number of times during the operational life span of the power plant. Consequently the consumption of high-grade and exotic materials will be high.

Radioactive waste

Nuclear fusion generates large amounts of radioactive waste. Indeed, no fission products result from fusion, but large amounts of activation products do. The fast neutrons liberated by the fusion reaction irradiate the reactor vessel and surrounding constructions. By neutron capture non-radioactive materials become radioactive, the co-called activation reaction. A part of the liberated neutrons are needed to breed tritium from lithium in the blanket of the reactor, but by far the biggest part of the neutrons escapes from the reactor

Energy balance

Analogue to fission power nuclear fusion, if ever feasible, would have a process chain:

• front end, comprising construction, recovery of deuterium and the production of tritium,

• mid section: operation, maintenance and refurbishments, including replacements of the reactor

• back end: radioactive waste management, decommissioning and dismantling of the radioactive parts of the fusion power plant.

The first requirement the fusion system has to comply with in order to be an energy source is a positive energy balance, not only of the reactor itself including its containment, plasma confinement and ignition system, but of the complete system.

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