Question

in 6000 words Evaluates the current challenges and opportunities confronting the nuclear fusion technology, including technical and environmental aspects

Answer 1

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The benefits of fusion as an energy source.

*The fuel for fusion is abundantly available. Two isotopes of hydrogen are well suited for fusion ,deuterium and tritium. Deuterium is available from seawater (and can be extracted by electrolysi) and it is expected that tritium can be produced within a fusion power station from small quantities of lithium-4. The long-term fuel security of fusion would appear to exceed that of fission power and hence far exceed that of fossil-fuel energy. A fusion

station would use about 100kg of deuterium and 3 tonnes of lithium to produce the same amount of energy as a coal-fuelled power using 3 million tonnes of fuel.

*Fusion has a low environmental impact. Whereas fission stations produce spent fuel with half-lives of thousands of years, the only radioactive wastes produced from a fusion station would be from the intermediate fuel, tritium, and any radioactivity generated in structural materials. The radioactivity of tritium is short-lived, with a half-life of around 12years, and if chosen appropriately the structural materials have a half-life of around 100years.

* Fusion is inherently safer than fission in that it does not rely on a critical mass of fuel. This means that there are only small amounts of fuel in the reaction zone, making nuclear meltdown impossible.

*Fusion power stations would present no opportunity for terrorists to cause widespread harm (no greater than a typical fossil-fuelled

station) owing to the intrinsic safety of the

technology. Fusion in a tokamak relies on a

continuous supply of fuel, without which the process soon dies away. Furthermore, the process is only sustained via careful use of the controlling magnetic fields. While the magnets contain some limited stored energy, the fusion reactor does not. This is in

contrast with other low-carbon electricity sources,

fission and conventional hydropower, which require the safe control of large amounts of stored energy, even when not operating.

*As with fission, fusion power stations would

provide energy at a constant rate, making them suitable for base-load electricity supply. Fusion electricity will be similar to fission electricity in its cost structure; a power station will require complex and expensive engineering, while fuel costs will be negligible in comparison. Staffing levels will be roughly constant whether or not the plant is enerating. As such, the majority of costs will be capital costs and almost all will be fixed. The marginal cost of electricity generation will be very small.

*Fusion power stations would not produce fissile materials and make no use of uranium and plutonium, the elements associated with nuclear weapons. This reduces proliferation concerns associated with these elements, although fusion is not completely free from proliferation risks.

6 challeges for fusion

*Planned availability

The requirement for continuous power at high avail-

ability is particularly demanding for the tokamak’s

essentially pulsed output, albeit possibly operating in

very-long-pulse mode. Although researchers suggest

that plasma motion and stability can be maintained for

many hours after the initiating voltage sweep, there is

a significant availability difference between long-pulse

operations and a truly continuous operation. Much

consideration has been given to the challenge of con-

tinuous operation.

*Reliability

An even greater challenge than availability is the

need to achieve very high levels of reliability. That is,

unscheduled and unanticipated interruptions to power

generation must be avoided. A fusion-based electric-

ity company in a modern competitive electricity market

will need to enter into long-term bilateral contracts with

electricity suppliers to provide the necessary business

stability..

*Structural integrity

As a consequence of its basis as a transformer driven

by a single sweep of the primary, a tokamak is inher-

ently a pulsed device. A power station will operate with

very long pulses, but during its life it will still be subject

to many tens of thousands of pulses. Given the very

large magnetic fields associated with plasma confine-

ment and drive, each pulse will place significant mag-

netic stresses on the structure of the power station. The station must withstand repeated cycling of these

structural loads.

* Helium supply

While the fuels for fusion power are abundant and eas-

ily obtained, this does not mean that a fusion power

station would be free from energy security risks. Cen-

tral to such risks must be the long-term availability of affordable helium used for tokamak pumping, purging

and, above all, cooling superconducting magnets.

*High-temperature plasma-facing

materials – the divertor

Components directly facing the very hot fusion plasma

include the first wall of the blanket on the outer edge of

the torus and the divertor, which is usually placed round the bottom of the torus. In all MCF the plasma must at some point touch the vacuum vessel. This could be

using a device dedicated to that purpose (a limiter), but

more conventionally that role is played by the divertor.

*Problematic materials

It is often rightly stressed that, if properly developed,

a fusion power programme need not lead to a legacy

of long-lived radioactive waste. The waste of the fusion

process is harmless helium gas in small quantities. The

main issue of concern for waste is the radioactivation

of the tokamak. It is possible to manufacture the device

from materials known only to activate into short-lived

radioisotopes.

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