Fusion for the future

In the summer, the Government will publish the recent review of its energy policy. The review has generated much debate on the role renewable energy sources and nuclear fission reactors should play alongside fossil fuel-burning power stations in the UK's energy supply mix for the next 30 years. But what other technologies might play a part in powering our future? One long-term option is nuclear fusion.   

Fusion, plasmas and the Sun   

In a nuclear fusion reaction lighter atoms are fused together to form a heavier atom with a release of energy. This process powers the Sun and stars. When a gas is heated to more than 10,000°C atoms have enough energy to ionise (break up) into their charged constituents, ie negatively charged electrons and positive nuclei. This hot ionised gas - ie plasma, is the fourth state of matter. The Sun is a plasma comprised mainly of hydrogen and its core temperature is ca 15 million°C. At these temperatures the positively charged hydrogen nuclei have enough energy to approach each other, overcome the repulsive forces between them, and fuse together to form helium nuclei.   

 

Fusion reactions release lots of energy because the total mass of the more stable products are slightly less than the total mass of the reactants. This small 'loss' of mass is converted into huge amounts of energy:
E = mc², where E is the energy produced, m is the mass lost and c is the speed of light.   

Hot research   

Fusion research on Earth focuses on the reaction between deuterium and tritium - two heavy isotopes of hydrogen. In the Joint European Torus (JET) fusion experiment, hosted at the UK Atomic Energy Authority's (UKAEA) Culham site, a large volume of a dilute mixture of the two gases (ca 1/100gram total material) is heated to 150 million°C to form a plasma. To trigger fusion, this plasma must be contained with a sufficiently high density of nuclei and retain its energy long enough for the reaction to start (in JET this time is ~1s). Under these conditions the deuterium and tritium ions fuse together to form helium and high-speed neutrons with a release of energy:   

²₁H + ³₁H  ⁴₂He + ¹₀n + energy 

Achieving and maintaining these extreme conditions in the plasma so that fusion can occur are what scientists have been grappling with over the past 50 years. 'You need   to put a lot of energy in to get even more out from fusion', explains Chris Warrick, a fusion experimental scientist and education outreach manager at Culham. 'Reaching temperatures greater than 100 million°C poses a big challenge'. To do this the plasma must be isolated from its surroundings to stop any cooling. The JET reactor is based on the most advanced confinement vessel - a tokamak, a doughnut-shaped vacuum vessel. Since the plasma's components are charged particles, magnetic fields are used to influence their behaviour and control the plasma in the vessel. A conducting coil passing through the centre of the reactor induces a current in the plasma which heats it up to about half the required temperature. Magnetic fields produced by large D-shaped copper coils surrounding the reactor vessel, and by the induced current, shape the plasma, keeping it away from the sides of the vessel in a 'magnetic bottle'. Additional heating of the plasma is provided in two ways: by injecting a beam of high-energy neutral particles into the plasma; and by irradiating the plasma with microwaves of a specific frequency.   

 

Built in 1983, the JET experiment holds the world record for power generated from fusion: 16MW (megawatts - 10⁶ watts) of fusion at a rate of 4MW per second set in 1997; and is capable of sustaining plasma for ca 1 minute. 'Currently JET's power output almost matches the power input required to heat the plasma to fusion temperatures', says Warrick, 'but when you factor in the electricity required to power the magnetic coils, ca 600MW, JET is inefficient'.   

So if achieving fusion still needs more power than it produces, will it ever offer a viable electricity supply? According to Warrick, the scale of the reaction greatly affects this energy balance. 'Size really matters with these devices. By increasing the volume of the reactor the fusion reaction can really get going, producing energy and helping to heat the plasma itself'. To this end Europe and its six international partners (Japan, Russia, China, Korea, India and the US) announced in June last year the go ahead for JET's successor to be built in Cadarache, France.   

ITER - the way forward   

ITER (International Thermonuclear Experimental Reactor) will be a similar design to JET but twice the size - an experimental power station-scale device. Costing €4.5 billion, ITER aims to demonstrate that a fusion power station is feasible. The reactor is expected to amplify the power output by a factor of 10, producing 500MW of power from fusion for periods of initially up to 10 minutes. ITER's design will incorporate niobium-tin superconducting coils to reduce the energy required to confine the plasma. Cooled using liquid helium, these coils should produce magnetic confinement fields for caone month from a single charge.    

ITER © ITER

 

ITER will also test technologies for harnessing the power generated from fusion, another challenge for scientists to overcome. 'In a power station fusion power will be harnessed   by slowing the high-energy neutrons, which escape the plasma, in a blanket of lithium-containing material', explains Ian Cook, manager of the materials development and fusion technology programme at Culham. 'As the neutrons are slowed and absorbed, the blanket heats up which in turn can heat steam to drive a turbine to pro-duce electricity. Tritium is a byproduct of the interaction between neutrons and lithium, and this would be recycled and burnt in the reactor vessel'.    

¹₀n + ⁶₃Li  ⁴₂He + ³₁H

Among other current blanket designs are a solid solution of lithium and lead, and pebble beds comprising beryllium and lithium ceramics, Cook told InfoChem. 

Alongside ITER will run an international fusion materials radiation facility (IFMIF) to test new materials for use in next-generation fusion power stations. Materials used in a commercial power station will have to withstand bombardment from energetic fusion neutrons. 'Continued exposure to high-energy neutrons alters a material's physical structure, which can lead to weakening processes such as swelling, creep or embrittlement. For future fusion power stations to be economic and successful they must be reliable', says Cook. 'Using a particle accelerator to fire a beam of deuterium nuclei at a lithium target to produce neutrons with energies similar to those emitted from the fusion plasma, scientists at IFMIF will test promising structural materials, such as ferritic (magnetic) steels with body-centred cubic crystal structures'. 

Why pursue nuclear fusion?   

Beyond the UK's future energy needs, world energy demand is expected to at least double in the next 50 years driven by global population growth. Nuclear fusion offers the possibility of a safe, emission free and sustainable electricity supply. 'Fusion is inherently safe because the reaction is so difficult to maintain. Without a constant supply of fuel and the correct conditions the reaction will not go', says Warrick. The fuels consumed by fusion, deuterium and lithium, are abundant - one litre of water contains 33mg of deuterium, which when fused with tritium generates energy equivalent to ca 340litres of gasoline. Lithium deposits are found in the Earth's crust, in sea water and in the sea bed. The fusion process emits no greenhouse gases such as CO₂- the only emission from the process is helium gas. And unlike nuclear fission reactors, fusion reactors produce no spent radioactive waste - tritium is a radioactive gas but this is recycled and burnt in the reactor. According to Cook, though materials in the reactor exposed to high-energy neutrons will become radioactive, the legacy lifetime of a decommissioned power station could be limited to 50-100 years through materials research and development done at ITER and IFMIF. After this time the materials could be recycled or reused in another fusion power station.   

The ITER/IFMIF facilities are expected to be operating within 10 years. Data from ITER/IFMIF experiments will inform the design of demonstration fusion power stations, which Warrick and Cook are confident we could see in place within 30 years - provided there is political will. James Berressem