Methods, advantages and disadvantages of generating energy from nuclear fusion, the energy generation mechanism that sustains stars
With a fast growing demand for energy to fuel our daily lives, the shortcomings of our reliance on archaic methods of generating this power are clear. Oil, natural gas, coal and even the uranium fuel for nuclear fission are finite resources, and whilst great improvements have been made in utilising renewable resources, their efficiency is just not sufficient to completely provide the energy generation requested by human society. Additionally, with concerns over climate change ever rising, there is increasing attention given to the possibility of electricity generation by nuclear fusion.
How nuclear fusion works
The critical criteria for a fusion reaction is for the atoms involved in the process to be at very high temperatures, several millions of degrees. This results in the elctrons of the atoms separating from the nucleus, creating a plasma. However, this plasma would interact with and damage the reactor due to its extreme temperature. To prevent this, the first and one of the most common reactor designs make use of the fact that plasma is electrically conductive, and utilise powerful magnets to contain it and preserve the reactor, a system called ‘magnetic confinement’. In magnetic confinement fusion, the gas is injected into the chamber and is held in the centre by the magnets. Strong electric currents are used to heat the gas until it becomes a plasma. Further heating to fusion temperature generally comes from a combination of injection of high-speed hydrogen atoms and high-frequency electromagnetic waves.
Another approach is known as ‘inertial confinement fusion’. This involves
heating the exterior of a pellet of fuel using lasers. The rapid expansion of the exterior of the pellet sends shockwaves to its core, which can heat it to fusion temperature.
Advantages and disadvantages of fusion power
Unlike alternatives such as fossil fuels or nuclear fission, the only direct waste product of fusion is helium: a noble gas and hence non-pollutant. Fuel for fusion is abundant; deuterium is extracted from water and tritium is produced from lithium. Today’s lithium stockpiles could create enough tritium to meet current energy demand for 3000 years, and, by extracting lithium from seawater, we could produce 60 million years’ worth of fuel. Fusion is also safer than fission. Due to the minimum amounts of fuel in use at any one time a dangerous nuclear accident isn’t possible.
However, the neutron radiation that would create the tritium in a reactor would have an adverse effect on the construction of the reactor. When a commercial fusion reactor comes to the end of its lifespan, the reactor core and several other components would be highly radioactive, perhaps even more so than waste from a fission reactor. On the other hand, the half-life of these freshly radioactive components would be shorter than that of fission waste, and would be completely decayed after 500 years, posing no threat to future generations.
Current state of research
Only some of the energy generated in the fusion reaction can be recaptured by the plasma. As a result, a fusion reactor must reach a Q factor (that is the ratio of fusion power produced to the power required to maintain the plasma in steady state) of around 5 before the self-heating of the plasma exceeds the external input. Currently it is thought that a commercially viable fusion reactor would have to attain a Q factor exceeding 25.
The International Thermonuclear Experimental Reactor, or ITER, is a planned tokamak style fusion reactor which, when completed, will be the largest ever built. ITER will be a purely experimental reactor, there are no plans to capture the power produced. One of the main goals of ITER will bve to heat the plasma to over 150 million degrees Celsius and produce 500MW of power from an input of 50MW for 10 minutes straight. This will be the longest time plasma has ever been held stable during fusion, representing a key milestones in nuclear energy research and more generally in our quest to clean and virtually unlimited energy sources.
References:
J. Ongena, G Van Hoost. Energy for Future Centuries: Prospects for Fusion Power as a Future Energy Source. Fusion Science and Technology Vol. 61 3-16 (2012)
G.Vayakis et al. Fusion Engineering and Design 53, 221-227 (2001)
Cover image: Inside the European JET Tokamak, both during (right) and after operation. Credits: CCFE, JET.
