Culham Centre for Fusion Energy (CCFE) is the UK’s national laboratory for fusion research. CCFE (formerly known as UKAEA Culham) is based at Culham Science Centre in Oxfordshire, and is owned and operated by the United Kingdom Atomic Energy Authority.
Fusion – Energy for the future?
Dr Anthony J. Webster, Culham Centre for Fusion Energy (CCFE)
The Sun is the source of nearly all our energy on Earth and is responsible for wind, hydroelectric, tidal, biofuels as well as the fossil fuels, coal, oil and gas. Its own internal energy source is fusion and Culham is researching into the possibility that fusion could be the answer to the Earth’s energy needs.
The world’s population is growing.
But so is the global energy demand
Unfortunately we are still dependent on fossil fuels for our energy needs and there are problems with fossil fuels. The two biggest problems are that they are running out and that they produce carbon dioxide and other pollutants.
Even though there are new sources of oil being discovered and peak production keeps moving to a later date we will run out.
Coal supplies will go the same way too.
Decreasing fossil fuels will have an effect on food production. In the US 6-7 times the energy in food is used in its production.
In the UK the sustainable population without fossil fuels would be only 17 million people. Before the widespread use of fossil fuels the population peaked at 5-6 million on 3 separate occasions in the past 2000 years. The current population exceeds 60 million.
Alternative Energy Sources
Renewable energy sources (including wind, wave, solar and hydro) are the most attractive option at present and offer long term, clean energy reserves. However they have a low energy density and they have a low reliability which means storage systems are required.
Solar sources take their energy directly or indirectly from the Sun. The Sun radiates light and heat but it is quite dilute by the time it reaches the Earth. This dilute radiation must be captured and concentrated. The difficulty here is that this requires “geologically scale” facilities. Examples of this include:
Mountain ranges to capture and concentrate precipitation for hydroelectric and pumped storage;
Large fractions of the country to be covered by solar panels and wind turbines;
Agriculture and forests covering a large fraction of arable land;
Phytoplankton in the oceans uses the Sun’s energy which then gets concentrated into larger sea life.
Long term options
Nuclear fusion, Advanced Nuclear Fission (Fast breeders) and large scale Solar power all have the potential to supply our current energy needs unfortunately none of these are sufficiently developed yet.
Fusion will occur when the combining of small nuclei gives rise to a large nucleus with higher binding energy per nucleon than the starting nuclei.
Fusion of deuterium with tritium creating helium-4, freeing a neutron, and releasing 17.59 MeV of energy, as an appropriate amount of mass changing forms to appear as the kinetic energy of the products, in agreement with kinetic E = Δmc^2, where Δm is the change in rest mass of particles. The energy represents 338xE12J/kg and 3.5MeV is about million times more than the energy produced in chemical processes.
1kg of pure Uranium 235 is equivalent to nearly 3000 tonnes of coal but 1kg of Deuterium and Tritium is equivalent to more than 10,000 tonnes of coal.
Fusion requires small quantities of fuel so costs are negligible.
Why is fusion difficult?
Energy is required to overcome the repulsion between the positive nuclei. The coulomb energy barrier is 288keV and temperatures of 3000MK are required. However quantum tunnelling can lower this barrier to give a maximum reaction rate of 70keV with a temperature of the order of 700MK.
Repulsion and fusion of nuclei
Fusion in the Sun can occur at low energies but for fusion to occur on Earth, you need a temperature of at least 100 million degrees Celsius—six times hotter than the core of the sun. The sun is a natural fusion reactor which makes up for its measly 15 million degrees (0.5eV to 1.4keV) with the intense pressure created by its core’s gravity.
The high temperatures on Earth are required to ionise the gas to produce the necessary plasma. The energies required are between 10 and 100keV. That is 1000 x the ionisation potential.
The ionization energy (potential) of an atom or molecule describes the amount of energy required to remove an electron from the atom or molecule in the gaseous state.
Strategies for controlled nuclear fusion
In a fusion reaction, energy is released when two light atomic nuclei are fused together to form one heavier atom. This is the process that provides the energy powering the Sun and other stars, where hydrogen nuclei are combined to form helium.
To achieve high enough fusion reaction rates to make fusion useful as an energy source, the fuel (two types of hydrogen – deuterium and tritium) must be heated to temperatures over 100 million degrees Celsius. At these temperatures the fuel becomes a plasma. This incredibly hot plasma is also extremely thin and fragile, a million times less dense than air. To keep the plasma from being contaminated and cooled by contact with material surfaces it is contained in a magnetic confinement system.
Magnetic confinement uses strong magnetic fields to confine the plasma and confine the energy sufficiently well that modest heating of the plasma produces net fusion energy.
A tokamak is a device using a magnetic field to confine a plasma in the shape of a torus. Achieving a stable plasma equilibrium requires magnetic field lines that move around the torus in a helical shape. Such a helical field can be generated by adding a toroidal field (traveling around the torus in circles) and a poloidal field (traveling in circles orthogonal to the toroidal field). In a tokamak, the toroidal field is produced by electromagnets that surround the torus, and the poloidal field is the result of a toroidal electric current that flows inside the plasma. This current is induced inside the plasma with a second set of electromagnets.
The above pictures show the Tokamak magnetic field and current. It shows the toroidal field and the coils (blue) that produce it, the plasma current (red) and the poloidal field produced by it, and the resulting twisted field when these are overlaid.
A stellarator is a device used to confine a hot plasma with magnetic fields in order to sustain a controlled nuclear fusion reaction. It is one of the earliest controlled fusion devices, first invented by Lyman Spitzer in 1950 and built the next year at what later became the Princeton Plasma Physics Laboratory. The name refers to the possibility of harnessing the power source of the sun, a stellar object. Current is not needed through the plasma and each coil is different.
Spitzer suggested extending the torus with straight sections to form a racetrack shape, and then twisting one end by 180 degrees to produce a figure-8 shaped device. When a particle is on the outside of the centre on one of the curved sections, by the time it flows through the straight area and into the other curved section it is now on the inside of the centre. This means that the upward drift on one side is counteracted by the downward drift on the other.
To allow the tubes to cross without hitting, the torus sections on either end were rotated slightly, so the ends were not aligned with each other. This arrangement was less than perfect, as a particle on the inner portion at one end would not end up at the outer portion at the other, but at some other point rotated from the perfect location due to the tilt of the two ends. As a result, the stellarator is not “perfect” in terms of cancelling out the drift, but the net result is to so greatly reduce drift that long confinement times appeared possible.
Electromagnetic waves – Plasma is conducting so this would normally prevent electromagnetic waves from entering the plasma. Electromagnetic waves do bounce but natural oscillations allow energy propagation. This process is made possible by coupling to modes of plasma oscillation. Plasma oscillation results from the effects of the long-range correlation of electron positions brought about by Coulomb interactions.
NBI is injection of high energy neutral particles. The particles need to be neutral to pass through the magnetic fields and not be deflected.
Driving current – resistivity of the plasma. A reduction in the resistivity at increased temperatures limits the extent to which the plasma may be resistively heated.
How close is Culham to harnessing fusion?
JET (Joint European Torus)
There is a new experiment every 20-30 minutes. It confines 100m^3 of plasma with a central temperature 10 times that in the centre of the Sun, for 10-20 seconds per experiment. It enables the fusion of deuterium and tritium to be studied and it has demonstrated the production of 16MW of power from fusion reactions.
The light seen in the video occurs when electrons re-combine with ions. Flecks of light are due to neutron instabilities.
JET is an example of a donut shaped Tokamak. So far it is the world’s most successful device . In 1997 it produced 16MW of energy from fusion for about a second and 4MW of fusion energy for 4-5 seconds.
Fusion Power Plants
A power plant must produce by fusion, more power than that used to confine and heat the plasma.
The fusion triple product
The triple product is a figure of merit used for fusion plasmas, closely related to the Lawson Criteria. It specifies that successful fusion will be achieved when the product of the three quantities – n, the particle density of a plasma, the confinement time, t and the temperature, T – reaches a certain value. Above this value of the triple product, the fusion energy released exceeds the energy required to produce and confine the plasma. For deuterium-tritium fusion this value is about: ntT ≥ 5×E21 m^-3 s KeV. JET has reached values of ntT of over 1xE21 m^-3 s KeV.
To increase the confinement time you need to increase the machine size. This is the first of two reasons why magnetic fusion devices must be large.
Why is magnetically confined fusion difficult?
There are three requirements:
1) Stability /Control of the plasma limits the pressure
2) Sufficient confinement of energy and particles limits the confinement time
3) Materials required needs to have sufficient lifetime – Fusion plasmas are heated by a combination of various systems to reach temperatures of around 100 million degrees Celsius. However, these heating systems must be able to survive the extreme conditions that they have themselves created.
1 + 2 determines the minimum size and 3 determines the lifetime. Together they determine the cost.
Why is stability a problem?
Plasma instabilities can be divided into two general groups:
1. hydrodynamic instabilities
2. kinetic instabilities
They can occur due to changes in the characteristics of the plasma (eg. temperature, density, electric fields, and magnetic fields).
The above picture shows instabilities in the plasma such as filaments.
Filamentation (or filamentary structure) is often seen in plasmas. It is created because plasma contains free electrons, making it highly electrically conductive — even more than metals, and even in tenuous cosmic plasmas. As charged particles readily move in a plasma, a ring of magnetic field forms around the current that can pinch it into filamentary current strands (i.e. pinched filaments).
Example of a kink instability
The above picture on the right is one of the earliest photos of the kink instability in action – the 3 by 25 cm pyrex tube at Aldermaston. The discharge initially formed a symmetric toroidal ring, but then developed a kink instability.
Courtesy of EFDA-JET
Stability also requires a sufficiently strong (externally imposed) toroidal field.
In classical electromagnetism, Ampère’s circuital law, discovered by André-Marie Ampère in 1826, relates the integrated magnetic field around a closed loop to the electric current passing through the loop.
For plasma stability you need energy to bend field lines, energy to compress field lines, energy to compress the plasma and the correct magnetic geometry to produce good field line curvature, however strong currents, strong pressure gradients and poor magnetic geometry, producing bad field line curvature, can drive instabilities.
JET loses energy from the copper coils so using superconducting coils would be needed or a bigger plasma.
A power plant will either require superconducting coils or a more compact geometry.
The above picture shows the plasma in the MAST reactor. Mast is an example of a spherical tokamak. Note the almost circular outer profile of the plasma. The extensions off the top and bottom are plasma flowing to the ring diverters, a key feature of modern tokamak designs. The high “elongation” is also evident, notably the filaments extending off the top and bottom near the central conductor.
The engineering challenge is to optimise for as large a current as possible through a small centre column.
A spherical tokamak is a type of fusion power device based on the tokamak principle. It is notable for its very narrow profile, or “aspect ratio”. A traditional tokamak has a toroidal confinement area that gives it an overall shape similar to a donut, complete with a large hole in the middle. The spherical tokamak reduces the size of the hole almost to zero, resulting in a plasma shape that is almost spherical, often compared with a cored apple. The spherical tokamak is sometimes referred to as a spherical torus and often shortened to ST.
MAST is exploring an alternative MCF route to fusion
Confinement/Transport – Modes of operation
The confinement of particles and energy is not total. Energy escapes by turbulent processes. There is an internal transport barrier.
Tritium is radioactive with a half-life of roughly 12.5 years. It may be “bred” from Lithium using the neutron source provided by D-T fusion.
Natural Lithium is made up of Li 6 (7.5%) and Li 7 (92.5%)
Blanket and Shield
A reliable and efficient breeder-blanket technology is vital for heat transfer and fuel generation in future fusion power plants. The high-energy neutrons released from fusion reactions do not interact with the plasma.
Neutrons are captured in the blanket and used to “breed” Tritium from Lithium.
The number of neutrons are multiplied and slowed to thermal energy levels by the breeding blanket where the energy is recovered for generating electricity. 14MeV kinetic energy is transformed to heat which is transferred to a coolant. This vapourises and is used to run a turbine to produce electricity.
Heat is removed and the magnets are shielded from gamma rays and spare neutrons.
Cross sections for slowing the neutrons, breeding Tritium and shielding require a blanket thickness between 1 and 1.5 metres.
This is the second reason why fusion power plants must be large!
Key Issues for Materials Selection
Safety and environmental considerations: Reduced volatility, gas emission Low specific radioactivity Low radioactive decay heat Small half-life radio nuclides Controlled paths for dispersion of radioactivity Reduced biological hazard potential Easy waste disposal
Materials and neutron damage
The fusion of deuterium and tritium produces a high energy neutron. The benefit is that the majority of the fusion reaction’s energy is evenly deposited but the structure of the irradiated materials is damaged and neutron damage can cause materials to become radioactive. The fusion neutrons produce atomic displacement cascades and transmutation nuclear reactions within the materials. Impurities such as hydrogen and helium can also form.
Neutron activation is the process in which neutron radiation induces radioactivity in materials, and occurs when atomic nuclei capture free neutrons, becoming heavier and entering excited states.
Some elements, such as tungsten, vanadium, chromium, iron, tantalum, carbon and silicon are very difficult to activate, because the capture of a neutron by the most common isotopes of those elements converts the atom into another, heavier stable isotope. Therefore these elements can be used in the fusion reactor as everything else useful is transmuted by high-energy neutrons to something very radioactive.
The requirement for only short half-life radioactive waste means that Ni and other high-activation elements such as Co are undesirable and this has resulted in the development of a range of reduced activation steels targeted at fusion applications.
Experimental reduced activation steels have tantalum replacing niobium, vanadium replacing titanium and chromium sort of replacing manganese (nothing much replaces molybdenum)
F82H: Fe – 7.7%Cr – 2%W – 0.2%V – 0.04%Ta – 0.09%C
Eurofer: Fe – 8.9%Cr – 1%W – 0.2%V – 0.14%Ta – 0.12%C
These will be “cool” enough to be recycled and re-used after about 50 years storage after 5 years’ service.
Damage to Materials
It isn’t just the production of radioactive materials which could be a problem. Neutrons have more energy than fission neutrons (14MeV) and in a power plant environment, roughly 20MW per m^3 can be deposited into the walls (“blanket” modules). Under these conditions, every atom will be displaced from its lattice position approximately 10 times each year and small irradiation induced defects are produced such as vacancies and interstitials.
2nm stacking fault tetrahedron.
In addition, Helium (and hydrogen) will be produced in the lattice at a rate of approximately 100 atomic parts per million per year.
These effects can cause swelling, embrittlement, and a general degradation of the material’s properties.
Evolution of the microstructure
The microstructure of the irradiated material results from interactions between the various irradiation-induced defects. It can be formed of: Small defect clusters; Dislocation loops; Stacking fault tetrahedral; Precipitates; Voids; He bubbles.
The next stage is ITER
Linear dimensions of ITER are roughly twice those of JET. It is designed to operate for up to 30 minutes and designed to produce 410MW of fusion power. There is ten times more fusion power, than heating and over half the world’s population is represented in the ITER project.
In ITER the main contenders for first wall structural materials are reduced activation ferritic/martensitic (RAFM) steel, oxide dispersion strengthened (ODS) RAFM steels, oxide dispersion strengthened RAF steels, refractory metals and alloys (W, Cr), titanium-base alloys, vanadium base alloys and SiCfibre/SiC composite materials.
Why is it always 30 years to fusion?
There isn’t enough funding. In 2004 fusion only received 1.5% of a combined energy subsidy and R&D spend of about 30 billion euro per year (coal received 44.5% and oil and gas received 30%).
New materials need to be developed. However the larger the device the more fusion research is expensive, the more difficult it is to obtain sufficient funding and the longer the time to design and construct. If funding is too low then magnetic confinement fusion cannot be developed. All of this slows research.
Both nuclear fission and nuclear fusion produce radioactive waste but fusion will be far less dangerous.
The neutrons in fusion would be quite dangerous to humans, but when the plant is turned off the production of neutrons ceases within milliseconds. The neutron bombardment does affect the vessel itself, and once the plant is decommissioned the site will be quite radioactive. However the radioactive products are short lived (50-100 years) compared to the waste from a fission powerplant (which lasts for thousands of years). Also, the radioactivity in a fusion powerplant will be confined to the powerplant itself; there will not be any waste needing to be transported for disposal, storage or reprocessing.
In fusion there is no equivalent to a fission reactor’s core, and no actinides (long lifetimes).
International Conference Nuclear Energy for New Europe 2009 Bled /Slovenia / September 14-17
Looking at the above charts full fusion plant recycling should be possible one hundred years after final shut down, something that wouldn’t be possible with fission.
Categorisation of all material arising from the operation and decommissioning of PPCS model B. NAW: non-active waste (to be cleared); SRM: simple recycling material (recyclable with simple remote handling procedures); CRM: complex recycling material (recyclable with complex remote handling procedures); PDW: permanent disposal waste (not recyclable).