Author: Graham Brennan, transport programme manager, SEAI The first article examined the physics, the inherent safety and environmental benefits of fusion power over its rival fission power. The challenges of confining and heating plasma to 150 million K were reviewed. In this second article on fusion power, the progress made with experimental reactors will be reviewed and a summary of work under way to scale up fusion power to power station scale will be given. This will ultimately provide us with a timeline for when fusion power could finally be wired in to our electricity grids.

Experimental reactors

The Soviets demonstrated very promising fusion results in the late 1960s with their “tokamak” magnetic confinement reactor. It soon became the standard for many researchers and it is estimated that there are about thirty tokamak experimental reactors in operation worldwide today. Fusion plasma, a mix of electrons and ions formed when the input fuel atoms are heated to millions of degrees kelvin, can be established in a matter of seconds. So a reactor designed to study the physics of plasma only needs to run for a short duration pulse. As the plasma requires substantial heat to reach fusion temperatures and the magnets must produce a steady field over a large volume, significant amounts of DC power are required to run these reactors. Entering service in 1983, one of the largest test reactors is the Joint European Torus (JET) which is based in Oxfordshire in the UK (Fig. 1). It runs only for a pulse of 40 seconds which places a substantial demand on the local electricity grid which is offset somewhat by a giant flywheel system. In 1997 JET achieved 16MW of fusion power output (the current world record) for an input power of 24MW. It has a volume of 100m3 and a D-shape cross section with a maximum section radius of 2.1m. To begin operation, the reactor is pumped down to vacuum levels in order to remove air molecules and any other impurities which might disrupt the plasma. While temperatures approach 150 million K at the core of the plasma, the pressure is only close to atmospheric pressure giving it a wispy density of 20x10-5 g/m3. The magnets keep the plasma away from the walls, however, the enclosing field forms a cut-off point near the bottom at the diverter channel (see Fig. 1). Here a major challenge has been to develop materials and cooling systems which can operate under sustained high temperatures while preventing trapped waste materials and impurities from re-entering the plasma field. [caption id="attachment_26823" align="alignright" width="300"]Fig. 1 A composite picture of the JET tokamak reactor with a fusion run superimposed. The bright pink bits are the plasma cooling and returning to a non-ionised state. The fusion plasma in the middle is not visible in the light spectrum. A diverter channel at the bottom is used to extract waste helium gas Fig. 1 A composite picture of the JET tokamak reactor with a fusion run superimposed. The bright pink bits are the plasma cooling and returning to a non-ionised state. The fusion plasma in the middle is not visible in the light spectrum. A diverter channel at the bottom is used to extract waste helium gas[/caption] As discussed in Part 1, following a nuclear fusion reaction, when the nuclei of Deuterium (D) and Tritium (T) (which are isotopes of hydrogen) join together, the energy of fusion is contained in the kinetic energy of the resulting helium (20 per cent) and neutron (80 per cent) components. Plasma 'ignition' is said to occur when the plasma, via the helium component only, produces enough heat to heat itself entirely and the external energy supplied is reduced to zero. However, most of the energy is contained in the neutron which flies out of the reactor and is captured in a water blanket to produce steam and then electricity. Therefore it is not necessary to achieve ignition in order to have a fusion process which can generate a net positive energy yield. Therefore, researchers defined a parameter called the energy gain rate (or Q) to measure the ability of a reactor to produce a useful amount of power. Q is calculated to be the total fusion power divided by the external energy supplied which for JET is 0.7. This value is representative of the performance of the current range of test reactors. Fig. 2 illustrates the progress which has been made to date with experimental reactors. While the next major goal is to build a reactor which can achieve a Q of 10, it is considered that a commercial reactor would need a Q of 20 or more. Note that if ignition were achieved, Q would approach infinity as no external heat would be required. [caption id="attachment_26824" align="alignright" width="300"]Fig. 2 Triple product for a range of experiments with Q gain bands for D-T fuel. ITER target gain is Q = 10, Q at “ignition” approaches infinity as the external heat supplied is reduced to zero Fig. 2 Triple product for a range of experiments with Q gain bands for D-T fuel. ITER target gain is Q = 10, Q at “ignition” approaches infinity as the external heat supplied is reduced to zero[/caption] Two important lessons have been learned from all of the test reactor work. Firstly, better computer models have enabled scientists to design reactors with better plasma stability. Secondly, by parameterising each of the test Tokamak reactors, an empirical design rule has been found which shows that the energy gain is strongly related to the size of the device. This is somewhat intuitive when we consider that the helium particle resulting from a D-T fusion reaction provides direct heating to the plasma. Given the great velocities involved, the more space the particle has to travel through (i.e. the bigger the volume of the reactor), the greater the chances of its kinetic energy being transferred to the plasma rather than lost in the walls of the reactor and waste gas.

The International Thermonuclear Experimental Reactor (ITER) project

The ITER project began following a meeting between Reagan and Gorbachev in 1985 and included the EU and Japan. It has since grown to include China, India and South Korea. Its main goal is to demonstrate that fusion power heat of 500MW can be produced by the reactor with an external energy input of 50MW resulting in a Q of 10. It is designed to operate continuously for periods of 6 to 50 minutes depending on the target power level which is far in excess of the experimental reactors to date. Importantly, no external power generating equipment will be attached so it will not be able to generate electricity. It is intended to be used to demonstrate heating, plasma control, diagnostics and remote handling maintenance procedures necessary for a future fusion power station. While the temperatures will be similar, the plasma densities and pressures will reach up to twice that of the JET experimental reactor. The ITER torus will have a volume of 840m3 and a maximum cross section radius of 6m. The smaller experimental reactors are designed to run for very short bursts. The high magnetic fields required to contain the plasma are fed by powerful DC currents. These currents cause the coil wires to become dangerously hot because of the ohmic heating. In order to avoid this, the designers of ITER have opted to use superconducting wires in the electromagnets which also results in very high magnetic flux densities. As a superconductor coil produces no resistance, it means that the magnets can be run at very high currents. However, it does mean that the wires must be cooled to cryogenic temperatures of -269°C before they become superconducting. The wires themselves are an alloy of niobium and tin and the coils are cooled internally with liquid helium. However, if the coil contains impurities or experiences excessive eddie currents, it could suddenly stop superconducting and become normally resistive again causing 'magnetic quenching' and significant disruption to the process. The coils require a special refrigeration plant to cool the helium to the required temperature. The helium is pumped directly through the core of the coils. The magnet assemblies sit inside thermal blankets to reduce the heating effect of the neutrons. In order to produce continuous operation, the magnets and the external heating systems will require large DC currents to be provided to them. ITER has a 400kV electrical supply available to it. [caption id="attachment_26825" align="alignright" width="300"]Fig. 3 Illustration of the ITER reactor with water cooling system used to absorb 500MW of heat from the fusion reaction. This steam could be used to generate electricity in later versions. A separate refrigeration plant is used to cool liquid helium which is pumped inside the superconducting coils of the magnets Fig. 3 Illustration of the ITER reactor with water cooling system used to absorb 500MW of heat from the fusion reaction. This steam could be used to generate electricity in later versions. A separate refrigeration plant is used to cool liquid helium which is pumped inside the superconducting coils of the magnets[/caption] Another important goal of ITER will be to demonstrate the sustained production of tritium which is one half of the D-T fuel required for the fusion reaction. When an isotope of lithium is struck by a neutron, it produces tritium which can be collected and stored near the reactor. ITER will include pipes to pump a fluid mixture containing lithium around the outside of the reactor exposing it to neutron bombardment. The resulting tritium will be extracted from the fluid and stored for later use to fuel the fusion process. The majority of the neutrons pass through the reactor walls and are stopped by a surrounding water blanket which then absorbs the heat from the fusion process. In an electricity generating power plant, this steam would be passed through a turbine to generate electricity. However, ITER will simply measure the heating power available and cool the water before returning it to the reactor (see Fig. 3). The total cost is expected to be over €15 billion with benefit in kind contributions made by each country. Construction has begun at the site in Cadarache in France and the first plasma runs are expected in 2020 (Fig. 4). Ireland participates in ITER via the EU’s Fusion for Energy group. We are involved in plasma research and Irish companies can bid for ITER contracts.

Pre-commercial power station

Stepping up from ITER, a pre-commercial power plant will be required to demonstrate continuous electricity production. This plant will need to show that the reactor materials can survive intact for many years. By colliding with the atoms of the metal matrix, continuous neutron bombardment can cause the formation of discontinuities which lead to the creation or enlargement of structural cracks. This is referred to as neutron embrittlement. Secondly, the plasma must remain stable and avoid scorching the walls of the reactor. The reactor walls may require liquid metal cooling systems in order to allow continuous operation for a longer life. If significant damage occurs to a wall, remote handling equipment will be required to remove the wall segment and replace it with a new component causing significant downtime. Several years of operation at power station representative output levels will be required. The operating costs and reliability must be demonstrated satisfactorily before commercial investment can happen. The EU is considering a demonstration power (DEMO) plant to follow on from ITER. Early design studies suggest a target output of 1,500MW of heat with a Q = 25. From this heat power, the goal of DEMO would be to generate 500MW of electricity. In order to achieve this, it must be 15 per cent larger than ITER with a 30 per cent greater plasma density. Detailed design could be completed by 2024 and operation could begin by 2033 assuming ITER goes well. It is considered that commercial reactors could be built for a quarter of the price of the DEMO project. [caption id="attachment_26826" align="alignright" width="300"]Fig. 4 ITER fusion reactor pit under construction in France Fig. 4 ITER fusion reactor pit under construction in France[/caption] In looking ahead to commercial reactors, researchers are trying to establish component test laboratories which would allow the reactor materials to be selected which would resist neutron embrittlement and produce reactor casings with short lived radiation levels following long term neutron activation. In Japan the International Fusion Materials Irradiation Facility (IFMIF) project has begun which will be able to test advanced reactor materials in environments that mimic those inside a fusion power reactor.

Conclusions

This article focused on fusion research associated with the tokamak reactor which is the most developed fusion option. There are of course other methods and approaches being studied and breakthroughs are always possible. ITER will do most of its work in the period of 2020 to 2030 and will be a critical stepping stone towards delivering a reactor capable of producing electricity continuously. A pre-commercial reactor will be needed to show 10 to 20 years of continuous operation before a commercial investor could take a risk on this new technology. Therefore, if this electricity demonstrator can begin operations in the 2030s, it could see the first commercial reactors begin operations in the early 2050s. The European Union has committed to reducing emissions from its electricity sector to near zero by 2050. Therefore, converting more and more of our energy demand, such as heat and transport, to electrical demand allows us to reduce our emissions from those energy modes also. While small countries like Ireland have an abundance of renewable energy to exploit, big industrial countries like Britain and Germany will require technologies such as carbon capture and sequestration together with nuclear fission to meet this goal of zero emissions. The UK recently announced the go ahead for the 3.2GW Hinkley fission reactor costing €25billion. Fusion power may arrive just at the right time to plug in to this clean energy network and power us with greater certainty into a future of virtually limitless pollution free energy. Human society could progress unrestricted by energy resources leading possibly to a new phase in the development of our world.