Scientists move a step closer to lighting Europe’s fusion fire

Paul Boughton

The aim of fusion research is to reproduce in a power plant the way that the sun generates energy – from the fusion of atomic nuclei.

On earththe fuel for this is an ionised low-density gasa plasmacomposed of the two hydrogen isotopes deuterium and tritium. This fuel is confined in a magnetic field and heated to ignite the fusion fire. Above a temperature of 100m°C the plasma starts to burn and hydrogen nuclei fuse to form heliumreleasing neutrons and large quantities of energy.

The recently completed European Fusion Power Plant Conceptual Study investigates the technical feasibility of deriving power in such a wayalong with its expected safety and environmental propertiesand the cost of a future fusion power plant.

The study investigates four different fusion power plant concepts. All have an electric power output of about 1500 MW and are of the tokamak type. To illuminate a wide spectrum of physical and technical possibilitiesthey are each based on different extrapolations of present day plasma physics and technology reaching far into the future.

Models A and B are the least far reaching. The assumptions on plasma behaviourfor example its stabilityare only about 30 per cent better than the very cautious estimates for the 500 MW ITER international fusion test device. Unlike in ITERthe building material is a low activation steel now being investigated in the European Fusion Programme.

The biggest differences relate to technical components of the power plant such as the so-called ‘blanket’. This is the lining of the plasma vessel that decelerates the fast neutrons resulting from the fusion process. These transfer their entire kinetic energy to a coolant in the form of heat and also produce tritium as the fuel component from lithium.

For these purposesmodel A is furnished with a liquid metal blanket. It uses a liquid lithium lead mixture for tritium productionand the fusion heat is absorbed and transferred with water. In contrastmodel B is fitted with a blanket filled with pebbles of lithium ceramic and beryllium. The helium coolant chosen here allows higher temperatures than does water – up to 500°C instead of 300°C – and hence higher efficiencies for the subsequent power production. Both blanket versions are being developed in the European Fusion Programme; test versions are to be investigated in ITER.
Unlike models A and Bthe more far reaching model C and the rather more futuristic model D are based on major progress being made in plasma physics.

Improved plasma states are combined with more powerful blanket conceptsbut ones that are already being developed in Europe. In the dual coolant blanket of model C the first wall is cooled with helium and most of the heat generated is transported to the heat exchanger by circulation of liquid metal. Silicon carbide inserts insulate the structure from the flowing liquid metal.

The higher coolant temperature of about 700°C allows more efficient conversion of fusion heat to electricity. Even more advanced in model C is the use of a self-cooling blanket – liquid metalup to 1100°Cserving for both cooling and tritium production.

Safetywaste and cost

Safety considerations concern mainly the radioactive tritium and the high-energy neutrons that activate the walls of the plasma vessel.

The consequences of all serious accidents were clarified in the study by analysing the more contemporary models A and B in greater detail. Sudden and total failure of the cooling system is assumed to cause the accident and the power plant is then left to its own devices without any intervention. This results in plasma instabilities impairing the operating conditions and immediately extinguishing the burning process. The residual heat in the walls is not sufficient to impair components severely or even melt them. The power plant does not contain any other energy source that could destroy its containmentwhich therefore always remains intact.

It was then investigated how much tritium and activated material could be mobilised by the temperature rise and escape from the plant. Finallythe resulting radioactive exposure at the power plant perimeter was determined for the most adverse weather conditions. Models A and B have values well below one to two orders of magnitude of the dose necessitating evacuation of inhabitants in the vicinity of the power plant. This also applies to model C; the values for model D are much lower still. In this respectthe new study reinforces seemingly attractive safety properties that have been found in previous investigations.

In terms of wastethe material activated by fusion neutrons was found to lose its radioactivity relatively quickly in all four models. In 100 years it drops to a ten thousandth of its initial value. In model Bfor examplealmost half of the material is no longer radioactive 100 years after shutdown and can be passed for any other use. The other half could – with the advent of appropriate technology – be recycled and reused in new power plants. So permanent storage would not be necessary. And this also applies to the other three models.

The first large production order for the Wendelstein 7-X fusion experimentconstruction of the plasma chamberhas been successfully completed. 20 sectors of the bizarrely shaped 35-tonne vessel were assembled from several hundred individual components.

Installation of the whole complex devicewhich started in spring 2005 at the Greifswald branch of Max Planck Institute of Plasma Physics (IPP) in Germanywill take about six years.

Wendelstein 7-Xwhich on completion will be the biggest fusion device of the stellarator type in the worldis aimed at investigating the suitability of this concept for application in power plants.

MAN DWEbased in DeggendorfGermanymanufactured the plasma vessel in 20 sections. The assembled ring-shaped plasma chamber with a diameter of about 12 metres will eventually be used to confine the heated plasma. The shape of the vessel is designed to match the twisting of a plasma ring and posed a major production challenge for MAN DWE. For exampletolerances of less than three millimetres were demanded in some places.

Production was accompanied from the outset by three-dimensional measurement with a laser trackerensuring that the prescribed shape had indeed been exactly attained.

Eventually the plasma vessel will be enclosed inside a wreath of 70 super-conducting magnet coils that generate the magnetic cage keeping the plasma suspended away from the interior walls of the plasma vessel.

Installation should be complete in 2011. 

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