Energy modelled on the sun: GRS prepares accident analyses for fusion reactor

04.07.2019

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The sun is man's most important source of energy. In its interior, 564 million tons of hydrogen fuse each second to form helium. For this to happen, positively charged atomic nuclei must overcome their electrical repulsion and combine to form helium nuclei ("fusion"). In the sun, this happens thanks to a sufficiently high level of energy in the form of heat. In the process, an unimaginable output of approx. 3.81026 watts is released and radiated into space; this corresponds roughly to the thermal output of 1017 nuclear power plants. On average, around 1.4 kilowatts of this power arrive on Earth per square metre in the form of radiation (so-called solar constant).

Since the 1950s, researchers have been trying to translate the phenomenon of nuclear fusion into a technical concept for power generation (e.g. "Z-Pinch", "mirror machine", "stellarator" and "Tokamak") on Earth. There are currently a number of research facilities in operation or under construction. The best-known and largest international fusion facility in the world is called ITER (International Thermonuclear Experimental Reactor); it is currently under construction at Cadarache in France. In the 2030s already, ITER is expected to supply more energy than is needed for its operation. All these projects have in common that heavy hydrogen (deuterium) and superheavy hydrogen (tritium) are to be fused. The difference between the various technical concepts lies on the one hand in the way the fuel is confined in principle (inertial confinement, magnetic confinement) and on the other hand in how confinement is implemented (e.g. annular closure of the magnetic field lines or reflection at the ends of the machine).

DEMO project: Demonstrating the feasibility of power generation

GRS is currently involved in a possible successor project to ITER called DEMO (Demonstration power plant). DEMO is to contribute to testing different technologies and to prepare or qualify them for use in commercial plants. The closed tritium fuel cycle, remote maintenance and the safety of the plant are also part of the project. On behalf of the Karlsruhe Institute of Technology (KIT), GRS analyses the safety of the plant for DEMO by means of deterministic accident analyses. Experts estimate that DEMO will be capable of permanently feeding with an electrical output of 300 to 500 megawatts into the grid.

To date, there is no fusion power plant for electricity generation. This is mainly due to the fact that a fusion power plant places high demands on the technology at various levels. The materials used must withstand high temperatures, sometimes high pressure in the cooling medium, and an intensive radiation field (e.g. from neutrons). Furthermore, a large amount of energy has to be applied to heat up the plasma.

How do fusion facilities like ITER and DEMO work?

ITER and DEMO are fusion facilities of the Tokamak type (Russian for "toroidal chamber in a magnetic field coil"). With this concept, a strong magnetic field is generated in a vacuum vessel that resembles a ring-shaped closed cylinder - a so-called torus. A thin gas mixture of deuterium and tritium is filled into this chamber and then heated up until it becomes plasma that is 150 million degrees Celsius hot.

Plasma is often referred to as the fourth aggregate state alongside solid, liquid and gaseous. It is a conductive gas consisting of positively charged ions and free electrons. The so-called "Lorentz force" and the ring-shaped closing of magnetic field lines can effectively trap charged particles of a plasma. The confinement by the magnetic field ensures that the plasma does not touch the walls of the vacuum vessel. This would lead to an immediate collapse of the plasma and thus of the fusion and endanger the integrity of the walls.

During nuclear fusion in ITER and DEMO, the hydrogen isotopes deuterium and tritium fuse to form helium, additionally a free neutron is also produced. The energy that is released is carried by the helium nucleus and the neutron as kinetic energy. The charged helium nucleus is available to the plasma as a heater. The uncharged neutron is not trapped by the magnetic field. It leaves the plasma and loses its energy in the inner lining of the vacuum chamber - the so-called blanket - and is finally captured by atomic nuclei. This allows the energy of the neutron to be used as heat. On the other hand, the capture of neutrons in lithium can be used to produce new tritium.

The blanket has a special role here. On the one hand, it serves to generate and remove heat. This requires a coolant, e.g. helium. On the other hand, it is responsible for the breeding of the fuel component tritium. Since tritium does practically not occur in nature, this substance must be continuously recovered for nuclear fusion. Deuterium, on the other hand, is abundant in the world’s oceans.

By means of fusion, around 100 megawatt hours (MWh) of thermal energy could be won from just one gram of the deuterium-tritium mixture. This corresponds approximately to the calorific value of 12 tons of hard coal.

The ITER fusion facility is currently being built in France (Photo: ITER Organization)

GRS looks into the safety of fusion facilities

The task of GRS is to examine the safety of the plants for the DEMO project by carrying out accident analyses. In fusion reactors, different types of accidents can occur. Every major event will lead to a collapse of the plasma and thus to an immediate stop of the nuclear fusion process. During operation, however, radionuclides are formed by the released neutrons. The main safety objective for a nuclear fusion facility is therefore to confine radioactive materials inside the plant so that they will not end up in the environment.

Compared with nuclear power plants, the risk potential of fusion facilities is lower by many orders of magnitude. This is mainly due to the substantially lower radioactive inventory. The radioactive inventory consists mainly of tritium. Tritium belongs to the "soft" beta emitters and has a relatively manageable half-life of 12.3 years - compared to uranium or plutonium - from which a radiologically relevant activity ensues. Since tritium, being a hydrogen isotope, behaves chemically like hydrogen, it may enter the organism of humans and animals through food or water and thus pose a health hazard. Safety-related studies and measures to counter any possible incidents and accidents are therefore important in order to avoid negative radiological effects on man and the environment.

Accident analyses with the MELCOR code

GRS uses calculation codes for its accident analyses that can simulate thermal-hydraulic problems and the behaviour of radionuclides. In the current project, GRS is investigating the safety of the DEMO facility by using the simulation code MELCOR.

With the help of design data, input data records that model the facility are created for MELCOR. Due to its complexity, this model is limited to a few areas of the torus. Then the conditions of the accident scenarios are determined. After the simulation of the scenarios, these are analysed by means of various aspects, such as pressure and temperature build-up in the vacuum vessel or in the blanket, and then compared with the safety requirements for the facility. In the project, GRS is responsible for the entire modelling of a newly designed blanket in MELCOR.

First results of the one-year project are expected this year.

 

Sectional view of the ITER reactor as Tokamak (Image: ITER Organization)

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