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Saturday, June 9, 2012

Nuclear fusion

Science and techno world topic: Physics

Nuclear fusion in stars

The "nuclear fusion" also called "fusion" is the union of two light atomic nuclei to form a single heavier and more stable. During this fusion reaction, the mass of the core product is less than the sum of the masses of the light nuclei of origin. However, under the famous relationship established by Albert Einstein "E = mc2", the difference in mass is converted into energy. One can observe this particular fusion in stars in which a colossal amount of energy is released. The phenomenon of nuclear fusion is therefore different from that of nuclear fission in which a heavy atom splits into two lighter atoms with an energy release much lower.

The process of nuclear fusion can take place only under conditions of specific temperature and pressure. For example, in the heart of the sun the pressure is 200 billion times atmospheric pressure and temperature terrestrial plant reaches about 15 million degrees. Under these conditions the light nuclei of hydrogen (75% of the composition of the Sun) merged into helium nuclei (24%) approximately twice as heavy, creating light and heat we receive. According to calculations, 620 million tons of hydrogen are converted into 615.7 million tons of helium every second.

Diagnostic checks inside the experimental chamber of the prototype facility of the future Laser Megajoule © P.Stroppa / CEA

Nuclear fusion on Earth
Very large amounts of energy are released by the nuclear fusion process. Able to reproduce this phenomenon on Earth would theoretically satisfy the energy needs of permanently humanity. This is precisely the major issue of research on nuclear fusion "controlled". Fuels necessary for fusion are two isotopes of hydrogen, deuterium, available in virtually unlimited quantities in sea water, and tritium that is produced from lithium relatively abundant in the crust.
The thermonuclear bomb - commonly called H-bomb - is now the only practical application of nuclear fusion. This was tested for the first time in 1952 in the United States in the wake of the mastery of the A-bomb (nuclear fission). Thermonuclear weapons have played a key role in the deterrent balance between the two blocs during the Cold War.
Research efforts are conducted for over 50 years to recreate the conditions of nuclear fusion in a reactor. However, mastering a controlled melting process is not yet demonstrated and the technologies and materials adapted to these extreme temperatures and pressures are not yet available for industrial use. Recreate a nuclear fusion process is much more complex than exploiting the fission chain reaction.

Issues of domestication of nuclear fusion

The advantages of this process
If the innovative principle of nuclear fusion power plants is validated scientifically and technologically it will develop a new rich source of additional energy from nuclear fission.

The environmental benefits 
The fusion generates some radioactive waste in addition to short-lived and no greenhouse gas emissions. In addition, it eliminates any risk of a runaway nuclear reaction and therefore any threat of explosion. Contrary to the process of nuclear fission, the slightest disturbance in a fusion reactor magnetic confinement would lead a cooling then a spontaneous cessation of fusion reactions.

The economic benefits
Nuclear fusion involves fuel (deuterium or lithium) present in large amounts on our planet, enough to supply any fusion reactors for many millennia. The risks of energy shortage would therefore be discarded. Few grams of fuel is sufficient to trigger and maintain the fusion reactions. A fusion power plant of 1000 MWe and would need 125 kg of deuterium and 3 tons of lithium (against 2.7 million tons of coal for thermal power plant of the same power) to operate a full year.

The limits

Technological limits
The current state of scientific knowledge today does not extract enough energy from fusion reactions to produce electricity. Moreover, it is not known yet produce materials that can resist long enough to radiation and neutron flux released during these reactions. Scientists estimate that the technologies needed to implement controlled nuclear fusion for energy production will not be available for many decades.

The financial limits
The financial cost of research facilities is worth billions of euros over several decades. This cost is most important for the potential benefits far apart in time. The investment in the ITER program for example, was initially measured at least 10 billion euro. The latest estimates of 2010, the estimated cost of construction of the machine now will be around 15 billion euro. Moreover, production cost of fusion energy remains an unknown until the process has not reached a mature science and technology.

Technical or scientific operation

Merge atoms on Earth is not simple. Must be melted two atoms and cause the fusion of their nuclei while their electrical charges tend to separate them. Why nuclei should be in a state of intense thermal motion. This is the case when heated to very high temperatures of the order of hundreds of millions of degrees.

Fusion Deuterium-Tritium

For thirty years, almost all research focuses on the fusion of two hydrogen isotopes: deuterium and tritium. The first exists in nature (present in seawater up to 33g / m 3) and the second can be produced within an industrial reactor fusion by interaction with lithium. The latter is present on Earth at 20 g per ton in the crust and 0.18 grams per cubic meter in the oceans. The temperature required for fusion of these two isotopes is about 150 million degrees, ten times the temperature of the heart of the Sun. This forms a plasma, the fourth state of matter in which atoms are fully ionized that is to say that their nuclei and electrons are no longer linked. By meeting of light atomic nuclei to form a single heavier and more stable then takes place. The neutrons released during this reaction irradiate the reactor vessel that stores thermal energy.

There are two ways of development of fusion reactors:

The inertial confinement reactor: the deuterium-tritium mixture is enclosed in microspheres. They are brought to very high pressure and temperature for an extremely short time using powerful lasers. The obtained micro thermonuclear explosion produces a pulse hyperpuissante agenda terawatt on a very short time, about 10 picoseconds.

The Laser Megajoule, under construction at CEA / CESTA, near Bordeaux, is one example. It aims to reproduce in the laboratory of physical conditions similar to those created during operation of nuclear weapons. This laser is primarily intended for scientific use and simulation of explosions of atomic weapons, as his fellow American, the NIF (National Ignition Facility) located in California.

The magnetic confinement reactor: the nuclei are brought to more than 100 million degrees in machines of a large volume called Tokomak. Since no material can withstand such temperatures, the plasma containing the mixture of low density deuterium and tritium is confined by a magnetic field generated by coils located around the room and by a strong electric current flowing in the plasma. The merger is initiated when the temperature on one hand and the density, and time of thermal insulation of the mixture on the other hand reach critical thresholds of ignition.

There are several prototype tokomak in the world, including the installation Tore Supra at Cadarache. ITER, being built on this site belongs to the same family.

Both methods have already yielded brief fusion reactions. However, they currently require more energy than they create. Therefore the main axes of research in the decades ahead will focus on the extension and optimization of the fusion process.

Past and present

History of nuclear fusion: the discovery of physical principles to the development of the first reactors.
20 Years in the British Francis William Aston and Arthur Eddington discovered the phenomenon of nuclear fusion that takes place in the sun. In 1946 a patent was registered in the UK by Thomson and Blackman for a reactor. In 1952, the United States detonated the first hydrogen bomb, thus achieving the first human thermonuclear fusion reaction. This demonstrated the superiority of the energy of nuclear fusion process that of fission.

Laboratory, Russian scientists are the first to obtain conclusive results in the quest for control of fusion energy. In 1968, they manage to fuse hydrogen, producing a plasma of about ten million degrees in a fusion reactor called “Tokomak” . It is with this machine that the technique of magnetic confinement has been developed. The plasma is created by electric shock and strong magnetic fields can isolating the walls of the machine, preventing it from cooling.

In the 80s, many countries are embarking on the construction of tokomaks. In Europe, under the aegis of the Association Euratom-CEA, Tore Supra was launched in 1988 on the center of the CEA / Cadarache, the only tokomak in the world with superconducting magnets capable of producing plasmas of long duration. As for the JET (Joint European Torus) in England, he holds the world record of fusion power (16 MW for a second) since 1997. The previous record had been obtained in the United States with TFTR, based in Princeton, which had produced a power of about 10 MW in 1991. Japan has also performed well with JT-60, similar to those of JET in the field of plasma physics.

ITER (International Thermonuclear Experimental Reactor) is currently under construction on the Cadarache site. It aims to demonstrate the technical feasibility of energy production from a nuclear fusion reactor magnetic confinement. It is considerably larger than its predecessors.

Units of measurement and key figures

Temperature: absolute zero to tens of millions of degrees

In a fusion reactor, the plasma temperature should reach 150 million degrees Celsius. Simultaneously, the temperature of the superconducting magnet coil creating the magnetic field and located a few meters from the plasma must remain as close to absolute zero, that is to say, the lowest temperature that can exist in the universe (- 273.15 degrees Celsius).

Available resources of Deuterium and Tritium
With 33 grams of deuterium per cubic meter of seawater resources exceed 10 billion years of annual world consumption of energy (reference year 2000). One gram of deuterium, tritium combined, could provide as much electricity as three tons of oil.
Tritium is a radioactive material to short lifetime (12 years). It is obtained from lithium. The average content of lithium in the earth's crust is about 20 parts per million (ppm). Lithium can also be derived from seawater (0.18 g / m 3) which represents a potential reserve of 230 billion tones.
Zone of presence or application

Main fusion reactors with magnetic confinement in a tokomak
ITER is being built in Cadarache. 6 countries (USA, China, India, South Korea, Japan, Russia) and the European Union contribute to this innovative project in the global framework of the ITER Organization.
KSTAR (Korean Superconducting Tokomak Advanced Research) in Daejeon.
Joint European Torus (JET) at Culham in the UK.
JT 60 in Japan.
Tore Supra at Cadarache in France.
Asdex and improvement Asdex-Upgrade, Germany.
Doublet and its improvement DIII-D, United States.
Main reactors inertial confinement fusion laser
The Center for Scientific and Technical Studies of Aquitaine is home Megajoule programs under the direction of Atomic Energy Commission and the PETAL laser in the framework of the European HiPER.
The proposed National Ignition Facility in the United States.

The various steps specified by the Commissariat a l'Energie Atomique (CEA) for the next century as part of the magnetic confinement fusion.

2020: Implementation of ITER operation to demonstrate the scientific and technological feasibility of fusion energy.
The reactor is currently under construction. This should take about ten years. As research reactor, ITER not produce electricity, only heat and steam.
Beyond 2050: First demonstration reactor DEMO
Research and optimization of energy efficiency and materials used; this demonstrator will generate several hundred megawatts.
Then: Test the industrial prototype.

The power of this prototype should be around 1,500 megawatts or as much as the EPR fission.
Finally: Deployment of industrial reactors
Large-scale production of energy from nuclear fusion.
Did you know?

Plasma is considered the fourth state of matter after the liquid, solid and gaseous. When you arrive at very high temperatures, the components of the atom separate, nuclei and electrons move independently and form a mixture broadly neutral: it is a plasma. This fourth state of matter found in stars and the interstellar medium, constitutes the majority of our universe (around 99%).On Earth, we do not meet except in the lightning or the Aurora Borealis. However, it is produced artificially by applying electric fields strong enough to separate the core of its electrons in gases. Application Examples: flat screen TVs or illuminating neon tubes.

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