IYSC
INVESTIGA I+D+i 2014/2015
Specific working guide on “Nuclear fusion”
Text by Mr Juan A. Jiménez
October, 2014
Introduction
Controlled Thermonuclear Fusion is one of the few energetic options
with enough supply potential on a large scale for the 21st century
and a far future.
Reasonable estimations predict that the world population will have
grown up to 10.000 million of people by the mid if this century.
In 1900, primary energy consumption per inhabitant and year in
industrial countries, was 2.2x1011 Joules, that is to say 5.1 t.e.p.
(equivalent tones of petrol) and 10 times less in developing countries.
Depending on the considered scenarios for the evolution of the
world's energy demand, consumption of primary energy may be
multiplied by 2 or 3 by 2050.
The sources of energy capable of meeting a substantial part of the
predicted energy demands are the following;
•
Fossil fuels: mainly coal, because petrol and natural gas will
have been diminish substantially
•
Nuclear Energy: fission and fusion
•
Renewable energy: hydraulic, solar, wind, tidal, geothermal,
biomass, etc.
Fossil fuels had environmental-polluting components like acid rain
and excess CO2. Renewable energies, although they have been
covering the increasing energy demand, are disperse sources and of
a low concentration for the industrial use. Nuclear plants have
additional problems like the storage of high-level radioactive waste. It
is necessary to develop new optional energies and pay special
attention on finding environmentally safe and economical energies.
Controlled nuclear fusion became one of these options, despite the
fact that there is still some work to be done regarding the
complicated technology needed for the devises used in nuclear fusion.
The challenge is to reproduce on Earth the reactions that take place
inside the stars. We aim to, therefore, build fusion reactors that are
capable of satisfying a substantial part of the world's energy demand
in the medium term and ensure its use for the future generations.
Nuclear Fusion
Back in the 20s of last century, scientists understood for the first
time, the true origin of the vast energy emitted by the Sun and since
then, humanity has dreamt to control that source or energy on Earth.
Last century, during the 50s, Man was able to produce the first fusion
reactions, although in an uncontrolled way, by the well-known
hydrogen bomb.
At the beginning of the studies of nuclear fusion for the production of
controlled energy (they also started in the 50s), it was predicted that
a fusion reactor could be possible in 20 years time but this prediction
was quite optimistic. Nowadays, knowledge of this source of energy is
broader, therefore we can estimate more accurately the time for a
successful development of a fusion process.
During the fusion reactions the nuclei of atoms join (fuse) releasing a
lot of energy. It is a different process from the nuclear reactions of
fission (taking place in the current in nuclear reactors). During a
fission process the nuclei of heavy atoms are divided in lighter atomic
nuclei. Fusion reactions in the Sun are mainly produced among
hydrogen isotopes (protium, deuterium and tritium), particularly
protium, the isotope of hydrogen with only one proton in its nucleus
and the most common type in nature.
In the centre of the Sun, temperature reaches 15 million grades and
density of matter is 150 times the density of liquid water. These
extreme conditions cannot be easily reproduced in lab conditions on
Earth. So far, it has been possible to reach temperatures above 150
million grades but with hydrogen densities lower than the air. Under
these conditions, the most efficient fusion reaction happens between
deuterium and tritium, producing helium (He) and one neutron.
Fusion Fuels: deuterium and tritium
We can state that the necessary fuels are varied. Deuterium is found
in sea and ocean water and its extraction is easy. However, tritium is
a radioactive hydrogen isotope with a disintegration period of 12,3
years. The product of its disintegration is helium (precisely 3He), the
helium isotope which is less frequent. This is why tritium is not
available in nature and it needs generating artificially. Fortunately,
high-energy neutrons that come from fusion reactions can be used to
bombard lithium in such a way that, by means of another nuclear
reaction, tritium is generated in the so-called "fertile blanket" of the
reactor. This fertile blanket consists of a layer that has lithium and
produces enough tritium to be used as a fuel. We can observe that
the products if this reaction are safe and the process is
environmental-friendly as it does not produce any polluting gases.
Plasma, the fourth state of matter
When matter is heated at a temperature capable of producing a
fusion reaction, matter is not in a liquid, solid or gas state but in its
fourth state. If we heat gas progressively, there is a moment that the
electrons of the atom surface gather enough energy so as to detach
their nucleus and the result is a bunch of electrically-charged
particles (electrons and ions). Most of the matter known in the
universe is in its plasma state. On Earth we can find plasma in nature
(northern lights (Aurora Borealis) and lightning) or generated by man
(fluorescent light, welding arcs ). In nuclear fusion the main focus is
on high-temperature plasmas. Due to the need to overcome
electrostatic repulsive forces, it is necessary to apply a lot of energy;
by heating it at high temperatures (thermonuclear fusion).
Plasma Confinement
The objective of research on nuclear fusion is to hold or "confine"
plasma in a "vase" (reactor) that can hold the high temperatures
needed for the reaction. Besides, there should be enough number of
nuclei to increase the possibility of collision (enough density of
particles).
If one could achieve the desired conditions of temperature and
density during a time which is long enough to facilitate the reaction,
we can say that we have achieved the desired confinement- this idea
is expressed by the Lawson criteria:
Methods for confinement
Gravitational
It is the only method that is proven to be a 100% efficient and the
starts are the evidence. In starts energy is produced by fusion
reactions achieving confinement times of thousands of years. A
gravitational force is capable of confining particles in such a restricted
space(even in big stars) so as to hold fusion reactions. However, this
method is impossible on Earth because we don't know of any
laboratory method to generate such a strong gravitational field.
Magnetic
It is the method that has been more efficient in the lab so far. It is a
method that confines the particles in a reduced space (magnetic
vessel) by the action of very powerful magnetic fields (50 thousand
times Earth's magnetic field). Plasma gets trapped in the magnetic
field (it has also been called magnetic trap) and this is possible
because the plasma particles are charged , which makes it possible
for the magnetic field to apply a force on them. In this method the
magnetic field is a substitute to the gravitational field mentioned
before. The magnetic confinement fusion devises that have been
more successful so far are those which have a doughnut or a toroidal
shape. The fact that a toroidal-shaped plasma should have no ends (
compared with that of a cylindrical shape) reduces the loss of
particles. The two main designs of experimental devises for the
studies on fusion plasmas through magnetic confinement (tokamak
and stellarator) are of a toroidal geometry. The main difference
between both designs is that in the tokamak, one part of the
magnetic fields is generated by an electrical current that goes
through the plasma whereas in the stellarator, all the magnetic field
is generated by currents in coils that are external to the plasma itself.
In the magnetic confinement, the confinement time depends, among
other factors, on the intensity of the magnetic field and the size of
the vase where the plasma is held.
In this confinement method, what is produced in first place is the
heating of a small capsule with the fuel (D-T) that is at a very low
temperature (to increase density). Heating can be produced by many
varied methods, for example, high-intensity laser light (step 1).
Sudden evaporation (ablation) of the superficial layer of the capsule
generates a crashing wave towards the outside and also inside of the
capsule (step 2). The crashing wave compresses the fuel confining it
to the smallest space possible (step 3), unchaining a fusion reaction.
Once the reaction has started, the fuel is allowed to explode and the
energy released is stored (step 4). In this case, the confinement time
is determined by the inertia of the matter on expanding after
compression; that is why it takes name "inertial". During fusion by
inertial confinement, due to its explosive nature, confinement time is
the least of the three methods.
Nuclear fusion: advantages and disadvantages
Fusion as a energy-generating method has important environmental
and safety advantages. Due to the fact that a fusion reaction is not a
a chained reaction it is not possible to loose control over it. In any
moment, the reaction can be stopped by closing the supply of fuel.
The matter for the fuel (deuterium and lithium), is widely available in
any part of our planet and there is plenty of matter to generate
energy for a million of years. Besides, a fusion process does not
provide gases that may contribute to the green house effect. The
reaction only produces helium, a safe gas, lighter than air, therefore
used to inflate balloons.
A safety aspect to especially consider in a fusion reactor is the
presence of one of the reactants: the tritium, a radioactive gas. Due
to the fact that it can be produced in situ, there is no need of
transportation of radioactive material from outside of the reactor. The
quantity of tritium needed in each moment is very little; therefore a
fusion electrical plant would contain a great deal of it.
Another aspect to consider is the activation of the materials that form
the inferior part of the fusion reactor. The materials exposed to the
bombarding of high-energy neutrons, coming from the reactions that
take place in the plasma, become radioactive (are activated) after a
certain time. This becomes a problem when the plant needs to be
dismantled. However, it is expected that the structural materials that
are currently being under research may have insignificant levels of
radioactivity after one hundred years after the shut down. This way
the fusion reactors will not be any load for future generations.
Nuclear Fusion Research experimental devises
The aim of international research in the filled of nuclear fusion is to
design a prototype of plant capable of generating fusion energy,
which meets the requirements of our society: safe, sustainable,
reliable, environmentally-friendly and economically viable. Since the
50s, there has been major progress regarding the scientific and
technical knowledge that is necessary in this field. For all these years
the performance of the fusion plasmas magnetically confined has
been multiplied by 10.000.
The JET experiment (Joint European Tours), near Oxford in the UK, is
right now the world's major experiment on fusion and belongs to the
European Union. The JET is a unique installation capable of working
with the fuels of the future fusion plants, deuterium and tritium and
have the highest energy production generated by fusion reactions,
having reached a power of 16,1 MW in 1991.
The next devise that will represent a new qualitative stage is ITER
(International Thermonuclear Experimental Reactor), an experimental
tokamak- type reactor whose aim is to prove the technical and
scientific viability of a nuclear fusion reactor. It is expected that ITER
generates plasmas capable of producing a power of 500 MW, ten
times higher than the energy supplied. In ITER, there will also be
experiments to try components and technologies that are essential
for a future plant of industrial fusion.
ITER will start working at the beginning of next decade and its
construction cost a 15.000 million euros. The current members of the
ITER project, on a large scale, are the European Union, Japan, China,
Russia Federation, India, the US and South Korea. The European
Union leads this project.
The following step after ITER will be DEMI, a demonstration plant of
the production of electrical energy based on nuclear fusion.
In Spain, the only devise for the study of fusion plasmas takes place
in the CINEMAT (National Laboratory of Fusion). It is a stellaratortype machine with a magnetic field of a Tesla and can reach
temperatures of 25 million grades.
Likely topics for discussion
•
When do you think that fusion energy will be available to
produce electrical energy?
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What advantages and disadvantages does fusion have as a
source of energy compared to other energies?
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How would an electrical plant based on a fusion reactor work?
•
What other fusion reactions could be used in the future?
•
Why is it difficult to reduce the size of a fusion reactor?
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Are there any other sources of energy that can be used to
colonize the outer space?
Resources
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http://fusionsites.ciemat.es/
http://www-fusion.ciemat.es
www.iter.org
www.euro-fusion.org
http://phdcomics.com/comics.php?f=1716 What is Fusion?
http://fusedweb.llnl.gov/cpep/Translations.html