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Now let's begin by introducing the basic concept of a Fusion reactor.
This is intended to give you an insight into the working principles
and the main parameters of a reactor.
Here's a picture of the ITER reactor.
It will be the first ever reactor that intends to demonstrate
that the output power generated by the nuclear fusion
is at least ten times the input power.
That input power is needed to reach the conditions
that allow enough fusion reactions to take place.
This reactor, ITER, has a torus-shaped vacuum
vessel surrounded by magnetic field coils.
Inside the torus, the fuel gas is heated to about 150 million degrees,
the temperature needed to reach fusion.
This hot gas, which we call a plasma,
is coloured pink in this picture.
To understand why the machine looks like this,
some basic physics background is needed,
which we will introduce in the next few slides.
A few questions might pop-up when looking at ITER:
Why is the machine so big?
Couldn't we make it smaller and cheaper?
I already mentioned that we need a temperature of 150 million degrees.
Why this high temperature?
And can we even reach that?
How do we keep a fuel of this temperature in place?
Which fuel to use?
Well, here I can already answer that we'll use Deuterium and Tritium;
I'll explain in a moment why this is the easiest.
However, this choice immediately has a far-reaching consequence:
Tritium handling and the neutron radiation
resulting from the fusion process makes it a nuclear facility.
That's why nuclear safety culture has a high priority.
And finally: why don't we have a working reactor yet?
What are the main remaining physics and technology challenges?
Now, let's start to discuss some fusion basics,
to be able to find the answers to these questions.
Don't worry if you don't follow all the
technical detail in the next few slides,
because at the end I'll summarise the
main points that we need to remember.
The starting point in our discussion
will be to find out which fuel to use.
For this we first ask the question: what
is the condition for fusion to work?
Obviously, we need to get net energy out of the reactor.
The energy set free in a fusion process originates
from the release of the binding energy of the nuclei.
Inside the nucleus the strong nuclear
force keeps the particles together,
meaning it costs energy to break up
the nucleus into individual particles,
and vice versa, energy is released by bringing
these particles together: the binding energy.
The picture shown here shows how this binding
energy varies according to the mass of the nucleus.
It's clear that for light elements this binding energy
increases, whereas for heavy elements it decreases.
This is where nuclear fusion is different from nuclear fission.
In fission, energy is obtained by breaking up
heavy elements like uranium into smaller ones,
but with fusion we gain energy by
combining light elements into heavier ones.
As an example, this picture allows us to calculate
the energy released from the fusion reaction
between the two hydrogen isotopes, Deuterium and Tritium,
leading to the production of Helium and a neutron.
In the formula, deuterium is denoted by 2H and tritium
by 3H, because they are the isotopes 2 and 3 of hydrogen.
When we talk about energy in atoms and their
nuclei, we use a unit of energy called MeV.
It stands for a million electron volts.
Now, the binding energy of a helium nucleus, with
its two protons and two neutrons, is 29 MeV,
and that's 17.6 MeV higher than deuterium and tritium together.
So we've gained some energy, and this is the energy released.
This energy is typically 10 million times
more than the energy in a chemical reaction.
So, to get the same amount of energy, we
need about 10 million times less fuel.
By the way, the energy released can also be
calculated using Einstein's famous equation E=mc2,
which says that the change in energy is just proportional to
the change in mass between the fuel and the reaction products.
So the requirement for a working fusion reactor
is that the power produced should be bigger
than the power needed to reach the
condition for fusion, to heat the fuel.
The power produced is equal to the
number of fusion reactions per second
multiplied by the energy per reaction, which we have just calculated.
The next step is to find out how many
fusion reactions per second we can make.
The number of fusion reactions of two species of nuclei
is proportional to the number of particles
of both species, which we call N1 and N2.
The relative velocity between the particles
V and the so-called reaction cross-section,
represented by the Greek signal sigma.
This is a measure of the probability that fusion can take place.
The probability depends on the kinetic
energy (or the temperature) of the particles.
In the figure, this fusion cross-section
is shown for several different processes.
From this figure it is obvious that the probability of
fusion reactions between Deuterium (labelled D in the figure)
and Tritium (T) is at least an order of magnitude
larger than for any other fusion reaction.
So, we can conclude that: the fusion reaction
between Deuterium and Tritium is the easiest
(it has the highest probability and peaks at the
lowest energy, so needs the lowest temperature)
The products of Deuterium and Tritium
fusion are Helium and a neutron.
The consequence of this reaction is that
we have to deal with nuclear safety
because of the tritium handling and the neutron radiation.
It was clear in the previous picture of the cross-section that no fusion
will happen if the kinetic energy of the fusion particles is too low.
The reason for this is that the nuclei are
electrically positively charged and repel each other.
Only when they come very close, a distance of around
10-15 metres, the nuclear force dominates, and the particles will fuse.
Now, we can calculate what energy is needed to overcome this
electrical charge repulsion, which is due to the Coulomb force.
It turns out that it's best to do this
at a temperature of 150 million degrees,
since then sufficient particles have enough
kinetic energy to overcome this Coulomb barrier.
Now that we know which optimal temperature is needed, we can
try to estimate how much heating power we need to reach this.
Here we have to realise two issues:
Firstly, the heating power depends on the insulation of the system:
how well is the energy we put in confined and not lost from the system.
This is quantified by defining the energy confinement
time, for which we use the Greek symbol tau_E,
the ratio of the thermal energy of the
plasma to the power lost from the system.
In the steady state situation this power loss is
equal to the heating power applied to the plasma.
The larger this confinement time, the better we confine
the energy, and the less heating power is needed.
The second issue is that for a working reactor this applied
heating power has to be less than the fusion power produced.
For ITER, the goal is that this fusion power
is ten times larger than the heating power.
Now, let's summarise where we are:
we found that it's easiest to use the deuterium-tritium fusion reaction
since this has the largest reaction rate at the lowest temperature.
We need a fuel temperature of 150 million degrees, since
that is the optimised condition for fusion energy production
and to minimize the energy needed to heat the plasma.
Several questions are still left on our list.
The most crucial one at this stage is to
find a way to confine this hot plasma:
to make sure the particles and energy
will not be able to get lost from our system.
For this, a magnetic field has to be applied.
A plasma at 150 million degrees consists only of charged particles:
the bare nuclei of deuterium, tritium and the
reaction product helium, and the free electrons.
If charged particles move in a magnetic field a
force acts on the particles, called Lorentz force,
proportional to the charge q of the particle, the
velocity v perpendicular to the magnetic field direction
and the strength of the magnetic field B.
The direction of this force is oriented such that
it results in circular orbits of the
particles around the magnetic field direction.
If now the magnetic field is made into a torus
shape, by the grey coils in this picture,
then in principle the charged particles will be confined
in the reactor, circling around the magnetic field.
This principle of magnetic confinement is the crucial
ingredient for the reactor of the type
of ITER, called a tokamak reactor.
The energy confinement time, which we saw two slides ago,
is increased by many orders of magnitude
by applying this magnetic field.
And the stronger the magnetic field the better the confinement.
That's why for ITER we've chosen to have
the strongest magnetic fields possible,
which are created by superconducting coils and can
be as high as 15 Teslas at the inside of the machine.
Apart from the superconducting magnetic
field coils to generate the torus-shaped field
(the coils coloured grey in this picture),
several other magnetic field systems are needed
to keep the plasma in a stable configuration:
like the blue coils depicted here.
They have several functions: mainly to shape the
plasma and to drive an electric current in the plasma.
But basically, it all comes down to
designing the magnetic field structure
to optimize the confinement of the plasma and keep it stable.
All these coils are made from superconducting material
since this leads to the lowest power losses inside the coils,
it allows for continuous operation and it
can reach the high magnetic fields needed.
And the strength of the magnetic field is important for two reasons:
Firstly, since the magnetic pressure balances the plasma pressure:
the higher the magnetic field, the
higher the pressure of the plasma can be.
The plasma pressure is related to the temperature and to the density
(and so the total number of particles in the reactor).
Secondly, as we saw earlier, the larger the magnetic field,
the better is the confinement, represented
by the energy confinement time tau_E.
With the information we now have, we can begin to understand
why ITER and future fusion reactors are so big:
the fusion power produced depends on the volume of the reactor:
a larger volume at the same temperature and density
simply means that there are more particles
that will fuse, so more output power.
But the power needed to heat the fuel also
increases if the machine becomes bigger.
However, this applied power is to compensate
for the energy lost from the system
and this power is lost over the outer area.
Since the ratio of volume to area scales with
the size of the machine, the bigger the better.
That's why the presently operating Tokamak JET is
just not big enough to reach a net power output.
ITER will be the first reactor that is big enough for this milestone.
You may wonder how the heating of the fuel
to the required 150 million degrees is done.
Well, there are two main methods for this:
The first is by the injection of neutral
deuterium or tritium particles at high energy.
These particles have been accelerated to
very large kinetic energy (around 1 MeV),
which is a much larger energy than the plasma particles.
These particles need to be electrically
neutral before they enter the reactor,
so that they are not deflected by the magnetic field.
Once inside the reactor they will
collide with the other fuel particles
and in this way transfer their energy to the plasma.
In ITER we can deliver 33 MW of power to the plasma in this manner.
The second heating method is by the injection
of electromagnetic waves into the
reactor, rather like a microwave oven.
As we saw before, charged particles will
circle around the magnetic field lines.
Ions do this at a typical frequency of 50 MHz,
and electrons do it at about 170 GHz;
it depends only on the strength of the magnetic field.
By injecting radio and/or microwaves with the same frequency,
the charged particles will be accelerated through a resonance;
they'll gain more energy and so increase
the temperature of the plasma fuel.
The energy produced in the fusion reaction is released
in the form of kinetic energy of the two products:
80% of the energy is in the neutron and 20% in the helium nucleus.
Since the neutron is not electrically charged,
it is not influenced by the magnetic field.
It will not be confined in the reactor
but will end up in the reactor wall,
which we'll refer to as the 'blanket'.
This blanket is thick enough to stop the neutrons escaping
and, in the process, their kinetic energy is converted into heat.
By removing this heat with cooling water and heat exchanges,
the heat could be used to generate electricity
using conventional generator turbines.
That will be done in future fusion power plants,
but in ITER this final step of generating electricity will not be done.
The other fraction of the fusion power, the 20% in
the helium particles, follows a different route.
First, since the helium nuclei are
charged, they are confined in the plasma.
The kinetic energy of the helium nuclei
will be transferred to the plasma fuel
by collisions with the plasma particles.
In fact, this helps to keep the plasma at the required temperature.
However, in the end this power will also be
lost from the plasma by a diffusion process.
Now the magnetic field structure is made
so that the power lost from the plasma
ends up at the bottom, in a structure called the 'divertor'.
This is depicted by the yellow arrows in the picture shown here,
which is a cross-section of the inside of the ITER reactor.
The power load at the divertor, introduced in the previous slide,
poses one of the major challenges to a fusion reactor.
During normal operation, this load
will be around 10 MW per square metre.
This amount is already very close to, or even beyond,
the maximum load that known materials can withstand.
However, under certain conditions in a fusion
reactor the situation will be much worse:
regularly occurring instabilities in the plasma
will increase this power load for a short time
by a factor of about 100, yielding power
loads close to 1 GW per square metre.
Now the challenge is how to cope with this extreme interaction
between the hot plasma and the material of the divertor.
This divertor material -in ITER it's tungsten-
will be exposed to large erosion rates,
but must not melt under the intense power load,
and evaporation must be minimised to avoid
tungsten getting into the plasma and diluting it.
Well, a final solution to this problem is not yet available,
but the ITER experiment hopes to demonstrate that the
divertor can survive reactor operation for several years.
Most fusion physicists regard this as the
biggest challenge in present fusion research.
To finish this basic introduction, let's consider tritium.
One part of the fuel, deuterium, is in
almost unlimited abundance on Earth
(there's 33 mg in every litre of ordinary water).
But the other fuel ingredient, tritium, is almost absent.
The solution to this shortage of tritium is that
a fusion reactor should generate its own tritium.
This can be done if the blanket contains a quantity of lithium,
since the reaction between the neutron from the
fusion process, which ends up in the blanket anyhow,
and the lithium in the blanket will make a
tritium nucleus, as well as some more helium.
So, the overall fusion reaction is actually deuterium
plus lithium, resulting in 2 helium-4 nuclei.
So far, that's the theory.
Perhaps it's obvious that this fuel cycle
requires a very high efficiency, close to 100%.
This still has to be proven or demonstrated in ITER.
This introduction has now given you the basic answers to be
able to understand why the ITER reactor looks as it does,
why there is the need for tritium, the high
temperatures, the magnetic fields, and so on.
To link this knowledge to the aim of the course,
about the nuclear aspects and the radiation,
the following things are worth remembering:
The fuel consists of deuterium and tritium.
However, in a power plant the tritium
will mainly be bred inside the reactor.
The fusion reaction produces a large number of highly
energetic neutrons, which will have several consequences:
thick blanket is required to stop the neutrons,
structural properties of materials will be changed
and radioactivity is induced in the
materials upon bombardment by neutrons.
These nuclear aspects have a substantial impact on the
safety culture and work procedures, as we will discuss next.
Before we proceed, you may like to answer some short
questions, to check your understanding of what we have learnt.
Thank you for your attention.
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