Dr.
John K.C. Leung
Department of Physics, The University of Hong Kong
The Einstein's mass-energy relation ()
is marvelous in that it tells us energy can be created if
we can destroy mass. But be careful! We are not talking about
production of energy by burning coal or gasoline, both of
which do not lead to reduction of the overall mass. Change
in mass only occurs when one heavy nucleus splits into two
(nuclear fission) or two light nuclei merge into one (nuclear
fusion). This is because the energy that holds the nucleons
(protons and neutrons) together, the binding energy, varies
from one element to another. This is quite easy to understand
if you remember that protons are positively charged and therefore
they repel each other strongly inside a nucleus, hence a heavier
nucleus requires a larger binding energy. So, where comes
this energy? It is found that there is a difference between
the mass of the nucleus and the sum of the individual masses
of the nucleons. For example,is
the element that is used to produce fission energy in a nuclear
power plant. It contains 92 protons and 143 neutrons. If
and
are the masses of free and unbounded protons and neutrons
respectively, then the mass of the nucleus
is less than 92
+ 143 .
The difference is called "mass
deficit", which is converted into the binding
energy according to Einstein's mass-energy relation.
In a fission process, a
nucleus, after absorbing a slowly moving neutron (called a
thermal neutron),
will split into 2 lighter and more stable nuclei (which are
still radioactive!). Any 2 lighter nuclei can be produced
as long as the total number of protons is 92 and the total
number of neutrons is 144 (143 + 1). An example of a fission
reaction is shown below where 3 high energy neutrons are emitted.
The 3 neutrons almost carry all the released energy Q because
of their small masses compared to the fission products. The
energy of the neutrons will be utilized to heat up water to
produce steam for conventional electricity generation. The
slowed down neutrons can be absorbed by another
to release energy again. This repetitive process is called
a "chain reaction".
Similarly in a fusion process, if the resultant nucleus is
more stable than the two fusing nuclei, the surplus energy
will be released. Figure 1 illustrates this point. You may
wonder how much energy can be produced by a single nuclear
reaction. Try to guess how many fusion reactions are required
to bring a cup of tea to boil. I bet you can't guess it. It's
talking about hundred million billion fusion reactions.
(Figure 1. Release of energy through
nuclear fusion and fission (from ITER web page))
Artificial nuclear reactors nowadays (e.g. nuclear power
plants) utilize nuclear fission to produce energy, while natural
nuclear reactors nowadays (e.g. the sun) utilize nuclear fusion
to produce energy. So, can we make a sun? In a star, the huge
mass of hydrogen is held together by their own gravitational
force so that a chain of fusion reactions can occur which
converts hydrogen to helium . The reaction rate is extremely
slow, but it nevertheless drives the universe due to the star
size and huge mass. Attempts to produce fusion reactions on
Earth started about 40 years ago and experimental fusion reactors
around the world commenced operations since the 1980s. In
the United Kingdom, the Joint European Torus (JET) commenced
operation in 1983. The JET is a Joint Undertaking of the European
Fusion Programme coordinated by the European Atomic Energy
Community (EURATOM). In the JET, deuterium (D or
)
and tritium (T or ),
both are isotopes of hydrogen, are fused to produce helium
()
and a neutron with a release of energy as shown in Figure
2. This reaction occurs only at temperature around 100 million
K, several times hotter than the sun.
(huge mass of hydrogen is held together
by their own gravitational force so that a chain of fusion
reactions can occur which converts hydrogen to helium)
(deuterium (D or )
and tritium (T or ),
both are isotopes of hydrogen, are fused to produce helium
()
and a neutron with a release of energy )
(Figure 2. Deuterium and tritium fusion
reaction (from ITER web page))
Most of the released energy is carried away by the high-speed
neutrons. In a fusion reactor, a jacket or blanket around
the reactor region would slow down the neutrons, converting
their energy into heat. This heat could then be extracted
to raise steam for conventional electricity generation. Figure
3 shows a schematic of a fusion reactor.
(Figure 3. Schematic
of a fusion reactor)
D ()
is an isotope of hydrogen and is a common and readily separated
component of water, therefore its supply is virtually inexhaustible
(
is normal water while
is called heavy water). In contrast, T (),
another isotope of hydrogen, does not occur naturally in any
significant quantities and must be manufactured through the
following breeding reactions by utilizing the neutrons generated
in the fusion reaction.
Therefore the fuels for a fusion reactor are D and
Li(lithium). It is noted that the reserves of lithium
on Earth is sufficient to produce electricity through fusion
reactions for the world's need for several thousand years.
Ultimately, it is hoped that the conditions would be reached
to enable a reactor to be built utilizing the reactions below.
In this case there would be no need to manufacture tritium
and a virtually inexhaustible reserve of energy would become
available! Isn't that great?
Let's first look at some fundamental problems.
Do you know it is not difficult to attain the temperature
of the sun? When a nucleus is accelerated to 1 keV, its temperature
is approximately equal to 10 million ,
almost the temperature of the sun! Achieving that temperature
is not difficult, but maintaining it is very difficult (think
of the reasons before you go on). Why is the temperature of
the nuclei related to its kinetic energy? You may think of
heating a solid material as an example. In the solid state,
the atoms or molecules are tightly bounded and they have little
kinetic energy. When it is heated, the temperature rises as
well as the kinetic energy of the atoms and molecules. At
sufficiently high temperature, the solid melts. The atoms
or molecules are now so energetic that they become loosely
bounded and are free to flow in the liquid state. When the
liquid is further heated, it boils and becomes a gas where
the atoms and molecules are free to move around at high speed.
Therefore temperature is actually a measure of the kinetic
energy of the atoms or molecules. If by some means, energy
can be given to an atom to raise its temperature to million
,
the orbital electrons would be completely separated from the
nucleus, forming an ionized gas, or "plasma"
(freely moving nuclei and detached orbital electrons). At
such high kinetic energy, two nuclei can overcome the repulsive
coulombic force between them and fuse together.
(The JET machine during the 1985 construction
phase
Photograph Courtesy: EFDA-JET)
The fact that plasma is a mixture of charged particles means
it can be controlled and influenced by magnetic fields (try
to recall the motion of a charge under the influence of a
magnetic field). With a suitably shaped field it should be
possible to confine the plasma with a high enough density
and a sufficiently long time to obtain net nuclear fusion.
The configuration that has so far advanced furthest towards
achieving reactor conditions and on which most data is available
is the tokamak, originally developed in the USSR. The whole
idea of the tokamak is to keep the hot plasma revolving at
high speed inside the torus so that it will not cool down
by touching the surrounding and therefore allows fusion reactions
to occur. But up to now, JET can only maintain fusion reactions
that last for a few seconds at most. A new and larger tokamak
is therefore required to continue the fusion research, and
ITER was put forward.
ITER means "the way" in Latin and is the next major
step for the development of fusion. It is an international
collaboration between Canada, Europe, Japan and Russia and
it will be the first fusion device to produce thermal energy
at the level of an electricity-producing power station. To
meet its objectives, ITER will be more than twice the diameter
of the largest existing tokamak. The Agreement for the joint
development was signed in July 1992. However the site for
building the ITER has not yet been determined and the four
competing sites are Cadarache in France; Clarington in Canada;
Rokkasho in Japan and Vandellos in Spain.
There were a number of major nuclear accidents in the past
that resulted in contamination to the environment. The latest
and the most severe one was the core meltdown in the Chernobyl
nuclear power plant on April 1986. In a nuclear fission reactor,
the chain reactions, once started, require external devices
to stop them. This makes the fission reaction inherently unstable
and accidents could occur. On the contrary, nuclear fusion
reactions could only occur under specially conditioned environment
and once the environment breaks down, the plasma will cool
rapidly and fusion reactions will stop immediately. So the
fusion reaction is inherently stable and safe.
However, radioactive wastes are also generated from fusion
reactors. Radioactive materials arising during operation and
remaining after final shutdown include activated materials
(materials that have been turned into radioactive after interaction
with the fusion neutrons) and contaminated materials, activated
corrosion products, tritium, and their mixtures. Due to decay
and decontamination, about 80% of the estimated 30,000 tons
of radioactive materials that will be produced at shutdown,
can be cleared within 100 years. The rest will require long-term
disposal in repositories. From the biological radiotoxicity
standpoint, which takes account of the different biological
damage potential of various waste materials and radioactivity
levels, ITER is increasingly less dangerous after 100 years
than the total ash from a large coal-fired power plant.
)
is marvelous in that it tells us energy can be created if
we can destroy mass. But be careful! We are not talking about
production of energy by burning coal or gasoline, both of
which do not lead to reduction of the overall mass. Change
in mass only occurs when one heavy nucleus splits into two
(nuclear fission) or two light nuclei merge into one (nuclear
fusion). This is because the energy that holds the nucleons
(protons and neutrons) together, the binding energy, varies
from one element to another. This is quite easy to understand
if you remember that protons are positively charged and therefore
they repel each other strongly inside a nucleus, hence a heavier
nucleus requires a larger binding energy. So, where comes
this energy? It is found that there is a difference between
the mass of the nucleus and the sum of the individual masses
of the nucleons. For example,
is
the element that is used to produce fission energy in a nuclear
power plant. It contains 92 protons and 143 neutrons. If 
and 
are the masses of free and unbounded protons and neutrons
respectively, then the mass of the 
)
and tritium (T or 
),
both are isotopes of hydrogen, are fused to produce helium
(
)
and a neutron with a release of energy as shown in Figure
2. This reaction occurs only at temperature around 100 million
K, several times hotter than the sun. 
is normal water while 
is called heavy water). In contrast, T (
,
almost the temperature of the sun! Achieving that temperature
is not difficult, but maintaining it is very difficult (think
of the reasons before you go on). Why is the temperature of
the nuclei related to its kinetic energy? You may think of
heating a solid material as an example. In the solid state,
the atoms or molecules are tightly bounded and they have little
kinetic energy. When it is heated, the temperature rises as
well as the kinetic energy of the atoms and molecules. At
sufficiently high temperature, the solid melts. The atoms
or molecules are now so energetic that they become loosely
bounded and are free to flow in the liquid state. When the
liquid is further heated, it boils and becomes a gas where
the atoms and molecules are free to move around at high speed.
Therefore temperature is actually a measure of the kinetic
energy of the atoms or molecules. If by some means, energy
can be given to an atom to raise its temperature to million
