Fusion Energy: The Ecstacy, The Agony and the Alternatives

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To begin with, let me ask you a question. How is man different from animals?ITER1

Well, I would say that man is unique because man is the only animal that uses fire. Although other animals are afraid of fire, man uses it; with fire he guards himself from the attack of wild animals, obtains warmth and cooks his food. We express this more generally by saying that man is the only animal to use energy constructively in the normal course of his way of life. So, if we say man is what he is because he uses fire, what we are really saying is that man depends on free energy. He requires extra energy which he can manipulate to perform tasks for him and grant him extra power. Energy is what has made man the king of all life on earth. Even among nations, the most powerful nations are those who are the largest consumers of energy.

Thermodynamically speaking, energy is a source of negative entropy. Energy sources supply human societies with heat fluxes of higher temperature. These energy fluxes are then thrown away, being released to the environment at low temperatures. In the process, however, human society increased negative entropy. Up to 1850, energy consumption was very low, and human society was almost closed with respect to energy. If a system is closed, it has maximum entropy and the distribution of energy is Maxwellian. Before the industrial revolution, wealth and power were distributed among the people according to such a Maxwellian distribution: the lives of the few who were rich and aristocratic were supported by their many poor retainers. After the industrial revolution, as energy consumption gradually increased, the difference in income between the rich and poor decreased. In modern economically advanced countries, it now approaches equality. The energy consumption of a country may now be considered directly proportional to its gross national product and to its average standard of living.

Thus as more and more undeveloped countries become developing countries, and more and more developing countries join the developed countries, the demand for energy will drastically increase.

The fossil fuels are non-renewable and will be over by the end of this century if not earlier. Renewable sources like sunlight, wind and rivers are not enough. Even nuclear fission is not a solution to our problem. If we use breeder reactors, we can make our fission fuel last a few more centuries. But then where can we hide the deadly radioactive wastes if the entire earth is going to depend on fission for energy? Add to that the danger of an accident. No densely populated country can withstand another Chernobyl. In fact, Germany has shut down almost all its nuclear power plants due to these dangers.

With this background , I’m sure you will be able to appreciate the potential and importance of nuclear fusion energy.

Fusion – the release of nuclear binding energy from light nuclei and its practical exploitation has been a major world research discipline for the past six decades. It promises to be an energy resource capable of indefinitely sustaining humanity under all conceivable scenarios of population growth and energy demand. Deuterium is present in all natural waters, and is cheap to extract. Tritium can be produced by the neutron bombardment of lithium which is present in plenty in the earth’s crust and in sea water. In fact fusion is the only energy indigenous to earth that will last as long as our planet exists.

That’s the ecstasy, so what’s the agony? The problem is that although we have made enormous progress in our scientific understanding of fusion, we have, as yet, no clearly identified route to an attractive commercial fusion power plant which will sell in the energy market place of the 21st century and beyond.

So, what prevents us from harnessing this nuclear fusion energy? The problem is, that this thermonuclear reaction can take place only when a mixture of isotopes of hydrogen is heated to a temperature of hundreds of millions of degrees and subjected to a pressure of the order of thousands of millions of atmospheres. Such conditions, which exist continuously inside the Sun and the stars can be created on earth only for 2 or 3 millionth’s of a second inside the shell in which an atom bomb is exploded. So, using an atom bomb, it was fairly easy for the scientists to create the hydrogen bomb which is a destructive use of nuclear fusion energy. Thus, so long as nuclear fusion energy remains bound to the atomic bomb, it is doomed to be a ‘giant’ capable of splitting a whole mountain range but incapable of moving a spaceship even a millimeter without destroying it.

Our everyday experience, limited to the surface of a dense and not too hot planet and its immediate surroundings, has led to the belief that matter can exist only in three states – solid, liquid and gaseous. But when we consider the conditions required for nuclear fusion, we get a fourth state of matter called plasma, which is fully ionized gas, full of mysterious properties. Its discovery has led to a new branch of physics called Plasma Physics.

Plasma consists wholly or partly of charged particles that either repel or attract each other, and at the same time rush about with terrific velocity. Their kinetic energy makes plasma considerably hotter than any chemical flame.

Furthermore, plasma can have at least 3 different temperatures at the same time:
the temperature of fast electrons, which is highest;
the temperature of neutral, non-ionized atoms, which is the lowest; and
an intermediate temperature corresponding to the motion of the variously ionized atoms.

At a temperature of 100,000 oC, plasma is considered ‘super cold’. It is considered cold at a temperature of million oC. Above 100 million oC it is considered ‘hot’ and above 5000 million oC it is considered ‘super hot’. Now, a successful fusion reactor has three basic conditions to meet:

  1. The plasma temperature must be high so that an adequate number of the ions have the speeds needed to come close enough together to react despite their mutual repulsion. The minimum temperature for  igniting a D-T plasma is about 100 million K.
  2. The plasma density n (ions/m3) must be high to ensure that collisions between nuclei are frequent.
  3. The plasma of reacting nuclei must remain together for a sufficiently long time τ. How long depends on the confinement quality parameter.

This requirement is known as the Lawson criterion.

Even if we find some ways to create such temperatures in the plasma, the nuclei and the electrons will continuously bombard the walls of the vessel containing the plasma, transferring to them all the heat being formed in it. And the toughest material on earth will not withstand a temperature above 4000 oC. Further, the pressure of the plasma would rise with its temperature; and even at 100,000 oC, the pressure would amount to over 1 million atmospheres. Again, no material on earth can withstand such pressures. The situation seems utterly hopeless.

Yet, in 1950, Sakharov and Igor Tamm of the Soviet Union and Lyman Spitzer of U.S.A independently struck upon the wonderful idea of confining and taming the plasma by using magnetic fields. In the presence of magnetic fields, charged particles follow a spiral trajectory along the field lines. Clearly, one way to isolate the fuel while it reacts would be to close these lines onto themselves in a dough-nut shaped or toroidal chamber. In the absence of collisions, the particles would be confined in this torus.

Unfortunately, in this case, nature does not favor simplicity. Many problems kept arising. But fortunately, their solutions too arrived, and this scheme has been used after some modifications by the magnetic confinement devices like the tokamak and the stellarator. However the two geometries differ in the way in which the poloidal field is produced. Tokamaks create the main confinement (toroidal) fields using coils located around the torus. They rely on the transformer effect to induce a toroidal current in the plasma that in turn, creates the poloidal field and also heats the plasma. Stellarators, on the other hand rely on external coils to create all of the fields. Both concepts have advantages and disadvantages that are mainly linked to the different ways of producing the poloidal field and heating of the plasma.

The most powerful tokamaks today have attained plasma temperatures of 30 KeV and confinement quality n τ  values of 2 x 1019 sm-3. In 1993, the Tokamak Fusion Test Reactor in Princeton,produced a record 6.2 MW of fusion power for 4 s with a D-T plasma. The input power was 28 MW. Breakeven and ignition will probably have to wait for the planned International Thermonuclear Experimental Reactor (ITER). It represents the final step before practical fusion power stations become a reality. Sponsored by the U.S., the European Community, the Commonwealth of Independent States and Japan, ITER is expected to cost about $7.5 billion and to generate 1GW from D-T reactions. The tokomak torus will be around 12m in diameter and have an elliptical cross-section 8.4m high. Superconducting magnets will confine the plasma with fields as high as 11 T, and the plasma current should reach 25 million A. About 80% of the  energy  released in the D-T reactions is carried off by the neutrons they produce, and these neutrons will be absorbed in ITER by lithium pellets inside stainless-steel tubes that surround the torus. Circulating water will carry away the resulting heat from the tubes, and the tritium that is formed will be flushed from the tubes by streams of helium gas.

However, it is not clear that the conventional tokamak approach will lead to a practicable commercial power plant that anyone will be interested in buying. This is a consequence of its projected low power density, high capital cost, high complexity and expensive development path. The same applies for other magnetic confinement devices like the stellarator, though the spherical torus, the spheromakand the field reversed configuration could lead to much cheaper, more compact fusion power core.

But our main alternative for achieving a commercial fusion reactor is Inertial Fusion Energy (IFE). Inertial Confinement uses energetic beams to both heat and compress tiny D-T pellets by blasting them from all sides. The result is, in effect, a miniature Hydrogen bomb explosion, and a succession of them could provide a steady stream of energy. It ten 0.1 mg pellets are ignited every second, the average thermal output would be 1 GW and could yield 300 MW or so of electric power, enough for a city of 175,000 people.

Laser beams have received the most attention for inertial confinement, but electron and proton beams have promise as well. The beam energy is absorbed in the outer layer of the fuel pellet, which blows off outward. Conservation of momentum leads to an inward shock wave that must squeeze the rest of the pellet to about 104 times it original density to heat the fuel sufficiently to start fusion reactions. The required beam energy is well beyond the capacity of today’s lasers, thought perhaps not of future ones. Particle beams are closer to reaching the needed energy but are much harder to focus on the tiny fuel pellets.

Another alternative method for achieving nuclear fusion, but without the need for monstrous temperatures and pressures currently being researched, is by using a Mesonic atom. In a Mesonic atom, a negative µ meson orbits round the nucleus (proton) instead of an electron. On encountering an atom of deuterium, it forms a mesonic molecule which results in the two nuclei fusing to form a helium nucleus releasing 5.4 MeV of energy.

To conclude, I would like to mention that from the ancient Greek legend of Icarus, we know that for millennia man yearned to fly before actually discovering the means to do so. Nature had provided the tantalizing example of the bird and left it to human ambition and intelligence to eventually find the solution. That mankind would some day fly was certain.

During human history, he has also admired and depended upon number of nature’s marvels; limitless energy in forms that include life-sustaining light and heat. Only in this century, however, has man deciphered how the Sun works. With that understanding was born the ambition to make a Sun. The tantalizing example of endless energy greets him each morning. That man will some day create his own Sun on earth seems certain.

The middle of this century will probably see the start of a new era in energy supply for the world.

References

1.      Physics World : Nov 1997, Mar 1996, Oct 1996.

2.      The Powerhouse of the Atom by K.Gladkov(chapters – 16, 17)

3.      Fusion, The Search for Endless Energy by Robin Herman

4.      Concepts of Modern Physics, 5th edition by Arthur Beiser (chapter 12 – 12.11, 12.12)

5.      Sourcebook on Atomic Energy by Samuel Glasstone(chapter 14 – pages 536 to 560)

6.      Nuclear Fusion by Keishiro Niu

7.      www.iter.org for details and picture of ITER

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