Nuclear Fusion

 

Nuclear fusion is the process of releasing energy from matter which occurs in the sun, the stars and the hydrogen bomb. During fusion, atoms of light elements combine to form heavier elements. The binding energy per nucleon for light elements is less than that for elements of intermediate mass. Thus the fusion of two light nuclei results in a more stable nucleus, and a large amount of energy is liberated. Both fusion and fission are methods of releasing a large amount of energy. In fusion, light atoms are combined to give heavier elements; while in fission heavy radioactive atoms are split into atoms of intermediate mass.

The simplest nuclear fusion reaction involves the isotopes of hydrogen: deuterium ²H₁ and tritium ³H₁. A large amount of energy is required to overcome the repulsion between positively charged nuclei to get them close enough (1-2 fm) to react. One way of producing high energy particles is an accelerator. This is not appropriate in this case. The other way to produce high energy nuclear is to raise them to a very high temperature (roughly 10⁸ K). If deuterium and tritium are heated to a temperature of over a million degrees a gas plasma is produced (a plasma is a 4th state of matter, which is composed essentially of gaseous ions and a matrix of free electrons.). Since the atoms have been stripped of their electrons, collisions will occur between nuclei, allowing them to approach closely enough to experience each other's strong attraction, and a fusion reaction occurs. The mass lost in this reaction is converted into energy according to Einstein's equation

³H₁ + ²H₁ → ⁴He₂ +¹n₀ + energy

This fusion reaction has a relatively low ignition temperature, and produces a large amount of energy. Deuterium is available from natural sources, but tritium is difficult to obtain and is extremely expensive. Tritium could be generated in a fusion reactor by bombarding a blanket of lithium with neutrons:

¹n₀ + ⁶Li₃ → ³H₁ + ⁴He₂

¹n₀ + ⁷Li₃ → ³H₁ + ⁴He₂ +¹n₀

Similar reactions are carried out using only deuterium, but these reactions require a temperature of several million degrees. Several reactions could occur, of which the simplest are:

²H₁ + ²H₁ → ³H₁ + ¹H₁ + 4.0 MeV

²H₁ + ²H₁ → ³He₂ + ¹n₁ + 3.3 MeV

Fusion is in principle a thermal reaction not inherently different in its kindling from an ordinary fire. Unlike fission, it does not require a critical mass. Once ignited its extent depends on the amount of fuel available. However, for fusion to occur, extreme physical conditions must be achieved:

1. A very high temperature must be attained.

2. Sufficient plasma density is required.

3. The plasma must be confined for an adequate time to allow fusion to occur.

These and other ' hydrogen burning' processes occur at the centre of the sun. This provides the enormous amount of solar energy which is radiated to earth and the rest of the solar system.

Characteristic features of nuclear fusion

The nuclear binding energy card indicates that the stable nuclides are characterized by B̄ =8 -8.5 MeV. The heavier nuclides through fission move to table region why the lighter elements move to this table region through fusion. Among the lighter elements ⁴He₂ is probably the most striking one, as it is having a comparatively high B̄ value (≈ 7 MeV). For example ²H₁, the B̄ value is 1 MeV. This is why, such lighter nuclides having very low nuclear binding energies will have a tendency to undergo fusion to reach the station, ⁴He₂. This driving force can be well understood from the calculation of mass defect in forming the respective nuclides.

2He1 (i.e. deuteron): Isotopic mass = 2.01419 amu; mass of the constituents (i.e. one proton and one neutron) = 2.01653 amu.

Hence mass defect = (2.01653 – 2.01419) × 931 MeV = 2.0 MeV

Therefore B̄ = 1 MeV

⁴He₂ (i.e. helium) : Isotopic mass = 4.00298 amu. Mass of the constituent nucleons = 4.03328 amu.

Hence, mass defect = (4.03328 – 4.00298) × 931 MeV = 28 MeV; B̄ = 7 MeV.

To consider this process, i.e. fusion, only very light nuclides (e.g. ¹H, ²H) are important. When the nucleus proceed to fuse with each other, experience the Coulombic potential barrier (E_Coul) which is proportional to Z₁Z₂/(R₁+R₂). The height of the potential barrier depends on the product of the nuclear charges of the nuclides to be fused. Classically, to surmount the barrier, the required kinetic energy of the lighter nuclides can be attained at the temperature of the order of 10⁷-10^{8 o}C which prevails only in the interior parts of the stars. For the relatively heavier nuclides the potential barrier is still higher. Thus, it appears that to initiate the nuclear fusion, the temperature should be tremendously high (10^{7 o}C) which can only be attend on earth through nuclear fission.

This is why, the nuclear fusion reactions are referred to as the thermonuclear reactions. However, once the process is started, the energy liberated can sustain the process automatically. Thus, the process can be defined as:

The nuclear fusion is a process which occurs at tremendously high temperature through the combination or fusion of two or more light and nuclides (e.g. ¹H, ²H) to produce relatively heavier and more stable nuclides (e.g. ⁴He₂).

That temperature at which the hydrogen nuclides undergo fusion makes the hydrogen atoms completely ionized producing free electrons and positively charged nuclei. This state of matter is referred to as plasma. Example

Hydrogen and Cobalt Bomb

In the thermonuclear bomb known as the hydrogen bomb, the basic reaction involves the fusion of hydrogen nuclei to form helium nuclei. The required temperature for this purpose is created by the explosion of a fission bomb placed at the centre. Thus in the hydrogen bomb, there is a small core of ²³⁵U or ²³⁹Pu bomb to function as a detonator. This is surrounded with a mixture of deuterium and tritium. Through the explosion of the fission bomb at the centre, the fusion process is started and continued with the liberation of a tremendous amount of energy. That destructive power of this thermonuclear bomb is frightening to the whole mankind.

The probable reactions set in hydrogen bomb are given below.

In the hydrogen bomb, there is no critical size in contrast to the atom bomb. This is why, hydrogen bomb of any size can be prepared. The hydrogen bomb has been modified to the cobalt bomb which is more alarming to the mankind. In the cobalt Bomb, the hydrogen bomb is encapsulated within a thick sheath of cobalt. The flux of neutrons emitted from the fusion process produces the radioisotope ⁶⁰Co through the (n, γ) process. The intense radioactivity emanated from ⁶⁰Co is alarmingly lethal to the living kingdom, and the cobalt bomb covers a larger area of destruction compared to that by a hydrogen bomb.

Controlled fusion reactions

Many attempts have been made to build apparatus in which control fusion reactions will occur. So far none have given good results. The problem is how to handle very hot gas plasmas. If the plasma touches is solid (e.g. a steel vessel), the solid is vaporized and the plasma cools down rapidly. The two main methods of confinement are magnetic and inertial. It can be deflected by a magnetic field. Plasma can be content inside a doughnut shaped 'magnetic bottle'(the extremely high magnetic fields required are obtained with electromagnets using a superconducting nobium/ titanium alloy cooled in liquid Helium to about 4K). Inertial confinement involves the Rapid collapse of the fuel container to make the fuel so dense that the fusion reactions occur. Alternatively laser fusion can be used, when high powered, pulsed laser beams are used to heat and compress small 'pellets' of fuel.

It is just possible that fusion maybe achieved by some totally different technique without using plasma to attend the high energy conditions. There was great excitement in March 1989 when Fleischmann and Pon claimed to have achieved 'cold fusion' in the laboratory at the University of Utah, USA. They electrolyzed 99.5% enriched heavy water D₂O made conducting by dissolving in it some LiOD (D is ¹H₁). Heat appears to be generated and this was attributed to D-D fusion. In a similar experiment Jones at Brigham Young University claimed neutrons were released. Unfortunately they were wrong.

Another interesting technique which may show promise is to replace an electron in a D₂⁺ molecule by a negatively charged muon which weights 207 times as much as an electron. This should reduce the D-D spacing by a factor of 200 which should make fusion easier.

Nuclear fusion holds the promise of being and important future source of energy. World energy consumption is high, and fuel resources are finite and limited. Oil and natural gas reserves may well be exhausted in 50 years. Coal may last rather longer, perhaps 200 years. Uranium resources are finite and the use of nuclear power electricity generating stations will only delay and eventual energy shortfall. All these fuel sources pose environmental problems. Fossil fuels (oil, gas and coal) contribute to the greenhouse effect and acid rain. A long term energy replacement needs to be found. There is concerned over the safety of nuclear power stations, and even greater concern over the shortage of nuclear waste products. If fusion can be fully developed:

1) The fuel for fusion (hydrogen) is almost infinitely available.

2) The nuclear processes in fusion are inherently safer than those of fission.

3) Promises to have minimal pollution problems.

4) Difficulties with spent fuel rods and reaction by-products are far less then with fission.

Fusion is an advancing research program, but many breakthroughs are required. The severe and demanding conditions for control fusion in the laboratory have yet to be achieved. If a controlled fusion reactor can be built it will supply almost unlimited power.

Reference

1) Concise inorganic chemistry by J. D. Lee.

2) Inorganic Chemistry by James E. Huheey, Ellen A Keither, Richard L. Keither, Okhil K. Medhi.