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Science of Nuclear Fusion

posted Jun 7, 2012, 8:28 AM by Administrator NLA   [ updated Jun 7, 2012, 8:30 AM ]
Dr Jaiby Joseph Ajish*

Presently, nuclear reactors use nuclear fission to generate power. In a conventional nuclear reactor, high-energy neutrons split heavy atoms of uranium, yielding large amounts of energy. Another fundamental source of energy is nuclear fusion, where energy is obtained when two atoms join together to form one. In a fusion reactor, hydrogen atoms fuse together to form helium atoms, neutrons and vast amounts of energy. The final mass of the combination is lower than the sum of the mass of the individual nuclei. This difference in mass is converted into energy according to Einstein’s Mass-Energy equivalence relation. Nuclear fusion is the process that powers stars as well as hydrogen bombs. As far as energy release is concerned, fission reaction is favorable for heavier elements, which have greater number of neutrons and protons, and fusion for lighter elements. Fusion reactors would be a cleaner, safer, more efficient and more abundant source of power compared to fission reactors. Conceptually, harnessing energy from nuclear fusion in a reactor is simple and straight forward. But in actual practice, it has been extremely difficult for scientists to come up with a controllable, non-destructive way of doing it. To understand why, we need to look at the necessary conditions for nuclear fusion.

W­hen hydrogen atoms fuse, the nuclei must come together. However, the protons in each nucleus will tend to repel each other because they have the same charge (positive). Achieving fusion reaction requires special conditions in order to overcome this repulsion. The conditions that make fusion possible are 1) high temperature and 2) high pressure. The high temperature gives the hydrogen atoms enough energy to overcome the electrical repulsion between the protons. Fusion requires temperatures of about 100 million Kelvin, approximately six times hotter than the Sun's core. This high temperature is achieved by the Sun due to its large mass and the force of gravity compressing this mass in the core. To achieve fusion on earth, we must use energy from lasers and ion particles to achieve these temperatures. Pressure squeezes the atoms together. The hydrogen ions must be within 1×10-15 meters of each other to fuse. The Sun uses its mass and the force of gravity to squeeze hydrogen atoms together in its core. We must squeeze hydrogen atoms together by using intense magnetic fields, powerful lasers or ion beams.

There are several types of fusion reactions. Most involve the isotopes of hydrogen called deuterium and tritium. With current technology, we can only achieve the temperatures and pressures necessary to make deuterium-tritium fusion possible. Deuterium and tritium combine to form helium-4 atom and a neutron as given below. Most of the energy released is in the form of the high-energy neutron.

The heat from fusion will be passed to a heat exchanger to make steam for producing electricity. Deuterium-deuterium fusion requires higher temperatures that may be possible in the future. Ultimately, deuterium-deuterium fusion will be better because it is easier to extract deuterium than tritium. Also, deuterium is not radioactive, and deuterium-deuterium reactions will yield more energy.

The strategies for creating fusion reactors are largely dictated by the fact that the temperatures involved in nuclear fusion are far too high to be limited in any material container. Reactors for nuclear fusion are of two main varieties, 1) magnetic confinement reactors and 2) inertial confinement reactors. The magnetic confinement reactor confines the fusion plasma by means of magnetic fields, which keep it perpetually in looping paths, and they do not touch the walls of the container. This method uses magnetic and electric fields to heat and squeeze the hydrogen plasma. The ITER project in France is using this method. In the case of inertial confinement reactors, a very high energy density is placed into a small pellet of deuterium-tritium and it fuses in such a short time that it can’t move appreciably. Inertial confinement uses laser beams or ion beams to squeeze and heat the hydrogen plasma. Scientists are studying this experimental approach at the National Ignition Facility (NIF) of Lawrence Livermore Laboratory in the United States.

Nuclear fusion provides a clean source of energy for future generations with several advantages over current fission reactors. Deuterium can be readily extracted from seawater and excess tritium can be made in the fusion reactor itself from lithium, which is readily available in the Earth’s crust. Uranium for fission is rare, and it must be mined and then enriched for use in reactors. The amounts of fuel used for fusion are small compared to fission reactors. This is to avoid uncontrolled releases of energy. Most fusion reactors make less radiation than the natural background radiation we live with, in our daily lives. Fusion reactors will not produce high-level nuclear wastes like their fission counterparts, so disposal will be less of a problem.

*Dr Jaiby Joseph Ajish is a Knowledge Contributor to the Nuclear Law Association. Dr Joseph Ajish, is an Experimental Nuclear Physicist, and holds a Ph.D from Kent State University, Ohio, USA.