The topic of nuclear fusion has been in news following the success of the Chinese Scientist at Hefei Institute of Physical Science, where they came a Step Closer To Creating An Artificial Sun.
The process in which two nuclei of light atoms (like that of hydrogen) combine to form a heavy, more stable nucleus (like that of helium), with the liberation of a large amount of energy is called nuclear fusion.
It takes huge amount of energy to put together two nucleus of even a lighter element like hydrogen, which can only be generated through intense heating, so much, so that the atoms turn into plasma state of matter and experience intense acceleration, required to overcome the large force of repulsion between two nucleus.
Heating the light atoms to an extremely high temperature carries out the nuclear fusion; the process is thus called thermo-nuclear reaction. There is some loss of mass during the fusion process, which produces a tremendous amount of energy.
The energy produced in nuclear fusion reaction is much more than that produced in a nuclear fission reaction, because the energy that holds a nucleus together is far greater than the one that holds electrons within the atom.
There are three difficult requirements for a sustained nuclear fusion reaction, and they must all be met simultaneously. Scientists must
- Heat a small quantity of fusion fuel to about 100 million °C,
- Contain and push the resulting plasma together long enough and at a high enough density for the fuel atoms to fuse,
- Recover enough net useful energy to make fusion profitable.
The two major approaches are magnetic confinement and inertial containment. The magnetic confinement approach is called tokamak approach, while the inertial confinement is called the laser bombardment approach. An experimental reactor working on laser bombardment approach in Princeton University has been named after the Hindu God, Shiva. However most of the recent researches have been based on the tokamak approach.
ENGINEERING PROBLEMS SO FAR ENCOUNTERED IN ITS DEVELOPMENT
- A critical temperature must be built and maintained and sustained.
- Even if the ignition and confinement problems are solved, scientists still face formidable problems in developing in a workable nuclear fusion reactor and plant.
- At the centre of the reactor the plasma may be 100 million °C, but only 2 meters away, around the magnets, the temperatures must be near absolute zero (—273°C) to be achieved by using liquid helium—a substance that may soon be scarce.
- The entire massive chamber must also be maintained at a near perfect vacuum.
- The inner wall of the reactor must resist constant baths of highly reactive liquid lithium (at 1,000°C) and steady bombardment by neutrons (which destroy most known materials) for 10 to 20 years. A wall of any known metal would have to be replaced every 2 to 10 years at such an enormous cost that fusion may never be economically feasible
- In addition, repairs would have to be made by automatic devices since no human worker could withstand the radiation.
- Fusion reactors, though much less dangerous than conventional or breeder reactors do have some potential radioactivity hazards. Worst would be the release of radioactive tritium (hydrogen-3), either as a gas or as tritiated water, which in turn could enter the human body through the skin, mouth, or nostrils.
- Tritium is extremely difficult to contain, because at high temperatures and high neutron densities it can diffuse through metals. The long-term disposal of worn-out radioactive metal parts from fusion reactors could also create a problem. Worn-out lithium blankets from fusion reactors would create ten times the volume of wastes created by fission plants.
In late 80s and early 90s, a new phenomena was reported called cold fusion
COLD FUSION OR HYPOTHETICAL FUSION
Two scientists, Martin Fleischmann of the University of Southampton, England, and B. Stanley Pons of the University of Utah, U.S.A., did a simple experiment by which they claimed to have found that it would be possible to duplicate in the laboratory what occurs in the sun at temperatures of millions of degrees. The reported results received wide media attention and raised hopes for a cheap and abundant source of energy. Many scientists tried to replicate the experiment with the few details available. Hopes fell with large no of negative replications, withdrawal of many positive replications, discovery of flaws and sources of experimental error in the original experiment, and finally the discovery that the scientists had not actually detected nuclear reaction by products.
THE INDIAN APPROACH
‘ADITYA’ is a medium-sized Tokamak conceived, designed and fabricated indigenously and it is commissioned and operational at the Institute of Plasma Research, Gandhinagar. The chief scientific objectives of ADITYA are:
- Investigation and control of edge phenomenon for improving confinement properties;
- Investigation of density and current limits of a Tokamak, with special emphasis on interesting phenomena, like MARFES, detached plasma, disruptive instabilities and their control and
- Study of novel regimes of operation. e.g. H mode obtained using bulk/localized heating by RF.
THE RECENT BREAKTHROUGH IN CHINA
Scientists at the Hefei Institute of Physical Science produced hydrogen gas more than three times hotter than the core of the Sun. They did this inside the Experimental Advanced Superconducting Tokamak (EAST) fusion device. The scientists managed to maintain the extremely high temperature – 50 million C°– for 102 seconds, a feat not achieved anywhere in the world yet.
The scientists were aiming for 100 million Kelvins for over 1,000 seconds (nearly 17 minutes).
MECHANISM OF THE NEW PROCESS
Inside the EAST device, a large metallic doughnut-shaped chamber, using magnetic confinement approach or in a tokamak reactor, hydrogen isotopes ‘deuerium’ and ‘tritium’ are collided at high speeds to produce helium. This produces a large amount of energy, akin to almost a medium-scale thermonuclear explosion. The heat produced inside the EAST device is 8,600 times that of the Earth’s core.
To keep the helium gas suspended in the fusion chamber the scientists created a magnetic field using superconducting coils fitted across the structure.
THE PROBLEMS CHINESE FACE
- While, theoretically the temperatures could be sustained for 102 seconds, it will take a while for the core of a device to sustain the high temperatures for a long period of time.
- The costs involved in building commercial power plants, which have a core that can sustain the extremely high temperatures, are very high, consequently, iy raises questions on its economic viability.
- The apparatus and temperatures needed to ensure high-speed collision of nuclei are not easy to put together. It is difficult to get the positively charged hydrogen isotopes to come close enough to collide.
- At times, it can also be difficult to contain the high amount of energy produced.
Many safety layers need to be put in place, so make it radiation protected, disposal of waste, etc.