18 Nuclear Fusion for Energy

Dr. Dhanya M.S M.S and Dr. Sandeep Kumar

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  1. Introduction

The world is getting warmer because of rising earth surface temperature mainly due to greenhouse gases emitted by fossil fuel burning.The nuclear energy is considered as clean energy as it is not emitting any greenhouse gases. Nuclear fusion is the nuclear reaction that produceshuge energy in sun. It is expected that fusion energy can play a significant role in mitigating greenhouse gases in decades to come by its use in energy production. The nuclear fusion and fission were considered as potential processes from the mid of 20th century as energy sources. The International Atomic Energy Agency (IAEA) has been the leader in nuclear fusion research since the 1950s and focused on facilitating the coordination of international fusion undertakings and enhancing the interaction among member countries for meeting the global energy demand. The depletion of fossil fuel reserves and imbalances arisen from its consumption lead to unsustainability in energy sector. India with rich thorium reserves, was thinking of utilizing nuclear reactions for energy production to address this issue.A challenge prevails in production of energy by nuclear fusion on the earth.

  1. Nuclear fusion

Fusion is the opposite of nuclear fission. Nuclear fusion is the phenomenon in which two lightweight atomic nuclei joined into a single, heavier nucleus along with production and release of high energy. Here two fusing atoms overcome their natural repulsion and fuse forwarding a reaction producinglarge amount of energy.

 

The fusion reaction mainly occurs between two hydrogen isotopes. The reactions are as follows:

 

  1. Nuclear fusion in stars

A large quantity of hydrogen and helium nuclei is present in and around the stars. The nuclear fusion produces heat energy while formation of Helium from hydrogen nuclei. The ultimate energy source, sun produces energy by the fusion of hydrogen nuclei to helium nucleus at a solar core temperature of 15 million Kelvin. The heat inside the sun and gravitational pressure compress the nuclei of particular atoms into heavier nuclei which help in releasing large amount of energy. In the sun single proton nuclei of two H-isotopes (Deuterium and Tritium) fused together to form heavy nucleus of helium and neutron. So in the fusion reaction, 4 protons merge to form 1 alpha particle, with the release of 2 positrons, 2 neutrinos and and enormous amount of energy.

 

Several chain reactions are continuously going on and depending on the mass of the star. For the stars having a size equal or smaller to the sun, the proton-proton chain reactions are dominated (Fig 2). In heavier stars, the CNO cycle is more prominent.

 

  1. Conditions for nuclear fusion reactions

 

The conditions required for nuclear fusion reactions are

 

Optimum and very high temperature: This will help to separate the electron from the nucleus and overcome electrostatic repulsion forces. This is possible by highly ionized plasma, a gaseous mass consisting of electrons and free atoms.

 

Confinement time: The minimum time required for the plasma to maintain at elevated temperature for the nuclear fusion reaction to occur without losing energy content after the energy sources feeding it are cut suddenly.

 

Adequate plasma density – for nuclear fusion reaction between nuclei.

 

For the fusion reaction to be energetically viable, relationship between the fusion power generated and the external power supplied to the plasma is given by Lawson criterion:

 

n. τ E > g (T) . f (Q)

where

n = particle density

τ E= energy confinement time

g(T) = variation of the reaction rate with temperature T

Q = energy amplification factor.

There are two characteristic values of Q:

Q= 1 , indicates that power produced by the plasma is equal to the power coupled to it from the

exterior. This is referred to as the ‘‘break-even’’ point.

Q = Infinity, indicates that external power contributed to the plasma is zero. The plasma is self sustaining and is said to be in ignition.

The f(Q) deuterium and tritium plasma is equivalent to 1 for Q=1 and it rapidly toward 5 for higher values of Q. At this condition and at a temperature of 10 keV, the Lawson criterion is

n. τ E ≈ 1020 (m-3.s)

  1. Confinement nuclear fusion reactions

 

The two important methods of confinement are

 

Inertial confinement fusion (ICF): In this radiation of laser beam is used to implode a small pellet of fusible fuel (deuterium and tritium) by confining high temperature. The energy is directed from different directions on the fuel till the fusion occurs. Usually one mg of Deuterium- Tritium makes a pellet ant it generate 350 MJ and 10 micro explosions per second produce 3.5 GW.

 

Magnetic confinement fusion (MCF): The charged plasma particles are trapped and confined by applying a specially configured magnetic field space. The confinement with magnetic field by using a magnetic mirror or a closed geometry (toroidal field)

  1. Fuel used for nuclear fusion reactions

 

Usually light nuclei mainly deuterium and tritium are used for nuclear fusion.

 

Deuterium (1H2): is stable isotope consisting of a proton and neutron. It is present in sea water (1 atom per 6500 atoms of hydrogen). 34 grams of deuterium is present in every cubic meter of sea water. Hence, nuclear fusion is an inexhaustible source of energy because on earth three fourth (71%) part is covered by water.

 

Tritium (1H3): composed of a proton and two neutrons by beta emission decay. The neutron capture reactions with the isotopes of lithium produce tritium. Lithium is an abundant material in the earth’s crust and in seawater. But tritium is limited in nature.

  1. Nuclear fusion reactor

Fig 3 shows the schematic diagram of a fusion tokamak-based nuclear fusion reactor. The injection of deuterium/tritium (gaseous or frozen ice pellets form) fuel mixture to a vacuum chamber (1) is converted to plasma state and burns continuously (2). The energy is produced by plasma in the form of particles and radiation (3). Afterwards the energy is lost in the first wall after absorption of charged particles and radiation. The kinetic energy is converted to heat in the blanket (4) after first wall within vacuum chamber. The steam generated is used to turn turbine and alternator for electricity production.

9.Scale up of Nuclear fusion to commercial range

 

The nuclear fusion was successful in production of 16 MW energy in a small reactor i.e. Joint European Torus (JET) and capable of sustaining 5 MW in 1997. The scale up of this is being installed in France. The International Thermonuclear Experimental Reactor (ITER) is a joint research project of the US, EU, Japan, Russia, China, South Korea, and India designed for 500 megawatts power generation. ITER will work on magnetic confinement approach in device tokamak. The fusible fuels will be introduced into and confined in a vacuum chamber and heated to temperatures exceeding 100 million degrees to convert it to plasma and confinement of plasma. The ITER was stared constructing in 2009 with a plan to complete it by 2025. The project will address several safety issues, scientific and technical problems that need to be overcome for making nuclear fusion a feasible method of energy production.

  1. Indian efforts for energy from nuclear fusion

Institute for Plasma Research (IPR), Gujarat is doing the R& D activities of Nuclear fusion for power generation in India.

 

Aditya Tokamak6 was a fusion reactor developed and has been operating since 1980s. Steady-state Superconducting Tokamak [SST-1] was developed in 2002.

 

India is also a partner member in the International Thermonuclear Experimental Reactor [ITER] project underway in France, contributing 10% of the hardware sub-systems needed to build it.

 

Planned to produce 2 x 1GWe power plant by 2060

  1. Advantages of nuclear fusion process

 

The energy production from nuclear fusion has benefits like Production of high amount of energy from a small mass of nuclei It is a clean energy.Fusion reactions are not radio-active like nuclear fission. Lithium and deuterium are not radioactive. Tritium is radioactive but with a short half-life of 12.6 years. As tritium is generated and used inside the reactor, no transport of radioactive fuel is required. The radiotoxicity in the reactor chamber and additional structural and waste substances will decay quickly within life of fusion reactor.

 

No greenhouse gases are emitted by this process.

 

Availability of abundant fusile fuel (deuterium from sea water and tritium from lithium). It is easy to control or stop the reaction in comparison to nuclear fission due to absence of ‘chain reaction No nuclear wastes are produced.

  1. Constraints in nuclear fusion

 

Commercial production of energy by nuclear fusion is not completely proved yet. Highly expensive.The fusion process requires extremely high temperatures. It requires energy to start the fusion reaction Building full-scale fusion-generating facilities will need engineering advancements like better superconducting magnets and advanced vacuum systems.

 

Conclusion

 

The nuclear fusion process is a clean and safe energy production from Deuterium and Tritium. Since the Deuterium is abundantly present in seawater, power by fusion may offer a sustainable source of energy after overcoming constraints of confinement of high temperature.

 

you can view video on Nuclear Fusion for Energy

References

  • Lilley, J. (2006). Nuclear Physics -Principles and Applications, John Wiley & Sons, Chinchester, England, p 385.
  • Kenneth S. Krane. (1988). Introductory Nuclear Physics, John Wiley & Sons, Chinchester, England. P 858.
  • Girard, J.-Ph., Garin, P., Taylor, N., Uzan-Elbez, J., Rodr´ıguez-Rodrigo, L. and Gulden,W. (2007). ITER, safety and licensing. Fusion Engineering and Design 82(5-14): 506-510.
  • Holtkamp, N. (2007). An overview of the ITER project. Fusion Engineering and Design. 82(5-14): 427-434.
  • Magaud P, Marbach G, and Cook I. (2004.) Nuclear Fusion Reactors. Pp. 365-381 in Encyclopedia of Energy, Volume 4, ed. C.J. Cleveland. Elsevier Science: Oxford, U.K.
  • http://ec.europa.eu/research/energy/euratom/index_en.cfm?pg=fusion&section
  • John Sheffield (2003) Report Number: JIEE 2003-03, Fusion Energy in India’s Future – isse.utk.edu, http://isse.utk.edu/pdf/jieepubs/2003_03india.pdf
  • http://www.engineeringchallenges.org/9079.aspx