Nuclear energy
Nuclear energy is the fundamental source of energy in Universe. Aided by Gravitational force, it plays a fundamental role in creation of planets, stars and galaxies. Sun provides more than 99% of energy on Earth and it derives its energy from nuclear reactions.
Each nucleus of every element has a certain binding energy pre nucleon i.e. proton or neutron, which is the energy released when the nucleus is formed. If a nucleus is formed by fusing lighter elements, the excess binding energy is released. This is the basis of nuclear fusion reactions. Relatively less amount of binding energy is again released when a heavy nucleus breaks up thus creating nuclear fission reaction.
Nuclear bombs
The unleashing of energy of atomic bombs was first done during World War II when 2 atomic bombs were dropped at Hiroshima and Nagasaki respectively and they almost destroyed the 2 cities. Their yields were between 10 to 20 kilotons of TNT and this was largely due to fission of nuclei of Uranium and Plutonium respectively. Fission bombs result in shock waves and radioactive by products besides intense heat. Fission based bombs of yields upto 500 KT e.g. Ivy King have been tested since.
Nuclear fusion delivers more energy per unit mass as compared to nuclear fission. This has led to development of fusion based Hydrogen bombs whose yields are in megatons of TNT. The largest nuclear fusion test bomb, i.e. Tsar Bomba had a yield of 50 megatons of TNT. However fusion needs original nuclei to be moving fast enough to overcome the Coulombic repulsion of their protons and hence all Hydrogen bombs use fission to generate the required heat and initial energy for collision. This is followed by 1 or more stages of fusion or fission. Their higher yields and hence smaller sizes imply that they are nuclear weapons of choice for missiles as well as preferred nuclear weapons for higher energies.
As per IEA, electricity’s share in overall energy consumption will grow and hence nuclear energy can play a crucial role in energy consumption. However generating nuclear energy in a more controlled way to create electricity or for other uses has different set of challenges.
Nuclear fission reactors
Nuclear reactors are based on breaking up nuclei e.g. of Uranium-235 or Plutonium-239 by absorption of neutrons. Naturally occurring Uranium contains 99% U-238 which is not easy to break up and proportion of U-235 has to be increased (enriched) to make energy generation viable. The majority of reactors slow down neutrons to increase the absorption of fission by U-235 nucleus using a moderator which is mostly water or to a lesser extent, Graphite or Heavy water (D2O). Water at high pressure in a separate system as used in Pressurised Water Reactors is also a common coolant and is used to drain the energy out of the reactor. Less commonly in Boiling water reactors water directly enters and leaves the reactor.
The biggest problem with nuclear reactors is production of nuclear waste which is radioactive. Tools, cloths, paper, rags etc used constitute the Low Level Waste (LLW) and they also produced by hospitals and industries besides nuclear reactors. In reactors they constitute more than 90% of the waste by volume but contribute to less than 1% of radioactivity. Used filters, resins, chemical sludges, metal fuel cladding and steel components of the reactor represent Intermediate Level Waste (ILW). The ILW needs shielding while LLW do not. Both types of wastes may be compacted and then buried in shallow sites.
The used fuel roads are highly radioactive though they constitute only 3% of the waste by volume and they are categorised as High Level Waste (HLW). Its volume can be reduced by removing Uranium and Plutonium. High Level waste is also hot and has to be cooled down by putting it under water or concrete for a few years. Thereafter it can be buried in deep geological sites. However the radioactivity of some of its constituents can last for millions of year and creating safe sites faces public resistance as people do not want such places near to where they live.
Accidents at nuclear plants can release radioactive substances with devastating effect. 2 recent cases being Chernobyl disaster (1986) and the Fukushima Daiichi nuclear disaster (2011). The former was due to a human error and the latter a natural disaster. These accidents have turned public opinion against nuclear energy especially in developed countries e.g. Japan, Germany, Switzerland, Italy, Spain etc who either do not want to build any new nuclear reactor or even want to shutdown the existing ones by a certain date.
Nuclear reactors not only generate electricity but also radioactive materials used in medicine and industry. Submarines and ships especially used for military purposes also use nuclear energy to avoid constant refuelling. These reactors are more rugged than land based reactors and use more enriched Uranium. Nuclear energy does not add to carbon emissions but carries safety concerns. As per IEA’s World Energy Outlook 2018, nuclear energy contributes to 10% of world’s electricity generation and the number will remain roughly the same for the next decade even as fossil fuels give way to renewables.
Nuclear fusion reactors
In theory, nuclear fusion offers a solution to radioactive waste problem as here relatively little radioactive waste is produced. The reaction that is researched the most is fusion of Deuterium and Tritium to form Helium.
But is requires onerous conditions of density and temperature for a significant amount of time to generate net output of power. In fact temperature requirements exceed that of stars as Gravitation aids in confinement in stars and it is a highly energy intensive process to increase the temperatures to this level. China’s Experimental Advanced Superconducting Tokamak (EAST) claims to have achieved the required temperature of 100 million Kelvin. At these temperatures, the fuel would be in plasma state and its confinement is a technical challenge.
There are 2 approaches for confinement i.e. using toroidal shaped magnetic fields called Tokamak and using lasers on small fuel pallets. The former is used in an international project called ITER (International Thermonuclear Experimental Reactor) and JET (Joint European Torus). National Ignition Facility (NIF) at USA uses lasers to heat and compress Hydrogen. Variants of these approaches including using magnetic or electric fields along with lasers or using only electrostatic fields are also being researched. So far, no fusion reactor has been created that generates more power than consumes.
Besides there is additional effort to convert generated energy to electricity as the neutrons carrying energy would be super heated to much higher temperatures than 100 million Kelvin. This may need multiple heat mechanisms to lower temperatures to use steam based electricity generation. Also due to these neutrons the reactor walls themselves may become radioactive and hence need to be replaced.
Conclusion
Nuclear energy has the potential to solve perhaps all energy needs of humans. However while humans have been able to use it for military purposes, they have been only partially successful to use it in a more controlled way to generate electricity. The technologies still need to mature to deliver the promised potential.
