Generation IV Reactors and Fusion
The power of the Sun at our finger tips - fusion is one of the techno-fix options for the sustainable energy economy. Billions are being invested, but are the results promising?
Fusion is a nuclear reaction which, as the name suggests, fuses two atomic nuclei together, rather than splitting one large one, as is the case with fission.
Fusion is the reaction which occurs in stars, including our Sun, and which results in the enormous amount of energy we experience as heat and light. Most fusion in our Sun is the formation of helium (atomic weight 4) from hydrogen (atomic weight 1). However, there are two isotopes of hydrogen: deuterium (atomic weight 2) and tritium (atomic weight 3). These heavier isotopes are used in the fusion experiments.
Physicists have invested a great deal of effort and money in the attempt to make fusion a viable source of energy, for the production of electricity. The main problem is not how to cause the fusion to occur, but how to contain the enormous heat it generates. This is the containment problem.
Many designs of containment chambers have been proposed. The first successful controlled release of fusion energy was achieved in 1991, at the Joint European Torus facility (JET), England: the Preliminary Tritium Experiment.
Current experimental designs include the tokamak and the inertial confinement fusion laser. These experiments are still not producing enough energy to be commercially viable.
A helium nucleus has a lower mass than the composite lighter nuclei. During the fusion of deuterium and tritium to a helium atom, the missing mass is converted to energy, according to Einstein's famous equation: E = mc2
However, to get the hydrogen protons to join together to form a helium nucleus, requires the electrostatic forces (like charges repel) of the two positive protons to be overcome, so that the strong nuclear force can hold the protons together. The strong nuclear force is much more powerful than the electrostatic forces, but only operates over very short distances. Beyond its effective range, the electrostatic force is dominant, and forms a Coulomb barrier.
Therefore, a large amount of energy is needed to cause fusion. This energy can be kinetic, such as can be obtained by a particle accelerator. Or it can be provided by heat. The heat strips off the single electron from the hydrogen atom, ionising it. The cloud of ions, with separated electrons, is the plasma. The ions are controllable, because they are positive. By placing them in a magnetic field, they can be positioned and held in place as they are heated under pressure.
21H + 31H → 42He + 10ν
Fusion of deuterium and tritium to helium releases 17.6 MeV of energy.
Clean And Environmentally Safe Advanced Reactor
The Clean And Environmentally Safe Advanced Reactor, CAESAR, is a design still in development, which proposes to utilise an initial quantity of LEU (low-enriched uranium) to start the reactor, which will then continue with only U-238 as fuel. Neutrons will be controlled by means of a steam moderator.
Thorium (Z = 90) is a radioactive element of the actinide series. It is a potential fuel for nuclear reactors.
Thorium is more abundant in the Earth's crust than uranium, and is found in monazite deposits. Monacite is thorium diphosphate Th(PO4)2, and thorium can be extracted through a process involving nitric acid.
Thorium-232 can be transmuted to U-233 by bombardment with neutrons. U-233 could be used as a fissile fuel for power generation. Technical difficulties have so far prevented the use of this alternative to U-235, but India has large reserves of thorium, and is carrying out a programme to develop it.
The use of thorium would reduce the quantities of dangerous nuclear waste, and avoid the generation of plutonium, which is problematic with the use of enriched uranium.
Thorium is weakly radioactive and has seven naturally occurring isotopes, all of which are unstable to very varying degrees (half-lives vary from 25.5 hours for Th-231 to 14 billion years for Th-232!). Th-232 is by far the most abundant thorium isotope in the crust. It is 3 or 4 times as abundant as uranium.
There is a great deal of interest in utilising thorium as a fuel for nuclear reactors, especially in India, which has no uranium resources, but abundant thorium.
23290Th + 10n → 23390Th + γ →β- 23391Pa →β- 23292U
One atom of U-233 releases 197.9 MeV (3.171 × 10−11 J), or 19.09 TJ/mol = 81.95 TJ/kg, of energy during fission. The half-life of U-233 is 160,000 years, and alpha decays to Th-229.
Th-233 has a half-life of 22 minutes, and decays into Pa-233 (half-life = 27 days), which beta decays to U-233 (half-life = 160,000 years). U-233 usually fissions, releasing an alpha particle to form Th-229, when impacted by a neutron, but a portion of it keeps the neutron and becomes U-234. The overall capture-fission ratio is lower for U-233 than for U-235 and Pu-239, which means the chain reaction requires a higher neutron density to reach sustainable levels.
When U-233 undergoes fission, it emits neutrons, which can impact another thorium-232 nucleus, causing the decay to start again. The chain reaction would be self-sustaining at a critical mass and geometry. The cycle is similar to that in fast-breeder reactors, which produces highly-fissile Pu-239 from low-fissile U-238. However, thorium is more abundant than uranium, and offers a more sustainable supply, especially since the fissile U-233 it uses can be 'bred' from natural ore thorium. Most uranium reactors are 'burner' type, and its fuel is enriched natural uranium ore.
Thorium also has the advantage that it can be mixed with U-238, and therefore has no weapons-grade potential, which is not the case with U-235 enriched fuel. It also has a higher neutron yield, produces fewer long-lived transuranium elements, and makes better-performing reactor cores.
Thorium needs to be neutron irradiated before it can be used as a fuel, and this presents greater technological challenges, which is why it has not been adopted extensively to date.
One of the three cores in the US Shippingport Atomic Power Station was a 60MWe thorium breeder reactor. It operated from 1977 till 1982, when the experimental core was removed, to reveal that the quantity of fissile material had actually increased by 1.4%, demonstrating the effectiveness of the breeding system. It used pellets made up of a combination of thorium dioxide and uranium-233 oxide.
For an account on the Indian Advanced Heavy Water Reactor (AHWR) project, designed to utilise Thorium-232: BARC Report on AHWR development programme
Advanced Heavy Water Reactor
Advanced Heavy Water Reactor, whose development may include thorium-based fuel cycles by the 2040's.
For an account on the Indian Advanced Heavy Water Reactor project, designed to utilise Thorium-232: BARC Report on AHWR development programme