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.
Fusion of deuterium and tritium to helium releases 17.6 MeV of energy.
There has been much investment in different types of reactor, whether to exploit different fuels, or the new generation of reactors, employing new technologies which were previously unknown or considered
The nuclear power that has been used since the 1950s utilises the fission of uranium. The large uranium atoms, a mix of the 235 and 238 isotopes, is used as fuel. A mass of uranium fuel is allowed to enter a controlled chain reaction, and uranium atoms decay into smaller atoms, releasing neutrons and heat. The heat is created by some of the mass during the decay process being converted to energy, according to Einstein's E = mc2, where E is the energy released, m is the mass deficit between the initial mass of uranium and the final products, and c is the speed of light, 3.0 × 108 m/s.
There are a large number of fuels used in the various types of nuclear reactors. The most common fuel is uranium, whereby the naturally abundant U-238 has been partially enriched to more fissionable U-235.
RGPu = reactor grade plutonium; WGPu = weapons-grade plutonium; MOX = Mixed Oxide Fuel
|U-235||704||enriched natural uranium ore, from 0.720% to 3-4%|
|U-238||4468||99.27% of natural uranium|
|Pu-239||0.0241||weapons decommissioning, neutron bombardment of U-238|
The energy released by one uranium-235 atom decaying is: E = mc2 = 3.24 × 10-11 J (83.14 TJ/kg)
The half-life of U-235 is 704 My. The half-life of U-238 is 4.468 billion years. This difference explains why U-238 has a natural abundance of 99.27%. U-235 has halved its quantity at least 8 times since its formation in the supernova which formed the solar system, 6 billion years ago. The quantity of U-238 has halved only once.
Uranium can be caused to decay at higher than natural rates in nuclear reactors. This releases huge amounts of heat which can be used to generate electricity. Uranium does not release pollution of the type fossil fuels do.
10n + 23892U
→ 23992U + γ →β- 23993Np
+ 23592U → 23692U → 14054Xe
A nuclear reactor controls the rate of fission by two means: the moderator and the control rods.
Control rods are made of a neutron-absorbing material, such as boron or cadmium. They can be inserted or removed from a nuclear reactor core to vary the degree the neutrons generated by the chain reaction of the nuclear fuel are absorbed. If fewer rods are present, more neutrons will collide with more uranium fuel atoms, causing the chain reaction to increase.
By inserting all the rods fully, the reactor can be shut down entirely. If too many of the rods were removed, the reactor could go critical, a condition at which the chain reaction is at a dangerous level.
Generation IV nuclear reactors are expected to be in operation in the 2030s. They offer improvements in economic viability, safety, waste reduction and are more proliferation resistant.
Most reactors in operation today are 2nd generation. Generation I reactors were the early prototypes built in the 1950s and 60s.
Generation II: these are the majority of commercial power reactors, and include LWR, PWR, BWR, CANDU, WER/RMBK.
- PWR = pressurised water reactor
- LWR = light-water reactor
- BWR = boiling water reactor
- CANDU = CANada Deuterium Uranium
- RMBK = Reaktor Bolshoy Moshchnosti Kanalnyy (High Power Channel-type Reactor)
Generation III: these reactors were introduced from the mid-1990s till 2010. They include advanced LWRs, ABWR, System SO+, APeco, and EPR. Generation III+ reactors are currently being employed, and are evolutionary in design, offering better ROI.
- ABWR = Advanced Boiling Water Reactor
- EPR = Evolutionary Power Reactor
Generation IV Reactors:
- VHTR = very high temperature reactor
- MSR = molten salt reactor
- SSR = stable salt reactor
- SCRW = supercritical water reactor
- GFR = gas-cooled fast reactor
- SFR = sodium-cooled fast reactor
- LFR = lead-cooled fast 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.
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
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-
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.
Thorium-232 undergoes a complex series of transmutations to arrive at stable lead
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
European Energy Technology 4