Nuclear Power

the conversion of small amounts of matter into energy as atomic nuclei split or join e=mc2 e - energy released m - the mass of matter lost c - the speed of light

when small amounts of matter from the nuclei of atoms are destroyed, producing large amounts of energy

Fission involves the splitting of the nuclei of large atoms, such as those of the isotopes U-235 and Pu-239 Fusion is the joining of the nuclei of large atoms, such as those of the isotopes H-2 and H-3 • only fission is used commercially now, fusion being a research project

• a neutron is released from a previous fission • it is absorbed by a U-235 nucleus • the result produces 1) heat 2) fission products 3) neutrons • the heat can be used to boil water, and the steam produced will drive turbines to generate electricity • fission products are fragments of the U-235 nucleus, e.g. I-131, CS-135 and Sr-90. These become high-level waste • the neutrons released have two paths: for a stable chain reaction one will cause a further fission. The other will miss uranium nuclei and be absorbed by control rods

• the power output of nuclear reactors normally changes quite slowly • therefore nuclear power is usually used for ‘base-load’ electricity supplies that are needed all the time

• tech is very complex, so difficult to use in less tech advanced societies which cannot support the industrial infr needed • the complex tech involved is very expensive • strong public opposition in some areas over safety concerns, especially after reactor incidents such as Chernobyl and Fukushima • concerns over possible links between nuclear materials for civil uses and military or terrorist uses • uncertainty over the permanent disposal of radioactive waste • uncertainty over the total costs of nuclear power since no commercial reactor has been fully decommissioned

• nuclear reactors = generation of electricity • also used to propel ~150 ships • fission products present in used fuel includes isotopes that have other uses such as caesium-137 for food irradiation and americium-241 in smoke alarms

• reactors require very little fuel because nuclear fuel = high energy density • this means power stations can be located where the transport of large amounts of fuels with a lower energy density would be a problem e.g. renewables • a reactor only needs to have a few tens of tons of fuel replaces each year, compared with 10,000t coal that would be burnt every day in a coal fired power station with a similar electricity output

• polymer adsorption • phosphate mining • coal ash

• uranium dissolved in seawater exists as uranyl ions • these adsorb onto certain polymers that are placed in the sea, normally charged plastic strands • dipping plastic fibres containing amidoxime compounds into seawater adsorbs the uranyl ions • the strands become saturated • the plastic can then be chemically treated to free the uranyl • the uranyl can be washed off using acids then collected and concentrated • the uranyl then has to be refined for use in reactors just like ore from a mine

uranium is often present in phosphate deposits and can be separated from the material extracted in phosphate mines

uranium can be extracted from coal ash. This will become economic if the price of uranium rises enough

• molten salt reactors • plutonium reactors • thorium reactors

• using molten salt as a reactor coolant increases the efficiency of electricity generation because the reactor can operate at a much high temperature without needing high pressure to prevent the coolant boiling • liquid-cooled reactors are much smaller than gas-cooled reactors, so they are cheaper to construct

almost all nuclear reactors harness the energy released by the fission of U-235, but this only makes up 0.7% of uranium in the mined ore - the rest is U-238 = not fissile - U-238 can be converted into fissile U-239 by neutron bombardment within a reactor - so the reactor is using up fissile fuel, but also producing more fissile fuel from the U-238 - U-238 = a fertile fuel plutonium reactors = breeder reactors - releases energy for elec - uses more fissile fuel than what is consumed - allows much more energy to be harnessed from the original uranium that was mined, however BR are more complex and expensive to operate - the fission of plutonium doesn’t require the neutrons to be slowed by a moderator so they are often called ‘fast reactors’ or ‘fast breeder reactors’

an isotope that is not itself fissile but can be converted into a fissile fuel by neutron bombardment e.g. U-238 into U-239

• neutron bombardment of U-238 turns it into U-239 • beta decay of U-239 turns it into Np-239 • beta decay of Np-239 into Pu-239

• higher energy output per fission event • ability to sustain nuclear fission chain reactions more efficiently BUT waste management is more complex and therefore more hazardous because of longer half-life, and is rarer than uranium so has to go through breeder reactors

• thorium-232 not fissile = doesn’t release energy when bombarded with neutrons • however, is fertile: can be converted to fissile U-233 • reactor designs include fuel rods of uranium-233 which release energy and neutrons to maintain the chain reaction • there are also rods of thorium-232 in the reactor core which ‘breed’ U-233 as they are bombarded with neutrons • the U-233 can be extracted to make new fuel rods

• thorium = 3x more abundant than uranium • much more difficult to make weapons materials than it is using uranium • much less radioactive waste is produced • the radioactive waste has shorter half-lives • no fuel enrichment is required

• the breeding rate for U-233 is slow, so the fuel is expensive • U-233 releases alpha radiation so it it very hazardous • being a less-developed technology than uranium reactors, the remaining development costs will be high and the governments skeptical

• the joining of the nuclei of small atoms • the source of energy of all stars e.g. isotopes of hydrogen such as deuterium and tritium

• deuterium (hydrogen-2) extracted from water • tritium (hydrogen-3) produced by neutron bombardment of lithium

producing controllable fusion on a small scale on Earth has proved difficult, and development of the methods to produce controlled fusion on Earth has proved complex