1. Chain reaction:

• Nuclear fission reactions can only continue in a reactor if the number of nuclei involved in the fission reaction stays constant or increases. This occurs if, on average, one or more neutrons is produced and absorbed per fission reaction.
• This type of self-sustaining reaction is called a chain reaction (Figure 1).
• Figure 1 Chain reaction grows if more neutrons are produced at each stage than are absorbed.
• Chain reactions are only sustainable with a minimum amount of fuel, called the critical mass. This is because neutrons lost from the surface are no longer involved in the chain reactions. The shape as well as the mass of the sample affect the critical mass.

• ⇒Role of neutrons in nuclear power stations:

• Neutrons that induce fission reactions in nuclear reactors are called thermal neutrons.
• Their mean kinetic energy is equivalent to [math]\frac{3}{2} kT [/math], where k is the Boltzmann constant and T is the absolute temperature of the reactor core.
• Typically, thermal neutrons travel at between 2.5 and 3.0 km.s^-1, relating to a reactor core temperatures of about 290-350 K.

2. Moderation of neutrons:

• Neutrons produced by nuclear fission move so fast that they are unlikely to be absorbed in uranium nuclei, so they must be slowed down.

  
  Figure 2 Nuclei in the moderator absorb energy from neutrons through elastic collisions, slowing the neutrons down.

• The role of the moderator is to slow down fast neutrons as they pass through materials like graphite or water (Figure 2).
• Fast neutrons repeatedly collide with nuclei in the moderator, exciting the nuclei to higher energy levels.
• The fast neutrons lose energy during these collisions, and further collisions between neutrons and nuclei are elastic, slowing the neutron down even more. The slower neutrons are called thermal neutrons.
• The excited nuclei lose their surplus energy as gamma radiation when they return to the ground level.
• Graphite and heavy water are suitable materials for the moderator because they do not absorb neutrons. Also, energy is transferred more efficiently during elastic collisions if the mass of the nucleus is close to the mass of a neutron.
• Consider a snooker ball colliding head-on with a second, stationary ball. One ball stops as its energy is transferred to the other ball, which carries on at the same speed.
• 
  Figure 3 a snooker ball colliding head-on
• A much lighter table tennis ball will bounce off a snooker ball, which keeps most of its energy.

3. Control rod:

• Control rods control the rate of reactions in the reactor. Materials such as boron, steel and cadmium absorb neutrons without undergoing fission. Other materials such as silver, are also suitable but are rare and expensive.
• Boron is particularly useful because about 20% of the boron in control rods is boron-10, which absorbs neutrons to become boron-11.
• When a control rod is lowered into the reactor (Figure 4), the control rods absorb neutrons, so the rate of the reaction slows down because fewer neutrons are available to trigger fission reactions.
• The position of the control rods can be adjusted to maintain the chain reaction at a steady rate, or to shut the reactor down completely.
• Figure 4 Moving control rods deeper into the reactor core absorbs more neutrons and slows down the fission reactions.

4. The coolant:

• Coolants are fluids that absorb heat from the reactor, and transfer this heat away to drive the turbines that generate the electricity and to prevent the reactor from overheating.
• Most of the UK’s nuclear reactors use carbon dioxide as a coolant, but some use pressurized water.
• The coolant circulates through tubes inside the reactor core, absorbing heat from the reactor.
• This hot coolant then passes through a heat exchanger or boiler where its heat is transferred to water in a secondary cooling system (Figure 5).
• Figure 5 Schematic system in a nuclear power station.
• As the water in the secondary cooling system heats up, it changes to high- pressure steam and is used to drive the turbines and generator.
• Any steam remaining in the secondary cooling system is condensed back into water before it circulates through the heat exchanger again.
• To achieve this, the steam passes through pipes in a condensing unit, which is another heat exchanger that uses cold water-filled pipes.
• The water in the condensing unit is usually taken from a nearby sea or river.

Safety aspects

5. Nuclear fuel:

• Nuclear fuel, in particular the spent fuel rods, and the nuclear reactor are highly radioactive.
• Workers and the community must be protected from exposure to radioactive materials to reduce the damage caused by ionising radiation.
• Exposure to ionising radiation can damage DNA in cells, and increase the long-term risk of cancer.
• The risk of harm is higher if people are exposed to higher doses of radiation, or if the time or intensity of exposure increase.
• Workers involved in a nuclear accident may receive very high doses, causing radiation sickness which can be fatal in a few days. Many steps are taken to reduce or prevent exposure.
• The reactor is surrounded by shielding, which protects workers from exposure to radiation.
• In many nuclear power stations, this is a steel pressure container that also contains the high-pressure coolant.
• This container is surrounded by 5 m of concrete to absorb neutrons and gamma radiation, and this is surrounded by a steel and concrete building, designed to contain radiation even if there is an accident.
• Cost and effectiveness are important factors to consider when choosing a material for the shield.
• Common materials used for shielding are lead, concrete, steel and water.
• Concrete is one of the most cost-effective materials used in nuclear power stations.
• In an emergency, nuclear power stations are designed to shut down automatically.
• During a shutdown, the control rods drop into the reactor core, absorb the neutrons and slow down or stop the nuclear fission reactions.
• In many nuclear power stations, the control rods are held vertically above the reactor core using electromagnets. If there is a power failure, the rods drop automatically into the reactor.

6. Nuclear waste:

• Nuclear waste is produced from nuclear power stations. It is grouped into three categories-low, intermediate and high-level wastes.
• Nuclear waste is handled remotely to protect workers from exposure to radiation. This includes tele-operation, where workers manipulate equipment remotely, and the use of robotic machinery.

• ⇒Low-level waste:

• Low-level waste, including clothing worn by workers, paper and rags, accounts for 90% of the volume of nuclear waste, but only 1% of the radioactivity.
• Low-level waste is compacted and encased in cement and stored on licensed sites until the radioactivity decays away and it can be disposed of in normal waste.
• Isotopes in low-level waste have different half- lives and activities, so their exact disposal procedures vary.
• Figure 6 Low-level nuclear waste

• ⇒Intermediate-level waste:

• Intermediate-level waste is mainly produced when a nuclear power station is decommissioned, and occurs in chemical sludges and resins.
• Intermediate-level waste accounts for 7% of the volume of nuclear waste, and 4% of the radioactivity.
• Intermediate-level waste with long half-lives is encased in cement in steel drums and stored securely underground, for example in caverns or in near-surface facilities.
• A near-surface facility holds drums containing isotopes with half-lives of less than a few years, which are placed in deep trenches and then covered by several metres of soil.
• Figure 7 Intermediate-level nuclear waste

• ⇒ High-level waste:

• The main source of high-level waste is spent fuel rods. High-level waste accounts for 3% of the volume of nuclear waste, but 95% of its radioactivity.
• The spent fuel rods are so radioactive that they continue to emit heat and have to be cooled as well as stored.
• Initially, spent fuel rods are stored under water which acts as a coolant as well as a shield from ionising radiation.
• For long-term storage, high-level waste is mixed with molten glass, then solidified inside stainless steel containers. This process is called vitrification. 
• These stainless-steel cases are stored in specially designed facilities, either above or below ground.
• The half-life of high-level radioactive waste depends on the isotopes present, but several fission products have half-lives of several thousand years.

7. Risks and Benefits:

• Nuclear power stations generate electricity using fission reactions. No smoke particles or greenhouse gases are released, so generating electricity by nuclear power does not contribute to acid rain or to global warming. By using nuclear power, many countries have reduced the amount of coal and oil burned to generate electricity, which reduces their greenhouse gas emissions.
• The death rate in coal mining and in the oil and gas extraction industries is high, partly because the regulation and safety legislation of mining in different countries varies. For example, many thousands of coal miners have died worldwide since 2000. Oil extraction has one of the highest death rates for workers, even with the improved safety measures introduced in recent decades.
• Hydroelectricity also kills: when the Banqiao hydroelectric dam (China) collapsed in 1975, the accident killed thousands of people directly, and more also died as a result of the famine and epidemics caused by the resulting displacement of people.
• The quantity of waste produced during nuclear power generation is small in comparison to the amounts from other methods of generating electricity, because the energy source, uranium, is very concentrated.
• Nuclear power is a very reliable way of generating electricity, and the output from many nuclear power stations can be controlled to match changes in demand.
• However, there are significant drawbacks to our use of nuclear power. As with any natural resource, there are limited supplies of uranium, although supplies are likely to last for thousands of years, especially if fast breeder reactors are used to change U-238 into Pu-239, another nuclear fuel.