Using Nuclear Energy for Power (College Board AP® Environmental Science): Study Guide

Nuclear fission

• The nucleus of an atom contains a huge amount of nuclear energy

• The amount of potential energy is 10 million times greater than coal, oil or gas

• When harnessed safely, nuclear energy can significantly reduce our dependency on fossil fuels

• However, it also has the potential to be highly destructive (nuclear weapons, for example)

 Nuclear fission

• Nuclear fission is when one large nucleus splits into two smaller nuclei

• The large nucleus that splits is often referred to as the parent nucleus

  • The smaller nuclei that are produced are referred to as the daughter nuclei

• This is the process used to generate electricity in nuclear power stations

• Nuclear fission is defined as:

  The splitting of a large, unstable nucleus into two smaller nuclei

• Some isotopes of uranium and plutonium are known as fissile materials

• This means they can undergo fission under the right conditions

• This makes them ideal to use as fuels in nuclear power stations

Spontaneous and induced fission

• It is rare for nuclei to undergo fission without additional energy being put into the nucleus

  • When nuclear fission occurs in this way, it is called spontaneous fission

• Usually, for fission to occur, the unstable nucleus must first absorb a neutron

• This makes a nucleus more unstable so that it decays almost immediately

  • When nuclear fission occurs in this way, it is called induced fission

Fission of uranium-235

• Uranium-235 is commonly used as a fuel in nuclear reactors

• It has a very long half-life of 704 million years

  • This means that it has low activity and releases energy very slowly

  • This is unsuitable for producing energy in a nuclear power station

• Therefore, the fission of uranium-235 must be induced

• During induced fission, the uranium-235 nucleus absorbs a neutron and becomes uranium-236, which is very unstable

• During fission, when a neutron collides with an unstable nucleus, the nucleus splits into:

  • two smaller nuclei (daughter nuclei)

  • two or three neutrons

  • gamma rays are also emitted

• The products of the fission reaction move away very quickly

• This is because energy is transferred from the nuclear potential energy stored in the original nucleus into the kinetic energy of the products

• In a nuclear power station, this energy can be harnessed and converted into electrical energy

Generating electricity

• The process by which electricity is produced in a nuclear power station is the same as for any other fuel-powered station; the only difference is the process used to produce the heat

• Nuclear fission produces a large quantity of heat, which is carried away from the reactor by a coolant (usually pressurised water)

• The coolant is then used to heat a separate water source, turning the water into steam

  • A separate source is used in order to reduce the risk of contamination

• The steam is then used to drive turbines, which then turn generators, producing electricity

• The nuclear reactor is:

  • The part of the power station that provides thermal energy from fission chain reactions

  • It contains control rods and moderators

    • control rods decrease or increase the rate of fission by absorbing neutrons

    • lowering the rods decreases the rate of fission as more neutrons are absorbed

    • raising the rods increases the rate of fission as fewer neutrons are absorbed

    • the moderator slows down the fast-moving neutrons produced by fission

• The water pump brings in cool water to be turned into steam and also to prevent the reactor from overheating

• The boiler uses thermal energy to boil water to create steam

• The turbine uses the steam to transfer thermal energy into kinetic energy

• The generator transfers kinetic energy into electrical energy

• The condenser is within the cooling tower, allowing steam from the turbines to condense back into liquid and cool before being reused

Shielding in a nuclear reactor

• The purpose of shielding is to absorb hazardous radiation

• The entire nuclear reactor is surrounded by shielding materials

• The daughter nuclei formed during fission and the neutrons emitted are radioactive

• The reactor is surrounded by a steel and concrete wall that can be nearly 2 metres thick

• This absorbs the emissions from the reactions and ensures that the environment around the reactor is safe for workers

Radioactivity

• Radioactivity is the energy emitted by the nucleus of a radioactive isotope such as uranium-235

• The emission of radiation from a nucleus can be spontaneous and random in direction

  • This random radioactive decay means:

    • Any nucleus can decay at any time

    • Deterioration is not affected by environmental factors

    • The probability of a nuclei decaying in a given time period can be estimated

• A nucleus with an imbalance of protons or neutrons, or is too heavy, is more likely to decay into several smaller nuclei until stable

• Radioactive nuclei decay (breakdown) will emit (give off) energy (radiation) even without fission; nuclear fission just releases masses of energy all at once

Disposal of nuclear waste

• The biggest problem concerning nuclear power is the waste that it produces

• This waste comprises of the unusable fission products from the fission of uranium-235 or from spent fuel rods

  • This is because each fission of a uranium-235 nucleus results in two smaller nuclei being produced

• This is by far the most dangerous type of waste, as it will remain radioactive for thousands of years

  • These smaller (daughter) nuclei are both highly radioactive—more radioactive, in fact than the original fuel

• As well as being highly radioactive, the spent fuel roads are extremely hot and must be handled and stored much more carefully than the other types of waste

• Whilst the amount of waste produced (relative to the amount of energy generated) is fairly small, the waste is extremely dangerous

  • Therefore, it must be stored underground till they are no longer harmful

• Mine tailings left over from mining uranium may have radioactive elements that can contaminate water or soil nearby

• Nuclear powerplants need lots of water and can deplete local surface or groundwater sources

• Thermal pollution water from the power plant is hot and when released back into surface water, it can cause thermal shock

  • It can reduce oxygen levels and disturb aquatic species balance, which harms aquatic ecosystems

  • Warm waters increase bacterial growth

  • Increased growth of algae in warmer waters further decreases dissolved oxygen levels

Advantages & disadvantages of nuclear power

• Nuclear power is a low-carbon, low-emission, non-renewable resource

• However, it is controversial due to the radioactive waste it produces and the potential scale of any accident

Advantages of nuclear power

• No pollution released into atmosphere

• Nuclear reactors are perfectly safe as long as they are functioning properly (rigorous safety checks must be routinely carried out and rigorous safety procedures followed)

• Nuclear power stations can generate electricity reliably on a large scale to be available as needed

• Small amounts of uranium are needed, and large reserves are available

• Reduces reliance on fossil fuels

• Increases energy security

Disadvantages of nuclear power

• There is a finite supply of uranium ore, so nuclear power is a non-renewable resource

• Nuclear fuels produce radioactive waste, which needs to be stored for thousands of years

• Safe ways of storing radioactive waste are very expensive

• If an accident occurs at a nuclear reactor, radioactive waste can leak out and spread over large areas

• The cost of decommissioning (shutting down) nuclear power plants is very high

• Risk of use of nuclear fuel by terrorists or rogue states