Nucler Power

Fission

The word fission means to divide or split. In nuclear fission, the nucleus of a large isotope is split to release energy.

Heavy isotopes, such as uranium are bombarded with free slow moving neutrons. When these combine with the uranium nuclei the resulting isotope of uranium is highly unstable and it very rapidly divides into 2 smaller more stable nuclei. These will still be radioactive, in the process 2 or 3 neutrons are released to initiate further fission reactions. Importantly the change in structure reduces the mass and this mass is converted into energy in accordance with Einstein’s equation E = m__c²

Example:

Uranium²³⁵ + neutron → Krypton⁹² + Barium¹⁴¹ + 3_ neutrons + _energy

²³⁵U +¹n → ⁹²Kr +¹⁴¹Ba +3¹n + energy (ɣ__ and thermal)

This reaction liberates approximately 7.4x_10¹³ _J per kg _of _²³⁵U

Chain Reaction:

A single fission reaction, such as the one above gives rise to 2 or 3 additional neutrons, each one of which can cause a further fission reaction. If left uncontrolled this will quickly escalate and a chain reaction will occur. This would result in the instantaneous release of all the energy. This is what happens in a nuclear explosion.

In a nuclear reactor, the excess neutrons are absorbed allowing the reaction to be controlled or even stopped.

The is achieved by a moderator material around the uranium, normally this a form of water made with an isotope of hydrogen, so it is called heavy water.

To control the rate of the reaction - control rods are used. These are lowered into the reactor to absorb neutrons and slow the rate of reaction down. They can also be pulled out to speed it up and produce more energy.

Fusion

The term fusion means to join or combine. In nuclear fusion, small light nuclei are forced together under high pressures and temperatures to form larger nuclei. This process releases large amounts of energy. It is the process that produces the energy in stars.

● ¹H + ¹H → ²H + e^- + energy

● ²H + ¹H → ³He + ɣ

● ³He + ³He → ⁴He + 2¹H + energy

The process begins with Hydrogen which is eventually fused to form Helium. This has to happen at high temperatures and pressure to overcome the proton repulsion force that tries to prevent the fusion of the nuclei.

At present, the technology to achieve this with commercially viable amounts of energy produced is not possible.

Nuclear Reactors

Nuclear reactors are used to generate electricity. They are used in power station, ship, submarines, satellites and space probes.

Currently, all nuclear reactors use nuclear fission. The conditions needed for fusion make it difficult to contain and at present fusion reactors are only experimental.

In a fission reactor, the thermal energy from the fission reaction is used to superheat steam. This is then blasted onto turbines to turn the generators and make electricity. Technically the only difference between a nuclear power station a conventional fossil fuel power station is the means by which heat is produced to superheat the steam. All other parts of the power plant are identical.

Nuclear power has many advantages over the use of fossil fuels. At the point of power generation, they do not produce carbon dioxide. However, carbon dioxide is produced in the construction of the plants and in the delivery and removal of fuel and waste. Fission can produce large amounts of energy for only a small amount of fuel compared to fossil fuels

1 kg of uranium produces 7.4 x 10¹³ J 1 kg of coal produces 2.88 x 10⁷ J

So Uranium can produce 250 million times as much energy per kg of fuel.

However, the uranium like the fossil fuels in a non-renewable resource. The uranium and the waste products are highly radioactive and have to be stored safely for thousands of years. Once the reactor plant is decommissioned the site is also radioactive for thousands of years. The material used as fuel and the waste can be processed into weapons, so safety and security are serious considerations too.

Advantages and Disavantages

However, the uranium like the fossil fuels in a non-renewable resource. The uranium and the waste products are highly radioactive and have to be stored safely for thousands of years. Once the reactor plant is decommissioned the site is also radioactive for thousands of years. The material used as fuel and the waste can be processed into weapons, so safety and security are serious considerations too.

The key advantage is that nuclear reactors can produce very large amounts of energy for very little fuel. They also do not produce any carbon dioxide at the point of electricity production. Unlike burning a fossil fuel there is no smoke or other particulate matter pumped into the atmosphere.

Explain the differences between fusion and fission and why at present only fission is used commercially for electricity generation.

Your answer should include: joining / light / nuclei / produced / produce / large / amount / amounts / energy
Explanation: Fusion is the joining of light nuclei to make a new larger nucleus, fission is the splitting of a large nucleus to make two smaller nuclei. Fission is used as it produced large amounts of energy, fusion requires high temperature and pressures that are not currently possible on a commercial scale.

How is a chain reaction in fission reactor controlled?

Your answer should include: absorb / absorbing / excess / neutrons
Explanation: The reactor is surrounded by a moderator and has control rods that can absorb excess neutrons to control the chain reaction.

Explain why some people are against the establishment of new nuclear reactors in the UK.

Your answer should include: security / issue / issues / radioactive waste / risk / accidents / carbon dioxide