Nuclear reactors

Nuclear reactors for A2 AQA Physics. From required knowledge to safety aspects.

What does the moderator do? What is it?

• reduces the speed of neutrons

• They slow down by colliding with the molecules of the moderator, resulting in a loss of momentum

• Neutrons slowed down so they are in thermal equilibrium with the moderator

• Ensures neutrons are more likely to be absorbed by uranium fuel

What do the control rods do?

• absorbs neutrons

• depth of control rods in the fuel rods is varied in order to reduce or increase rate of fission.

• lowering rods decreases fission rate, more neutrons are absorbed

• Also a safety precaution, rods can be lowered to ensure no reaction can take place.

What does the coolant do in a nuclear reactor?

• transfers thermal energy between the water systems of a nuclear power plant

How does the coolant work in a nuclear reactor?

• heat exchanger mediates thermal energy exchanges between different water systems

• Coolant is pumped into the reactor at a cold temperature to extract the heat released by the fission reactions

• In the heat exchanger, the coolant transfers the heat to water that is pumped in externally to produce steam

• This steam then goes on to power electricity-generating turbines

What are the different water systems in a nuclear reactor?

• Coolant (usually but not always water) used in the reactor vessel

• Water and steam that drives the turbine

• Condenser that cools the steam

What is shielding, what does it do?

Prevents ionising radiation from leaking into the immediate surroundings.

What materials are used for the moderator in a nuclear reactor?

materials that are poor at absorbing neutrons;

• water

• graphite

What materials are used for the control rods in a nuclear reactor?

• boron

• cadmium

What materials are used for the coolant in a nuclear reactor?

• water

• helium

• molten salt

What materials are used for shielding in a nuclear reactor?

• lead

• concrete

What kind of waste does a nuclear reactor produce?

low, intermediate and high level waste.

What is the high-level waste produced from nuclear reactors?

• fission fragments from fission of U-235 or spent fuel rods

• Most dangerous type of waste as it has the longest half-life (thousands of years)

• spent fuel rods are extremely hot so require additional care when being handled/stored

How is high-level waste disposed of/stored?

• The waste is initially placed in cooling ponds of water close to the reactor for a number of years

• Isotopes of plutonium and uranium are harvested to be used again

• Waste is mixed with molten glass and made solid (this is known as vitrification)

• Then it is encased in containers made from steel, lead, or concrete

• This type of waste must be stored very deep underground

What is the intermediate level waste produced from nuclear reactors? How is it disposed of/stored?

• Everything between daily used items and the fuel rods themselves

• Usually, this is the waste produced when a nuclear power station is decommissioned and taken apart

• This waste will have a longer half-life than the low-level waste, so it must be encased in cement in steel drums and stored securely underground

What is the low level waste produced from nuclear reactors? How is it disposed of/stored?

• lightly contaminated waste such as clothing, gloves and tools

• will be radioactive for a few years so may be encased in concrete and stored several metres underground until can be disposed of with regular waste

• Has a shorter half-life than intermediate and high-level waste.

What is the critical mass?

What is a thermal neutron?

A neutron that is slow moving (has collided with the moderator?)

What is a chain reaction?

Which two factors must be controlled in a nuclear reactor to produce energy at the required rate?

• the number of free neutrons in the reactor

• the energy of the free neutrons

benefits /disadgvantages of usinf water as a moderator in nuclear reactors>

Risks of nuclear power

• The production of radioactive waste is very dangerous and expensive to deal with

• A nuclear meltdown, such as at Chernobyl, could have catastrophic consequences on the environment and to the people living in the surrounding area

Benefits of nuclear power

• Nuclear power stations produce fewer polluting gases, little greenhouse gas emissions

• Nuclear power can be produced continuously (not dependent on wind or sunlight) so are reliable

• They require far less fuel as uranium provides far more energy per kg compared to coal and other fossil fuels

• produces medical isotopes

• some nuclear power stations can adjust their output quickly to account for demand

Points that affect nuclear energy in society

• Nuclear power can scare people if they do not understand it

• It is dangerous if not handled properly, yet it is invisible which can be difficult for some people to comprehend

• increased education on nuclear energy, society can use this knowledge to inform their own decisions and opinions