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?

the minimum mass of fuel required to maintain a steady chain reaction

What is a thermal neutron?

A neutron that is slow moving

What is a chain reaction?

where each fission goes on to cause at least one more fission

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/disadvantages of using water as a moderator in nuclear reactors.

Inexpensive and not very reactive. Contains hydrogen nuclei which are most similar to the size of a neutron.

The water must be highly pressurized to remain liquid at high temperatures

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

Why is water a good material to use for coolant? What are its disadvantages?

It has a high specific heat capacity meaning it can transfer large amounts of thermal energy. Water is also inexpensive.

It has a high potential for radioactive contamination, and needs high pressure to achieve the right temperatures, which is an explosion risk.

Why is helium a good material to use for coolant? What are its disadvantages?

Helium has a higher specific heat capacity than water and molten salt, so it can transfer large amounts of thermal energy.

More expensive than molten salt or water.

Why is molten salt a good material to use for coolant? What are its disadvantages?

Can transfer large amounts of thermal energy due to a high heat capacity. It does not have to be highly pressurised so is less of an explosion risk. However its specific heat capacity is lower than that of water and helium.

Properties that must be considered when choosing a coolant for a nuclear reactor

• Ability to absorb neutrons (should not absorb neutrons)

• Stability under high temperatures and high levels of radiation

• Non-corrosive/unreactive

• Should have a high boiling point

• (low) viscosity

• (high) specific heat capacity