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 no polluting gases
They are highly reliable for the production of electricity
They require far less fuel as uranium provides far more energy per kg compared to coal and other fossil fuels
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