3.8.1.7-8 Induced fission pt2 and Safety aspects Flashcards
What are the components of a nuclear reactor?
Moderator
Control rods
Coolant
What is the purpose of the moderator?
Material that surrounds the fuel rods and control rods inside the reactor core.
Fast-moving neutrons produced by the fission reactions slow down by colliding with the molecules of the moderator, causing them to lose some momentum.
The neutrons are slowed down so they are in thermal equilibrium with the moderator (hence thermal neutrons).
This ensures they can react efficiently with the uranium fuel.
What is the purpose of the control rods?
They absorb neutrons.
The number of neutrons absorbed is controlled by varying the depth of the control rods in the fuel rods.
Lowering the rods further decreases the rate of fission, as more neutrons are absorbed. Raising them increases rate of fission as fewer neutrons are absorbed.
This is adjusted automatically so that exactly one fission neutron produced by each fission event goes on to cause another fission.
What is the purpose of the coolant?
To remove heat released by fission reactions.
It carries the heat to an external boiler to produce steam.
The steam then goes on to power electricity-generating turbines.
How does moderation of fission reactors work?
The first few collisions of a neutron with the moderator transfer sufficient energy to excite nuclei in the moderator with the neutrons being absorbed.
The subsequent collisions of a neutron with the moderator are elastic - momentum is transferred to the moderator atoms. With each collision, the neutron slows down until the average kinetic energy of the neutrons corresponds to that of the moderator nuclei. Eventually, neutrons reach speeds associated with thermal random motion, so can cause fission.
What materials are used for moderators?
Graphite and water
- must be made from light nuclei which are not fissionable and will not absorb neutrons, but will absorb their energy
What material is used for the control rods?
- non fissionable materials, so they can absorb excess neutrons without decaying themselves.
Boron and cadmium
What material is used as a coolant?
Water
- has a high specific heat capacity meaning it can transfer large amounts of thermal energy.
(inert gas or molten salt e.g. He can also be used)
What is shielding?
Lead is used.
To prevent radiation leakages
What measures can be taken to reduce a worker’s exposure to radiation?
Fuel rods are handled remotely - by machines
Nuclear reactor is surrounded by lead or concrete shielding
In an emergency, control rods are fully lowered into the reactor core to stop fission reactions by absorbing all free neutrons (emergency shut-down)
What is low-level nuclear waste?
Waste such as clothing or tools which is lightly contaminated.
It will be radioactive for a few years, so must be encased in concrete and stored a few metres underground before being disposed of normally.
What is intermediate-level nuclear waste?
Everything between daily used items and fuel rods themselves.
Usually produced when a nuclear power station is decommissioned.
Has a longer half life, so stored in steel drums and stored securely underground.
What is high-level nuclear waste?
From unstable fission products or fission of Uranium-235 or spent fuel rods.
Most dangerous, radioactive for thousands of years.
Fuel rods are extremely hot and must be handled and stored more carefully than other types of waste.
How is high-level waste treated?
Initially placed in cooling ponds close to the reactor for a number of years.
Isotopes of U and Pu are harvested to be reused.
Waste is mixed with molten glass and made solid (vitrification)
It is encased in containers made from steel, lead or concrete.
It is stored very deep underground.
What are the benefits of nuclear power?
No polluting gasses
Highly reliable
Less fuel required compared to fossil fuels (uranium provides more energy per kg)
