Safety Issues in Nuclear Power
Nuclear Fuel
- The fuel used in nuclear reactors is called enriched uranium
- This is U-238 enriched with U-235 as U-235 is the isotope that undergoes fission
- The U-238 isotope absorbs fission neutrons which helps to control the rate of fission reactions
- Several measures are in place to reduce the worker’s exposure to radiation
- The fuel rods are handled remotely ie. by machines
- The nuclear reactor is surrounded by a very thick lead or concrete shielding, which ensures radiation does not escape
- In an emergency, the control rods are fully lowered into the reactor core to stop fission reactions by absorbing all the free neutrons in the core, this is known as an emergency shut-down
Nuclear Waste
- There are three main types of nuclear waste:
- Low level
- Intermediate level
- High level
- Low-level waste
- This is waste such as clothing, gloves and tools which may be lightly contaminated
- This type of waste will be radioactive for a few years, so must be encased in concrete and stored a few metres underground until it can be disposed of with regular waste
- Intermediate-level waste
- This is 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 must be encased in cement in steel drums and stored securely underground
- High-level waste
- This waste comprises of the unusable fission products from the fission of uranium-235 or from spent fuel rods
- This is by far the most dangerous type of waste as it will remain radioactive for thousands of years
- As well as being highly radioactive, the spent fuel roads are extremely hot and must be handled and stored much more carefully than the other types of waste
- How high-level waste is treated:
- 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
Risks & Benefits of Nuclear Power
- Benefits
- 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
- Risks
- 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
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
- However, with increased education on nuclear energy, society can use this knowledge to inform their own decisions and opinions
Moderation of Fission Reactors
- During fission, neutrons are released with high energies and must be slowed down by water moderation to maintain the chain reaction
- 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
- In these subsequent collisions, 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 (after about 50 collisions), the neutrons reach speeds associated with thermal random motion (hence the name thermal neutron)
- At these speeds, neutrons can cause fission rather than rebound off of the uranium nuclei
The process of moderation changes fast neutrons to slow neutrons
Materials Used for Nuclear Reactor Components
- Moderators must be made from light nuclei which are not fissionable and will not absorb neutrons but will absorb a large amount of energy from them
- Graphite and water are commonly used for moderators
- Control rods must be made with non-fissionable materials
- This is so that they can absorb excess neutrons without decaying themselves
- Boron and cadmium are commonly used for control rods
- Often water is used as both the coolant and moderator
- This is because it has a high specific heat capacity meaning it can transfer large amounts of thermal energy
- Other materials such as molten salt or inert gas (e.g helium) are sometimes used as a coolant
- Another important component of a nuclear reactor is shielding
- Alpha and beta radiation can be stopped by a few cm of material, however, gamma radiation is much more penetrating
- Therefore, lead or concrete is needed to ensure there are no radiation leakages
- A summary of materials used are shown in the table below:
Materials for Moderators Table
