Nuclear Fission Reactors
- Nuclear reactors use rods of uranium that are rich in \(^{235}\)U (or sometimes plutonium rods rich in \(^{239}\)Pu as fuel for fission reactions. (The rods also contain other isotopes, but they don't undergo fission.)
- These are placed into the reactor remotely, which keeps workers as far away from the radiation as possible.
- These fission reactions produce more neutrons which then induce other nuclei to fission - this is called a chain reaction.
- The neutrons will only cause a chain reaction if they are slowed down, which allows them to be captured by the uranium nuclei - these slowed down neutrons are called thermal neutrons.
- Fuel rods need to be placed in a moderator (for example, water) to slow down and/or absorb neutrons. You need to choose a moderator that will slow down some neutrons enough so they can cause further fission, keeping the reaction going at a steady rate.
- The moderator slows down neutrons through elastic collisions (kinetic energy is conserved) with nuclei of the moderator material. When neutrons collide with particles that are of a similar mass, they are slowed down more efficiently.
- Water is often used as a moderator since it contains hydrogen, which is of a similar mass to a neutron.
- You want the chain reaction to continue on its own at a steady rate, where one fission follows another. The amount of 'fuel' you need to do this is called the critical mass - any less than the critical mass (sub-critical mass) and the reaction will just peter out.
- Nuclear reactors use a supercritical mass of fuel (where several new fissions normally follow each fission) and control the rate of fission using control rods.
- Control rods control the chain reaction by limiting the number of neutrons in the reactor. They absorb neutrons so that the rate of fission is controlled.
- Control rods are made up of a material that absorbs neutrons (e.g. boron), and they can be inserted by varying amounts to control the reaction rate.
- Coolant is sent around the reactor to remove heat produced by fission. The material used should be a liquid or gas at room temperature, and be efficient at transferring heat.
- Often the coolant is the same water that is being used in the reactor as a moderator. The heat from the reactor can then be used to make steam for powering electricity-generating turbines.
Reactor shielding
- The nuclear reactor is surrounded by a thick concrete case, which acts as shielding. This prevents radiation escaping and reaching the people working in the power station.
Emergency shut-down
- In an emergency, the reactor can be shut down automatically by the release of control rods into the reactor. The control rods are lowered fully into the reactor, which slows down the reaction as quickly as possible.
Handling and storing fission waste products
- Unused uranium fuel rods emit only alpha radiation, which is weakly penetrating and so is easily contained.
- Spent fuel rods are more dangerous, since fission waste products usually have a larger proportion of neutrons than nuclei of a similar atomic number - this makes them unstable and radioactive.
- The fission waste products emit beta and gamma radiation, which are strongly penetrating. The products can be used for practical applications such as tracers in medical diagnosis. However, their handling and storage needs great care since they are highly radioactive.
- When material is removed from the reactor, it is initially very hot, so it is placed in cooling ponds until the temperature falls to a safe level. This is done remotely - just like the handling of fuel - to limit the radiation that workers are exposed to. The radioactive waste should then be stored in sealed containers until its activity has fallen sufficiently.
- Many countries, including the UK, use nuclear power to generate electricity. There are some huge benefits to generating electricity using nuclear power.
- Unlike with burning fossil fuels such as coal, oil and gas, there is enough fuel for us to keep generating electricity using nuclear power for centuries to come. The process also doesn't release greenhouse gases which can affect our atmosphere.
- The biggest benefit of nuclear power is its efficiency (the energy produced per unit mass of fuel) - it generates many thousands times more electrical energy per kg of nuclear fuel than you would get per kg of fossil fuel. However, all these benefits need to be weighed up against the risks of nuclear power.
- Nuclear reactors have to be designed and built extremely carefully to minimise the danger of a nuclear disaster. One of the biggest risks comes from how we deal with the waste produced and making sure it doesn't endanger people or the environment.
- It's important that our society properly understands the science, benefits and risks of nuclear power to be able to make informed decisions, e.g. on whether to build and develop more nuclear power stations, where to build power stations and store waste etc. It will never be a risk-free way of generating electricity, but there are lots of measures in place to make it as safe as possible.