Uranium or plutonium isotopes 25 April 2017 Today’s Title: CW: nuclear power Learning Question: how does a nuclear reactor work? Starter – What two elements are usually involved in nuclear fission? Uranium or plutonium isotopes

Key words… Nuclear reactors Core Control rods Moderators Radioactive waste

Nuclear reactors Nuclear energy from the nucleus of uranium or plutonium atoms are transferred into thermal energy using nuclear fission. Uranium pellets are inserted into long, hollow fuel rods. The rods are then placed inside the reactor core.

Nuclear reactors The rate at which nuclear energy is converted to thermal energy has to be kept constant by controlling the chain reaction. If just one of the neutrons released by decay of a uranium nucleus is absorbed by another uranium nucleus, this will cause it to decay. This will make the chain reaction keep going at a constant rate, as the number of neutrons will not increase or decrease. If too many neutrons are released, too many chain reactions will go on at the same time and the reaction will go too fast.

Controlling fission In order to control fission, the extra neutrons that released by the decay of each nucleus have to be absorbed. Control rods in the reactor core carry out this job. Control rods contain elements that absorb neutrons. These rods are placed between the fuel rods in the reactor core.

Controlling fission If the rate of fission needs to be increased, control rods are moved out This means fewer neutrons are absorbed, so the chain reaction goes faster. When the control rods are fully lowered into the core, the chain reaction will stop and the nuclear reactor will shut down.

Testing your knowledge What energy transfer takes place in a nuclear reactor? A: Nuclear energy to thermal energy. 2. Why must the chain reaction in a nuclear reactor be controlled? A: To maintain a steady rate of fission – if this does not happen, the reactor could explode like a nuclear bomb.

Moderators Neutrons produced from nuclear fission of uranium-235 nucleus move very fast. These neutrons have to be slowed down to ensure that they will be absorbed by another uranium-235 nucleus (if this didn’t happen, no power would be generated) Another material is put into the reactor core to make sure that this happens. This material is known as a moderator.

Testing your knowledge In a nuclear reactor, what are the functions of: a the control rods? A: To absorb neutrons. b a moderator? A: To slow down neutrons. 4. What materials in a reactor core can affect the speed of the neutrons emitted in nuclear fission?

Fuel produces heat and heat is used to boil water and is turned into steam Steam turns a turbine Turbine turns a generator and the generator makes electricity Electricity goes to the transformers to produce the correct voltage

Testing your knowledge 5. Draw a flow diagram to show how nuclear energy is transformed into electrical energy in a nuclear power station. A: Flow diagram should include the following stages: nuclear energy → kinetic energy → thermal energy → kinetic energy → electrical energy

Radioactive waste After a period of time, radioactive waste builds up in the reactor core. This waste is made up of radioactive daughter nuclei and radioactive isotopes formed when the materials in the core absorb neutrons At present the most dangerous waste is sealed in glass-like blocks which are buried deep within rocks. Careless disposal of waste in the past has led to pollution of land, rivers and the ocean.

Testing your knowledge 6. Explain what is done to increase the temperature of the core of a nuclear reactor. Raise the control rods to increase the rate of the chain reaction. 7. Describe how control rods are used to control the rate of the chain reaction in a reactor core. The rate of reaction needs to be maintained at only one neutron from each fission going on to cause another fission. If the reactor core is cooling down, the rate of reaction needs to be increased. Control rods are raised to increase the reaction rate and hence the temperature. If the reactor is heating up too much, the rate of reaction needs to be decreased. Control rods are lowered to reduce the reaction rate and hence the temperature.