Fission and Fusion
LEARNING OBJECTIVES 7.17 know that nuclear reactions, including fission, fusion and radioactive decay, can be a source of energy 7.18 understand how a nucleus of U-235 can be split (the process of fission) by collision with a neutron, and that this process releases energy as kinetic energy of the fission products 7.19 know that the fission of U-235 produces two radioactive daughter nuclei and a small number of neutrons 7.20 describe how a chain reaction can be set up if the neutrons produced by one fission strike other U-235 nuclei 7.21 describe the role played by the control rods and moderator in the fission process 7.22 understand the role of shielding around a nuclear reactor 7.23 explain the difference between nuclear fusion and nuclear fission 7.24 describe nuclear fusion as the creation of larger nuclei resulting in a loss of mass from smaller nuclei, accompanied by a release of energy 7.25 know that fusion is the energy source for stars 7.26 explain why nuclear fusion does not happen at low temperatures and pressures, due to electrostatic repulsion of protons
Fission = splitting the atom ATOMIC STRUCTURE Fission = splitting the atom Stray neutron Uranium 235 nucleus A neutron strikes a nucleus of Uranium 235 – this becomes unstable, splits into two lighter nuclei and releases 2 or 3 neutrons.
Fission = splitting the atom ATOMIC STRUCTURE Fission = splitting the atom Stray neutron Uranium 235 nucleus The emitted neutrons go on to split other nuclei, and so on … the result is a chain reaction, releasing huge amounts of energy A neutron strikes a nucleus of Uranium 235 – this becomes unstable, splits into two lighter nuclei and releases 2 or 3 neutrons.
Fission = splitting the atom In a nuclear power station we want these chain reactions to produce lots of energy, but the reactions need to be controlled. How can we achieve this?
CONTROLLING THE REACTION The energy from the nuclear chain reaction is used to heat water to make steam. This steam drives turbines, which in turn drive the generators and so make electricity. Hot water out Nuclear fuel in core Cold water in Steel pressure vessel
CONTROLLING THE REACTION The energy from the nuclear chain reaction is used to heat water to make steam. This steam drives turbines, which in turn drive the generators and so make electricity. In many reactors the fuel is uranium dioxide (natural uranium enriched with extra U-235.) The fuel is in sealed tubes. Hot water out Nuclear fuel in core Cold water in Steel pressure vessel
CONTROLLING THE REACTION The energy from the nuclear chain reaction is used to heat water to make steam. This steam drives turbines, which in turn drive the generators and so make electricity. To slow down the rate of reaction, water acts as a moderator, slowing down the neutrons. In many reactors the fuel is uranium dioxide (natural uranium enriched with extra U-235.) The fuel is in sealed tubes. Hot water out Nuclear fuel in core Cold water in Steel pressure vessel
CONTROLLING THE REACTION To slow down the rate of reaction, water acts as a moderator, slowing down the neutrons. Control rods, containing boron or cadmium, can be lowered between the nuclear fuel tubes. These rods absorb neutrons, so controlling the rate of reaction in the core. Hot water out Nuclear fuel in core Cold water in Steel pressure vessel
CONTROLLING THE REACTION To slow down the rate of reaction, water acts as a moderator, slowing down the neutrons. Control rods, containing boron or cadmium, can be lowered between the nuclear fuel tubes. These rods absorb neutrons, so controlling the rate of reaction in the core. Hot water out Nuclear fuel in core Cold water in Steel pressure vessel
CONTROLLING THE REACTION To slow down the rate of reaction, water acts as a moderator, slowing down the neutrons. Control rods, containing boron or cadmium, can be lowered between the nuclear fuel tubes. These rods absorb neutrons, so controlling the rate of reaction in the core. Hot water out Nuclear fuel in core Cold water in Steel pressure vessel
CONTROLLING THE REACTION To slow down the rate of reaction, water acts as a moderator, slowing down the neutrons. Control rods, containing boron or cadmium, can be lowered between the nuclear fuel tubes. These rods absorb neutrons, so controlling the rate of reaction in the core. Fuel rods must be replaced every three or four years. This is a fully automated process. Hot water out Nuclear fuel in core Cold water in Steel pressure vessel
CONTROLLING THE REACTION To slow down the rate of reaction, water acts as a moderator, slowing down the neutrons. Control rods, containing boron or cadmium, can be lowered between the nuclear fuel tubes. These rods absorb neutrons, so controlling the rate of reaction in the core. The safe disposal of spent nuclear material is still a huge environmental problem. Hot water out Nuclear fuel in core Cold water in Steel pressure vessel
Fusion = joining small nuclei ATOMIC STRUCTURE Fusion = joining small nuclei
Nuclear Fission and Nuclear Fusion are opposites! ATOMIC STRUCTURE Fusion = joining small nuclei Nuclear Fission and Nuclear Fusion are opposites! Nuclear Fission Nuclear Fusion
NUCLEAR FUSION Fusion = joining small nuclei Nuclear Fission involves splitting a large unstable nucleus Nuclear Fusion involves joining small nuclei together
Fusion = joining small nuclei NUCLEAR FUSION Fusion = joining small nuclei For example, look what happens when two light hydrogen nuclei collide at high speed. H 2 1 Fusion 1 H 1
Fusion = joining small nuclei NUCLEAR FUSION Fusion = joining small nuclei The hydrogen nuclei fuse (join together) to produce a larger, heavier helium nucleus. H 2 1 Fusion 1 H 3 He 1 2
Fusion = joining small nuclei NUCLEAR FUSION Fusion = joining small nuclei The helium nucleus has less mass compared with the two separate hydrogen nuclei. Energy is released as radiation. Energy H 2 1 Fusion 1 H 3 He 1 2
Fusion = joining small nuclei NUCLEAR FUSION Fusion = joining small nuclei For any given mass of fuel fusion produces far more energy than nuclear fission! Nuclear Fusion
Holy Grail Safe, clean energy supplied by nuclear fusion Fusion = joining small nuclei Holy Grail Safe, clean energy supplied by nuclear fusion This is what we want!
NUCLEAR FUSION Nuclear fusion is the energy source of stars, such as our Sun Fusion = joining small nuclei Why can’t we re-create this energy source on Earth?
NUCLEAR FUSION Problems, problems, problems…. Fusion = joining small nuclei The two hydrogen nuclei will try to repel each other (both are positively charged)
NUCLEAR FUSION Problems, problems, problems…. Fusion = joining small nuclei The two hydrogen nuclei will try to repel each other (both are positively charged) Very high temperatures (100 million oC) must be contained within a very strong magnetic field
NUCLEAR FUSION Problems, problems, problems…. Fusion = joining small nuclei The two hydrogen nuclei will try to repel each other (both are positively charged) Very high temperatures (100 million oC) must be contained within a very strong magnetic field To increase the chances of fusion between nuclei the pressures must be very high
NUCLEAR FUSION Problems, problems, problems…. Fusion = joining small nuclei The two hydrogen nuclei will try to repel each other (both are positively charged) Very high temperatures (100 million oC) must be contained within a very strong magnetic field To increase the chances of fusion between nuclei the pressures must be very high Nuclear fusion reactors are really hard and expensive to build. It’s like producing a model Sun in the laboratory! Perhaps one day …..
LEARNING OBJECTIVES 7.17 know that nuclear reactions, including fission, fusion and radioactive decay, can be a source of energy 7.18 understand how a nucleus of U-235 can be split (the process of fission) by collision with a neutron, and that this process releases energy as kinetic energy of the fission products 7.19 know that the fission of U-235 produces two radioactive daughter nuclei and a small number of neutrons 7.20 describe how a chain reaction can be set up if the neutrons produced by one fission strike other U-235 nuclei 7.21 describe the role played by the control rods and moderator in the fission process 7.22 understand the role of shielding around a nuclear reactor 7.23 explain the difference between nuclear fusion and nuclear fission 7.24 describe nuclear fusion as the creation of larger nuclei resulting in a loss of mass from smaller nuclei, accompanied by a release of energy 7.25 know that fusion is the energy source for stars 7.26 explain why nuclear fusion does not happen at low temperatures and pressures, due to electrostatic repulsion of protons
Fission and Fusion