Fission & Fusion (Edexcel IGCSE Physics (Modular): Unit 2): Flashcards
Exam code: 4XPH1
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Define nuclear fusion.
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Define nuclear fusion.
Nuclear fusion is when two small nuclei join together to produce a larger nucleus.
Define nuclear fission.
Nuclear fission is when one large nucleus splits into two smaller nuclei.
What are the three sources of nuclear energy?
Nuclear fusion, nuclear fission and radioactive decay.
What nucleus is formed when deuterium and tritium nuclei fuse, and what else is released?
A helium nucleus is formed, with the release of energy.
The energy from 1 kg of hydrogen that undergoes fusion is equivalent to the energy from burning about ______ of coal.
The energy from 1 kg of hydrogen that undergoes fusion is equivalent to the energy from burning about 10 million kilograms of coal.
In nuclear fission, what are the parent nucleus and daughter nuclei?
The parent nucleus is the large nucleus that splits; the daughter nuclei are the smaller nuclei that are produced.
True or False?
Nuclear fusion occurs naturally on Earth.
False.
Nuclear fusion does not happen naturally on Earth; it occurs naturally in stars, although fusion reactors are currently in development on Earth.
Define nuclear fission.
Nuclear fission is the splitting of a large, unstable nucleus into two smaller nuclei.
Define a fissile material.
A fissile material (such as certain isotopes of uranium and plutonium) can undergo fission under the right conditions, making it suitable as a fuel in nuclear power stations.
What is the difference between spontaneous and induced fission?
In spontaneous fission, an unstable nucleus splits without extra energy being added. In induced fission, the nucleus first absorbs a neutron, which makes it unstable enough to decay almost immediately.
Why must the fission of uranium-235 be induced rather than left to occur spontaneously?
Uranium-235 has a very long half-life (700 million years), so it has low activity and releases energy too slowly to be useful in a nuclear power station.
When a uranium-235 nucleus absorbs a neutron, it becomes uranium-236, which is very ______ and splits almost immediately by nuclear fission.
When a uranium-235 nucleus absorbs a neutron, it becomes uranium-236, which is very unstable and splits almost immediately by nuclear fission.
What three things are produced when a uranium-235 nucleus undergoes induced fission?
Two smaller daughter nuclei, two or three neutrons, and gamma rays.
True or False?
In fission, energy is transferred from the nuclear potential energy of the original nucleus into the kinetic energy of the products.
True.
This kinetic energy of the fast-moving fission products can be harnessed and converted into electrical energy in a nuclear power station.
Define a chain reaction.
A chain reaction occurs when a neutron emitted from the splitting of a nucleus causes further nuclei to split, and the neutrons emitted from these cause further fission reactions.
Define critical mass.
The critical mass is the minimum mass of fissile material required for a chain reaction to be maintained.
What is the purpose of the control rods in a nuclear reactor?
To absorb neutrons. Lowering the rods further decreases the rate of fission (more neutrons absorbed); raising the rods increases the rate of fission (fewer neutrons absorbed).
What is the purpose of the moderator in a nuclear reactor?
To slow down the fast-moving neutrons produced by fission, so they reach thermal equilibrium with the moderator, becoming thermal neutrons that can react efficiently with the uranium fuel.
What is the purpose of the shielding around a nuclear reactor, and what is it made from?
To absorb hazardous radiation emitted by the daughter nuclei and neutrons produced during fission. It is made from a wall of steel and concrete, nearly 2 metres thick.
If the mass of fissile material exceeds the ______, the rate of reaction accelerates, which can cause a huge, uncontrolled release of energy.
If the mass of fissile material exceeds the critical mass, the rate of reaction accelerates, which can cause a huge, uncontrolled release of energy.
True or False?
Lowering the control rods further into a nuclear reactor increases the rate of fission.
False.
Lowering the control rods further decreases the rate of fission, because more neutrons are absorbed.
Define nuclear fusion.
Nuclear fusion is when two light nuclei join together to form a heavier nucleus.
Define the fuel required for nuclear fusion.
Isotopes of hydrogen, which are found in water.
Why has nuclear fusion proven very hard to reproduce on Earth?
It requires extremely high temperatures to maintain, which are very difficult to achieve and sustain outside of stars.
Where does the energy released during nuclear fusion come from?
A very small amount of the reacting particles' mass is converted into energy.
Give three advantages of fusion reactors over fission reactors.
Fusion generates more energy than fission per kilogram of fuel, the fuel (hydrogen isotopes from water) is more abundant than uranium or plutonium, and fusion produces no long-lived nuclear waste.
Compared with fission reactors, the conditions required for nuclear fusion are much ______ to achieve and maintain on Earth.
Compared with fission reactors, the conditions required for nuclear fusion are much harder to achieve and maintain on Earth.
True or False?
Nuclear fusion reactors are currently a commercially viable method of generating electricity.
False.
Fusion reactors are not yet commercially viable for generating electricity, although they are in development.
Define nuclear fusion, as it occurs in stars.
In the centre of a star, hydrogen nuclei undergo nuclear fusion to form helium nuclei, releasing a huge amount of energy.
Define electrostatic repulsion, as it applies to nuclear fusion.
The repulsive force between two positively charged protons. Nuclei must have very high kinetic energy to overcome this repulsion and get close enough to fuse.
What prevents a star from collapsing under its own gravity?
The heat produced by nuclear fusion provides an outward pressure that balances the inward pull of gravity, keeping the star in equilibrium.
What happens in larger stars once the core temperature is hot enough?
Helium nuclei can fuse together to form heavier elements.
Why are extremely high pressures (densities), in addition to high temperatures, required for nuclear fusion to occur?
High densities increase the number of collisions between nuclei, which increases the chance that fusion reactions take place.
Nuclear fusion requires extremely high ______ and extremely high ______.
Nuclear fusion requires extremely high temperatures and extremely high pressures.
True or False?
On Earth, under normal conditions, fusion reactions between nuclei occur very frequently.
False.
Under normal conditions on Earth, the possibility of collisions between nuclei resulting in fusion is very low; the necessary high temperatures and pressures are usually only found in the cores of stars.
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