Energy From The Nucleus (P20) Flashcards
By which process is energy released in a nuclear reactor?
Nuclear fission
What is nuclear fission?
The splitting of an atomic nucleus into two smaller nuclei, two or three neutrons and energy.
When does nuclear fission happen?
As a result of an atom absorbing a neutron and becoming unstable.
Why can nuclear fission be a chain reaction?
Each atom splitting releases two or three neutrons, which can each cause another fissionable nucleus to split.
What are the products of nuclear fission?
- Two smaller nuclei
- Two or three neutrons (at high speeds)
- Energy
What form is energy released in during nuclear fission?
Radiation and the kinetic energy of the neutrons and nuclei.
Which fuels are used in fission reactors?
- Enriched Uranium - Mostly U-238, which is not fissionable, and 2 or 3% of U-235, which is fissionable.
- Other types of reactor use plutonium-239.
What happens to the non-fissionable uranium-238 in a reactor?
The nuclei absorb neutrons and become other heavy nuclei, such as plutonium.
What does a nuclear reactor look like?
There are FUEL RODS evenly spaced in a steel REACTOR CORE. There are also CONTROL RODS and a COOLANT (water). The reactor core is surrounded by thick concrete.
How does a nuclear reactor work?
- The fission neutrons are slowed down by the atoms in the water molecules. This allows them to cause more fission reactions. The water is called a moderator.
- Control rods absorb surplus neutrons, keeping the chain reaction under control.
- The depth of the rods is adjusted to maintain a steady chain reaction.
- The water acts as a coolant. The molecules gain kinetic energy as it is pumped through the core. It then goes to a heat exchanger, transferring the energy to it.
- The reactor core is made of thick steel to withstand the high temperature and pressure. The core is surrounded by concrete to absorb escaping radiation.
Why is the reactor core made of steel?
To withstand the high pressure and temperature.
Why is the reactor surrounded by thick concrete?
To absorb escaping radiation.
How is a chain reaction controlled in a nuclear reactor?
The control rods absorb fission neutrons, ensuring that on average only 1 neutron per fission goes on to cause another fission reaction.
What is nuclear fusion?
The joining of two atomic nuclei to form a larger one, while releasing energy.
What happens when H-1 collides with H-2?
He-3 is formed.
When can nuclei fuse together?
- If they collide at high speeds
* If the relative mass of the resulting nucleus formed is no more than 55 (i.e. Approx. Iron)
Why can nuclei not fuse together to form nuclei heavier than iron?
It requires energy.
How does the Sun work?
- The Sun is made up of 75% hydrogen and 25% helium.
- The core is so hot that is only contains nuclei, with no electrons.
- By nuclear fusion, 2 hydrogen nuclei (a single proton) join together to form heavy helium (1 proton, 1 neutron).
- Another proton collides, making an nucleus with 2 protons and 1 neutron.
- Two of these collide to form helium (2 protons, 2 neutrons). This also, releases 2 protons.
How does the Sun release energy?
Nuclear fusion. This releases energy in the form of kinetic energy of the product nucleus and other particles emitted.
How are light elements formed?
Nuclear fusion of hydrogen nuclei in stars.
How are heavy elements formed?
When a massive star collapses and explodes.
Why is it difficult to make a fusion reactor?
• High temperature
• Plasma must be contained within magnetic field
This means that a lot of energy is required, and the energy produced must be greater than this to make it worthwhile.
What are the benefits of nuclear fusion (as an energy source)?
- No radioactive waste
- Clean energy -> No greenhouse gases
- Fuel is readily available
- Low fuel cost
What are differences between nuclear fusion and fission?
- Fission produces radioactive waste (which must be contained and stored), while fusion doesn’t.
- Fission requires rare radioactive material, while fusion fuel is readily available as heavy hydrogen in sea water.
- Fission is already used as an energy source, while fusion isn’t.
What are the similarities of nuclear fusion and fission?
- The energy released can be used to generate electricity.
* No burning of fossil fuels -> No atmospheric pollution
What is the Big Bang Theory?
A theory of how the universe began. It states that entire universe and everything in it was at one point compressed into one infinitely small point.
How old is the universe?
Approx. 13 billion years
What three things were created in the Big Bang?
Space, time, radiation
What happened at time zero (after the BB)?
- No concept of time
- Universe is infinitely small and hot
- No forces (e.g. gravity)
What happened at 0.1 seconds after the BB?
• Quarks and electrons are created from radiation
What happened at 100 seconds after the BB?
• Neutrons and protons form
What happened at 100,000 years after the BB?
• Hydrogen and helium atoms form
What is a galaxy?
A collection of billions of stars held together by their own gravity.
What happened during the Dark Age after the BB?
- Universe becomes transparent as radiation passes through the empty spots between atoms
- Microwave background radiation is released
What happened a few billion years after the BB?
• Gravity pulls uncharged particles together into clumps, which eventually become galaxies.
What was the Dark Age of the universe?
From the time when radiation could pass through the empty space between atoms up to the formation of stars.
What happened at 10 billion years after the BB?
The Sun and solar system was born.
What was the universe like billions of years before stars and galaxies formed?
A patchy cloud of hydrogen and helium.
What force is responsible for the formation of stars and galaxies?
Gravity (pulling matter into clumps)
Summarise the history of the universe.
• Quarks and electrons form - 0.1 seconds
• Neutrons and protons form - 100 seconds
• Hydrogen and helium atoms form - 100,000 years
(• Dark Age)
• First galaxies and stars form - Few billion years
What are stars formed out of?
A cloud of dust and gas
What is a protostar?
A gas and dust cloud that can go on to form a star.
What happens to a protostar?
- As it becomes denser, it gets hotter. If it becomes hot enough, nuclear fusion can happen, releasing energy and making the star even hotter.
- Objects that form but are too small to become stars can be attracted by a protostar and become planets.
What is a main sequence star?
A star in the main stage of its life, before it runs out of fuel for nuclear fusion.
What allows nuclear fusion to keep happening in a star?
The energy released in the core keeps it hot.
Why is a main sequence star stable?
The force of gravity acting towards the core is balanced by the force of radiation flowing outwards from the core.
What happens when a star becomes a red giant (in terms of elements)?
Helium and other light elements fuse to form heavier elements (but still lighter than iron).
What causes a main sequence star to become a red giant or red supergiant?
It runs out of hydrogen nuclei to fuse.
What causes a red giant or a red supergiant to collapse?
It runs out of light elements in its core to fuse.
What is a white dwarf?
A red giant that has collapsed and heated up, becoming white. It is a dense white star, much smaller in diameter than it was.
What is a black dwarf?
A white dwarf that has faded out, gone cold and turned black.
What is a supernova?
When a red supergiant collapses and compresses, it reaches a point where the compression suddenly reverses and explodes. The explosion is called a supernova.
What happens after a supernova?
If the red giant is big enough, it becomes a black hole. If it becomes a neutron star.
What is a neutron star?
A very dense object made of only neutrons.
What is a black hole?
A region of space with a gravitational field so strong that not even light can escape it.
What will eventually happen to the Sun?
It will become a red giant, then a white dwarf, and then a black dwarf.
What are the stages in the life of a star with a similar mass to the Sun?
Protostar -> Main sequence star -> Red giant -> White dwarf -> Black dwarf
What are the stages in the life of a star with a greater mass than the Sun?
Protostar -> Main sequence star -> Red supergiant -> Supernova -> Neutron star OR Black hole
When are light elements formed?
From fusion in stars. Main sequence stars fuse hydrogen nuclei (protons) into helium and other small nuclei. When these stars become red giants, they fuse helium and other small nuclei into larger nuclei.
When are heavier elements formed?
In supernova (when a red supergiant collapses and explodes). The force fuses the smaller nuclei together.
What does the debris of a supernova contain?
All of the known elements, both light and heavy.
How was the Sun and Solar System formed?
The debris from a supernova clumped together to form a star. Then, the debris around this clumped together to form orbiting planets.
Why are the rocky planets nearer to the Sun than the gaseous planets?
The Sun’s heat evaporated ice and drove gas away from the inner Solar System as it formed.
How is naturally-occurring uranium evidence that the Earth was formed from the remnants of a supernova?
It is the heaviest known natural element, and since it is heavier than iron, we know that it must have been made in a supernova.
How can nuclei be made to fuse together?
By making two light nuclei collide at a high speed.
What happened in Chernobyl?
In 1986, a nuclear reactor exploded in Ukraine. This resulted in fires and a cloud of radioactive material.
What is radon gas and why is it dangerous?
An alpha-emitting isotope which seeps into houses in certain areas through the ground.
Are nuclear reactors safe?
Yes. There are thousands in use around the world. None of them is the same type as Chernobyl.
What happens to nuclear waste?
Unused plutonium and uranium are removed and then the waste is stored is secure conditions for many years.
What are the risks of alpha radiation?
Inside the body - Very dangerous: Affects all the surrounding tissue
Outside the body - Some danger: Absorbed by skin, damages skin cells, damages retinal cells
What are the risks of beta and gamma radiation?
Dangerous - Reaches cells throughout the body
What are the risks of high doses of radiation?
It kills living cells.
What are the risks of low doses of radiation?
A risk of cancer developing in cells.
