7 Fission and fusion Flashcards
What do nuclear reactions involve?
They involve a change in the qualities of atoms.
What is fission?
Heavy atoms may split into lighter atoms and other pieces in a process called fission.
What is fusion?
Lighter atom may be forced to join together to make heavier atoms in a process called fusion.
What happens in both fission and fusion, in relation to the mass of the products?
The mass of the starting atoms is greater than the mass of the products.
Where does this missing mass go?
It has been converted into energy.
What provides a large proportion of the heat within the Earth itself through radioactive decay?
Within the core of the earth, radioactive isotopes of elements like uranium, thorium and potassium provide this large proportion of heat.
Where is hydrogen converted into helium?
In the sun.
Through what process is hydrogen converted into helium?
In a fusion reaction.
What is a fusion reaction?
When the centers of atoms (nuclei) join together.
What does this fusion reaction provide us with?
It provides us with a continuous supply of energy in the form of heat and other electromagnetic radiation.
Why is uranium-235 used as fuel in a nuclear reactor?
It is used because its nuclei can be split by a neutron.
What is this process of splitting an atom called?
Fission.
What kind of material is uranium-235?
A fissile material.
Why is uranium-235 called a fissile material?
This is because it goes through the splitting process easily.
What happens in this fission reaction?
A slow-moving neutron is absorbed by a nucleus of uranium-235.
What is a diagram which shows an example of fission of uranium-235?
What happens to the resulting nucleus of uranium-236?
It is unstable and splits apart.
What are the fragments of this decay?
The fragments of this decay are the two daughter nuclei of barium-144 and krypton-89.
What else does the decay produce?
The decay also produces gamma radiation and three more neutrons.
What is the equation for this decay?
236 144 1
92 U –> 56 Ba + 3 0 n + γ radiation.
What does fission produce in terms of energy?
The fission reaction produces a huge amount of energy.
Why does fission produce a large amount fo energy?
This is because of the mass of the products, the barium and krypton nuclei and the three neutrons is slightly less than that of the original uranium-236 nuclei, and the lost mass is converted into energy.
Where does this energy go?
- Most of this energy is carried away as the kinetic energy of the two lighter nuclei.
- Some is emitted as gamma radiation.
What are the two reasons as to why the fission process would repeat?
- The three neutrons produced by the fission may hit other nuclei of uranium-235, so causing the process to repeat.
- if one neutron from each fission causes one nearby uranium-235 to split, then the fission reaction will keep going.
How would fission get faster and faster?
If more than one neutron from each fission causes fission in surrounding nuclei, then the reaction gets faster and faster - a bit like an avalanche.
What does the process of fission causing additional nuclei to split cause?
A chain reaction.
What does each fission result in?
In more nuclei splitting apart.
What happens if the amount of uranium-235 is small?
If it is small, then many of the neutrons released do not hit other uranium nuclei and the reaction does not get faster and faster.
What is needed for a chain reaction?
For a chain reaction to happen there must be a minimum amount of the uranium-235.
What is this minimum amount called?
The critical mass.
How do we make sure that atomic bombs undergo chain reactions?
In an atomic bomb two pieces of fissile material (isotopes that can be triggered into splitting apart) that are smaller than the critical mass are forced together under high pressure to form a mass greater than the critical mass.
What is the result of forming a mass greater than the critical mass in atomic bombs?
The result is a chain reaction with the rapid and uncontrolled release of huge amounts of energy.
What is an image of a chain reaction in uranium-235?
Where is another place where chain reactions take place in controlled conditions?
In a nuclear reactor.
What happens if this is allowed to take place in a nuclear reactor?
The reactor core overheats, resulting in a nuclear explosion with the sudden release of enormous amounts of heat energy and radiation.
How is the nuclear fission controlled in a nuclear reactor?
In a nuclear reactor the process is controlled so that the heat energy is released over long periods of time.
What is done with the heat produced in the core?
The heat produced in the core or heart of the reactor is used to heat water.
Where does the steam produced from the water go?
The steam produced then drives turbines (engines) to turn generators.
What is an image of the basic parts of a nuclear reactor?
What does the reactor core contain?
The reactor core contains fuel rods of enriched uranium.
What is enriched uranium?
Enriched uranium is uranium-238 with a higher proportion of uranium-235 than is found in the natural reserves of uranium.
Where is graphite found?
It is used as a moderator.
What is a moderator?
A material used in nuclear reactors to make neutrons move more slowly.
What is the job of the moderator?
It is to absorb some of the kinetic energy of the neutrons to slow them down.
Why do we need to slow down the neutrons?
This is because slow neutrons are more easily absorbed by uranium-235.
What can start the fission process in relation to neutrons and their speed?
A neutron slowed in this way can start the fission process.
What is another component of the nuclear reactor?
Control rods.
What are the control rods?
Rods used in a nuclear reactor to slow down or stop a nuclear chain reaction.
What are these control rods usually made of?
Boron or cadmium.
What do these control rods do?
These absorb the neutrons and take them out of the fission process completely.
What happens when the control rods are fully inside the core?
When the control rods are fully inside the core, the chain reaction is almost completely stopped.
What happens along with the chain reaction being completely stopped in terms of heat?
The rate of production of heat is low.
What happens when the control rods are withdrawn?
The rate of fission increases producing heat at a greater rate.
What prevents any radiation from escaping?
The reactor vessel.
What is this reactor vessel usually made out of?
The reactor vessel is made of steel and surrounded by a concrete layer about 5 meters of thickness.
What does the nuclear process in a reactor produce?
It produces a variety of different types of radioactive material.
What are the characteristics of one type of radioactive material produced in a nuclear reactor?
Some have relatively short half-lives and decay rapidly.
What is an advantage of these radioactive materials with short half-lives that decay rapidly?
These soon become safe to handle and do not present problems of long-term storage.
What are the characteristics of the second type of radioactive material produced in a nuclear reactor?
Other materials have extremely long half-lives.
What is the problem with these radioactive materials which have extremely long half-lives?
These will continue to produce dangerous levels of ionising radiation for thousands of years.
What problem do these waste products bring about?
These waste products present a serious problem for long-term storage.
Where are these waste products usually stores?
They are usually sealed (closed) in containers which are then buried deep underground.
What is important about where they are stored?
The sites for underground storage have to be carefully selected.
What is the most optimal site to store this waste?
The rock must be water resistant and the geology of the site must be stable
Where is a bad idea to store waste?
Storing waste in earthquake zones or areas of volcanic activity would not be sensible.
What are some reactors designed to produce?
Some reactors are designed to produce plutonium.
What is plutonium?
Plutonium is a very radioactive artificial element.
What is the problem with plutonium?
Small amounts of plutonium represent a serious danger to health.
What kind of material is plutonium?
Plutonium is another fissile material.
How can a chain reaction start from plutonium?
If a large enough mass of plutonium is brought together a chain reaction will start.
What can be plutonium be used for?
Plutonium can be used in the production of nuclear weapons.
What is an advantage of nuclear power stations?
Nuclear power stations do not produce carbon dioxide or acidic gases as fossil fuel power stations do.
What is good about the fact that nuclear power stations don’t produce carbon dioxide in terms of the climate?
This means that nuclear power does not contribute to global warming or acid rain.
What is another advantage of nuclear power stations and their longevity?
Only small amounts of uranium are needed for a chain reaction and the supply of nuclear fuel will last many hundreds of years - unlike some fossil fuels that could run out in the next fifty years.
What is a diagram that shows how a nucleus of deuterium collides with a nucleus of tritium? They undergo fusion to form the nucleus of helium, a neutron and a large amount of energy.
What is a short explanation of what is going on in terms of energy?
They undergo fusion to form the nucleus of helium, a neutron and a large amount of energy.
What is an explanation of the whole diagram?
Two isotopes of hydrogen, deuterium (H-2) and tritium (H-3), collide at very high speed. The result is the formation of a helium nucleus, a neutron and a large amount of energy.
What is the energy source for our sun and all stars?
The fusion process.
What is a diagram that shows how the mass of the products of fusion is smaller than the two hydrogen nuclei?
What are the materials used in fusion?
This process uses materials that are more readily available than the uranium used in a conventional fission reactor
What is an advantage of fusion?
It has the advantage of producing no radioactive waste.
What is a disadvantage of fusion?
The problem is creating the very high temperatures needed to make the deuterium and tritium nuclei collide.
Why do deuterium and tritium need very high temperatures to collide?
This is because a lot of energy is needed to overcome the repulsive (pushing away) force between the positive electric charge in the nuclei of each isotope.
Where should the high temperature (100 million*C) liquid be contained?
With a very strong magnetic field or ‘magnetic bottle’.
How can you increase the chance of fusion between the light nuclei in the hot liquid?
The pressure within the liquid must be very high.
