Module 6 Fission and Fusion Flashcards
Why is there a slight increase in mass when water at 100º is boiled to steam?
Gain in potential energy to separate particles. This is latent heat.
Plug into E=mc^2
Why is there a gain in mass during ionisation?
Energy is gained as electrostatic potential energy to separate opposite charges. Energy is converted to mass by E=mc^2.
Why can the mass gained during ionisation be ignored?
Negligible when compared to the original mass.
What can be said about the masses of particles during radioactive decay?
Total final mass is less than total initial mass.
What happens to the mass lost during nuclear decay?
Increases binding energy and gained as kinetic energy in products.
What is the symbol for a mass defect?
Δ m.
Why does a proton plus a neutron have higher mass than a deuterium nucleus?
There is a mass defect due to energy put in, to overcome the strong nuclear force.
What can you do when showing a diagram with three forces where RF=0?
Put it into a closed vector triangle.
How do you calculate the nuclear binding energy?
Find mass of original nucleus, find mass of separate protons and neutrons, find change in mass, plug into E=mc^2.
Define binding energy.
Minimum energy required to split a nucleus completely into its separate protons and neutrons.
Define mass defect.
[mass of protons + mass of neutrons] – mass of nucleus.
What is used to measure nuclear stability?
Binding energy per nucleon.
How do you calculate binding energy per nucleon?
Calculate binding energy, divide by A
(A is nucleon number)
What is the nucleon number of iron?
56.
What is the proton number of iron?
26.
What is the approximate value for the binding energy per nucleon of iron?
8.72 MeV.
What is the most stable nucleus?
Iron-56.
On binding energy per nucleon (y axis) against nucleon numer (x axis)
To what side of iron does fusion happen?
To the left side.
What happens during fusion?
Two nuclei fuse to form a larger one.
Explain, using binding energies, why fusion releases energy.
Two small nuclei join to form a larger one. Total binding energy increases. E=mc^2. Higher mass defect and so energy is released.
When is energy released during nuclear reactions?
Total final mass is less than initial mass. Mass defect (mass is lost). Energy is released according to E=mc^2 as kinetic energy of the products.
What must be done to calculate the energy released when two protons combine to form a hydrogen-2 atom?
Write out an equation. Find mass defect (mass of two protons) - (mass of hydrogen-2 and positron).
Find E using E=mc^2.
Positron will annihilate with an electron. Double mass of one electron and plug into equation E=mc^2.
Why is usually a very high temperature required for fusion?
High temperature to provide the high kinetic energy to overcome electrostatic repulsion so closest approach can be less than 3 fm and the strong nuclear force can come into play and bind nucleons.
Why is a high density required for nuclear fusion?
So there is a high probability of a head-on collision so maximum kinetic energy is transferred to electrostatic potential energy.
How to find the gain in potential energy of colliding particles during fusion when a temperature is given?
Calculate average kinetic energy using KE=3/2kT.
Gain in potential energy = Loss of kinetic energy + Loss of kinetic energy
(both particles lose all their kinetic energy in a head-on collision). So multiply kinetic energy by 2.
Describe how you find out the temperature at which fusion of two particles can happen?
Find the electrostatic potential energy which is required for r to be 3 fm using the Coulomb equation for energy.
Divide value by 2 to get the kinetic energy of one of the original particles in the collision.
Plug into KE=3/2kT and rearrange for T (in Kelvin).
Given the temperature of the core in the sun is 1.5x10^7K, explain why it has a long life.
Find the potential energy and thus kinetic energy and temperature required for fusion (r=3 fm).
Value will be greater than temperature.
Plasma of the sun has a Maxwell-Boltzmann distribution.
Very few protons have higher kinetic energy and can fuse. Chance of protons fusing is therefore low and has a longer lifetime.
Explain why helium nuclei are much more difficult to fuse when compared to hydrogen nuclei.
Each He nucleus has double the charge. When plugged into the Coulomb energy equation, potential energy increases by a factor of 4. Therefore average temperature required increases by a factor of 4.
Why does fission have a lower final mass when compared to initial?
Mass defect due to neutrons being un-bonded which requires the input of energy, which is converted to an extra mass. Therefore there is a decrease in mass (mass of initial particles - final). Energy is produced during the reaction and is converted to kinetic energy of particles.
What is spontaneous fission?
A large nucleus splits into two smaller nuclei plus several neutrons and releases energy.
What is induced fission?
Where a large nucleus absorbs a neutron and splits into two smaller nuclei and several neutrons and releases energy.
What is the basic principle of chain reactions?
Fission produces neutrons which can induce further fission.
Explain what must be done so fission reactions can be controlled.
Fission reactions must only produce one neutron so that only one further fission reaction is induced.
What are fuel rods?
Rods which are enriched with a higher percentage of U-235 which can undergo induced fission.
What is the fuel usually used in nuclear reactors?
Uranium-235.
What does uranium-235 turn into once it undergoes fission?
Two smaller nuclei plus on average two neutrons.
What is a moderator rod usually made of?
Solid carbon graphite (can also be of water).
What is the function of a moderator rod?
Absorb neutrons from fission and causes them to lose kinetic energy through many collisions, forming thermal neutrons with low kinetic energy which can be absorbed by U-235 inducing fission.
Why must moderator rods be used?
To decrease kinetic energy of neutrons forming thermal neutrons.
Why must thermal neutrons be used for fission?
Fast moving ones will rebound and not join nuclei.
What are control rods made of?
Boron, Cadmium.
What is the function of control rods?
Absorbs excess neutrons so that on average each fission will induce one further fission reaction.
How can a stable temperature of a nuclear reactor be maintained?
Lowering or raising control rods.
How can the rate of fission be controlled in a nuclear reactor?
Lowering or raising control rods.
What is nuclear waste?
Radioactive byproducts of fission reactions.
Why does some nuclear waste need storing?
Products are radioactive isotopes with long half-lives so will remain radioactive and potentially dangerous for years.
What is a percentage mass conversion of a sample?
Mass defect (energy converted) over original mass.
Compare the reactions (in practice) of fission and fusion.
Fission is a self-sustaining chain reaction that’s easy to start and control. Fusion requires very high temperatures and densities to start; therefore, a high input energy is required to start.
Why would a fission reactor require a large input energy?
Requires high temperatures and pressures/densities. Strong magnetic fields are needed to contain plasma.
Why are fission reactors not used commercially?
Have only been shown to be used experimentally and have not been shown to operate continuously.
Using the idea of fuel, explain why fusion may be advantageous over fission reactors.
Fission reactors require uranium-235 or other elements and isotopes which must be mined and enriched. Thus a finite resource. Fusion reactors only require isotopes of hydrogen which are plentiful in seawater.
Using the idea of nuclear waste, explain why fusion may be advantageous when compared to fission.
Fission reactors produce large amounts of radioactive waste which must be treated and stored for potentially thousands of years. Fusion reactors produce minimal radioactive waste.
Why are fusion reactors considered safer?
Fusion reactors may produce a lot of radioactive contamination if shielding is damaged. If control systems fail, a nuclear meltdown may happen. Fusion reactors will shut down if the plasma is not confined and will cool down immediately so fusion stops. Minimal danger.
Explain how you show the energy of each photon during an annihilation between a positron and an electron.
Find total energy using E=mc^2 (add individual energies). Divide final value by 2.
