Nuclear Physics 2 Flashcards
How do you get the mass of a nucleus in this topic
Deal with protons and neutrons separately
Use the values to the full 4sf
What is mass defect
The mass of the nucleus is less than the mass of its constituents for all nuclei regardless of being ions or isotopes
Given in kg and usually very very small
Binding energy
Energy released when nucleons come together to form the nucleus (loss of mass means loss of energy)
Energy needed to split an atom into its constituent parts (nucleons) (increase in mass requires an input in energy)
Where has the mass gone when the nucleus is formed in nuclear fusion
Released as energy
How do you calculate the binding energy of an atom
Need to know the mass defect, found using the mass of the nucleons minus the mass of the nucleus
E=mc^2
E=Binding energy in Joules
m=Mass defect/difference in kg
c=Speed of light in a vacuum in m/s
Why is it sometimes more useful to use alternative units to SI units such as MeV instead of J
Dealing with very small number and standard form
1 atomic mass unit
The mass of 1/12 of a carbon 12 atom
1u=1.661x10^-27kg (formula sheet front page)
1u in MeV, mass of a proton and mass of a neutron in u
ALL ON THE FORMULA SHEET FRONT PAGE
How do you find binding energy when given in SI units
Find the mass defect in kg
Use E=mc^2 to find the binding energy in Joules
How do you find binding energy when given in atomic mass units
Find the mass defect in u and then multiply by 931.3Mev to get binding energy in MeV
Smaller nucleus binding energy per nucleon vs total binding energy compared to a larger nucleus
Smaller means a greater binding energy per nucleon but a smaller total binding energy
Greater binding energy per nucleon means …. stable because…
More stable
Releases more energy when it is being formed
So requires more energy to separate the particles within the nucleus
What is the peak of the average binding energy per nucleon against the number of nucleons in the nucleus
Iron 56
How does the average binding energy per nucleon against number of nucleons in the nucleus explain why fission and fusion work
Fusion; joining lighter nuclei to make heavier nuclei releases energy
Fission; splitting heavier nuclei into lighter nuclei releases energy
Fusion releases more energy than fission due to a steeper gradient
Explain nuclear energy states
Nucleus has an excited state which is unstable
Emitting one or more gamma photons after alpha or beta decay can lose the excess energy and return the nucleus to its ground state
Nuclear fission
A heavy nucleus splitting into two lighter and more stable nuclei with the release of energy and neutrons
Outline nuclear fission
Bombard heavy nucleus with neutrons
Absorbs neutrons
Splits into two light nuclei
Releasing two or three neutrons
In terms of the average binding energy per nucleon curve against time, what is the effect of nuclear fission
End products have a larger binding energy per nucleon than the starting nucleus
Which occurs due to them releasing energy
Because mass is lost
And this mass lost is converted into energy
Do you use mass difference or mass defect for nuclear fission calculations
Mass difference
Since not looking at a single nucleus
mass on the left>mass on the right
Energy released
Condition for interaction to release energy
What forms can energy be released in in nuclear fission
Kinetic energy of the two lighter daughter nuclei
Kinetic energy of the neutrons
Gamma rays given off at the time of the reaction
Energy carries away by neutrinos
Energy subsequently released by alpha or beta decay of the daughter nuclei since there is no guarantee they will be stable
Explain the containment structure for harnessing the energy released in a fission reactor
- Fuel rods made of uranium, which contains 0.7% U235 naturally, but enriched to 5%
- Fission reaction occurs in the rods, releasing energy and increasing the temperature of rods
- Neutrons released each fission go on to create further fissions, sustaining the reaction
- Coolant pumped around core which transfers energy from fuel rods to pipes containing water
- Water is heated by the coolant
- Turns to steam
- Steam drives a turbine which is connected to a generator
- Generator creates an alternating current
Purpose of coolant in nuclear fission reactors
In the centre of the reactor
Transfers heat from fuel rods to the pipes containing water that passes into reactor vessels
Passes through heat exchangers to maximise the efficiency of heat transfer to the water in order to make sure it boils and turns into steam before reaching the turbines
Suitable coolant for nuclear fission reactors
Carbon dioxide gas
Purpose of control rods in nuclear fission reactors
Affects the number of neutrons released
2.4 average neutrons released each fission
To maintain a steady rate 1.4 must be absorbed
Made of a material that absorbs neutrons
Putting control rods further into the reactor decreases the rate of reaction
Controlled by a computer to ensure the rate remains at a constant level
Control rod functions in emergencies
Inserted all the way in to stop the rate of reaction
Why are control rods necessary
If left alone a nuclear fission reaction will increase the number of fissions occurring per second since more neutrons released than used
Hence the amount of energy released increases
Dangerous because as temperature increases pressure will keep increasing and could lead to a meltdown
Critical mass
When one fission creates one further fission
Purpose of the moderator in a nuclear fission reactor
Slows the neutrons down
Neutrons released are travelling with a very high energy and travelling too fast to be absorbed by another uranium nucleus to start another fission reaction
Each fuel rod is inserted into a material designed to reduce the energy of the neutrons without stopping them completely
Must be a material neutrons can pass though whilst undergoing collisions in the nuclei in the moderator
Each collision reduces the energy of the neutrons, slowing them down
Suitable material for a moderator
Water
Heavy water
Graphite
What are thermal neutrons
Neutrons that have been slowed to sufficient speeds
To be absorbed by a uranium nucleus
3 things safety features of nuclear fission reactions protect
Workforce
Wider community
Environment
Safety features of a nuclear fission reactor
Reactor core is a thick steel vessel designed to withstand high pressure and temp in the core. Absorbs beta radiation and some of the gamma radiation and neutrons from the core
Core in a building with very thick concrete walls which absorb the neutrons and gamma radiation that escaped from the reactor vessel
Every reactor has emergency shut down system designed to insert control rods fully into the core to totally stop fission then flood the system to remove the heat
Sealed fuel rods are inserted and removed from reactor by means of remote handling devices
Why are fuel rods more dangerous after than before
Before they only emit alpha radiation which can be absorbed
After they emit beta and gamma radiation due to the many neutron rich fission products that form
After contain plutonium isotope 239Pu which is a very active alpha emitter and can cause lung cancer if inhaled
Why is nuclear waste so dangerous
If in an excited state then they may emit gamma rays
If they are still unstable they may emit further beta particles
Beta and gamma are more penetrating so need to be carefully contained
How must the spent fuel rods be removed and stored
By remote control
Stored underwater in cooling ponds to remove excess thermal energy for up to a year since they continue to release heat and radiation
What happens to the remaining plutonium and uranium
Separated to be recycled
What happens to radioactive waste
Stored in sealed containers in deep trenches
High level waste may be made into solid pyrex glass (vitrification) then placed in stainless steel or lead barrels or concrete cylinders and stored deep underground
Must be stored safely for centuries since it contains long-lived radioactive isotopes which must be prevented from contaminating food and water supplies’
Issue of transporting waste
Provides a potential danger to the public
So transported enclosed in crash resistant, extra thick and strong casing or processed onsite nearby
Why is someone living near a nuclear reactor more of a danger to other people than someone living next to an X-ray
Nuclear reactor emits fast moving neutrons which the body cells absorb
Creating unstable isotope cells
Therefore emit alpha, beta and gamma radiation which can damage others
X-rays emit alpha and gamma and beta radiation which strip cells of electrons
Meaning you don’t emit radiation
So can’t damage others
Nuclear fusion
Two light nuclei combining to form one heavier nucleus with the release of energy
What reaction produces the biggest possible increase in binding energy
Deuterium + deuterium into a helium nucleus
Explain the outcome of nuclear fusion in terms of the average binding energy per nucleon against number of nucleons graph
Heavy nucleus after the reaction has a greater binding energy per nucleon therefore more stable
So must be a loss of mass
Heavy nucleus at the end has a lower mass than the sum of the masses of the lighter products at the beginning of the reaction
How does fusion work
For two nuclei to fuse they have to get close enough to each other to virtually come into contact (within 1fm)
So the strong nuclear force can act to hold the nuclei together
To get just the nuclei on their own they have to be isolated to form plasma
All nuclei have a positive charge so repel each other
So must have a high kinetic energy to overcome the repulsion
So plasma must be at a high temperature and pressure
Which is hard to achieve
At the moment more energy is put into the system to achieve these conditions than is gained from the fusion
Much easier to achieve in the centre of stars due to a high pressure and temperature caused by the large mass of the start
Explain why the mass of all nuclei is less than the mass of its constituent parts in terms of gluons
Total energy of hadron = quarks plus gluons
When brought together, they share gluons, meaning they share energy and so the energy decreases and hence mass decreases
