Mass and Energy Flashcards
What is the Mass and Energy Equation
E = mc2
Energy = Mass x Speed of Light2
What is the Mass Defect / Mass Difference
The difference between the total mass of all the nucleons sperately compared to the mass of the nucleus
Why is there a Mass Defect
- Because energy is needed to bring the constituent parts of a nucleus together
- The mass that is “lost” is converted into energy and released when the nucleons fuse to form a nucleus
What is Binding Energy
The energy required to separate the nucleus into its indivdual neutrons and protons
What is the Binding Energy per Nucleon
The binding energy per nucleon is the binding energy of a nucleus divided by the number of nucleons in the nucleus
Describe the graph of binding energy per nucleon against nucleon number
Which element has the highest binding energy per nucleon
- Iron has the highest binding energy per nucleon
- Nuclei smaller than iron can undergo fusion
- Muclei larger than iron can undergo fission
What is the one Atomic Mass Unit
1u is defined as 1/12th of the mass of a carbon-12 atom, which is 1 × 1 .661 x 10-27 kg
What is Nuclear Fission
- Nuclear fission is the splitting of a large nucleus into two daughter nuclei with the release of lots of energy
- It occurs in very large nuclei, which are unstable (such as uranium), and occurs completely randomly, howevever it can also be induced
Why is Energy released during Nuclear Fisson
The smaller daughter nuclei have a higher binding energy per nucleon
What is Nuclear Fusion
- Nuclear fusion is where two smaller nuclei join together to form one larger nucleus
- It only occurs in fairly small nuclei
Explain why energy is released during Nuclear Fussion
The larger nucleus has a much higher binding energy per nucleon
State two nuclei that are most likely to be used to form the plasma of a fusion reactor
Deuterium and tritium
Does Fusion or Fission release more energy
Fussion releases far more energy than fission however fusion can only occur at extremely high temperatures (for example in stars) because a massive amount of energy is required to overcome the electrostatic force of repulsion between nuclei
Describe how fission can be induced
Fission can be induced by firing a thermal neutron into the uranium nucleus causing it to become extremely unstable
What are the Advantages and Disadvantages of using Nuclear Fisson to generate electrcity
Advantages -
- No greenhouse gases are emitted reducing global warming
- The quanity of waste is small compared to other methods of generating electricty because far less fuel is used
- It is quite a reliable way of generating electricty as supplies of uranium are set to last thousands of years (not completely reliable though) and the output can be controlled to match demands
Disadvantages -
- The waste products need to be stored securely for many decades (though only small waste and some can be recycled)
- New nuclear power stations are extremely expensive
- Decommissioning nuclear power stations is very expensive as the safe disposal of intermediate-level waste adds to the cost
- Risk of nuclear meltdown could have catastrophic consequences
Why are thermal neutrons used to induce fisson
Thermal neutrons have a low energy meaning they can induce fission whereas neutrons with a higher energy rebound away from the uranium-235 after a collision and do not cause a fission reaction.
How is fission used in nuclear reactors
- Rods of uranium-235 absorb neutrons and become unstable and then split into two daughter nuclei
- They also then release 2/3 more neutrons
- The neutrons released during fission then have the possibility to go on to cause more fission reactions forming a chain reaction, where each fission goes on to cause at least one more fission
What is Critical Mass
- The critical mass is the minimum mass of fuel required to maintain a steady chain reaction
- Using exactly the critical mass of fuel will mean that a single fission reaction follows the last, while using less than the critical mass would lead the reaction to eventually stop
What are the key features inside a thermal nuclear reactor
- Moderator
- Control rods
- Coolant
What is the function of a Moderator
- To slow down the neutrons so they travel slow enough to be absorbed by the uranium
- They do this through elastic collisons between the moderator and the nucleus
Describe the best choice of material for the moderator
- The closer the moderator atoms are in size to a neutron, the larger the proportion of momentum which is transferred, therefore the lower the number of collisions required to get the neutrons to thermal speeds
- Because of this, water is often used as a moderator as it contains hydrogen, also it’s inexpensive and not very reactive
- Graphite is also used as a moderator
What is the function of the control rods in a thermal nuclear reactor
- They absorb neutrons so that only 1 of the neutrons released in each reaction can go on to be absorbed by another uranium
- This stops the chain reaction from being out of control and the reactor from overheating from too many reactions happening at once
Describe the best choice of material for the control rods
They are made of materials which absorb neutrons without undergoing fission such as boron and cadmium
