ib physics 12 nuclear physics 6 mr. jean. the plan: video clip of the day –example of fission...
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IB Physics 12
Nuclear Physics 6
Mr. Jean
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The plan:
• Video clip of the day– Example of fission energies– Example of fusion energies– Recap of nuclear physics
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Conservation LawsFor any nuclear reaction, there are three conservation laws which must be obeyed:For any nuclear reaction, there are three conservation laws which must be obeyed:
Conservation of Charge:Conservation of Charge: The total charge The total charge of a system can neither be increased nor of a system can neither be increased nor decreaseddecreased..
Conservation of Nucleons:Conservation of Nucleons: The total The total number of nucleons in a reaction must be number of nucleons in a reaction must be unchanged.unchanged.
Conservation of Mass Energy:Conservation of Mass Energy: The total The total mass-energy of a system must not change mass-energy of a system must not change in a nuclear reaction.in a nuclear reaction.
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Example 7: Use conservation criteria to determine the unknown element in the
following nuclear reaction:1 7 41 3 2
AZH Li He X energy
Charge before = +1 + 3 = +4Charge before = +1 + 3 = +4
Charge after = +2 + Z = +4Charge after = +2 + Z = +4
Z = 4 – 2 = 2Z = 4 – 2 = 2
Nucleons before = 1 + 7 = Nucleons before = 1 + 7 = 88Nucleons after = 4 + A = 8Nucleons after = 4 + A = 8
(Helium has(Helium has Z = Z = 22))
(Thus, (Thus, A =A = 44))
1 7 4 41 3 2 2H Li He He energy
1 7 4 41 3 2 2H Li He He energy
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Conservation of Mass-EnergyThere is always mass-energy associated There is always mass-energy associated with any nuclear reaction. The energy with any nuclear reaction. The energy released or absorbed is called the Q-value released or absorbed is called the Q-value and can be found if the atomic masses are and can be found if the atomic masses are known before and after.known before and after.
1 7 4 41 3 2 2H Li He He Q
1 7 4 41 3 2 2H Li He He Q
1 7 4 41 3 2 2Q H Li He He
Q is the energy released in the reaction. Q is the energy released in the reaction. If Q is positive, it is exothermic. If Q is If Q is positive, it is exothermic. If Q is negative, it is endothermic.negative, it is endothermic.
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Example 8: Calculate the energy released in the bombardment of lithium-7 with hydrogen-
1.1 7 4 41 3 2 2H Li He He Q
1 7 4 41 3 2 2H Li He He Q
1 7 4 41 3 2 2Q H Li He He
73 7.016003 uLi
42 4.002603 uHe1
1 1.007825 uH
Substitution of these masses gives:Substitution of these masses gives:
QQ = 0.018622 u(931.5 = 0.018622 u(931.5 MeV/u)MeV/u)
Q =17.3 MeV
Q =17.3 MeV
42 4.002603 uHe
The positive Q means the reaction is The positive Q means the reaction is exothermic.exothermic.
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Nuclear Fission:
• Critical Mass: if mass of uranium is too small, too many neutrons escape without causing further fission in uranium so the reaction cannot be sustained
• Thermal Neutron: low-energy neutron (≈1eV) that favors fission reactions – energy comparable to gas particles at normal temperatures
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Nuclear Reactions:
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Fission & Fusion Problems:
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Natural Isotopes:
• Naturally Occurring Isotopes of Uranium:
• Uranium-238: most abundant, 99.3%, very small probability of fissioning when it captures a neutron, not used for fuel, more likely to capture high energy neutron than low energy one
• Uranium-235: 0.3%, 500 times greater probability of fissioning when captures a neutron but must be a low-energy (thermal) neutron, used for fuel
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Fuel Enrichment:
• This process of increasing proportion of uranium-235 in a sample of uranium – 1) formation of gaseous uranium (uranium
hexafluoride) from uranium ores– 2) Separated in gas centrifuges by spinning –
heavier U-238 moves to outside– 3) increases proportion of U-235 to about 3%
to be used as fuel in nuclear reactors
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Fuel Enrichment:
• Advantage: more uranium is available for fission and reaction can be sustained
• Disadvantage: enriched fuel can be used in the manufacture of nuclear weapons – threat to world peace – 85% = weapons grade
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Inside the reactor:
• Moderator: material (water, graphite) used to slow down high-energy neutrons emitted from fission reactions to thermal levels for use in further fission reactions to sustain the chain reaction - slow neutrons by collisions
• Control Rods: inserted between fuel rods – made of neutron-absorbing cadmium or boron -used to control reactor temperature to prevent overheating – lowered if too many neutrons/reactions and excess thermal neutrons are absorbed
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Nuclear Waste:
• Low-level waste: radioactive material from mining, enrichment and operation of plant – must be disposed of – left untouched or encased in concrete
• High-level waste: disposal of spent fuel rods- some isotopes have ½ lives of thousands of years – plutonium 240,000 years
• stored under water at reactor site for several years to cool of then sealed in steel cylinders, buried underground
• reprocessed to remove any plutonium and useful uranium, remaining isotopes have shorter ½ lives and long-term storage need is reduced
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• Nuclear Weapons Manufacture: – Enrichment technology could be used to make
weapons grade uranium (85%) rather than fuel grade (3%)
– Plutonium is most used isotope in nuclear weapons and can be gotten from reprocessing spent fuel rods
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Example Question: Suppose the average power consumption for a household is 500 W per day. Estimate the amount of uranium-235 that would have to undergo fission to supply the household with electrical energy for a year. Assume that for each fission, 200 MeV is released.
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Nuclear Fission:
• http://www.youtube.com/watch?v=szpnRx7U41M (Yelling guy)
• http://www.youtube.com/watch?v=0kLXGTob9s8 (Non-yelling)
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Example Question #2:
• A fission reaction taking place in a nuclear power station might be– Estimate the initial amount of uranium-235 needed
to operate a 600 MW reactor for one year assuming 40% efficiency and 200 MeV released for each fission reaction.
235 1 141 92 192 0 56 36 03U n Ba Kr n
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Nuclear Fusion:
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Nuclear Fusion: • Nuclear Fusion: Two light nuclei combine to form
a more massive nucleus with the release of energy.
• Write the reaction equation for the fusion reaction shown below.
2 3 4 11 1 2 0H H He n
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• To calculate how much energy is released in this fusion reaction we would need to again use the change in mass vs. energy relationship.
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• Plasma: fuel for reactor – high energy ionized gas (electrons and nuclei are separate) – if energy is high enough (hot enough), nuclei can collide fast enough to overcome Coulomb repulsion and fuse together
• Magnetic confinement: charged particles are contained via magnetic fields – travel in a circle in a doughnut shaped ring (tokamak)
• Heating Plasma: accelerate nuclei by means of magnetic fields and forces = high temperatures (high kinetic energies)
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• Problems with current fusion technology: – Maintaining and confining very high-density and
high-temperature plasmas – very difficult to do – uses more energy input than output – not commercially efficient
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Fusion Reactions:
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