Physics 12 Nuclear Physics 7 – Fission Mr. Jean The plan: Video clip of the day Nuclear Physics.
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Transcript of Physics 12 Nuclear Physics 7 – Fission Mr. Jean The plan: Video clip of the day Nuclear Physics.
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Physics 12
Nuclear Physics 7 – Fission
Mr. Jean
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The plan:
• Video clip of the day• Nuclear Physics
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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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Summary
ParticleParticle Fig.Fig. SymSym MassMass ChargeCharge SizeSize
Electron e 9.11 x 10Electron e 9.11 x 10-31-31 kg -1.6 x 10 kg -1.6 x 10-19 -19 C C Proton Proton pp 1.673 x 101.673 x 10-27-27 kg +1.6 x 10 kg +1.6 x 10-19 -19 C 3 C 3 fmfmNeutron Neutron nn 1.675 x 101.675 x 10-27-27 kg 0 3 kg 0 3 fmfm
Fundamental atomic and nuclear Fundamental atomic and nuclear particlesparticles
The mass number A of any element is equal to the sum of the protons (atomic number Z) and the number of neutrons N :A = N + Z
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Summary Definitions:A A nucleonnucleon is a general term to denote a is a general term to denote a nuclear particle - that is, either a proton or nuclear particle - that is, either a proton or a neutron.a neutron.The The mass number mass number AA of an element is equal of an element is equal to the total number of nucleons (protons + to the total number of nucleons (protons + neutrons).neutrons).IsotopesIsotopes are atoms that have the same are atoms that have the same number of protons (number of protons (ZZ11= Z= Z22), but a ), but a different number of neutrons (N). (different number of neutrons (N). (AA11 A A22))
A A nuclidenuclide is an atom that has a definite is an atom that has a definite mass number mass number AA and and ZZ-number. A list of -number. A list of nuclides will include isotopes.nuclides will include isotopes.
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Summary (Cont.)
A Mass numberZ Atomic numberX Symbol
A Mass numberZ Atomic numberX SymbolSymbolic Symbolic
notation for notation for atomsatoms
D H nm Zm Nm M D H nm Zm Nm M Mass Mass
defectdefect mmDD
Binding Energy per
nucleon
MeV =
nucleonBE
A
EB = mDc2 where c2 = 931.5 MeV/u
BindinBinding g
energyenergy
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Summary (Decay Particles)
An An alpha particlealpha particle is the nucleus of a is the nucleus of a helium atom consisting of two protons helium atom consisting of two protons and two tightly bound neutrons.and two tightly bound neutrons.
A A beta-minus particlebeta-minus particle is simply an is simply an electron that has been expelled from the electron that has been expelled from the nucleus. nucleus. A A beta positive particlebeta positive particle is essentially is essentially an electron with positive charge. The an electron with positive charge. The mass and speeds are similar.mass and speeds are similar.
A A gamma raygamma ray has very high has very high electromagnetic radiation carrying electromagnetic radiation carrying energy away from the nucleus.energy away from the nucleus.
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Summary (Cont.)
4 42 2
A AZ ZX Y energy
4 42 2
A AZ ZX Y energy
Alpha Decay:Alpha Decay:
01 1
A AZ ZX Y energy 0
1 1A AZ ZX Y energy
Beta-minus Decay:Beta-minus Decay:
01 1
A AZ ZX Y energy 0
1 1A AZ ZX Y energy
Beta-plus Decay:Beta-plus Decay:
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Summary (Radioactivity)
Nuclei Nuclei RemainingRemaining
0
1
2
n
N N
0
1
2
n
N N
Activity RActivity R
0
1
2
n
R R
0
1
2
n
R R
Mass RemainingMass Remaining
0
1
2
n
m m
0
1
2
n
m m
Number of Half-Number of Half-lives:lives:
12
tnT
1
2
tnT
The The half-life Thalf-life T1/21/2 of an isotope is the time in of an isotope is the time in which one-half of its unstable nuclei will which one-half of its unstable nuclei will decay.decay.
The The half-life Thalf-life T1/21/2 of an isotope is the time in of an isotope is the time in which one-half of its unstable nuclei will which one-half of its unstable nuclei will decay.decay.
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Summary (Cont.)
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 decreased.decreased.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 mass-energy of a system must not change in a nuclear reaction. (Q-value = change in a nuclear reaction. (Q-value = energy released)energy released)
Nuclear Reaction:x + X Y + y + Q