Physics 12

Nuclear Physics 7 – Fission

Mr. Jean

The plan:

• Video clip of the day• Nuclear Physics

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

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

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

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

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

• 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

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.

Nuclear Fission:

• http://www.youtube.com/watch?v=szpnRx7U41M (Yelling guy)

• http://www.youtube.com/watch?v=0kLXGTob9s8 (Non-yelling)

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

Nuclear Fusion:

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

• To calculate how much energy is released in this fusion reaction we would need to again use the change in mass vs. energy relationship.

• 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)

• 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

Fusion Reactions:

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

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.

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

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.

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:

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.

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