Chapter 29: Nuclear Physics

Nuclear Fission

The type of nuclear reaction in which a large nucleus splits into smaller nuclei.

Nuclear Fusion

The type of nuclear reaction in which two or more small nuclei merge to form a larger nucleus.

Chain Reaction

A self-sustaining series of reactions in which the neutrons produced in one reaction trigger additional reactions.

Critical Mass

The quantity of material that contains the necessary concentration of nuclei for a chain reaction.

Energy from Nuclear Reactions

• When nuclear changes occur, there is a slight mass difference between the reactant nuclei and the product nuclei (mass defect).
• The mass difference represents the difference in binding energies of the reactants and products.

Fission Reactions

• Some materials undergo fission when they are bombarded by a neutron.
• The neutron produces a very unstable isotope, which then splits.

Example: Writing Nuclear Equations

Determine the missing product by completing the following nuclear equation:

235₉₂U + 1₀n → 142₅₄Xe + 90₃₈Sr + 4 1₀n

235 + 1 = A + 90 + 4(1)

92 = Z + 38

A = 142

Z = 54

The missing product is 142₅₄Xe.

Example: Calculating Energy Released

If the mass of uranium-235 is 235.043 929 9 u and the products have masses of 140.914 411 9 u (Ba) and 91.926 156 1 u (Kr), what is (a) the mass defect and (b) the energy released?

The mass of a neutron is 1.008 664 9 u. A mass of 1 u has an energy equivalence of 931.5 MeV.

Δm = Σm_reactants - Σm_products

Δm = (m_U + m_n) - (m_Kr + m_Ba + 3m_n)

Δm = (235.043 929 9 u + 1.008 664 9 u) - [(91.926 156 1 u + 140.914 411 9 u + 3(1.008 664 9 u)]

Δm = 0.186 032 1 u

Energy released: 0.186 032 1 u × 931.5 MeV/u = 173.2889 MeV

Chain Reactions

• A self-sustaining series of reactions in which the neutrons produced in one reaction trigger additional reactions.
• A condition in which the rate of fissions is constant is said to be a critical process.
• A situation in which the rate of fission is increasing is called supercritical.
• A reaction is subcritical if the rate of fission is decreasing.

Uses of Nuclear Fission

• Nuclear power plants
• Nuclear weapons

Nuclear Power Reactors

• Uranium-235 concentration is increased through a process called enrichment.
• The neutrons are “slowed down” with a moderator (usually water).
• A critical chain reaction must be established, so control rods are used to adjust the rate of the reactions.

Fission Bombs

• An uncontrolled fission reaction occurs using nearly pure U-235 or Pu-239.
• The fissile material is separated into two or more subcritical masses.
• The masses are forced together, and immense amounts of energy are released.

Fusion Reactions

• In the sun, a multi-step process begins with four hydrogen-1 nuclei and ends with a helium-4 nucleus and two hydrogen atoms.
• The enormous pressure within the sun is necessary for this to occur.

Uses of Fusion

• Fusion reactors: Man has not yet produced a self-sustaining fusion reaction that produces usable energy. A consortium of nations is working together to develop fusion reactor technology.
• Fusion bombs: Thermonuclear devices use deuterium and tritium nuclei under extremely high temperatures and pressures.

Questions for Students

1. Define nuclear fission and nuclear fusion. How do they differ?
2. What is a chain reaction and what is critical mass?
3. Explain the process of nuclear fission in a power reactor.
4. Describe the challenges of creating a working fusion power plant.
5. What are the uses of nuclear fission and fusion in modern technology?