Nuclear Stability and Reactions: A Comprehensive Overview

Posted on Oct 14, 2024 in Chemistry

Nuclear Stability and Reactions

From Two Viewpoints: Nuclear Power and Energy

A. Strong Nuclear Force

The strong nuclear force, theorized by Hideki Yukawa, is the strongest of the fundamental forces but has the shortest range. It is independent of charge and works only for particles within a distance of less than 10-16 meters. For distances between 10-16 and 10-14 meters, there is less repulsion. Nuclei with approximately equal numbers of protons (Z) and neutrons (N) are stable. As Z increases, the number of neutrons increases more rapidly than the number of protons. For Z < 20 and N > 20, stable nuclei have Z = N. For Z > 20, stable nuclei have more neutrons than protons, suggesting that an excess of neutrons provides stability. For Z > 82, nuclei are not stable, and isotopes do not exist.

By considering the position of stable nuclei, it is possible to predict and justify the type of decay a given nucleus might undergo.

To move from a zone with excess protons to the zone of stable nuclei:

  • Neutrons increase, protons decrease.
  • Both neutrons and protons decrease simultaneously. This is achieved by beta emissions.

To move from a zone with excess neutrons to the zone of stable nuclei:

  • Reduce the number of neutrons.
  • Increase the number of protons.
  • Simultaneously decrease N and increase Z. This is achieved by positron emission (the antiparticle of the electron).

B. Energy

When a nucleus is formed from protons and neutrons, the mass of the nucleus formed is less than the mass of the constituent nucleons. The difference between the two is known as the mass defect. According to Einstein’s mass-energy equivalence (E=mc2), this mass defect is converted to energy. This energy is called binding energy.

Binding energy represents how much energy is released during the formation of a more stable nucleus. It could also be defined as the energy required to break a nucleus down into its constituents. A small mass defect results in a large amount of energy released.

Formula: (Insert binding energy formula here)

Binding energy per nucleon is a good indicator of nuclear stability. The maximum value is for nuclei with mass numbers between 40 and 80. The greatest stability is at A = 56, which corresponds to iron. From there, stability slowly decreases. For smaller nuclei (H, He), joining them (fusion) to form larger nuclei increases stability and releases energy. Larger nuclei tend to undergo fission reactions, releasing energy and achieving greater stability. The binding energy of nuclei increases more rapidly for smaller nuclei and decreases slowly for larger nuclei.

5. Nuclear Reactions

Fission

Fission is the process where a heavy nucleus splits into two lighter nuclei, typically through the action of a neutron. This process can occur spontaneously, but it is rare and not productive. Therefore, fission is usually induced by slow neutrons. This process was discovered by Otto Hahn. Fission is primarily used with heavier nuclei, specifically isotopes of plutonium and uranium. The most commonly used isotope is uranium-235.

Fission is an asymmetrical process because the fragments formed do not have the same mass number, and the products formed are not always the same. Fission releases a large amount of energy in the form of kinetic energy of the fission fragments and emitted neutrons. Lise Meitner and Otto Frisch were key figures in understanding the physics of nuclear fission.

The fact that neutrons are released during fission and that fission can be induced by neutrons suggests the possibility of a chain reaction. The neutrons produced can induce new fission events, releasing more neutrons. If no control is exercised, the reaction can become explosive. However, if the reaction is regulated by partial absorption of neutrons, it can be kept under control in a nuclear reactor.

Fusion

Fusion is the process where two light atoms combine to form a heavier atom. Due to the repulsion between the positively charged nuclei, they must have a lot of energy to overcome this repulsion and get close enough for the strong nuclear force to take over. The repulsion increases with the number of protons, so fusion is only feasible for lighter nuclei. Fusion requires temperatures of around 100 million degrees Kelvin. This raises the challenge of finding a container that can withstand such temperatures.

At these temperatures, the nuclei are stripped of their electrons, and the protons and neutrons form a substance called plasma. Plasma can be contained by magnetic fields. Fusion also releases large amounts of energy.

Advantages of Fusion:

  • It is an almost inexhaustible resource, as deuterium and tritium are readily available and cheap raw materials.
  • It is chemically non-polluting.
  • It does not require the transport of radioactive material.
  • It presents no disposal problem.
  • It is safer than fission.

Disadvantages of Fusion:

  • Possible leakage of radioactive tritium gas.
  • Leakage of lithium, which can cause an explosion upon contact with air.
  • Induced radioactivity in the materials of the reactor wall.

Nuclear Reactor

A nuclear reactor is a device where a controlled fission chain reaction takes place to obtain energy. The heat generated in the reactor is used to produce steam, which drives a turbine to generate electricity.

Elementary Particles

Elementary particles are the smallest constituents of matter. Until 1932, only the proton and electron were known. Every particle has its antiparticle, or antimatter, which has the same mass, equal half-life, and equal spin but opposite charge and sometimes a different magnetic moment. When a particle and its antiparticle interact, they annihilate each other, producing energy.

Particles are classified into groups based on their mass and other properties. Protons, neutrons, and all hadrons are made of quarks. Unlike these particles, quarks have fractional charges. There are six types of quarks: up, down, charm, strange, top, and bottom. Each quark also has a color charge (red, green, or blue).

Fundamental Interactions:

  1. Strong Nuclear Interaction: The most intense but shortest-range interaction. It binds quarks together and holds protons and neutrons together in the nucleus. Without the strong force, protons would repel each other due to their positive charges.
  2. Electromagnetic Interaction: The second strongest interaction. It acts on electrically charged particles and can be attractive or repulsive, depending on the sign of the charges. It is responsible for the stability of atoms, molecules, and matter.
  3. Weak Nuclear Interaction: A very short-range interaction responsible for beta decay of atomic nuclei and transformations of leptons.
  4. Gravitational Interaction: The weakest of all interactions. It is attractive and acts on all bodies. It has an unlimited range and is responsible for the overall structure of the universe.