Nuclear Reactions: Fission and Fusion Explained

Posted on Jan 9, 2025 in Chemistry

Fission

This reaction is caused by the bombardment of certain nuclides of high atomic number with neutrons. After absorbing a neutron, the nucleus splits into two nuclides of lower atomic number and additional neutrons, releasing energy corresponding to the mass difference between initial and final particles. In the above example, this should be greater than 200 MeV. This energy appears as kinetic energy of product particles and gamma rays. The neutrons generated in the process can also interact with other uranium nuclides, producing a chain reaction. However, technically, it requires a critical mass of fissile material. The energy generated is enormous, making it the largest source of energy (nuclear reactors). This process occurs spontaneously in transuranic elements, atoms of mass number greater than 250, which are too big to be stable. Nuclei with mass numbers ranging between 220 and 250 do not undergo spontaneous fission, but this can be induced by bombarding them with slow neutrons, protons, fast neutrons, or accelerated particles.

Fusion

This is like the opposite of fission. Lower mass nuclides are combined. The total mass of particles produced is less than the reactants, and energy is released in the process. In the example, the smallest mass is 0.0189 amu, which gives Q = 17.6 MeV. This process requires much higher activation energies than the fission process. In this case, the activation energy is the kinetic energy required to overcome the repulsive interactions between the colliding nuclei or particles. Once the new nucleus is formed, a much larger amount of energy is released than the kinetic energy possessed by the nuclei before fusion.

Activation of Nuclear Reactions

When a certain amount of some material is placed in a “stack”, it may become activated when bombarded with neutrons in the pile. Radioactive elements can be made by several nuclear reactions. Some of them have already been described. The performance of a nuclear reaction depends on parameters such as:

  • The number of particles bombarded.
  • The number of target nuclei.
  • The probability of occurrence of the nuclear reaction.

The probability is called the “cross-section (σ)” for a nuclear reaction and is measured in barns, where 1 barn = 10-24 cm2/atom. It depends on:

  • The nature of the target material.
  • The kind of particles and their energy bomb.

From a geometric point of view, this is an oversimplification. A nucleus can have a large area for a certain type of reaction or a smaller area for another. For example, when a neutron approaches the nucleus, the following can occur:

  • A nuclear reaction immediately, such as fission.
  • A new stable isotope in an excited state can be produced, which will deliver its energy in the form of a gamma photon.
  • A radioactive nucleus can be produced.

For our purposes, we are interested in the activity of a nuclide produced by slow or thermal neutrons. The neutron flux (n) depends on the type of battery and the position within the reactor since the flow is higher in the center and weaker at the periphery. Its range is from 1010 to 1014 neutrons/cm2/sec.

Growth of the Activity

The above examples do not consider the decay of radioactive material formed. In a short time, it is right to use the above formula. But with longer times, produced atoms begin to decline, while new atoms are activated. Thus, the growth in activity is less than predicted by the equation above. It may be that the activity increases less rapidly than production by irradiation and finally reaches a maximum or “saturation activity” after several half-lives. Saturation activity occurs when the rate of production of atoms in the stack is equal to the rate with which they decay.