Fusion and Fission: Two Nuclear Reactions, Two Energy Paths

Nuclear Reactions: Fission vs Fusion

Nuclear reactions (transformations involving atomic nuclei) represent the most energetic processes in the universe. Two main pathways exist to release this energy contained in matter: fission (process by which a heavy atomic nucleus splits into two lighter nuclei) and fusion (process by which two light atomic nuclei unite to form a heavier nucleus). Although radically different in principle, both reactions obey Albert Einstein's (1879-1955) famous equation: \(E = mc^2\), which establishes the equivalence between mass and energy.

Nuclear Fission: Dividing to Exist

Fission involves the splitting of a heavy nucleus (such as uranium-235 or plutonium-239) into two lighter fragments upon neutron impact. The released energy comes from the mass difference related to the binding energies per nucleon. Typically, the fission of \(^{235}\)U releases about \(200\ \text{MeV}\) per individual nuclear fission reaction, after capturing a neutron.

Discovered in 1938 by Otto Hahn (1879-1968) and Fritz Strassmann (1902-1980), and interpreted by Lise Meitner (1878-1968) and Otto Frisch (1904-1979), this reaction releases considerable energy and several neutrons, which can in turn cause new fissions, thus creating a chain reaction.

The released energy comes from the mass defect: the sum of the masses of the fission products is less than the mass of the initial nucleus. This mass defect, although tiny, converts into colossal energy according to \(E = \Delta m c^2\), where \(\Delta m\) is the mass difference and \(c\) is the speed of light.

In summary, the nucleus splits because, beyond a certain size, it is energetically more favorable to exist as two medium nuclei rather than as a single heavy, unstable nucleus. Fission is the expression of this quest for stability, triggered by the addition of a neutron.

Nuclear Fusion: Unity is Strength

Conversely, nuclear fusion involves the union of two light atomic nuclei, such as hydrogen isotopes (deuterium \(^{2}\)H and tritium \(^{3}\)H), to form a heavier nucleus (helium \(^{4}\)He). This process, which powers stars like our Sun, releases even more energy per nucleon than fission. To overcome the electrostatic repulsion between positively charged nuclei (Coulomb barrier), extreme conditions of temperature (on the order of millions of degrees) and pressure are required. The energy released is about \(17.6\ \text{MeV}\) per D-T reaction.

Mastering fusion on Earth represents a monumental technological challenge, but its potential is immense: abundant fuel, low production of long-lived radioactive waste, and no risk of runaway reaction.

N.B.:
When it is said that D-T fusion releases 17.6 MeV, this refers to the total energy per elementary reaction, i.e., for the interaction between a deuterium nucleus (\(^{2}\)H) and a tritium nucleus (\(^{3}\)H). This energy is distributed between a helium-4 nucleus (≈ 3.5 MeV) and a neutron (≈ 14.1 MeV). Since the reaction involves 5 baryons (2+3), the energy per nucleon is: \[ \frac{17.6}{5} \approx 3.5\ \text{MeV per baryon}. \] This value is often compared to other nuclear processes: fission releases ≈ 0.9 MeV/baryon, while fusion reaches several MeV/baryon, hence its superior energy potential on the scale of reactive mass.

Comparison of the Two Energy Paths

The following table summarizes the main characteristics of these two nuclear reactions, highlighting their fundamental differences.

• References: International Atomic Energy Agency (IAEA) (new window), ITER (new window).