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The Hidden Weapon in Nuclear Reactors: Why is 10B the most important neutron absorber?
Neutron capture is a nuclear reaction process in which an atomic nucleus collides with one or more neutrons and merges to form a heavier nucleus. Unlike positively charged protons, neutrons are uncharged and can easily enter the nucleus, making neutron capture play an important role in the formation of heavy elements. Especially in stars, neutron capture can occur in a fast way (r-process) or a slow way (s-process). Nuclei with a mass greater than 56 cannot be formed through exothermic thermonuclear reactions (such as nuclear fusion), but can be achieved through neutron capture.
When neutrons are captured inside a star, they can quickly increase the mass without changing the properties of the element.
In small neutron flows, such as in a nuclear reactor, a single neutron can be captured by a single nucleus. For example, when natural gold (197Au) is bombarded with neutrons, an excited state of the isotope 198Au is formed, which then rapidly decays back to the ground state of 198Au by emitting gamma rays (γ). This process increases the mass number by 1, which can be expressed by a simplified formula as 197Au + n → 198Au + γ. The process of using thermal neutrons is called thermal capture. The isotope 198Au is a beta emitter and will eventually decay into 198Hg, an isotope of mercury, and increase its atomic number by one.
The r-process enters the picture when the neutron flow is very high (such as within stars), because the density of the neutron flow is so high that the nuclei do not have time to undergo beta decay between neutron captures. In this way, the mass number increases very quickly, while the number of atoms (i.e. elements) remains the same. When no further neutrons can be captured, these extremely unstable nuclei will transform into beta-stable isotopes of heavier elements through multiple beta-decays.
The neutron absorption cross section is an important indicator for measuring the probability of a nuclide being captured by neutrons. It is usually measured in barns. The size of the absorption cross section is often closely related to the neutron energy.
The thermodynamic significance of neutron capture cannot be ignored because it involves the formation process of isotopes of chemical elements. The energy captured by neutrons affects the standard enthalpy of formation of the isotope, which is a critical factor in the design and operation of nuclear reactors.
In terms of technical applications, neutron activation analysis technology can be used to remotely detect the chemical composition of materials, because different elements release different characteristic radiation when absorbing neutrons. This makes neutron activation analysis widely used in mineral exploration and security fields.
10B is considered the most important neutron absorber in nuclear reactors. It is often used as a boride control rod or as a boric acid additive in the coolant of pressurized water reactors.
In engineering, the most important neutron absorber is 10B, which is often used in control rods of nuclear reactors or as boric acid coolant in pressurized water reactors. In addition to 10B, there are other neutron absorbers including xenon, cadmium, hafnium, scandium, cobalt, yttrium, neodymium, uranium, etc., all of which exist in nature in different mixtures of isotopes. Among these neutron absorbers, hafnium can effectively absorb neutrons and can be used in reactor control rods, but it coexists in the same ore as zirconium. The two have similar chemical properties, but their nuclear properties are quite different. Strontium's ability to absorb neutrons is 600 times that of zirconium, so it is widely used in internal components of nuclear reactors, especially the metal cladding of fuel rods. For these elements to work individually, zirconium and hafnium need to be separated from their natural mixture, which can be achieved cost-effectively through ion exchange resins.
Faced with the continuously developing nuclear energy technology, we should think about: How to maximize the potential of neutron capture to promote the application of nuclear energy while ensuring safety?
