Damage to Graphite under Neutron Irradiation
Graphite has long been recognized as one of the most important materials in nuclear technology. It plays a versatile role in the design and operability of many nuclear reactors, including those in the first commercial nuclear reactors, such as those at Calder Hall and Windscale in the United Kingdom (UK). Graphite is used primarily as the moderating material within nuclear reactors, providing a wide variety of shades to absorb neutrons from fission processes, thus boosting the efficiency of the reactor. However, like most materials, graphite can be damaged by neutron irradiation.
Neutron irradiation can be damaging to graphite for several reasons. Firstly, neutron bombardment can cause the atomic nuclei of the graphite lattice to be so distorted that stability is lost, leading to a decrease in the strength of graphite as a structural material. Dissipation of energy dispersed as phonons can be increased by neutron bombardment, resulting in increased temperatures which can further weaken the material. Secondly, several types of defects can be created through neutron irradiation, including Frenkel defects, point defects and vacancies. These can cause an increase in the crystal lattice strain resulting in micro-cracking. This can lead to a reduction in the thermal conductivity of the graphite, which will reduce its effectiveness as a moderator. Finally, neutron bombardment can cause an increase in the intensity of the graphite crystal radiation. This can result in higher energy gamma radiation being produced, which can damage operating components or cause led to an increase in the leakage of fission products from the reactor core.
The damage caused by neutron irradiation is increased with the neutron flux density in the graphite. For example, at a flux of 7 × 10^15 n/cm^2s, the graphite can suffer irreversible damage to its structure after a few hundred hours of neutron bombardment, leading to localized melting and weakening of the material. At typical fluxes measured in nuclear power plants, the graphite can be expected to experience damage over a much longer time period.
In order to minimize the damage caused by neutron irradiation, manufacturers of graphite products have developed grades of graphite specifically tailored to withstand neutron irradiation. These graphites generally contain a low density of Frenkel defects, allowing them to retain their strength despite neutron bombardment. Additionally, these graphites are designed to have a high carbon content, and thus a higher heat resistance. Manufacturers also design graphite specially for nuclear applications. In particular, graphite grades designed for the powerful RT2 reactors in Russia are designed to withstand neutron fluxes of up to 1 × 10^16 n/cm^2s.
To monitor the effects of neutron irradiation on graphite, manufacturers use a variety of tests. These tests measure the decrease in the strength of the graphite material, as well as the presence of defects, like point or Frenkel defects, in the material. Additionally, samples of the graphite may be exposed to ionizing radiation in a laboratory to simulate neutron bombardment, in order to accurately measure their radiation resistance.
In conclusion, graphite is a vitally important material in nuclear technology, acting as a moderator of neutrons to boost the efficiency of the fission process in nuclear reactors, however it is vulnerable to damage from neutron irradiation. To combat the damage caused by neutron bombardment, manufacturers have designed graphite specifically for irradiation, with low levels of defect and increased heat resistance, as well as coming up with tests to monitor the effects of neutron bombardment on graphite.
