Grafiittiydin

Grafiittiydin (Finnish for graphite core) refers to a portion of a nuclear reactor where graphite material forms the neutron moderator and, in many designs, also serves as structural support around the fuel channels. In such systems, the fuel assemblies are surrounded by blocks or bricks of graphitic carbon, which slow fast neutrons to thermal energies that are more readily captured by fissile nuclei such as uranium-235.

Graphite is a crystalline form of carbon with a layered structure and relatively low atomic number. It has high thermal stability and good moderating properties, slowing neutrons to thermal energies with relatively low absorption. However, it is susceptible to oxidation at high temperatures in air and can become brittle under irradiation; it also becomes radioactive over time due to neutron activation, producing isotopes such as carbon-14.

Historically, graphite moderated reactors were common in the mid-20th century. The United Kingdom used graphite moderators in Magnox and the Advanced Gas-Cooled Reactors; the Soviet RBMK design also used a graphite moderator. The Windscale fire of 1957 highlighted safety concerns associated with graphite. Today most commercial reactors use alternative moderators, such as light water or heavy water, but graphite remains in use in some research reactors and certain gas-cooled designs.

Disposal and decommissioning of irradiated graphite pose long-term challenges due to activation and volume of waste. Proper handling, containment, and monitoring are required to ensure safety and environmental protection.