Helium atom scheme.

Chemical element with atomic number 2. Helium is a transparent noble gas that is lighter than air. It is the second lightest element, with the lowest boiling point (−268.9 °C) among all elements.

Helium can be used as a coolant in a fast neutron HTGR (high temperature gas-cooled reactor). Its usage increases safety (the gas does not boil) and energy yield due to the higher temperature.

Helium is a product of the deuterium-tritium fusion reaction. Specifically, the reaction produces helium nuclei, also called alpha particles. The alpha particles are also generated by radioactive decay and form alpha radiation.

Liquid helium cooled to a low temperature is used as a coolant for various devices and for superconducting materials that keep their superconducting properties only at temperatures close to absolute zero. These materials are used, for example, in the construction of magnetic coils for thermonuclear fusion research devices such as tokamaks or stellarators.