Breeder reactors need fast neutrons for breeding and fission. Water is not suitable for the cooling of the fuel elements. The high density of energy requires a coolant with a very good thermal conductivity. Liquid sodium is such a medium, and it is liquid between 100–1000°C at a pressure below four bars. Its viscosity is comparable to that of water and its compatibility with several materials is fairly satisfactory. Its neutron-physical properties lead to relative high breeding ratio. The activation of sodium according to 23Na(n,γ)  24 Na   or   23Na(n,  2n)  22Nahttps://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780429333774/638acc69-825a-40f0-a740-e0854ea3b8b5/content/eq23.tif"/>