Abstract: A nuclear fuel assembly has a plurality nuclear fuel pins which are extremely long and slender thin walled tubes containing heavy nuclear fuel pellets supported in exceedingly close pitch as a cluster. The end support for supporting these pins vertically locates each pin by a spigot and socket connection of special design. By other connection the pin is positioned and retained in the cluster whilst it can execute limited changes in length, and can when necessary be detached from its support by a force which may distort the connection but will not put the integrity of the fuel can as a fuel container at risk.