Fuel rod for a nuclear plant and a plenum spring arranged to be provided in such a fuel rod

A nuclear fuel rod for a nuclear plant, wherein the fuel rod comprises a cladding tube sealed by a first end plug at a first end and second end plug at a second end, a plurality of fuel pellets stacked on each other inside the cladding tube such that they form a column of pellets and a plenum spring arranged in a space in the fuel rod in order to, at least during transportation, hold the column of pellets with a spring force towards the second end of the cladding tube and wherein the plenum spring comprises a first length variable part arranged to abut towards the uppermost located fuel pellet in the column of pellets with an end portion and a second part arranged to allow engagement of the plenum spring against an inner surface of the cladding tube by a radially outwardly directed pressure, wherein the plenum spring comprises a third part arranged to allow releasing of the engagement of the second part of the plenum spring in the cladding tube during operation of the nuclear plant, wherein said third part allows such a releasing when the first length variable part of the plenum spring is compressed to be below a specific length, and wherein said third part comprises a stop portion arranged to obtain contact with the uppermost located fuel pellet in the column of pellets when the first part is compressed to said specific length.