Nuclear fuel element comprises a central core of nuclear fuel, partic. UO2 opt. mixed with PuO2, and an elongated composite sheath consisting of a substrate, partic. a Zr-alloy, with a protective layer on its inner surface of a matl. selected from alloys incorporating Cr resistant to oxidn., partic. stainless steel of high Cr content, a Cr-contg. coating, an Al-contg. coating or a silicon-contg. coating alloyed with the substrate. Protects the substrate from corrosion by fission prods. or attack by irradiated impurities or their decompsn. prods. The tubular fuel element has a free space at its upper end for the reception of a helical element e.g. a spring, for retaining the nuclear fuel tablets e.g. of UO2, in posn.