Nuclear fuel element

At least one of nitrogen, nitrogen oxide and carbon oxide is added to a gas to be filled in a nuclear fuel element. In this case, nitrogen oxide is nitrous oxide, and a fuel cladding tube comprises a base material comprising a zirconium-base alloy lined to the inner circumferential surface with a zirconium liner. Within a range of burn-up degree from about 15 to 30GWd/t, various kinds of gases are adsorbed or absorbed at a top end of each of fine cracks caused to the inner surface of the fuel cladding tube, and as the adsorption amount is increased, the embrittlement or corrosion of the top end of the crack of materials of the fuel cladding tube made of a zirconium-base alloy or zirconium material are moderated more, to inhibit the development of the cracks. Oxygen, nitrogen, nitrogen oxide or carbon oxide is suitable as the material to be adsorbed to the top end of the crack easily. A nuclear fuel element having long-life and high reliability and with improved stress corrosion cracking resistance can be attained by improving an atmosphere in the nuclear fuel element during operation. (N.H.)