Abstract: A nuclear fuel element comprises plutonium-containing fuel pellets filled in a cladding with both end sealed with end plugs. One end plug has a filter at the inside and a passage communicating with the space in the cladding through the filter. The passage is sealed at the surface part of the end plug by welding. The filter and the passage in the end plug are utilized for withdrawing a gas in the cladding filled with the fuel pellets in a process for fabricating the nuclear fuel element. The filter traps nuclear fissionable materials in the cladding to prevent its leakage to the outside. After the withdrawal of the gas, a helium gas is filled in the cladding through the passage in the end plug. After completion of these operations, the passage in the end plug is sealed as mentioned above.