Plenum gas checking type fuel rod

The present invention concerns an improvement for the safety of fuel rods in a reactor. The inner side of a fuel rod cladding tube is divided into a fuel pellet portion and a gas plenum portion, and a back-flow check valve is disposed at the divided portion. A heat insulation material is disposed below the fuel pellet so as not to directly conduct the heat of high temperature to the fuel pellet upon burning. Since the volume of the gases present in the gap of the fuel pellet portion is sufficiently smaller compared with the volume of the plenum, most of fission gases formed from the fuel pellets are accumulated in the volume of the plenum. Gaseous fission products are not leached out from the gas plenum by the effect of the back-low check valve. Accordingly, even if the cladding tube of the fuel pellet portion should be perforated or damaged during reactor operation or upon occurrence of accident, most of the fission gases are kept in the gas plenum, to reduce the releasing amount of radioactivity. (I.N.)