Gas pressure measuring method for the inside of nuclear fuel element

A heating device for rapidly heating a cladding tube in a region of a gas plenum portion of a nuclear fuel element is actuated to rise the temperature from room temperature to 327degC in about 10sec. Simultaneously, the temperature of not heated portions of the cladding tube is controlled to a predetermined level by a thermostable device. The time at each point of the cladding tube and the time dependency of the change of the outer diameter of the cladding tube after starting of the heating is recorded in a data recording device. In a typical BWR type fuel element, the relation between the inner pressure increasing amount ΔP of the cladding tube and the outer diameter expansion amount ΔD caused corresponding to the former is represented by ΔP = 14.1 x ΔD. In addition, the pressure P0 in the nuclear fuel element at a room temperature is approximated by P0 = 14.1 x ΔD x T/(T - T0) in which T0 is a room temperature and T is a temperature of the cladding tubes kept upon testing. With such procedures, gas pressure in the nuclear fuel element can be measured accurately in a non-destructive manner. (I.N.)