METHOD FOR NON-DESTRUCTIVE MEASUREMENT OF INTERVAL BETWEEN FUEL PELLET AND COVER PIPE IN FUEL ELEMENT

PURPOSE:To rapidly measure the interval between a fuel pellet and a cover pipe within a used nuclear fuel element in a non-destructive manner with high accuracy, by a method wherein an ultrasonic pulse is emitted to the nuclear fuel element and made reciprocating through the nuclear fuel element to be received by an ultrasonic pulse receiver and the reciprocating time thereof is measured. CONSTITUTION:A nuclear fuel element 6 being an object to be measured is constituted by laminating fuel pellets 8 within a cover pipe 7 and providing a plenum part provided with a compression spring 9 above said fuel pellets. The ultrasonic pulse generated by an ultrasonic pulse transmitter 2 is transmitted into the element 6 and, subsequently, the ultrasonic pulse reciprocating in the element 6 is received by an ultrasonic pulse receiver 4. Then, the reciprocating time of said pulse is measured to make it possible to measure the interval between the outer surface 10 of the pipe 7 and the surface 12 of each of the pellets 8 within the element 6 in a non-destructive manner without damaging the pipe 7.