AbstractIt is shown that an accurate determination of the fission rate in fuel elements can be obtained by the technique of solid-state track detectors. The method gives: (a) the radial variation and (b) the absolute value of the fission rate, by application of the measured value of the mean-path-length of the fission products in the fuel. The quality of the results is independent of the neutron spectrum during the irradiation.In a particular case (thermal flux), results deduced from measurements performed with solid-state track detectors are compared with those obtained by the fissile detector (activation) technique. The agreement is excellent.