[en] This paper describes the application of the Harwell Neutron Absorptiometer to routine analysis of the U-235 content of fuel element inserts manufactured at the Dounreay Nuclear Power Development Establishment for the use in Materials Testing Reactors. The instrument response, which is principally dependent on the ²³⁵U closely follows a logarithmic relationship. Neutron attenuation due to the aluminium matrix and the presence of ²³⁸U is less than 2% of the total attenuation. The absorptiometer can be used to estimate the weight of ²³⁵U in a single insert with a total error in the range 1 to 1.6%. (author)