A technique for determining the uranium concentration in the VVER-1000 primary coolant and natural fresh water was developed. Thin-layer inorganic sorbents on Ti and Ce hydroxides (thickness being tens of μg cm-2) were used for increasing the uranium concentration. After exposure in the experimental solution, thin-layer inorganic sorbents were irradiated by thermal neutrons when positioned in close contact with plastic track detectors; the latter were then etched electrochemically. A Lavsan film, 180 μm thick, was used as a detector for fission fragments. Detection sensitivity for uranium in ammonia-boron-potassium solution and distillate is a hundred or a thousand times higher than for the case of the track detector irradiated by neutrons directly in the solution. A detection limit for uranium in solutions is no worse than 3 × 10-12 g cm-3. It is defined, mainly, by a natural contamination of the sorbent by uranium. High sensitivity, a low detection limit, and simplicity of the technique in practice make it feasible for solving the problems of nuclear reactor radiation safety, radio-ecology and other research fields.