At the nuclear reprocessing plant of La Hague (France), spent fuel elements are left to cool in storage pools before reprocessing. These pools are housed in large buildings (typical dimensions 95 x 25 x 6.5 m), ventilated by twelve ceiling diffusers and sixty-two exhaust slots located near the surface of the pool. The operators of this facility raised several questions concerning this ventilation system: what are the movements of air in the building. What are the transit times and transfer coefficients between the surface of a pool and the air sampling system. Pool water temperature is normally kept at about 30 deg C. In case of perturbation of the cooling system, this temperature would rise to an estimated 40 deg C. How would the airflow in the building be affected. This paper shows how these questions could be addressed by the combined use of smoke injections, velocity measurements, tracer-gas experiments and the numerical predictions of a flow code. (author). 5 refs., 7 figs., 2 tabs.