[en] The reactivity effects of hydrogenous substances, such as the oil used in the primary coolant pumps, which could enter the core of a fast reactor in a hypothetical accident, have been studied in a series of experiments and calculations. Measurements to study the influence of the density of the hydrogen and its location on reactivity were made in two assemblies in the zero power reactor ZEBRA. The first of these was similar in size to PFR and the second was the large BZB assembly of the BIZET programme. The results of this work have been compared with calculations using the FGL5 nuclear data library. Calculations for a 1200 MW (e) CFR have been made using three quantities of material (8, 40 and 160 Kg of hydrocarbon, equivalent to 10, 50 and 200 litres of oil). The calculations have used different geometrical models and hydrocarbon distributions and have explored the influence of core temperature, fuel burn-up and the presence of control rods to estimate the maximum reactivity changes that can be obtained. The results have been analysed in terms of components of the change in neutron balance produced by the material and uncertainties in these have been derived from the ZEBRA work. (author)