 His mentor Gilbert Newton Lewis isolated the first sample of pure heavy water by electrolysis in 1933.Although a scientific curiosity from the start, considerable interest in heavy water was aroused in 1939 when Hans von Halban and Lew Kowarski suggested that heavy water could be used as a neutron moderator in a nuclear reactor using natural uranium.They carried out experiments on uranium using ordinary water, but had found that the hydrogen atoms absorbed neutrons, thus preventing the desired chain reaction.