One of the by-products at Rjukan was the production of heavy water—a key component in nuclear weapons, and necessary for the Germany designs as a moderator. The hydrogen plant at Vemork was the first mass producer of heavy water, and in 1939 IG Farben, who owned 25% of Norsk Hydro at the time, asked to import five liters of heavy water, but was denied due to lack of an export license. In 1939–40 production at Vemork was 20 kilograms, by 1942 production had increased to five kilograms per day.