From History Extra from Damien Lewis author of Hunting Hitler’s Nukes: The Secret Race to Stop the Nazi Bomb:
That Hitler’s Germany might win the race to build the world’s first atom bomb was arguably one of Winston Churchill’s greatest wartime concerns, and one that was shared with his good friend US president Franklin D Roosevelt. When Churchill flew across the Atlantic to meet Roosevelt to discuss this issue, they agreed no effort could be spared to stop Hitler from getting the bomb, for with it he would win control over the world. “We both felt painfully the dangers of doing nothing,” Churchill would later write. “We knew what efforts the Germans were making to procure supplies of heavy water – a sinister term, eerie, unnatural, which began to creep into our secret papers. What if the enemy should get the atomic bomb before we did! We could not run the mortal risk of being outstripped in this awful sphere.”
The term ‘heavy water’ referred to deuterium oxide, a refined form of naturally-occurring H2O that is used as a moderator in nuclear power plants, which can breed weapons-grade uranium and plutonium.
So great were the wartime leaders’ fears that Roosevelt demanded immediate action to sabotage Hitler’s nuclear efforts. Only Britain had the capacity to respond. Churchill ordered a series of dramatic raids by his ‘Volunteers for Special Duties’ – his commandos, Special Operations Executive [SOE] agents and Special Forces: unique fighting men and women ideally suited to such near-suicide missions. Those daring raids targeted the giant heavy water plant at Vemork, in German-occupied Norway, among other facilities, which formed a key element of the Germans’ nuclear programme, and was the only element deemed vulnerable to Allied sabotage.