The story is that a scientist or engineer is kidnapped by the Norse gods to help them. A tree that gave fruit that kept them powerful was dying, and the ice giants were on the war path. He used gnomes that could produce anything to build a nuclear device, and dropped it on the giants from a flying chariot. He also used some basic knowledge of trees to save the tree and was allowed to eat of its fruit.
I believe you are referring to "Day of the Giants" by Lester del Rey. It was originally published as "When the World Tottered" in the December 1950 issue of Fantastic Adventures before being published as a novel in 1959.