Sure, big fusion bombs could leave everything radioactive.

The 5.27 year half-life of the ⁶⁰Co is long enough to allow it to settle out before significant decay has occurred and to render it impractical to wait in shelters for it to decay, yet short enough that intense radiation is produced.

The Wikipedia entry shares this description:

At the time when the Second World War just got seriously going, the United States and Soviet Union had not yet become directly involved and the world’s attention was riveted on the unfolding Battle of Britain, Heinlein was four or five steps ahead of everybody. More than a year before Roosevelt authorized the Manhattan Project, Heinlein correctly foresaw that: a) The President of the US would initiate a secret project to develop nuclear weapons and employ scientist refugees from Nazi Europe; b) By 1945, the US would have a weapon able to destroy an entire city in one blow from a single airplane—and would use that weapon to end to war; c) That with the US having thus won the war, the world would become aware of the realities of a nuclear arms race—without using the term, Heinlein predicted and described in detail the doctrine of Mutual Assured Destruction; and d) Concretely, the main issue on the agenda in the post-1945 years would be whether the Soviet Union would obtain nuclear arms, and if it did—would the Soviets try to launch a surprise nuclear attack on the United States. For having predicted all that in 1940—even to accurately predicting the remorse and guilt feeling of the scientists involved—Heinlein deserves much plaudits.

‘What I thought more likely was that the Germans would use an ordinary explosive bomb containing radioactive material. If we were unable to neutralize the effects of such a weapon promptly, a major panic could easily sweep through the Allied countries. However, as the plans for the invasion of Europe began to take form, we considered very seriously indeed the possibility that the Germans might lay down some kind of radioactive barrier along the invasion routes. We could not calculate with any certainty the likelihood of their doing this, for we were truly in the dark then about their progress in atomic development.’

Groves, Leslie M.. Now It Can Be Told: The Story of the Manhattan Project (Franklin D. Roosevelt and the Era of the New Deal) (pp. 256-257). Da Capo Press. Kindle Edition.

Groves, Leslie M.. Now It Can Be Told: The Story of the Manhattan Project (Franklin D. Roosevelt and the Era of the New Deal) (pp. 210-211). Da Capo Press. Kindle Edition.