In the omnibus The Past Through Tomorrow, "Blowups Happen" is referred to as a 1940 story, but it mentions Hiroshima and Nagasaki, reflecting revisions made in 1946.
Heinlein's concept of a nuclear reactor was one of a barely contained explosion, not the steady-state thermal plants developed later.
Another consultant, Captain Harrington of the United States Naval Observatory, arrives as Lentz is preparing to return home.
The situation seems hopeless, as the energy produced by the reactor is sorely needed on Earth, oil having become too scarce and valuable to use for fuel.
The main reactor being in space removes the danger of planetary catastrophe but, by breeding the new fuel, still allows it to produce the needed energy.
In Heinlein's Future History, the next story sequentially is "The Man Who Sold the Moon", in which the reactor indeed explodes in space.