Before the test it was calculated whether the atmosphere would ignite leading to the apocalypse. Though nuclear chain reactions are said to grow exponentially they didn't calculate whether the Universe would explode as only a finite amount of fissile material was used. However, since not all that material was consumed by the explosion as seen in the fallout it must be the case that a nuclear chain reaction does not grow exponentially.
| 1945 AD | Manhattan Project | Trinity Test |