What evidence does the author give the reader that a nuclear explosion happened before the beginning of Ray Bradbury's short story "There Will Come...

There are two sections in Ray Bradbury's short story "There Will Come Soft Rains" where the writer indicates the city surrounding the house has been destroyed in a nuclear blast. The first two thirds of the story are broken up by the house indicating the time of day.

In the beginning of the story the reader may be curious that the automated house goes on with its daily routine despite the fact there are no humans around. The text tells us the house is "empty" and the prepared breakfast is left uneaten. At "ten o'clock" the text says the house is the only one standing in a city that has been left in ashes and permeated by radioactivity. 

At "ten fifteen" the text tells us one side of the house has been burned black except for five places where silhouettes remain on the wall. Silhouettes of a man mowing the lawn, a woman picking flowers, and a boy and girl throwing a ball are permanently marked on the wall. These people were presumably incinerated in the blast and only their shadows left on the scorched wall. These same types of images were found after the first atomic bomb leveled Hiroshima at the end of World War II.