3.5 seconds after detonation
Out to a distance of 0.6 miles from the center there will be nothing recognizable remaining, with the exception of some massive concrete bridge abutments and building foundations. At 0.6 miles some heavily damaged highway bridge sections remain, but little else until 1.3 miles, where a few very strongly constructed buildings will survive. A distance of 1.7 miles (12 psi ring) is the closest range where any significant structures will remain standing.
Of the people in this area during the night, there are virtually no survivors. Most deaths will occur from collapsing buildings. Although many fires will be started, only a small percentage of the buildings are likely to continue to burn after the blast wave passes.
The fireball will have a radius of 0.4 miles. However, the blast effects will greatly outweigh any direct thermal effects due to the fireball.