Satsuki sets off a trap that destroys the Great Cultural and Sport Festival Stadium, which she survives in her underwear. Previously, I calced the diameter of the Honnouji Academy grounds to be 410.32m across.
1 pixel = 410.32m/757 = 0.542034346m
0.542034346m X 1453 = 787.575905m
1 pixel = 787.575905m/792 = 0.994414021m
1016.29113m/2 = 508.145565m
Given our radius, we can find the energy of the fireball. Entering that into Nukemap and we have a yield of 98 kilotons. Alternatively, using the MIT nuclear explosion calculator gets us a yield of 448.1 kilotons.
Final Results
Grand Cultural and Sport Festival Stadium explodes (low end) = 98 kilotons
Grand Cultural and Sport Festival Stadium explodes (high end) = 448.1 kilotons
