Ash can withstand Charizard's Flamethrower, which reportedly can melt boulders. Ash can survive heat as high or even exceeding 2,400 degrees fahrenheit with only slight damage.

Well according to Pokémon Stadium, Charizard can quickly melt glaciers weighting 10 000 tons. Assuming metric tons, this would wield about 7.10 kilotons of TNT in term of energy. The nuclear bomb "little boy" was 12 kilotons.

TL;DR Ash survived what would be considered the energy of a small nuke on him.