The blast radius of a 250 gallon propane tank would be 100 yards

Little Boy was the nuclear bomb detonated over Hiroshima. It used uranium and had an explosive blast equivalent to 12,500 tons of TNT. A 1 megaton hydrogen bomb, hypothetically detonated on the earth's surface, has about 80 times the blast power of that 1945 explosion.

Considering the tonnage of a bomb to be contant, The blast radius varies dependent on whether it is a ground burst or an airburst. Further, the height of the airburst above ground affects the radius too. At a height of 1900 feet above ground, Little Boy produced a blast radius of 1 mile; an area of some 4.7 square miles.

standard blast radius is around 3-8 feet

about 7.7 percent

30 miles.