Volume of sphere=4/3 x Pi x r^3, r=radius
V =4/3 x 3.141592 x 4,000^3
V=268,082,517,333 cubic meters
Mass =268,082,517,333 kg
KE =1/2 x M x v^2
KE=1/2 x 268,082,517,333 x 30,000^2
KE=120,637,132,800,000,000,000 =~1.2 x 10^20 Joules
1 Megaton =4.2 x 10^15 Joules, therefore:
[1.2 x 10^20] / [4.2 x 10^15] =~28,723 Megatons of energy released.