How is it that when momentum is constant, energy always stays a constant.


For Example :- 
if $P = 0$ kinetic energy will also be 0. 

But in a explosion (that momentum is conserved) where the object was still before the explosion the momentum still should be 0. By using $E_k=\frac{P^2}{2m}$ energy also return as 0 Jules. 

If the $E_k=\frac{mV^2}{2}$ is used the kinetic energy comes as $E_k>0$ since kinetic energy is a scalar.

How is that?