Is there a minimum mass required to create an atomic bomb? I have heard that there is a minimum mass of fissile material required to produce an explosion.  If so why would this be?  Is it true North Korea found a way to overcome this?
 A: It is called critical mass. As Wikipedia defines it:

In nuclear engineering, a critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction.

Nuclear reaction is a chain reaction, that is the neutrons released in one decaying nucleus trigger more reactions in other nuclei and so on, resulting in avalanche growth, which may manifest itself as an explosion. However, if a neutron doesn't meet a target, it simply escapes from the fissile material - so there is the minimal amount of material that is required for the chain reaction to actually happen.
Note however, that critical mass is not an absolute - it depends on the shape of the material, presence of reflectors around, and so on. There have been multiple accidents in nuclear laboratories, (documented in Wikipedia, e.g.) where critical mass was assembled accidentally, e.g., when the experiment was done near a reflecting wall or due to incorrect manipulations of a scientist.
Note also that critical mass is not sufficient for creating an explosion - in many cases the material simply melts from heating, loses its shape and stops being critical. Nuclear bomb is thus more than just a piece of Uranium (which is why US needed to conduct the Trinity test, to see that the thing actually works.)
In other words, nuclear weapons are explicitly designed for the critical mass to be assembled and the reaction to generate huge amount of heat, before the weapon disintegrates (see the figure below):

On the other hand, nuclear reactors are designed to include moderators and control rods that slow down the nuclear reaction (e.g., by absorbing neutrons) and thus keep the amount of heat produced under control.
