Subcritical and critical mass
Subcritical masses are small enough to allow the neutron to escape from the surface of the fuel before colliding with a nucleus. Pieces of uranium about the size of a small tennis ball are known to be at subcritical mass.
A mass of fuel that is too small to support a nuclear fission reaction is called subcritical mass. The neutron travels through the fuel escaping from the surface before having a chance to collide into a nucleus. The chain reaction soon dies out .
Critical mass is a mass of fuel that, on average, each fission reaction produces products that split one other nucleus. This will not cause a chain reaction and therefore a nuclear explosion, but will maintain the reaction progressing steadily.