A nucleus in the middle emits a neutron in a random direction, if it hits another nucleus it is absorbed, that hit nucleus emits two neutrons and dies.
A full chain reaction, involving essentially all the nuclei, is much more likely to occur with a bigger piece of material. In real life, with trillions of trillions of nuclei (at least) there is a precise critical size (for a given shape) at which the reaction "goes critical" and explodes.
With our far smaller grids the random fluctuations are large, so we can just get the general idea. For the smallest grid, you'll find most times not much happens. For the largest grid, almost all the nuclei will explode. Try it!
The mean free path is how far the neutron gets on average before hitting another nucleus. Roughly it's the inverse of the ratio of the nuclear size (the 'cross section") to the atom size (meaning distance between nuclei). This is how many rows of atoms the neutron will get through, on average, before it hits a nucleus. For a real neutron in uranium, for example, the cross section of the nucleus is (now two-dimensional) less than a billionth the size of the atom, so the neutron can go through billions of rows (actually planes, in 3D) of atome, the mean free path is of order centimeters. This gives the scale of size needed for a successful chain reaction.
Code by Atallah Hezbor