> Why would the average distance a neutron has to travel to strike a nucleus increase? Because if the problem is that neutrons are escaping the object before h

> Why would the average distance a neutron has to travel to strike a nucleus increase? Because if the problem is that neutrons are escaping the object before hitting a nucleus, and we are adding more nuclei so the likelihood that they hit something increases, the new collision candidates will be further away than the old ones. In other words, adding material to the edge of the object does not affect the per distance probability of collision. It only affects the overall probability of collision. Since the per distance probability does not change while the overall probability does, the probability increase must lie outside of the average path length of a neutron through the original object.