A reactor physics term describing the state when the number of neutrons released by fission is exactly balanced by the fuel and poisons escaping the reactor core?

Answer 1

I think you must mean "criticality", which is a term applied to nuclear reactors meaning the neutron flux in the active core remains at a steady level, the actual level can vary from a low level equivalent to a few watts of power up to the reactor maximum allowable. At criticality the number of neutrons being produced every second by fission is just balanced by the number being captured in further fuel nuclei, in other core materials, the moderator itself, and escaping at the core boundaries.

Answer 2

Criticality is the term used to describe the state when the number of neutrons released by fission is balanced by the fuel and poisons escaping the reactor core. In a critical reactor, the neutron population remains stable without increasing or decreasing over time.