(a) Define the term “critical mass” for a fissile material.
(b) A quantity of fissile material is assembled into a sphere whose mass is lower than the critical mass for that material. Give two clear physical reasons, based on neutron behaviour, why this sphere cannot maintain a steady fission chain reaction even though each fission event releases more than one neutron.
(c) State any two engineering measures that are commonly employed in power reactors to ensure that the effective mass of fissile material remains sub-critical during shutdown or storage.