Self-sustained chain reactions depend critically on the value of the neutron multiplication factor k. Answer the following aspects related to neutron economy in a large block of 235U kept at normal atmospheric conditions.
(i) Define the neutron multiplication factor k for a fissionable assembly.
(ii) State clearly the conditions that correspond to a sub-critical, critical and super-critical assembly.
(iii) Identify and briefly explain any four independent physical processes that can decrease the value of k inside a reactor core.
(iv) Give two reasons why a moderator material of low atomic mass and low neutron absorption cross-section is required in a thermal reactor.
(v) With respect to the desired value of k and the generation rate of neutrons, differentiate between the design objectives of a nuclear power reactor and an atomic bomb.