Select from below the three statements that are true.

Question $16$Answer

The amount of energy released by a fission event is so large that if just a few such events occur each second the required heat output of the reactor will be attained.

The fission cross$-$section for U$-235$ increases with the energy of the free neutrons.

In the context of nuclear physics the term 'cross section' is a measure of the likelihood of a particular nuclear reaction occurring.

A reactor core should be run in 'sub$-$critical' mode as 'critical' mode indicates the existence of a chain reaction within the core and the likelihood of the reactor running out of control.

Criticality is maintained in a nuclear reactor by raising or lowering the control rods which control the numbers of free neutrons available for generating new fission events.

A chain reaction is sustained by free neutrons from one fission event going on to induce fission in further nuclei.

A moderator is used within a reactor to remove the slow neutrons which do not possess enough energy to cause fission.