In nuclear reactors, 'moderators' are substances: A. That slow down neutrons…

Concept: A moderator is a light-nucleus material placed in a reactor core so that the fast neutrons released by fission, born with roughly 2 MeV of energy, lose that energy through repeated elastic collisions and settle at thermal energies of about 0.025 eV. The fission cross-section of a fissile nucleus such as U-235 rises steeply as neutron energy falls, so bringing neutrons down to thermal energy is what keeps a thermal reactor's neutron economy self-sustaining. Restraining or shutting down a chain reaction is a separate function, and it is performed by neutron-absorbing control rods rather than by the moderator.

Application: Each statement in the stem is now tested against that definition.

1. Slowing neutrons down is the defining action of a moderator. Light nuclei such as hydrogen-1, deuterium and carbon-12 have masses close to that of a neutron, so each elastic collision removes a large share of the neutron’s kinetic energy. Statement A is true.

2. Limiting the chain reaction is the job of control rods made of boron, cadmium or hafnium, which are inserted specifically to capture neutrons and bring the neutron population down. A good moderator is chosen for the opposite property: it lowers neutron energy while absorbing as few neutrons as possible. Restraining the reaction is therefore not the function of a moderator, and statement B is false.

3. Helping to sustain the chain reaction follows directly from moderation: a thermal neutron is absorbed by U-235 with a fission cross-section hundreds of times larger than that of a fast neutron, which drives the multiplication factor k up towards 1. Statement C is true.

4. Most power reactors in service — pressurised water, boiling water and pressurised heavy water designs — are thermal reactors, and removing their moderator makes them sub-critical at once. The qualifier “most” is what makes the claim safe, because fast breeder reactors deliberately run with no moderator at all. Statement D is true.

5. Heavy water, D2O, is a standard moderator and is the moderator of the PHWR and CANDU family; deuterium slows neutrons efficiently while capturing very few of them. Ordinary light water and graphite are the other common choices. Statement E is true.

Cross-check: The three core materials of a reactor are separated cleanly by function, which is the distinction the item is testing.

Result: The statements that survive the test are the neutron-slowing action, the sustaining role, the indispensability in most reactors and the heavy-water example, that is A, C, D and E. The claim about limiting the chain reaction describes control rods rather than moderators, so the accepted set is A, C, D and E only.