Light Water Reactars (WR) are nuclear reactors

The correct answer is: Which use ordinary water as moderator

To understand why this is correct, let us first break down the key concepts involved in nuclear reactor design — specifically the roles of a moderator and a coolant, and then look at what defines a Light Water Reactor (LWR).

What is a Moderator?
In a nuclear reactor, fission reactions are initiated when neutrons strike uranium nuclei. However, the neutrons released during fission are fast neutrons and are not very effective at causing further fission in uranium-235. A moderator is a material placed in the reactor core to slow down (thermalize) these fast neutrons into thermal (slow) neutrons, which are far more efficient at sustaining the chain reaction. A good moderator must have a low atomic mass (to absorb kinetic energy from neutrons through collisions) and a low neutron absorption cross-section.

What does "Light Water" mean?
Water exists in two forms relevant to nuclear engineering:

• Light Water (H₂O) — ordinary water, composed of hydrogen (¹H) and oxygen.
• Heavy Water (D₂O) — water composed of deuterium (²H, or D) and oxygen. It is significantly more expensive and rare.

A Light Water Reactor (LWR) uses ordinary water (H₂O) — that is, "light water" — as its moderator. This is the defining characteristic of an LWR. The hydrogen nuclei (protons) in ordinary water are very effective at slowing down fast neutrons because their mass is nearly equal to that of a neutron, allowing maximum energy transfer per collision (similar to a billiard ball collision).

Why the other options are incorrect:

❌ "Which use heavy water as a coolant" — Heavy Water Reactors (HWRs), such as the CANDU reactor, use heavy water (D₂O). LWRs use ordinary (light) water, not heavy water. Also, in an LWR, ordinary water serves as both moderator and coolant — but the defining feature is its role as the moderator.

❌ "Which use graphite rod as moderator" — Graphite-moderated reactors are an entirely different class, such as the RBMK reactor (used in Chernobyl) or gas-cooled reactors. LWRs do not use graphite as a moderator.

❌ "Which use steam as moderator" — Steam is not used as a moderator in any standard reactor design. In Boiling Water Reactors (BWRs), which are a type of LWR, steam is generated inside the core, but the liquid ordinary water is what acts as the moderator, not the steam.

Key Takeaway:
The name "Light Water Reactor" directly refers to the use of ordinary (light) water — H₂O — as the moderator (and coolant) in the reactor. The two main subtypes of LWRs are:

• Pressurized Water Reactors (PWRs) — water is kept under high pressure to prevent boiling; it moderates neutrons and transfers heat to a secondary steam loop.
• Boiling Water Reactors (BWRs) — water is allowed to boil inside the reactor core; the steam generated directly drives the turbine.

Both types use ordinary water as the moderator, which is precisely why they are classified as Light Water Reactors.