CANDU Reactor Overview

The term CANDU stands for CANada Deuterium Uranium. It is a specific type of nuclear fission reactor developed in Canada. CANDU reactors are known as pressurized heavy-water reactors (PHWRs). Understanding their key components, especially the fuel and moderator, is crucial for comprehending their operation.

CANDU Fuel Type

One of the defining characteristics of a CANDU type nuclear reactor is its fuel. These reactors are designed to operate using natural uranium as their fuel. Natural uranium consists primarily of uranium-238, with a small percentage (approximately 0.7%) of the fissile isotope uranium-235. Unlike many other reactor types, such as light water reactors, CANDU reactors do not require the uranium fuel to be enriched. This means the fuel doesn't undergo a costly and energy-intensive process to increase the concentration of uranium-235.

CANDU Moderator Explained

For a nuclear reactor to sustain a chain reaction, the neutrons released during fission must be slowed down to thermal energies. This process is called moderation, and the material used for this purpose is called a moderator. In CANDU reactors, heavy water ($\text{D}_2\text{O}$) is used as the moderator. Heavy water is water in which the hydrogen atoms are replaced by deuterium, an isotope of hydrogen with one proton and one neutron. Deuterium nuclei have a very low neutron absorption cross-section, meaning they absorb very few neutrons. This property makes heavy water an extremely efficient moderator.

Why CANDU Reactors Use Heavy Water

The choice of heavy water as a moderator is directly linked to the use of natural uranium fuel. Since natural uranium has a lower concentration of fissile uranium-235, it produces fewer neutrons per fission that are available for sustaining the chain reaction. A highly efficient moderator like heavy water is necessary to conserve these neutrons by slowing them down without absorbing them significantly. This allows the chain reaction to be maintained even with the less enriched natural uranium fuel. In contrast, light water ($\text{H}_2\text{O}$), which is used in many other reactor types, absorbs too many neutrons to be effective with natural uranium, thus requiring enriched uranium fuel.

Analyzing CANDU Reactor Options

Let's evaluate the given options based on the characteristics of CANDU reactors:

• Option 1: Natural Uranium is used as fuel and water as moderator. This option is incorrect. While natural uranium is the fuel, "water" typically refers to light water ($\text{H}_2\text{O}$), which is not used as the moderator in CANDU reactors.
• Option 2: Natural Uranium is used as fuel and heavy water as moderator. This option accurately describes the core components of a CANDU reactor: it uses natural uranium as its fuel and heavy water ($\text{D}_2\text{O}$) as its moderator. This combination is essential for the reactor's operation.
• Option 3: Enriched Uranium is used as fuel and water as moderator. This option is incorrect. CANDU reactors specifically use natural, not enriched, uranium. Also, they use heavy water, not light water, as the moderator.
• Option 4: Enriched Uranium is used as fuel and heavy water as moderator. This option is incorrect. While heavy water is used as a moderator, CANDU reactors do not use enriched uranium as fuel; they rely on natural uranium.

Therefore, the statement that correctly describes a CANDU type of nuclear reactor is that natural uranium is used as fuel and heavy water as a moderator.