Question

In nuclear reactors, the most commonly used neutron absorber is:

• A. Uranium
• B. Hydrogen
• C. Boron
• D. Oxygen

Hint:

The two most common materials used for control rods in nuclear engineering are Boron ($B$) and Cadmium ($Cd$), because both elements have high neutron absorption cross-sections.

Answer 1

The Correct Option is C

Concept: Nuclear power reactors sustain control over the core's fission chain reaction by managing the population of thermal neutrons. The rate of fission depends directly on the neutron flux. To regulate or halt this process safely, control rods made of specialized materials called neutron absorbers (or neutron poisons) are inserted into the core. These materials must have an exceptionally high thermal neutron capture cross-section ($\sigma_c$), meaning they can absorb neutrons efficiently without undergoing fission themselves.

Step 1: Analyze the properties of the options provided.
* Uranium (U): A heavy fissile/fertile material used as primary nuclear fuel (e.g., $U^{235}$, $U^{238}$), not an absorber. * Hydrogen (H): Has a low mass number and tends to scatter and slow down fast neutrons through elastic collisions. It serves as an effective moderator (e.g., in light water $H_2O$), rather than a dedicated absorber. * Boron (B): The isotope Boron-10 ($B^{10}$) has a very high thermal neutron absorption cross-section (approximately $3837\text{ barns}$). The absorption reaction is: \[ _5B^{10} + _0n^1 \rightarrow _3Li^7 + _2\alpha^4 + \gamma \] This makes Boron highly efficient at absorbing neutrons and controlling the reaction rate. * Oxygen (O): Has a very low neutron absorption cross-section, allowing it to be paired with uranium in oxide fuels ($UO_2$) or used in cooling water without capturing neutrons excessively.

Step 2: Conclude the industry standard application.
Boron (often in the form of boron carbide $B_4C$ control rods or dissolved boric acid in pressurized water reactors) is widely used across the nuclear industry as a reliable neutron absorber. Therefore, the correct choice is Option (C).