In a nuclear reactor, the rate of the fission chain reaction is controlled by inserting rods made of which material that absorbs neutrons effectively?

Introduction / Context:
Nuclear reactors generate energy by sustaining a controlled chain reaction of nuclear fission, typically in fuel rods containing uranium or plutonium. To keep this chain reaction stable and prevent it from running out of control, reactors use control rods that absorb neutrons and moderate the number of neutrons available for further fission events. This question asks which material is used for such control rods in many reactor designs.

Given Data / Assumptions:

• The context is a nuclear reactor where a chain reaction must be controlled.
• Options include rods made of cadmium, iron, platinum and graphite.
• We assume a typical thermal reactor design used in basic physics and chemistry discussions.
• The key property we seek is strong neutron absorption.

Concept / Approach:
Control rods in nuclear reactors are made of materials with high neutron absorption cross sections, so they can remove neutrons from the system when inserted into the reactor core. Common materials include cadmium, boron and hafnium. Cadmium, in particular, is well known for its ability to absorb thermal neutrons. Graphite, by contrast, is used as a moderator in some reactor types to slow down neutrons, not primarily as a control rod material. Iron and platinum do not have the same combination of properties and are not standard choices for control rods in basic textbook descriptions.

Step-by-Step Solution:
Step 1: Recall that controlling a chain reaction requires adjusting the number of free neutrons available for fission.Step 2: Control rods must therefore be made of materials that absorb neutrons efficiently.Step 3: Cadmium is a classic neutron absorbing material used in control rods because of its high capture cross section for thermal neutrons.Step 4: Graphite is usually used as a moderator to slow down neutrons, not as the main control rod material in basic discussions.Step 5: Iron and platinum do not feature in standard lists of control rod materials for controlling chain reactions.Step 6: Therefore, the correct choice for controlling the chain reaction is the cadmium rod.

Verification / Alternative check:
Textbooks on nuclear physics typically list cadmium and boron as materials used in control rods. Illustrations of reactor cores often show thin rods of these materials being raised or lowered to adjust neutron flux. Graphite is mentioned separately in descriptions of graphite moderated reactors, where it surrounds the fuel and slows down neutrons to thermal energies. This clear separation of functions between control rods and moderators reinforces that cadmium rods are associated with direct control of the chain reaction.

Why Other Options Are Wrong:
Option b, iron rod, refers to a structural metal that does not have particularly high neutron absorption and is not used primarily as a control rod material. Option c, platinum rod, is expensive and not standard in reactor control applications. Option d, graphite rod, might be used as a moderator or structural component, but in basic nuclear reactor questions, graphite is associated with slowing neutrons rather than absorbing them to shut down the reaction. Therefore, these alternatives do not match the typical control rod material described in introductory courses.

Common Pitfalls:
Students may confuse the roles of moderator and control rod or assume that any material present in the reactor can control the chain reaction. Another pitfall is focusing on the mechanical strength or cost of the material rather than its neutron absorption properties. To avoid these mistakes, remember that control rods must "soak up" neutrons, and cadmium is one of the classic examples of a strong neutron absorber used for this purpose.

Final Answer:
In a nuclear reactor, the chain reaction is controlled by inserting cadmium rods, which absorb neutrons effectively.