Nuclear safety systems: the “safety” (shutdown) rods inserted during upsets such as earthquakes are commonly made from which strong neutron absorbers?

Introduction / Context:
Reactor shutdown systems must rapidly terminate the chain reaction under abnormal conditions (e.g., seismic events). Control and safety rods achieve negative reactivity insertion by absorbing neutrons efficiently. Material selection centers on elements with high neutron capture cross-sections and adequate mechanical/chemical compatibility.

Given Data / Assumptions:

• Control/safety rods must insert promptly and withstand radiation and temperature.
• Materials should have high thermal neutron absorption and acceptable structural properties.
• Common rod materials: boron (as B4C) and cadmium, sometimes hafnium, silver-indium-cadmium alloys.

Concept / Approach:
Boron-10 and cadmium isotopes have large capture cross-sections for thermal neutrons, making them ideal for scram systems. Zircaloy and structural steels are preferred for cladding/structures due to low absorption; using them as absorbers would be ineffective.

Step-by-Step Solution:
List candidate absorbers and their cross-sections.Match to widely used control-rod compositions: B4C pellets, cadmium sheets, or Ag-In-Cd alloys.Select “boron or cadmium.”

Verification / Alternative check:
Reactor handbooks and vendor specs consistently show B4C and cadmium as proven absorber media for shutdown rods.

Why Other Options Are Wrong:

• High-carbon steel, molybdenum, Zircaloy: structural materials with relatively low absorption; unsuitable as primary absorbers.
• Copper-nickel: not used for control rods.

Common Pitfalls:
Confusing cladding/guide tube alloys (low absorption) with absorber media (high absorption).

Final Answer:
Boron or cadmium