Purpose of a thermal (inner) shield in high-power nuclear reactors: what is its primary function inside the reactor vessel region?

Introduction / Context:
Large power reactors include several shielding systems with distinct purposes. In addition to biological shields that protect personnel outside the core, an inner “thermal shield” (often a thick steel or heavy-metal structure) is positioned to reduce neutron and gamma heating of the reactor vessel and internals. Understanding what this shield does—and what it does not do—helps avoid confusion with moderators, absorbers, and mechanical hardware supports.

Given Data / Assumptions:

• High neutron and gamma flux exists near the core.
• Excessive radiation heating can raise temperatures and cause embrittlement or swelling in structural materials.
• Thermal shield is located between core and vessel wall (design-dependent).

Concept / Approach:
Fast neutrons and prompt gammas deposit energy in surrounding structures. The thermal shield’s mass and composition attenuate this radiation before it reaches the vessel wall, thereby limiting temperature rise and long-term material damage. It does not function as a control system for reactivity, nor as a moderator. Instead, it is a passive protection component designed to intercept and reduce heat deposition and flux at sensitive structures.

Step-by-Step Solution:

Verification / Alternative check:
Design descriptions show thermal shields lowering vessel wall temperature gradients and fluence, extending component life and inspection intervals. Biological shielding, in contrast, addresses dose outside containment; moderators and absorbers serve different neutronic functions.

Why Other Options Are Wrong:

• Absorb fast neutrons to shut down: that is a control rod/poison role, not the thermal shield’s main purpose.
• Slow secondary neutrons: that is the moderator’s role.
• Prevent fuel-coolant contact: a mechanical design and grid/spacer function, not shielding.
• Steam generator corrosion: unrelated to the in-vessel thermal shield.

Common Pitfalls:
Confusing inner thermal shielding with biological shielding or control rod functions.

Final Answer:
Protect the reactor vessel and internal walls from radiation damage and excessive heating