Moderator
Material that slows fast neutrons to thermal energies through elastic collisions, enabling fission chain reactions.
Common moderators include light water (reduces energy by factor of ~10 per collision), heavy water, graphite, and beryllium. Moderating ratio measures slowing efficiency vs. absorption; water has ~70:1 for H-1. In LWRs, moderator also acts as coolant, doubling as safety feature via void coefficient.
Why it matters now
Moderator choice drives SMR designs like NuScale VOYGR (light water) vs. heavy water PHWRs, key for 2025 NRC approvals amid data center power demands.
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