What is inside the fuel boxes that allows for moderation?

What is inside the fuel boxes that allows for moderation?

• A. Distilled Water
• B. Liquid Nitrogen
• C. Deionized Water
• D. Heavy Water

Answer: (C)

Neutrons released by fission must be slowed to thermal energies to sustain the chain reaction, and a moderator like water provides frequent collisions with hydrogen nuclei to do that slowing. Inside the fuel boxes, the water needs to be extremely free of ions, because impurities can capture neutrons or cause corrosion, reducing moderation efficiency. Deionized water has most minerals and ions removed, so it minimizes unwanted neutron absorption and keeps the moderator effective. That makes it the best choice among the options: it reliably slows neutrons without introducing impurities that would steal neutrons or damage the fuel assemblies. Distilled water is very pure but can still carry dissolved gases or trace minerals; liquid nitrogen isn’t a moderator and would freeze; heavy water would also moderate well but is costly and not used in many common designs.

Neutrons released by fission must be slowed to thermal energies to sustain the chain reaction, and a moderator like water provides frequent collisions with hydrogen nuclei to do that slowing. Inside the fuel boxes, the water needs to be extremely free of ions, because impurities can capture neutrons or cause corrosion, reducing moderation efficiency. Deionized water has most minerals and ions removed, so it minimizes unwanted neutron absorption and keeps the moderator effective. That makes it the best choice among the options: it reliably slows neutrons without introducing impurities that would steal neutrons or damage the fuel assemblies. Distilled water is very pure but can still carry dissolved gases or trace minerals; liquid nitrogen isn’t a moderator and would freeze; heavy water would also moderate well but is costly and not used in many common designs.