Why is heavy water used as a moderator and not normal water? And why isn't hydrogen already the fuel everyone uses?

7 answers & replies

• [+][–]Ishita GoyalDropperSample· 6d ago

  I think heavy water is the moderator because deuterium is heavy so it soaks up neutrons, like a sponge, and that stops the chain reaction from getting out of control?

  Report

  • [+][–]Siddharth RaoDropperSample· 6d ago

    Ishita, careful, this is the opposite of the real reason. A moderator must slow neutrons without absorbing them. If it absorbs neutrons they're lost and the chain reaction fizzles. Absorbing neutrons is the job of control rods (like cadmium or boron), not the moderator.

    Heavy water is a good moderator because deuterium captures neutrons much less than ordinary hydrogen does. Ordinary water has a decent chance of absorbing neutrons, so a reactor with natural uranium and ordinary water as the moderator can't sustain a chain reaction. With heavy water as moderator, it can.

    Report

    • [+][–]Ishita GoyalDropperSample· 6d ago

      oh. Yes, I mixed up with control rods. Fixed in my notes, thanks siddharth.

      Report
• [+][–]Rahul DubeyDropperSample· 6d ago

  Your remembered facts are all correct. Add: it's also used as a tracer to study reaction mechanisms, and the other deuterated examples NCERT gives are $SO_3 + D_2O \rightarrow D_2SO_4$ and $Al_4C_3 + 12D_2O \rightarrow 4Al(OD)_3 + 3CD_4$. Properties are higher because of the stronger bonding with the heavier isotope: density at 298 K about 1.1 g/mL, higher than 1.0 for water.

  One more thing, high-purity heavy water is not good for living things as a replacement for water. It slows growth in higher organisms. So you can't just swap it.

  Report
• [+][–]Kavya NaikDropperSample· 6d ago

  Hydrogen fuel, the longer answer:

  • Storage: hydrogen is a very light gas, so a tank of it at ordinary pressure holds very little energy per litre. You need compression to high pressure or liquefaction at very low temperature, both of which cost energy and need expensive tanks. Even though per gram H2 has more energy than petrol, per litre it's much worse.
  • Safety: it is very flammable and leaks easily because the molecules are tiny.
  • Production: most hydrogen today is made from fossil fuels, so unless you make it by electrolysis with clean electricity, the "clean fuel" claim doesn't hold.
  • Infrastructure: petrol pumps exist everywhere; hydrogen stations don't.

  The good part is that fuel cells running on H2 and O2 give water as the only product, and they are used in space missions. The "hydrogen economy" is about storing and transporting energy as hydrogen.

  Report

  • [+][–]Farhan ShaikhDropperSample· 6d agoAuthor

    this is what I was looking for, "per gram vs per litre" is a very good one. thanks all, notes updated 🙏

    Report
• [+][–]Lakshmi TripathiDropperSample· 6d ago

  Short exam version: Moderator = D2O (slows neutrons, low capture). Control = Cd or B rods (absorb neutrons). Fuel = H2 in fuel cells (pollution free, product is water, high energy per mass, but storage is difficult).

  Report