Heavy water is more effective, but light (ordinary) water moderates neutrons too. That's how light water reactors work: https://en.wikipedia.org/wiki/Light-wate

Heavy water is more effective, but light (ordinary) water moderates neutrons too. That's how light water reactors work: https://en.wikipedia.org/wiki/Light-water_reactor https://en.wikipedia.org/wiki/Light-water_reactor