Why is heavy water used as moderator?

Related questions

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4. Which reactors melted down at Fukushima?
5. Why is d2o used as moderator?
6. What caused Fukushima?
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1. #types
2. #certain
3. #react
4. #likely
5. #water
6. #without
7. #where
8. #moderator
9. #fissioning
10. #neutrons
11. #fissile
12. #heavy
13. #candu
14. #which
15. #reactor
16. #neutron
17. #captures
18. #design
19. #nuclear
20. #reactors
21. #uranium