>>12127180Water can't be used as a MODERATOR NOT MEDIATOR unless your uranium enrichment levels are high enough.
This is because water also absorbs some incident neutrons, and if the reactivity from the fuel isn't high enough, the water will actually add negative net reactivity and the core can't run.
This is why:
1. the reactor used graphite with its very cheap low enriched uranium as the moderator
2. the reactor was so fucking big (mostly unfissile u-238 and graphite)
3. when the water in the coolant channels boiled away it actually RAISED reactivity instead of lowering it
