A heat engine needs a heat difference. If you operate the hot side, the reactor, at a very hot temperature, the cool side can be quite hot too and there is stil

A heat engine needs a heat difference. If you operate the hot side, the reactor, at a very hot temperature, the cool side can be quite hot too and there is still enough temperature difference to generate electricity. The hot running cool side can be air cooled, doesn't need water. Traditional nuclear power plants operate at low temperature, hence the cold side needs to be cool for enough temperature difference. For that cool you need water cooling. Thorium salts and other fluorides can go to high temperatures at low pressures. It's easy to make them high temperature. Other high temperature liquids used for cooling include sodium and lead.