This means they don't have to be in heavy, thick steel pressure vessels, they don't have to use water for coolant and there's nothing in the reactor that's going to make a big change in density, like water.
Det betyder at de ikke skal indkapsles i massive stålkonstruktioner, de behøver ikke vand til køling og der er intet i reaktoren der markant kan ændre massefylde, ligesom vand kan.