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.
No se necesitan esos recipientes a presión, de acero grueso y pesado, no se necesita agua para refrigerar, y nada en el reactor cambiará su densidad tan drásticamente como lo hace el agua.