Being sodium cooled, both a steam explosion and hydrogen gas generation via water is ac minimized. Lots of safety advantages to a coolant that is still liquid at operating core temperature with no added pressure. But of course liquid sodium does come with some other caveats that are of concern. Making it a sealed unit with minimum moving parts helps with some of those potential problems for sure. As does keeping the total thermal capacity relatively low, below the threshold where secondary fission products need active cooling to prevent meltdown even after the reactor is shutdown.