I like that it is not dumping heat into the local river or sea and doesn't have the high pressures involved. Compared to water-cooled, liquid metals can absorb a lot more heat while maintaining a consistent pressure. Liquid sodium has a boiling point more than 8 times higher than water so it can absorb all the extra heat generated in the nuclear core. The pitch is that the sodium doesn’t need to be pumped, because as it gets hot, it rises, and as it rises, it cools off. Even if the plant loses power, the sodium just keeps absorbing heat without getting to a dangerous temperature that would cause a meltdown.
If I understand the tech correctly, the Natrium design also includes an energy storage system that will allow it to control how much electricity it produces at any given time which I think is unique among nuclear reactors. This feature is important for integrating with power grids that use variable sources like solar and wind.
The downside is that it will take years to build. Getting a lot of them online to replace coal and natural gas plants would take over a decade.
Yes, keeping the streams cool is good.
The boiling point may be higher, but the heat capacity is significantly (>40%) lower, so that offsets it some. Moreover, pressure does not only build up at boiling, but at any temperature - it's dependent on the vapor pressure (as a function of temperature). Sodium's vapor pressure goes up quite a bit up to its boiling temperature. So the pressure will also be going up. I'm not sure what I'd like better, a steam explosion or a metal (liquid and vapor) explosion. In particular a metal that when coming into contact with water creates explosions (of hydrogen and oxygen, from splitting the water - nice youtube vids on that...).
I think the advantage of the sodium is that it works better when power is down.
Water also convects (pour boiling water in a cup, see the "snaky thingies" undulate: that's convective flow.
This is driven (just as the chimney!) by the differences in density of the liquid.
For Sodium there is a 2.7% difference in density between melting and boiling points. For water that's quite a bit less (0.6%), so this will work better for Sodium indeed, i.e. without power it will be easier to transport heat away than it is for water (and it stays liquid unless above 800 C or so, which is good too).
These two aspects (still liquid at high T, and more convection due to larger density differences) are I think the main advantages (over water).
I thought that these storage systems were molten sodium chloride (salt), not sodium. Imagine if something goes wrong and the sodium cools down and solidifies. How to get that out of your tank, a hunk of (soft) metal. Water is a no-no (see explosion above). For NaCl it's easy though: add water, dissolve it, done. That's how salt can be mined from deposits underground.
I agree that building generation with *integrated* storage is ideal.