> The sales pitch for salt-cooled reactors is the lack of any coolant that would become pressurized hot gas in an accident. Heat can stay in salt or in other low vapor pressure materials.

Sodium-cooled reactors and the upcoming lead-cooled reactor also have this property. It turns out to not be such a huge advantage, we have plenty of experience working with pressurized water.

> The problem with LWRs is the water goes to steam in accidents, and this steam must be contained. This drives the size of the containment building, and the containment building is costly.

No, it's really not a problem. The loop doesn't suddenly loose compression if something bad happens. If there's electric power, there's more than enough time to slowly cool down the reactor.

And a containment building (that also protects against external threats like an airplane ramming into the reactor) has more than enough volume if the primary loop is de-pressurized and the water flashes into steam.

> An alternative for LWRs would be to filter and vent the steam instead of trying to contain it.

The water in the primary loop is clean. It's constantly purified by filtration through ion exchange resins. Once the activated oxygen decays (in ~1 hour) you can swim in it (although I wouldn't drink it).

PWRs (actually, all thermal power plants) have areas where steam can be dumped. If you watched "Chernobyl" series, the ridiculous scene with divers was supposed to happen inside such an area ("barboter pool").

Modern PWRs are also designed to do that safely. There's plenty of capacity to condense all the water from the primary loop after the loss-of-cooling. Of course, after that the fuel will melt down, and chew through the reactor vessel.

The filtering system you linked is not strictly necessary for modern PWR designs. They will still be safe in case of an accident with total loss of cooling, but the containment building will be hopelessly contaminated internally. This filtering system can allow the steam to be vented into the atmosphere, perhaps giving more time to fix the emergency cooling systems.

> No, it's really not a problem. The loop doesn't suddenly loose compression if something bad happens. If there's electric power, there's more than enough time to slowly cool down the reactor.

It can in design basis accidents, for example a complete break of a main circulation pipe leading the loss of coolant (LOCA) into the containment. The emergency cooling system would then operate by spraying water into the core that would evaporate into steam that would go right out of the reactor vessel. The containment has to be sized for such an accident.

As an example of such an accident, consider what would have happened at Davis-Besse had the erosion of the lid of the reactor vessel progressed to an actual perforation. As it was, the steel was removed in an area down to the inner stainless steel liner, a liner that was never intended to be load bearing against the internal pressure.

> And a containment building (that also protects against external threats like an airplane ramming into the reactor) has more than enough volume if the primary loop is de-pressurized and the water flashes into steam.

Right, it does. That's why it's so big and expensive, with so much internal volume. If it didn't have to, it could be made much smaller. The airplane requirement doesn't change this; it's easier to make a smaller containment building resistant to aircraft impact than a larger one.

> The water in the primary loop is clean. It's constantly purified by filtration through ion exchange resins. Once the activated oxygen decays (in ~1 hour) you can swim in it (although I wouldn't drink it).

That's true in normal operation, where you might have some small number of fuel rods with cracks or perforations (but even that is getting pretty uncommon these days). It would not be true in a design basis accident, where some or all of the fuel may have partially or completely melted, and where cladding will have been compromised by high temperature reaction with steam. The design must assume essentially all the volatile fission products have gone into the water. At TMI, fission products carried in the water (and also noble gases) raised radiation levels in the containment building to 800 rem/h during the accident.

> The filtering system you linked is not strictly necessary for modern PWR designs.

I offered up the possibility that such systems could replace the large volume containment of modern systems (or at least reduce its size and cost). Sure, they're not obviously necessary if you have a large volume containment already (although some countries ended up requiring them anyway since some accident scenarios do involve venting, as happened at Fukushima, which admittedly had pre-modern designs.)