[pdonis]

> do you know anything about why the passive designs weren't built?

Because they're too recent. By the time these designs were developed, the nuclear industry in the US was basically dead, so there has been no real opportunity for them to be commercially deployed.

[Arnt]

I seem to recall reading about them in the earlyish nineties. Specifically a fine article about a Swedish gravity-based design, which used electric power to keep things running and many, many failures should result in things being pulled down by gravity and failing safely.