> Not in the US (because of the flammability), unless something's changed very recently.
I don't know when it changed but R-290 (propane) is allowed up to 13oz. My ice maker uses it. I believe it must also be sealed without service ports both to prevent leaks and prevent anyone from connecting normal A/C service equipment to the system. Service requires emptying the system then brazing service ports onto the fill pipes (which are left much longer than normal for this purpose).
In commercial applications you'll see R-290. It's often used in low temperature applications, like ice cream display freezers. I've seen them in gas stations and pizza shops quite a bit.
the volume in a pipe is a lot less than what you'd find in a tank, especially considering the pressure in said pipes is usually one tenth of a psi (.6 kPa)
A fuel-air explosion will not be contained by a reasonable amount of concrete or masonry. The real safety lesson is to control sources of ignition and prevent the buildup of released gases.
Notably, Propane has almost identical refrigeration characteristics to R-22, so it was an easy drop-in replacement for systems designed for R-22 when that was banned.
I don't know when it changed but R-290 (propane) is allowed up to 13oz. My ice maker uses it. I believe it must also be sealed without service ports both to prevent leaks and prevent anyone from connecting normal A/C service equipment to the system. Service requires emptying the system then brazing service ports onto the fill pipes (which are left much longer than normal for this purpose).