I do not know if the UL certifies industrial boilers, but I am sure
someone does.
It seems they do small scale generators, maybe not the biggies:
http://www.ul.com/global/eng/pages/offerings/industries/energy/power/microturbines/tests/
I am sure that Rossi reactors scaled for home use will require UL
certification. You will not be able to buy home insurance without it.
Certification is not a magic wand that automatically prevents problems.
Someone certified the Fukushima reactors and the seawall.
- Jed
