The USSR talked about it. The U.S. talked about it (there were even toy versions!). Modern Russia is giving it serious thought. The Red Chinese are supposedly thinking about it, too, but they're not sayin' much.
...No matter how safe or careful, a reactor-on-rails is going to be difficult to insure -- the rails can be fraught with hazard.
(The USSR did build the world's first civilian power reactor, then ended up running it 18 years past the planned 30-some year active life when Soviet checks started to bounce. It's a museum now, a memento of an earlier time. It's a tiny prototype of the big RBMK design -- yes, that RBMK.)
...All that, and I never even mentioned Supertrain! Well, until now.
INDIANAPOLIS HAMFEST, 2015
2 weeks ago