We have had mini reactors for decades. Its kind of stupid that we don't have them all over the place. Though. I think the point to that was to not allow other countries have time to study and steal the technology. What has probably happened is that enough of the tech has already been stolen that we are just like "fuck it, lets use it everywhere now". Every US sub and major battle ship is a mini-reactor that is mobile and has been for decades. Just think about it that way.