I was interviewed several months ago by the author for this article.
One form of MSR, the liquid fluoride thorium reactor (LFTR), has garnered particular enthusiasm among those who regard thorium as an attractive replacement for uranium and plutonium in the fuel cycle. (Thorium is both cheaper and more abundant than uranium.) According to Kirk Sorensen, an engineer at NASA who also runs a blog on the merits of the thorium cycle, natural thorium provides at least 250 times more energy per unit than natural uranium. However, unlike fissile uranium, natural thorium must be “seeded” with external neutrons in order to get it to fission. Another obstacle for the MSR is finding materials capable of withstanding hot, corrosive, radioactive salt.