Fragor isn't nuke level.
Fragor is mountain+ and calced to be 17 MT. That's not just a nuke but a super-nuke
However, that wouldn't be a problem for Hiei, who's tanked island+ punches from Sensui.
Hiei could "keep up" with Sensui yet the latter was not only toying with him, but Hiei wasn't able to do a thing.
Actually, Sensui was not toying against them, he was infact serious http://manga.animea.net/yu-yu-hakusho-chapter-147-page-9.html