The hut in question is Himmelsbach Hut, on the N shore of Russet Lake. Fissile Peak is just SE from the hut. No reservations needed, it is a pretty basic small shelter (no heat, bring your own stove and winter sleeping bag). It sleeps 4 comfortable although it is known to sleep 12.
www.ubc-voc.com/wiki/Himmelsbach_Hut