No, not necessarily.  There exist self-regulating reactor core designs that will tend towards a sub-critical reaction above a certain design tempeture, making a cascading reaction (i.e. meltdown) very unlikley.  A few such reactor designs have been around for some time.  I used to have a nuclear reactor training simulator around here somewhere, if I can find it I might post a link (can't remember if the simulator is classified).  One such reactor design, that does not claim that feature as a safety feature due to some other very bad effects, is the Candu reactor designed in Canada.  It's a great design that was stolen by the Chinese for their domestic designs after they bailed on Russian designs following Chernobyl.  That's actually probably for the best, but even they don't use it for their power reactors because it's not a presurized design, but an open top, deap pool design.  If it were to boil off about ten feet of it's water, the reduction in water pressure at the core would reduce the ability of the water to slow down the neutrons to capture range until after they had left the core, thus going subcritical.  Of course, those ten extra feet of water are also necessary for human safety as the water itself is the shielding.  Lose ten feet of cover water, and spectators start dying within a couple dozen feet of the pool's surface, so it's not exactly a good thing to advertise.