A little bit about the Chernobyl reactor:
Encyclopedia of Earth
"The combination of graphite moderator and water coolant is found in no other nuclear power reactors. The design characteristics of the reactor mean that it is unstable at low power levels, as was shown in the Chernobyl, Ukraine accident. The instability was due primarily to control rod design and a positive void coefficient. A number of significant design changes have now been made to address these problems."
Again, anytime nuclear reactions are involved it is appropriate to act with care and to carefully monitor the situtation. It seems that is being done.
Right now, the majority of the focus needs to be on rescuing, caring for, and otherwise assisting those injured or immediately threatened by the tsunami. Fortunately, unlike Haiti, Japan has a sufficiently resiliant infrastructure that will help limit the extent of the "collateral damage" caused by the earthquake/tsunami.