This is a good article: Power failure due to turbine shutdown, failure in backup diesel generators due to tsunami means no AC power and loss of instruments & co

This is a good article: Power failure due to turbine shutdown, failure in backup diesel generators due to tsunami means no AC power and loss of instruments & controls. The article states further that: "... According to technical documents translated by Aileen Mioko Smith of Green Action in Japan, if the coolant level dropped to the top of the active fuel rods in the core, damage to the core would begin about 40 minutes later, and damage to the reactor vessel would occur 90 minutes after that. ..."