Fukushima successfully SCRAMed their reactor. Previously produced fission byproducts continued to decay after criticality was lost. While the thermal power of t

Fukushima successfully SCRAMed their reactor. Previously produced fission byproducts continued to decay after criticality was lost. While the thermal power of this decay process was an order of magnitude below peak, it greatly exceeded passive cooling capability. Without backup power to actively cool the core, meltdown is unavoidable: this decay heat is far more than enough to destroy the core. This is a real risk for current reactor designs. Loss of backup power means a loss of any safe shutdown option.