Although there is no way to guarantee that a reactor will always be designed, built and operated safely, the nuclear power industry has improved the safety and performance of reactors, and has proposed safer reactor designs, though many of these designs have yet to be tested at industrial or commercial scales. Mistakes do occur and the designers of reactors at Fukushima in Japan did not anticipate that a tsunami generated by an earthquake would disable the backup systems that were supposed to stabilize the reactor after the earthquake. According to UBS AG, the Fukushima I nuclear accidents have cast doubt on whether even an advanced economy like Japan can master nuclear safety. Catastrophic scenarios involving terrorist attacks are also conceivable.