Starting with June 1, 2011, all the operators of nuclear power plants in the EU had to review the response of their nuclear plants to extreme situations, in particular operators had to check and improve mitigation measures available after a potential loss of safety functions, caused by any reason. That includes the loss of electrical power or loss of ultimate heat sink for heat removal from the reactor, the management of loss of core cooling functions in their reactors as well as in spent fuel pools and the maintenance of containment integrity.