In December 2014, the Swedish regulator required that all Swedish reactors should be equipped with an Independent Core Cooling System - ICCS - before 31 December 2020. The system should be able to cool the reactor during 72 hours without any supply of water, power, fuel or other consumables, and should survive external events (seismic events, hard weather etc) with an estimated probability of exceedance of 10−6/yr. The requirements are influenced by the so called "Forsmark-event" 25 July 2006 and the Fukushima accident. Design of and preparations for ICCS is (2017) already underway for R3 and R4, but will not be made for R1 and R2 since they were planned to be shut down before 2021. However, the Swedish Radiation Safety Authority stated that in case that the reactors were to be shut down when they were originally planned to be closed, i.e. around 2025, they could be operated without additional upgrades after 2020.