Japan's nuclear watchdog decided Wednesday to require operators of boiling water reactors -- the same type as those at the crisis-hit Fukushima Daiichi nuclear complex -- to install new emergency cooling systems that would activate in the event of serious accidents.

The Nuclear Regulation Authority approved at a regular meeting the revision of safety regulations to make mandatory the installation of cooling systems to circulate water inside reactor containers.

Such systems are designed to prevent temperature rises that could damage containers in the event of serious accidents. Boiling water reactors are housed in containers that are smaller than those for pressurized water reactors, making it easier for internal temperatures and pressure to rise when conventional cooling systems are damaged.

Due to the rule change, emergency cooling systems will need to be installed for a number of boiling water reactors that have been undergoing checks required for the resumption of operation. Tohoku Electric Power Co.'s Higashidori nuclear power plant in Aomori Prefecture and Onagawa complex in Miyagi Prefecture, and Chubu Electric Power Co.'s Hamaoka plant in Shizuoka Prefecture are among those affected.

The new cooling systems will be activated when severe accidents occur at nuclear plants, with water that has accumulated inside reactor containers flowing outside for cooling before returning.

If a reactor still cannot be brought under control, the operator will be allowed to release steam through a filtered venting system. Under safety requirements implemented following the 2011 Fukushima nuclear crisis, reactors must be equipped with such systems.

While several pressurized water reactors have cleared the stricter safety standards introduced after the Fukushima disaster, two boiling water reactors at Tokyo Electric Power Company Holdings Inc.'s Kashiwazaki-Kariwa power station in Niigata Prefecture became the first such reactors to be approved earlier this month.

Tokyo Electric, which runs the Fukushima complex, had already planned to install the new emergency cooling system for the Nos. 6 and 7 reactors at the Kashiwazaki-Kariwa plant and the nuclear watchdog has decided to make installation of the emergency system a requirement. It will formalize the decision after soliciting public comment on the rule change.