By Scott DiSavino
(Reuters) - Regulators on Friday told the owners of the nation's nuclear reactors to implement new safety rules based on the lessons learned from the earthquake and tsunami that crippled Japan's Fukushima Daiichi nuclear plant a year ago.
The Nuclear Regulatory Commission (NRC) said it authorized its staff to issue three immediately effective orders implementing some of the more urgent recommendations.
The NRC gave the plants until December 31, 2016, to complete modifications and requirements for the three orders.
Since an earthquake and tsunami damaged the Fukushima reactor last March, nuclear regulators around the world have carefully studied the causes of the accident and tightened scrutiny of reactor operations.
"The Commission has taken a significant step forward on our post-Fukushima efforts," NRC Chairman Gregory Jaczko said in the release.
Two of the orders apply to every U.S. nuclear power plant, including those under construction and to Southern Co's recently licensed new Vogtle reactors in Georgia.
The first order requires the plants to better protect safety equipment installed after the September 11 terrorist attacks in 2001 and to obtain sufficient equipment to support all reactors at a given site simultaneously.
The second order requires the plants to install enhanced equipment for monitoring water levels in each reactor's spent fuel pool.
The third order applies only to U.S. boiling-water reactors that have "Mark I" or "Mark II" containment structures, which are similar to the damaged reactors at Fukushima.
The NRC said these boiling water reactors must improve venting systems (or for the Mark II plants, install new systems) that helps prevent or mitigate core damage in the event of a serious accident.
Of the nation's 104 operating nuclear power reactors, 31 have the Mark I or Mark II type of containment, which was designed by U.S. multinational conglomerate General Electric.
EARTHQUAKE AND FLOODING
In addition, the NRC said it will require all reactors to reanalyze their earthquake and flooding risks and assess how their communications and equipment would perform during a prolonged loss of electrical power.
The reactors at Fukushima responded as designed by shutting down after the earthquake cut offsite power to the plant.
But the tsunami that hit an hour or so after the earthquake, damaged the plant's backup diesel generators, which ultimately left the plant with no power supply. Without power, known in the industry as a station blackout, the plant could not run the water pumps needed to keep the fuel in the reactors cool, which led to fuel meltdowns and radiation releases.
The biggest nuclear power operators in the United States include Exelon Corp, Entergy Corp, Dominion Resources Inc, Duke Energy, Progress Energy, NextEra Energy, Southern and the Tennessee Valley Authority.
(Additional reporting by Eileen O'Grady in Houston; Editing by Alden Bentley and Lisa Shumaker)