It's official: The United States will now have two fewer nuclear reactors, bringing the total down to 100.

The San Onofre nuclear power plant (Mike Blake/Reuters)

On Friday, Southern California Edison announced that it was permanently shuttering the San Onofre nuclear plant near San Clemente, Calif. The facility had been offline since January 2012 after a leak was found inside its steam generator.

My colleague Steve Mufson has much more detail on the decision — which will mean layoffs for 1,100 workers:

The decision to close the reactors comes less than a month after the Nuclear Regulatory Commission’s Atomic Safety and Licensing Board unanimously rejected the company’s arguments in favor of restarting one of the reactors at 70 percent of capacity, saying that a lengthier license amendment process was required.

Another big question, meanwhile, is how Southern California's going to generate enough electricity to keep the lights on without those two reactors. Back when it was running, the plant had provided enough electricity to power 1.4 million homes.

The Energy Information Administration says that the state has gone on a building frenzy ever since the reactors went down in January 2012, adding 2,502 megawatts of capacity over the past year, mainly natural gas and some solar:

The agency also notes that California has been ramping up its imports of electricity from other states, particularly Arizona, to cope:

California currently imports about 25 percent of its electricity from out of state — and those imports are responsible for roughly half of the state's greenhouse-gas emissions from the power sector. (California doesn't have any coal plants itself, but it does import power from five out-of-state coal facilities.) To the extent that nuclear power is being replaced by fossil fuels, the state's climate-change pollution will go up.

The EIA also suspects that electricity rates will rise as a result: "Replacing the power from a low-cost source of generation like [San Onofre] already has changed wholesale electricity prices in [California]. Rising natural gas prices are likely to increase that effect in 2013."

Right now, Southern California Edison says it can maintain power supplies unless there's an exceptionally hot summer or wildfires that disrupt transmission lines. Retiring a nuclear power plant might be technically doable. But that doesn't mean it's easy.

Further reading:

--The San Onofre closure means that four reactors will get retired in 2013, a record for a single year. Here's an earlier post on how the shale-gas boom has put pressure on the nuclear industry. (Note that four new reactors are currently in various phases of development in Georgia and South Carolina, so the total could rise again.)

-- A helpful FAQ on the San Onofre shutdown from the Los Angeles Times.

-- It will cost some $2 billion to close the reactor. Bloomberg breaks down the regulatory battle over who shoulders those costs.