Protesters and police outside the Sendai Nuclear Power Plant in Japan on Tuesday.JIJI PRESS / AFP / Getty Images

Two years ago, Japan shut down all of its nuclear reactors. On Tuesday morning, one of them kicked back into gear.

Japan imposed a ban on nuclear-power generation in September 2013 in response to the meltdown of several reactors at the Fukushima Daiichi plant following a devastating tsunami in 2011. The nuclear disaster spewed radioactive materials into the air and nearby water, and forced 100,000 people to evacuate their homes.

The disaster was the worst since the Chernobyl explosion in 1986, and it led Japanese regulators to rethink safety standards for the nation’s more than 40 commercial nuclear reactors. The reactor that restarted Tuesday, at the Sendai Nuclear Power Plant in southern Japan, is the first to come back online since officials announced new standards in June 2013.

The Sendai reactor will start generating electricity by Friday, according to the plant’s operator, Kyushu Electric Power Company. It will reach full capacity by the start of next month. Its relaunch opens the door for other utility companies to apply to restart reactors, and applications for 25 reactors at 15 plants have already been submitted. But they face a long and expensive process—more than $100 million was poured into the Sendai plant to meet regulation requirements.

More on the process, from The New York Times:

The plants need to be retrofitted with new ventilation systems and other protections, and the operators require the approval of local political leaders to switch them back on. The Sendai plant was declared safe by regulators nearly a year ago, in September 2014.

Again from the Times, on the standards introduced in 2013 by the Nuclear Regulation Authority, which was created specifically to replace existing regulation agencies:

In the future, nuclear plant operators must bolster their tsunami defenses and check for active earthquake faults under their plants. They must also set up emergency command centers and install filtered vents to help reduce the discharge of harmful radioactive substances from the reactors.

These safety standards are legally binding, unlike previous guidelines, which were not backed up by law and were adopted by nuclear operators on a voluntary basis. They also address, for the first time, the possibility of severe accidents like the Fukushima disaster, which set off multiple fuel meltdowns and forced more than 100,000 people from their homes. … It will take “many months” for the authority to conduct the necessary checks and approve bringing the reactors back online, authority officials said. Local news reports said the approval process would take at least six months.

The decision to reboot Japan’s nuclear energy sector is controversial. The nuclear energy industry—unsurprisingly—welcomed the restart of operations, as did Japanese Prime Minister Shinzo Abe. He said the reactors at Sendai had passed "the world's toughest safety screening.”

For the past two years, Japan has imported expensive natural gas and coal to meet its power needs, causing electricity prices to jump by 20 percent since the Fukushima accident. But the memory of the disaster remains fresh in the Japanese consciousness. Before the disaster, when 30 percent of Japan’s energy came from nuclear sources, a majority of citizens supported expanding nuclear power, according to polls. Now, a majority want to end it altogether.

Dozens of protesters attended the relaunch of Sendai’s nuclear reactor on Tuesday, including Naoto Kan, who was prime minister at the time of the Fukushima meltdown.

The cleanup of the Fukushima plant is expected to take about 40 years.

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