After a 9.0-magnitude earthquake and ensuing tsunami hit Japan's northeastern coast in 2011, leading to the infamous reactor meltdown at Fukushima, the country decided to hit the pause button on nuclear power. Over the next two years, Japan took all 54 of its reactors offline as regulators reevaluated their safety rules.

The adjustment turned out to be quite painful.

Before Fukushima, nuclear power supplied 27 percent of Japan's electricity. By 2014, that had dwindled to zero. To make up the gap, Japan has had to import more coal, oil, and natural gas from overseas:

That took a toll. Japan's trade deficit ballooned as energy imports rose. Household electricity rates have risen 19 percent since 2011, while factories and offices saw their rates go up 29 percent. The country has also had to grapple with the threat of looming blackouts by slashing energy use during summer months. And the surge in fossil fuel use wasn't great for the environment: Japan's carbon dioxide emissions leapt upward after 2011.

That all helps explain why Japan's central government is keen on bringing many of the country's 43 remaining operable reactors back online. This week, officials hailed the news that a reactor on the island of Kyushu was being restarted under new safety rules, the first to flip on since 2013.

The catch? Between widespread public opposition and various technical challenges, bringing back the bulk of Japan's reactors won't be as easy as it sounds.

Japan just restarted its first nuclear reactor — and more may follow

This week, Kyushu Electric Power announced that it had restarted one of two reactors at its Sendai plant, which is located in the country's southwest and had been closed since 2011. Sendai Unit 1 will reach full power in September. Sendai Unit 2 is scheduled to come online in October.

This is all part of a gradual effort to bring at least some of Japan's 43 remaining operable nuclear reactors back online. But it's a sluggish process. Restarting a reactor requires approval from both the country's Nuclear Regulation Authority (NRA) and from the local government. And both are serious hurdles.

For its part, the NRA is requiring new safety measures for any reactor applying to restart. Those steps can range from larger tsunami walls to backup control rooms. The agency has also been scrutinizing fault lines and potential vulnerabilities at various plant sites to ensure there's not a repeat of Fukushima. And even once that's all done, local prefecture governments can still prevent a reactor from restarting.

So where do things stand today? Five of Japan's reactors have received approval from the NRA thus far. But of those, only the two Sendai units have also gotten the go-ahead from their local government. By contrast, Takahama Units 3 and 4 in Fukui Prefecture have received the NRA's blessing, but the local government appears to be fairly skeptical.

Meanwhile, Japan has another 19 reactors currently under review by the NRA. And then there are another 19 that have yet to apply. (The remaining 11 have been permanently shut down post-Fukushima.) The US Energy Information Administration has a valuable summary:

One final, technical obstacle: Even after approval, it's no simple matter to restart a nuclear power plant that's been dormant for four years. The equipment hasn't been used and might be in disrepair. Operators are out of practice. That can potentially lead to problems. Bloomberg recently examined data on 14 reactors worldwide that had been restarted after being out of service for at least four years, and found that all of them suffered from various glitches (including one reactor in Sweden that had six emergency shutdowns in the year that followed).

So, yes, Japan just restarted one reactor. But it's not yet clear how quickly others can follow.

Japan's central government wants to ramp up nuclear — but there's plenty of opposition

Ultimately, Japan's central government envisions most of its remaining reactors reopening. The country's official energy plan calls for nuclear to provide 22 percent of electricity by 2030, only slightly lower than it was pre-Fukushima.

"It is important for our energy policy to push forward restarts of reactors that are deemed safe," said Yoshihide Suga, chief Cabinet secretary, on Monday. Without those reactors, the government argues, it will be harder to keep energy prices down and meet Japan's climate-change goals.

Yet there's also a fair bit of public opposition to any nuclear renaissance. Polls suggest that most people want to reduce the country's dependency on nuclear. In Kyushu, demonstrators protested the reopening of the Sendai plant, with opponents arguing that the region lacked an adequate evacuation plan in case of a meltdown.

Much of this skepticism traces back to Fukushima. It's true that there were no immediate deaths from the meltdown itself (although some people received heavy radiation exposure, and of course 15,000 people died from the earthquake and tsunami). But the aftermath has been messy. Some 80,000 residents were evacuated from the area, and the government has had to spend $10 billion on clean-up efforts. Importantly, the incident also undermined the public's long-standing faith in Japan's nuclear regulators. One 2012 survey found that 80 percent of respondents disapproved of how the government handled the crisis.

Can Japan meet its climate goals without nuclear power? Not easily.

The flip side is that, without nuclear, it's hard to see how Japan will be able to wean itself off fossil fuels — which, if you take the ongoing climate talks seriously, is the end goal for developed countries.

Nuclear, after all, was Japan's biggest source of carbon-free energy by far. In recent years, the country has enjoyed a small boom in solar and wind construction, but those two sources still provided a mere 1 percent of the country's electricity in 2012.

And unlike, say, the United States, Japan has one serious limitation on the growth of renewables: land. You need vast, vast arrays of wind farms and solar panels just to replace a single nuclear plant. As energy analyst Robert Wilson points out, Japan would probably need to cover all of its free land with solar panels to fully power the country (and that's ignoring the still-unsolved question of what you'd do at nighttime). It's tall order. Offshore wind could help — and the country has recently been experimenting with floating offshore solar plants — but the country does face unique challenges in this regard.

No surprise, then, that Japan has been backtracking on its climate goals of late. Before Fukushima, the country had hoped to reduce its greenhouse-gas emissions 20 percent below 1990 levels by 2020. Once its nuclear reactors got shut down and fossil-fuel use surged, Japan's government had to water that goal down to a mere 5 percent cut by 2020. And, as Roger Pielke Jr. of the University of Colorado has calculated, even that new, weaker goal will prove hard to meet unless Japan can bring at least 9 of its reactors back online.

So there's Japan's dilemma. Bringing back nuclear is hard. Staying off nuclear might be even harder.

Further reading

-- The rise and fall of nuclear power, in 6 charts