Japan on Tuesday turned on the No. 1 reactor at its Sendai nuclear plant, 1,000km south-west of Tokyo, a move that brought nuclear power back to the country after it was nuclear-free for two years.
The shutdown was triggered by fears from the public after a 2011 earthquake and tsunami cause the meltdown of the Fukushima nuclear plant and the evacuation of tens of thousands.
The nuclear restart came amid still-vociferous protests from the public.
Here are some answers to questions you may have about the restart:
1. How many nuclear reactors are operable in Japan?
Before the 2011 earthquake, Japan's 54 reactors were responsible for 30 per cent of the country's electrical needs.
The quake and resulting tsunami caused all six of the Fukushima No. 1 plant's reactors to shut down.
Five more reactors will also be shut down, as they are too old and costly to retrofit to meet new safety regulations issued in 2013.
This leaves 43 reactors that can be switched on in Japan.
2 . How many more reactors are set to be turned on?
Including the No. 1 reactor at the Sendai plant, which was restarted on Tuesday, five have passed checks by Japan's Nuclear Regulation Authority.
Sendai's No. 2 reactor is set to be restarted later this year.
The other three that have passed checks are reactors No. 3 and No. 4 at the Takahama plant in Fukui prefecture, and the No. 3 reactor at the Ikata power plant in Ehime prefecture. But the three have to pass further hurdles before being turned on.
More than 20 more reactors are being inspected to see whether they meet the new rules.
3. What do the government and businesses want?
Japan Prime Minister Shinzo Abe wants to get all the nuclear reactors back online, as their shutdown in the last few years meant the energy-hungry country had to rely on coal and other sources, as well as imported energy.
The power companies that own the reactors are also keen for them to get back to business as they have had to make up lost generating capacity with pricey fossil fuels.
Last month, Japan's Industry Ministry gave the nod to a target that for nuclear power to generate as much as 22 per cent of electricity by 2030.
4. How do ordinary Japanese feel?
A majority of the public do not want the reactors to be restarted, according to polls, and are sceptical of that the new regulations will help. Protests have been staged outside Mr Abe's residence and the Sendai plant, and the Premier's popularity ratings have suffered due to the criticism surrounding the issue.
In a poll conducted last week by the Mainichi Shimbun newspaper, 57 per cent were against the Sendai reactor's restarting, and 30 per cent were positive about it.
5. What's going on in Fukushima now?
The clean-up of the areas affected by radiation due to the Fukushima meltdown is expected to take decades, due to radiation leaks. Tens of thousands of residents have been evacuated from the area, and many may not see a return.
Source: The Wall Street Journal, Agence France-Presse