Story highlights With the shutdown, all 50 of the country's reactors will be offline

It's unknown when or if any of the reactors will come back on

In 2012, all reactors went offline as well

But then, fearing an energy crunch, two were restarted

Japan's only operating nuclear reactor has been shut down for maintenance, leaving the country with no nuclear power supply for the second time in 40 years.

Kansai Electric Power Co. confirmed reactor no. 4 was shuttered at midnight on Sunday at its Oi plant in Fukui prefecture in western Japan.

All 50 of the country's reactors are now offline.

The government hasn't said when or if any of them will come back on.

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The idling is not unprecedented.

In May 2012, Tomari Nuclear Power Plant's reactor 3 in Hokkaido was shut down -- leaving Japan without an operational reactor.

At the time, it was a much-watched move by the government, industry and environmentalists, who are waged in a public battle over the future of Japan's energy policy.

The public has been suspicious of nuclear energy and its regulatory bodies since a tsunami and earthquake triggered nuclear meltdowns at three reactors at the Fukushima Daiichi nuclear plant in March 2011.

That movement grew from the grass-roots level in the wake of the disaster, as the country watched tens of thousands of residents living within a 20-kilometer (12-mile) radius of the nuclear plant evacuated, and the remaining area turn into a contaminated wasteland.

Before the Fukushima disaster, Japan relied on nuclear for about 30% of its energy. As reactors have come off-line, the country has increased its imports of fossil fuels.

But what was to have been a historic move turned out not to be so.

Fearing that the void in nuclear power supply would result in an energy crunch -- and possibly rolling blackouts -- the government allowed two reactors to start back up at the Oi plant.

Since then, public fears have worsened as the crippled reactors at Fukushima continue to prove problematic.

After this recent shutdown, the government might wait until all safety checks are complete before deciding if it will bring any of the reactors back on.