Relatives react as they reunite at each other at a shelter in Kesennuma, Miyagi Prefecture, three days after northeastern coastal towns were devastated by an earthquake and tsunami. (AP)

As bodies washed ashore by the hundreds and an emergency deepened at a coastal nuclear plant, millions in Japan on Tuesday faced an unabating sense of apprehension, mourning and astonishment over the emerging scope of this nation-changing catastrophe.

The toll of Japan’s triple disaster — first an earthquake, then a tsunami, then a related nuclear crisis — is both visceral and hard to see. Officials in coastal towns say they are running low on body bags; homes and the people inside them have been pulverized. But Japan is also trying to quantify — and contain — the potential damage from a partial meltdown at the Fukushima Daiichi plant, where on Tuesday another explosion was heard at a damaged nuclear reactor, the third since Saturday.

Officials from Tokyo Electric Power Co., owner of the nuclear complex, said radioactive substances were emitted after the 6:14 a.m. explosion. A grave Prime Minister Naoto Kan told the nation that radiation had already spread from the reactors and there was “still a very high risk of further radioactive material escaping.”

Radiation fears took their toll on the markets. The benchmark Nikkei 225 stock average plummeted 10.6 percent to 8,605.15, after declining as much as 14 percent during the day.

Four days after the earthquake and resulting tsunami destroyed much of the northeastern coastline here, the U.S. Geological Survey updated the magnitude of the quake from 8.9 to 9.0, making it the fourth largest in the world since 1900.

More than 500,000 people have been removed from the hardest-hit areas and 15,000 have been rescued. But time was running low for rescuers to help those stranded by flooding or trapped in debris. Officials said about 2,000 bodies were found Monday along the coast of battered Miyagi Prefecture, and a survey of local governments conducted by the Kyodo news agency found that about 30,000 people in the devastated areas remain unaccounted for.

With some roads impassable and fuel almost nonexistent in the north, relief and rescue workers have struggled to reach the areas where they are needed most. Survivors in shelters say they are short of food and water. With the country’s power supply depleted by the damaged nuclear plants, many shelters have no heat, and on Monday, Japan began widespread efforts to curb nationwide energy usage.

As the government urged companies and residential complexes to keep lights off or cut down on time, Tokyo on Monday felt as though it had been put on pause. Millions stayed indoors. Train lines ran on limited schedules. At the iconic crosswalk in front of Tokyo’s Shibuya Station — usually a riot of lights and noise — the massive video screens were turned off. No Japanese pop music was blaring; only footsteps could be heard.

Many of these power reductions were voluntary. But the sudden downsurge in electricity use also caused confusion, as the Tokyo Electric Power Co. made on-the-fly changes to its planned series of rolling blackouts, announced Sunday. Tepco Executive Vice President Takashi Fuji­moto said that, at least Monday, lower-than-expected demand prompted the company to keep lights on in some areas — despite public announcements saying otherwise. As the plans unfolded with little correct information, chief government spokesman Yukio Edano criticized Tepco’s management, calling for a speedy release of accurate information.

Even so, Japan, a country of paradoxes, seems to be handling its greatest crisis since World War II with decorum, fighting chaos with order. A ferryboat is sitting atop a house in the tsu­nami-ravaged town of Otsuchi, but at shelters nationwide, shoes are neatly removed at the entrance and the trash is sorted by recycling type.

There has been virtually no evidence of looting or rising crime levels, and the Japanese have shown stoicism while waiting in long lines.

Also on display have been Japan’s unrelenting politeness and its love for group consensus. Twitter users told stories about the stranded and the homeless sharing rice balls. Travelers heading north reported 10-hour car rides — with no honking. At a convenience store in one battered coastal prefecture, a store manager used a private electric generator. When it stopped working and the cash register no longer opened, customers waiting in line returned their items to the shelves.

Even at Tokyo’s Kokubunji Station, with most train lines down, morning commuters waited hours just to board their trains. Lines reached out of the station, over crosswalks and along the streets for several hundred yards. Railway employees wearing suits and white masks directed commuters into lines — east going this way; west going that way.

But along hundreds of miles of coastline, there are biting concerns about safety.

The fuel rods in one of the Fukushima Daiichi plant’s reactors became partly exposed when water levels fell temporarily, raising the risk of overheating and meltdown.

Although government officials say that radiation levels around the plant are not dangerous, several thousand people have been tested for radiation exposure. On Monday, the U.S. 7th Fleet repositioned its ships — about 100 miles away from the nuclear plant — after 17 crew members were found to have trace amounts of radioactive material on their bodies and clothing.

U.S. Ambassador John V. Roos told reporters Monday that Nuclear Regulatory Commission experts are in Japan and have been consulting with their Japanese counterparts. “We are confident that the government of Japan is doing all it can to respond to this serious situation,” he said.

Yukio Sekiguchi, 64, lived in the shadows of two nuclear plants in Tomioka, located just a couple of miles off the Pacific Coast. He operated an izakaya — a popular after-work spot for drinks and food — about 500 yards from the coastline and about two miles from one of the plants. Although the tsunami damaged his home and his business, Sekiguchi knows that the power plants could be just as dangerous.

“We have mixed feelings,” he said. “I have a business and it’s supported by the people of the plant. . . . But the families with young children, that’s a main concern. You can’t visibly see the radiation.”

In the meantime, thinning amounts of basic supplies in the areas north of Tokyo have forced long lines that snake outside stores. Often the stores have empty shelves, with instant noodles and rice in high demand.

“We all know what the situation is, and we all feel each other’s pain,’’ said Hidenori Chonan, a supermarket manager in Fukushima, where several hundred people waited hours before the store was set to open.

Staff writer Rick Maese and correspondent Erin Cox contributed to this report.