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Major problems at a key Japanese nuclear plant following a massive earthquake and tsunami last week have led to a selling spree on global stock markets of nuclear energy-traded funds and the disaster has cast a chill among lawmakers who were already skeptical of President Obama's plan to invest in nuclear power as part of an overall energy policy.

The Dow industrials opened to a 250-point drop on Tuesday, after spooked sellers reacted to concerns about General Electric, Entergy Corp and other companies that operate nuclear reactors in the U.S. and abroad. The Japanese Nikkei dropped 11 percent Tuesday as Germany's DAX Index was trading down 12 percent from its high after a sell-off in utilities affected by a possible shutdown of German nuclear power sites.

Rep. Ed Markey, ranking Democrat on the House Natural Resources Committee, has said he wants the Nuclear Regulatory Commission to suspend approval of the AP 1000 nuclear reactor design by Westinghouse for a pressurized water reactor, warning that the containment structure for the design would not withstand a strong earthquake and could "shatter like a glass cup" under sufficient stress.

Tamping down some of the hysteria that has been building since Japan announced that it had lost coolant operations on three nuclear reactors at the Fukushima Daiichi plant, the hardest hit by Friday's devastation, White House spokesman Jay Carney on Monday claimed in his daily briefing that nothing has changed at home.

Nuclear power "remains a part of the president's overall energy plan. When he talks about reaching a clean energy standards it's a vital part of that. And as we get more information from Japan and what happened there, that can be incorporated, but right now we remain committed to the clean energy standards and the other aspects of the president's energy plan," he said.

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"From a policy perspective, we will continue to operate our reactors and ... operate them safely. We will continue to seek to build nuclear into a part of a responsible energy future, and we will uphold our confidence in the NRC to make sure that we always do so to the extent that it can be done safely," added Deputy Energy Secretary Dan Ponemon, who also briefed reporters along with Nuclear Regulatory Commission spokesman Greg Jaczko.

In the president's already contentious energy policy proposals, he has proposed $36 billion in loan guarantees for nuclear power. Though much of that is directed toward upgrading existing plants, opponents to nuclear power say a better investment is in solar or wind power research.

"We have put hundreds of billions of dollars of subsidies into the nuclear industry and we've spent a fraction of that trying to develop the kinds of energy sources that we desperately need. I think we need a major reordering of our financial priorities here," said Dr. Ira Helfand of Physicians for Social Responsibility.

More than 200,000 people have been evacuated from northeast Japan after cooling systems shut down at the Fukushima Daiichi plant, which led to explosions and radiation leaks following Friday's earthquake and tsunami. U.S. Navy ships sent to help with the humanitarian crisis were forced to move upwind after low-level radiation was detected on 17 helicopter crew members positioned there for relief efforts.

Back at home, Obama expressed his concern for the people of Japan, and pledged that the United States "will stand with the people of Japan" in the difficult days ahead.

On Monday, Japan officially requested U.S. assistance in helping to cool three nuclear reactors whose fuel rods Japanese officials say they believe had begun melting. Two NRC and two Energy Department experts are already on the ground consulting with their Japanese colleagues. Additional technicians may travel there soon.

With consistently bad news trickling out from Japanese government officials about the potential nuclear meltdown, much of the attention at home has turned away from the unfolding tragedy to the debate about whether a similar fate could await the U.S.

"This is nuclear power's Achilles' heel and shows why it is sheer folly to pour resources into building and maintaining nuclear reactors in the U.S.," Tyson Slocum, director of Public Citizen's energy program, said in a statement. 

The disaster in Japan "demonstrates the difficulty in planning for both the 'known unknowns' and 'unknown unknowns' that impact nuclear reactors from natural disaster and terrorism," Slocum added.

"It really comes down to luck," said Rita King, a freelance journalist specializing in nuclear issues.

King said nuclear facilities and other buildings aren't constructed for the "unthinkable." While the Fukushima Daiichi plant withstood the 8.9 magnitude earthquake, it was wiped out by a tsunami. Even as the containment walls of the facility withstood hydrogen explosions, hundreds of aftershocks bring additional uncertainties, she said.

But the American Council on Science and Health, a group of scientists and doctors that monitors the use of science in public policy debate, said dire warnings about U.S. insecurity are the work of agenda-driven groups.

"The anti-nuclear energy activists trying to exert pressure to stunt the progression of nuclear power do not really care about nuclear safety," said Dr. Gilbert Ross of ACSH. "Eliminating nuclear power as a source of energy is the wrong answer from any point of view. Nuclear power offers the safest and cleanest source of energy on Earth."

Senate Majority Leader Mitch McConnell, R-Ky., suggested the dire warnings only undermine U.S. energy independence.

"We ought not make a long-term decision about energy policy based on an environmental catastrophe in another part of the world," he said.

The NRC is not in a position to discuss any potential changes to safety and security measures at U.S. nuclear plants, Jaczko told reporters, particularly because no one has yet fully assessed the sequence of events in the Japanese plant failure. But people who worry about a repeat should take stock of the "very high standards" imposed on U.S. facilities to withstand earthquakes, tornadoes and tsunamis.

"We have a strong safety program in place to deal with seismic events that are likely to happen at any nuclear facility in this country," he said, adding that the NRC reviewed tsunami survival capabilities after the 2004 Indian Ocean quake.

"As an independent regulatory agency, we will always take whatever steps are necessary to ensure the safety and security of nuclear power plants in this country," Jaczko added.

Despite the debate roaring at home, the United States may not be in a position to replace nuclear power, which currently supplies 20 percent of U.S. electricity.

"Nuclear plants run at 100 percent capacity for about 18 months between refueling. They put massive amounts of power onto the grid. They are base load power source," said Alex Flint, a spokesman with the Nuclear Energy Institute, the nation's nuclear energy trade association.

Plus, Flint noted, other fuels include their own set of consequences.

"Whether we build more coal plants or gas plants or expanded hydro facilities we'd deal with the externalities associated with every one of those power sources," he said. "However one generates electricity, there are consequences associated with it. there are tradeoffs."