A top federal nuclear official last night warned that the ongoing crisis in Japan has grown more serious, but he defended the safety of U.S. nuclear power plants, including those similar in design to the crippled Fukushima Daiichi plant in Japan.

Nuclear Regulatory Commission Chairman Gregory Jaczko told reporters last night that the situation at the plant is likely more dire than originally thought.

He said the plant's Unit 4 spent fuel pool could be empty and the Unit 3 pool may have a crack. Without proper cooling -- which is typically provided by the water in the pools -- spent fuel rods can ignite, emitting radioactive elements.

Jaczko added that if a similar situation were to occur in the United States, his agency would recommend an evacuation of everyone within a 50-mile radius of the plant. That recommendation led the State Department yesterday to urge all American citizens to leave the region around the plant. The Japanese government is only requiring evacuations within a 12-mile radius.

"The Nuclear Regulatory Commission received information through individuals we have in Japan that the situation with some of the reactors and the spent fuel pool was becoming more serious over the last many hours," Jaczko told reporters in the Capitol last night. "As a result, we took a look at the information that we had and compared it to the actions that we would take here in the United States. As a result of that, we determined that in the United States we would make a recommendation to evacuate personnel to a much larger area than is currently being done in Japan."

But he added, "It is my great hope that the information that we have is not accurate. I would hope for the sake of everyone that the situation is not at the state that we think it is."

His comments came shortly after he received a verbal lashing from Democrats on Capitol Hill about the safety of U.S. plants.

"Japan thought their plants were prepared for a worst-case scenario, but they weren't," Sen. Frank Lautenberg (D-N.J.) told Jaczko during a briefing in the Environment and Public Works Committee yesterday afternoon. "How can we be sure in the United States that we don't have plants demolished and radiation released in a worst-case scenario?"

Jaczko and his staff told lawmakers that 23 of the nation's 104 nuclear reactors use the same General Electric Mark I boiling water containment design as those at the Japanese plant. They stressed that precautions have been taken at each plant to avoid disasters such as the earthquake- and tsunami-caused one in Japan.

But his comments did not appease Democrats on the panel.

"The problem with nuclear power is it cannot be just 99.99 percent safe, because its potential toxicity brings far-ranging and serious consequences as we are seeing right now in Japan," said Sen. Bernie Sanders (I-Vt.). His state is home to the Vermont Yankee nuclear power plant, which uses the same containment design as the Fukushima plant and went online a year after the Japanese plant became operational in 1972.

The regulatory commission has temporarily delayed its review of a 20-year relicensing of the 650-megawatt plant in Vermont.

Other lawmakers with nuclear power plants in their states raised concerns that NRC has not yet taken proactive measures to ensure the safety of the U.S. plants that use similar technology as the Fukushima plant. The Obama administration has said it would not stall the licensing process for existing or new plants because of the disaster.

Chairwoman Barbara Boxer (D-Calif.) cited examples in Switzerland and Germany where older nuclear plants have temporarily been shut down in the wake of the Japanese disaster to ensure their safety.

"This is an extraordinary event in history. ... I'm not asking you why, but I am saying to you: Is this not a warning that we should be humbled," she asked. "Wouldn't you immediately say as the lead person with all the American people behind you, 'Guess what, I'm independent, I represent the American people, and I want to go back to every plant, and not wait two years. ... We're not going to issue any relicense unless we make sure ... that we go back and retrofit.'"

"Is that too much to ask?" she added. "Just look at these plants that are on earthquakes, and look at these plants that are really old like Bernie Sanders. I think what we're asking is not for some major breakthrough but just can we take another look, and put some teeth behind it, and put a timetable behind it, and do these immediate inspections and not say, 'Well we're waiting to learn from Japan,' because here's what we know."

Jaczko said NRC plans to conduct a "systematic and methodical review" of the Japanese situation and will apply that to its review of the safety of U.S. reactors.

Boxer has particular concerns about the two plants located in her state, which is known for its seismic activity and has a stronger chance of being struck by a tsunami than other regions of the United States.

"Roughly 424,000 people live within 50 miles of the Diablo Canyon [Nuclear Power Plant] and 7.4 million live within 50 miles of San Honore Nuclear Generating Station," she and Sen. Dianne Feinstein (D-Calif.) wrote in a separate letter to Jaczko yesterday. "Although many safety measures have been taken to address potential hazards associated with these facilities, we need to ensure that the risk is fully evaluated."

Boxer and Feinstein are asking NRC to thoroughly inspect the two plants to evaluate their safety and emergency plans.

"We want to have those plants inspected. We want answers," Boxer said at the briefing. "I'm looking to you for more leadership than I've gotten. Maybe I've missed it; I can't mince words, but I've got two plants with millions of people. I need you to take a look."