One lawmaker called the five-member commission "the most secretive agency" in Washington.
Members of the House Energy and Commerce Committee described the nuclear panel as dysfunctional and accused its chairman, Gregory Jaczko, of acting unilaterally on the commission's behalf. They cited several examples, including Jaczko' s declaration in March that Japan's nuclear crisis constituted an emergency in the United States.
The panel is considering a request by the Energy Department to shut down the proposed Yucca Mountain waste site 100 miles northwest of Las Vegas, and has begun initial steps to implement the shutdown.
Even so, Jaczko said there has been no final vote on the issue and no timeframe to make a decision. He repeatedly declined to discuss the commission's internal deliberations.
"We don't yet have a final decision," he told an Energy and Commerce subcommittee on Wednesday.
But three NRC commissioners appearing before the committee appeared to contradict him, telling lawmakers that they have cast what they consider substantive votes on the matter.
"I consider that I took my position," said Commissioner Kristine Svinicki. Two other commissioners, William Magwood and William Ostendorff, said they had also made a public decision on the Yucca site.
"I did not view it as advisory. They were final votes," Ostendorff said, referring to a series of votes the commission took last year on Yucca.
None of the four commissioners who testified Wednesday disclosed how they voted, or the vote's outcome. They said the Yucca case is considered quasi-judicial and not subject to public review until is published.
Rep. Lee Terry, R-Neb., said he was mystified and even disturbed by the commission's silence.
"We have now found the most secretive agency on Capitol Hill," Terry said, adding that he used to award that distinction to the Federal Reserve Board.
"The level of non-information is frustrating," Terry said. "I'm very disturbed."
Rep. Joe Barton, R-Texas, accused Jaczko of "playing some kind of foot-dragging game" and intentionally delaying a decision on Yucca Mountain.
Before becoming NRC chairman, Jaczko served as a staff member for Senate Majority Leader Harry Reid, a Nevada Democrat who is a vocal critic of Yucca Mountain.
Last year, the NRC's Atomic Safety and Licensing Board ruled that the Energy Department didn't have the authority to withdraw its application to build the site. The board said it was up to the NRC to issue a "merit-based" decision.
Jaczko said Wednesday there's no timeframe for the commission to act. A budget approved by Congress and signed by President Barack Obama includes $10 million for the NRC to document its staff review of the Yucca case and "preserve it as appropriate for publication and public use," Jaczko said.
Yucca Mountain is a wild expanse of desert brush and red mountains 100 miles outside of Las Vegas where deer, coyote and antelope roam isolated fields and visitors must pass background checks before being allowed past heavily guarded fences.
Many lawmakers outside Nevada claim the stark landscape is the nation's best hope for a national nuclear waste dump, while others call it dangerous, noting that the half-built nuclear junkyard would require nuclear plants to ship their waste to rural Nevada along the nation's roadways and railways.
The Obama administration has shuttered the site and said there is no chance of reopening it. No money has been allocated for the project in this year's federal budget.