Another problem has surfaced in the scientific work that is supposed to ensure the safety of entombing the nation's most lethal nuclear waste in Yucca Mountain.

A spokesman for the project confirmed Wednesday that concerns by nuclear regulators about flawed humidity measurements in corrosion-rate studies of the metal waste disposal packages have prompted them to order a halt to that work.

The stop-work order took effect Jan. 30, about three weeks after inspectors from the Nuclear Regulatory Commission's safeguards office wrote to the project's licensing director to say that the work was based on humidity gauges that weren't calibrated. Project contractor Bechtel SAIC had claimed that the work was "technically sound" with "defensible results."

The revelation comes nearly a year after the Energy and Interior departments revealed that several U.S. Geological Survey hydrologists had exchanged e-mails discussing "fudge factors" and possible falsification of quality assurance documents on water infiltration research.

Bob Loux, executive director of Nevada Nuclear Projects Agency and a leading critic of the government's effort to dispose of spent nuclear fuel at Yucca Mountain, 100 miles northwest of Las Vegas, said the latest revelation means the project's entire quality assurance program is flawed.

"This strikes right in the heart of the whole corrosion issue. If some of the data is suspect, it's huge," Loux said.

Steve Frishman, a full-time consultant for the Nevada Nuclear Projects Agency, said that if they used improperly calibrated or uncalibrated equipment, government scientists might have underestimated corrosion rates of the nickel alloy, known as Alloy-22, that will be the outer cover of the stainless-steel waste packages. The packages are supposed to contain 77,000 tons of spent fuel assemblies and highly radioactive defense wastes in a maze of tunnels inside the mountain.

"They did not only not follow their quality assurance measures, they also didn't follow the scientific procedures for the experimental work. ... It isn't science if quality assurance isn't there," Frishman said.

"Now we have the corrosion rate of the container in question, and because of the USGS stuff we have the infiltration of water in question, and these are two critical pieces of the repository design," he said.

An investigation into the uncalibrated instruments used in corrosion experiments is under way to determine the root of the problem and what corrective actions must be taken, said Allen Benson, a Department of Energy spokesman for the Office of Repository Development in Las Vegas.

"We take these quality assurance concerns very seriously, and we will look into and address all the concerns raised by the NRC," Benson said.

He said the investigation will focus on high-temperature humidity instruments called "Vaisala probes," and "any other instruments at or beyond documented calibration ranges."

In August, observers from the NRC staff examined an audit by a Bechtel SAIC team into the quality assurance of waste package corrosion experiments conducted between 2002 and 2005 at Lawrence Livermore National Laboratory in California, 45 miles east of San Francisco.

The NRC observers found that:

• Experiments were started without calibrated instruments.

• Calibration was not documented.

• Lawrence Livermore scientists planned to conduct an "in-house" experiment to calibrate the probes and qualify data "after experiments were completed."

A Lawrence Livermore spokesman deferred comment Wednesday to DOE's Office of Repository Development.

Benson said the investigation is expected to be completed in late March. Until then, project officials won't know whether any or all of the Lawrence Livermore work on the corrosion studies will have to be redone.

Sandia National Laboratories currently is redoing the infiltration model, anticipating that the lack of traceable quality assurance documentation of the scientific work will lead to failure to pass the scrutiny of a license application review by the NRC.

As for the corrosion experiments, Frishman said lack of calibration of the humidity instruments would skew results of how dust containing minerals and salts could accelerate the corrosion of the waste packages' outer shell. Some of the experiments were exploring the impact of "deliquescence," in which some minerals and salts soak water vapor form the air, creating a corrosive solution.

At a Feb. 1 technical review panel meeting in Las Vegas, scientists estimated that corrosion will take its toll on waste packages after they have been in the mountain for 40,000 to 60,000 years. Water moving through the mountain at first would be driven away by heat from the decaying waste, but eventually moisture would condense and infiltrate the tunnels, carrying off deadly, long-lived radioactive materials.

While the news was breaking about the calibration issues Wednesday, state Attorney General George Chanos and members of the statewide environmental group Citizen Alert met in Las Vegas with Minnesota legislators who wanted to hear the state's concerns on Yucca Mountain before touring the site today.

"Our new attorney general said as long as he is attorney general, Yucca Mountain is not going to happen," said Peggy Maze Johnson, executive director of Citizen Alert.