More problems with quality control at Hanford's vitrification plant in the last year have Sen. Ron Wyden, D-Ore., calling for an explanation.

He sent a letter Tuesday to Energy Secretary Samuel Bodman asking for information on the cause of the most recent problem, a failure to fully inspect welds on pipes that could contain high level radioactive waste.

Bechtel National, the contractor building the $12.3 billion plant, has found about 1,800 piping sections that did not have every weld inspected with X-rays before they were shipped by vendors to Hanford. None of the sections have been installed in the plant.

But continuing issues about the quality oversight of components manufactured for the plant raise questions about the Department of Energy's oversight of work by contractor Bechtel National and whether the plant will operate safely, Wyden said in the letter.

DOE twice has fined Bechtel National for quality issues. The first fine was for $198,000 in spring 2006 and the second was for $165,000 in October 2007. Neither fine covered the piping which has more recently been a problem. But there have been some related problems, such as a failure to ensure processing tanks were fully inspected.

Both the tanks and the piping were manufactured for "black cells," the areas of the plant that will hold handle high level radioactive waste and will be too radioactively hot to be entered by humans after operations begin.

The subcontracts to manufacture the piping were awarded in early 2003 and later that year one of the manufacturers asked for clarification on which piping would be inside black cells and would require every weld to be inspected rather than the usual sampling of 5 percent of welds, said Drew Slaton, spokesman for Bechtel National. All manufacturers then were given revised specifications to show which pipes were planned for black cells.

But the issue came up again in January 2006, when a Bechtel employee assigned to oversee manufacturing at one of the subcontractor sites noticed that not all welds had been inspected on a black cell pipe section. Bechtel issued a corrective action report then to address any ambiguity in specifications, Slaton said.

However, in April 2007 when Bechtel reviewed the issue again it found more problems and issued a second corrective action report to address problems.

"Poor review of the extent of condition and corrective action management resulted in only recently understanding the scope of the problem," said a weekly report of the Defense Nuclear Facilities Safety Board issued in October 2007.

That month Bechtel started a more in-depth process to try to understand why suppliers were unable to identify which piping would be used in black cells and required every weld to be inspected. It identified 20 actions to improve controls and communications with vendors.

The review found about 1,800 pipe segments that would be used in the black cells had not had every weld inspected. All those pipes but one were planned for drain lines or similar uses that would not subject them to high temperatures and pressures. The one exception was the result of human error, Slaton said.

In November, Bechtel suspended work in piping until issues could be resolved. A month later it issued an analysis concluding that construction drawings did not clearly show which piping was inside black cells. Some of the piping previously was installed in modules for placement in black cells, but no module with the uninspected piping has been installed in the vit plant's Pretreatment Facility or High Level Waste Facility.

Bechtel has initiated a more broad-based review to make sure that similar problems are prevented throughout the plant as designs progress to drawings and then manufacturing, Slaton said. It also has placed additional inspection requirements on items before they are installed in the plant and when they are received at the plant.

The plant is being built to turn radioactive waste from the past production of plutonium for the nation's nuclear weapons program into a stable glass form for disposal. It is expected to begin full operations in 2019.