A panel of nuclear safety experts is warning that design flaws remain unresolved at the facility being built to process dangerous waste at the Hanford Site in southeastern Washington.

The Defense Nuclear Facilities Safety Board says the Waste Treatment and Immobilization Plant (WTP) could explode or cause an uncontrolled nuclear chain reaction. In both cases, dangerous levels of radioactivity would likely contaminate the Columbia River, the Tri-Cities and big swaths of productive agricultural land.

The $15 billion plant is designed to convert the worst nuclear waste at Hanford into glass rods for permanent disposal. Under construction for 15 years and plagued with problems, the WTP isn't projected to begin operations until 2036.

The DNFSB is urging the U.S. Department of Energy to fix flaws it says remain in the plant's design.

The waste that would be processed was created over a 40-year period, the result of plutonium production for the nation's nuclear arsenal. Fifty-six million gallons of highly radioactive and toxic sludge is stored in 177 underground tanks, some of which are known to be leaking. The government believes that converting the sludge into glass logs -- through a process known as vitrification -- is the only way to safely store it for the tens of thousands of years it will remain deadly.

The DNFSB report published last month is only the latest warnings issued by the panel about the WTP design. DNFSB's experts say Hanford scientists still haven't resolved many of the technical problems highlighted in previous assessments.