More than one-fourth of the Savannah River Site’s high-level atomic waste tanks have cracked, rusted or leaked since the 1950s and 1960s, federal inspection reports show.

U.S. officials have been unable to agree with environmentalists on how to stop future threats from the tanks. But neither side disputes the potential hazard these tanks present to the environment near Aiken.

Exposure to high-level waste can kill a person instantly. Some radioactive waste can linger in the environment for hundreds of thousands of years.

Recent federal inspection reports note hundreds of “leak sites,’’ or cracks, in 15 of the 51 aging steel tanks. In some cases, the cracks date to the 1950s; in others, leak sites have been found only in the past three years, according to the most recent tank inspection report.

To date, secondary containment systems from most tanks have kept radioactive poisons from reaching groundwater beneath the 300-square-mile federal weapons complex.

The government also does not know of any tanks that currently are leaking, said Dean Campbell, a spokesman for Westinghouse Savannah River Co., which runs SRS for the U.S. Department of Energy.

Still, a containment system failed in one case, when radioactive waste leaked into the ground. And federal officials say the tanks’ condition is a concern. A number of tanks are within 8 to 10 feet of the water table, according to Westinghouse.

“They obviously are getting older and will not last forever,” said Charles Hansen, an assistant waste disposition manager with the U.S. Department of Energy.

“This is highly radioactive, and there is a concern to get that waste out as soon as possible. There’s always some potential for inadvertent leakage into the environment.”

One of the most recent problems occurred in 2001, when 92 gallons of radioactive waste leaked through a 40-year-old tank and into a containment area, according to the government’s latest tank inspection report.

After finding the waste, inspectors located six leak sites on the 750,000-gallon, 24-foot high steel tank. The leak was stopped before it reached groundwater.

All told, the tanks at SRS contain about 37 million gallons of liquids, salts and highly contaminated sludge. The containers are more than 24 feet long and can hold 750,000 to 1.3 million gallons of liquid high-level waste.

Waste pumped into them came from the creation of materials for nuclear weapons during the Cold War. The tanks contain an assortment of radioactive waste, including uranium, cesium and plutonium.

Today, many tanks are less than half full, but environmentalists say the remaining radioactivity is among the highest at federal nuclear weapons facilities. DOE officials say some of the tanks at SRS have less radioactivity than others.

“These tanks are not designed for long-term storage, as you can obviously tell,’’ said Geoff Fettus, a lawyer with the Natural Resources Defense Council, which sued to stop the DOE’s cleanup plan in favor of its own.

Most of the tank problems center on age and the way they were constructed, according to the U.S. Department of Energy and its chief contractor, Westinghouse.

The 1950s- and ’60s-era tanks were not treated to handle stress as well as more recently installed tanks, the DOE and Westinghouse say. Carbon steel used at the time also is not as sturdy as that used later. Most of the tank cracks occur along weld seams, where stress builds up from the waste inside.

In 1960, tens of gallons of high-level liquid waste trickled into the ground at SRS after one tank cracked. The liquid flowed for about six hours and escaped a secondary containment pan, Campbell said.

Campbell said the tank had since been emptied of most of its contents, although a layer of waste remained on the bottom.

Other problems found in the latest Westinghouse tank inspection report include:

• More than a foot of dried waste in a containment area outside a 1-million gallon tank installed in 1957. Inspections have found 33 leak sites, but federal officials estimate the tank contains 50 cracks.

• Nearly one foot of dried waste in a containment area outside of a 750,000 gallon tank installed in 1955. The leak may have occurred the same year the tank was installed. DOE hasn’t determined the source of the leaked waste.

• Eighteen leak sites have been found on one tank, including two discovered in 2000 and one through-wall crack in 2002. Two cracks have been found near the tank’s bottom.

• Five gallons of waste were discovered leaking from a 45-year-old tank in 2001. Inspections found 15 leak sites never before detected.

In an attempt to attack the problem, energy department officials seek permission from Congress to empty most of the waste from the tanks, then fill the containers with grout to neutralize atomic refuse the agency says it cannot remove.

Agency officials say that will speed the tank cleanup before a serious accident occurs. Officials say the concrete-like grout will make the gooey waste less likely to move in groundwater. Of the 51 tanks at SRS, two have been formally closed in the manner the DOE proposes for the remainder of the tanks.

Critics of the DOE plan, including former President Jimmy Carter, say the tanks’ poor condition shows the need to empty the containers completely. They do not think concrete will neutralize the remaining waste in the tanks.

Waste removed from the tanks likely would wind up at the Yucca Mountain national disposal site in Nevada, if it opens.

So far, Congress has supported the DOE’s plan, but the issue likely will not be resolved for at least another month.

Reach Fretwell at (803) 771-8537 or sfretwell@thestate.com.