A tractor-trailer carrying 50 steel drums full of uranium oxide overturned on a southeastern Colorado highway last week, spilling at least 15,000 pounds of the poisonous, radioactive material.

Federal officials said the accident was the largest spill ever recorded of yellowcake, as the industry calls the fine powder used in the processing of nuclear fuel.

Had it occurred in an urban area, there would have been the potential of a major health hazard, accoridng to Paul B. Smith, the Environmental Protection Agency's radiation expert in Denver. When inhaled, minute amounts of yellowcake dust create lesions of the kidneys and liver.

The Sept. 27 crash occured in the a rural area 35 miles south of Lamar when the truck collided with three horses at 1 a.m. It was carrying 42,000 pounds of yellowcake from an Exxon Corp. uranium mill near Casper, Wyo., to a conversion plant in Gore, Okla.

More than 2 million pounds of the toxic material were shipped along the same route this year, passing through Denver, Exxon officials said.

Thirty-two of the 50 steel drums were thrown from the truck and damaged. "Some were squashed flat," said Charles Mattson, a health physicist with the Colorado Health Department. "It's a mess," he added. "But everything should be cleaned up by this weekend."

More than a dozen Exxon workers were on the site yesterday with respirators and special clothing, shoveling up the yellowcake and contaminated soil over a 5,000-square-foot area. The ground had been covered with plastic several hours after the accident by local police to prevent the powder from spreading.

Both Smith and Rep. Timothy E. Wirth (D-Colo.), who is investigating the incident, yesterday criticized the standards for packing of radioactive material of the Transportation Department and the Nuclear Regulatory Commission as inadequate.

"It this had happened in the middle of Denver, a lot of people would have been exposed to a very dangerous substance," Wirth said. "They don't have any standards for packaging. They have no plans to keep these trucks away from populated areas or require better containers."

NRC spokesman Frank Ingram, however, disputed Smith's and Wirth's assessment of the accident. "There is basically no health hazard involved here," he said. "Yellowcake is toxic, but it's a low-level radioactive material. You'd be sick to your stomach before you could inhale enough to do any damage to yourself."

Plain metal drums, similar to ordinary industrial shipping containers, are considered adequate, Ingram said. "You don't ship yellowcake in the same kind of shielded lead casks as plutonium because it's not that dangerous."

More than 25 policemen and other rescue workers who visited the accident site without respirators were given urine tests which showed no evidence of radioactivity or heavy metal poisoning, Mattson said.

Wirth, however, contended that evidence of a health hazard might not show up for as long as a year because the material would not travel so quickly from the lungs to the kidneys.

"NRC says yellowcake isn't dangerous," Wirth added. "But it is, or the people cleaning it up wouldn't be wearing respirators." Exxon officials said respirators and special clothing are also worn in uranium mills where the yellowcake is manufactured.

"Yellowcake," said EPA's Smith, "is toxic - I don't care what you say. If you spill it on a busy thoroughfare the cars would suck it up through their ventilation ducts. The potential for massive exposure is there."

Uranium oxide, Wirth said, "isn't the kind of stuff you'd want on your breakfast cereal."

Wirth, who has tried to pass a bill tightening air transportation of plutonium, called the yellowcake accident "bizarre. Isn't it extraordinary to have a truck ripping along the highway with 42,000 pounds of uranium in standard industrial containers? What else is being shipped through our cities?"

Ingram said NRC had no monitoring system of how much yellowcake is being transported where. He said there have been 97 other reported transportation accidents involving various types of nuclear materials, but said none involved as large an amount of yellowcake.