Federal officials conceded yesterday that they cannot account for 16 tons of weapons-grade uranium processed by the atomic bomb factories at Oak Ridge, Tenn., and Portsmouth, Ohio.

The officials insisted in testimony before a House Commerce subcommittee that the missing uranium was not lost or stolen but probably is in the thousands of miles of pipes that run through the massive diffusion plants in Oak Ridge and Portsmouth.

"The way these plants are built with pipes literally covering thousands of acres you're always going to have missing uranium," Gen. Edward B. Giller, associate deputy administrator of the Energy Research and Development Administration, told the House Subcommittee on Energy. "You try to recover it, but you can't."

Giller and other ERDA officials testified to the missing 16 tons of uranium when questioned by subcommittee Chairman John D. Dingell (D-Mich.), who asked why ERDA said last week it could not locate a little more than four tons of weapons-grade uranium and plutonium.

Acting ERDA Administrator Robert W. Fri told Dingell that the 16 tons was kept out of last week's report because it is only an estimate of the weapons-grade uranium bound up in the pipes of the two gas diffusion plants.

"It's impossible to pinpoint the missing uranium in the diffusion plants" Fri said. "The uranium goes in there as a gas uranium [hexafluoride] and passes through God-knows-how-many hundreds of miles of pipes. It spends four or five days in a cascade of pipes before it comes out an enriched product."

The gas diffusion plants that produce uranium for weapons move uranium through "barriers" that constantly strip lighter uranium atoms (the isotopes known as U-235) from their heavier kin, the isotopes called U-238 that are most abundant in natural uranium.It is the lighter atoms that are the fissionable ones and will drive a chain reaction that results in a nuclear explosion.

Fri said that about 80 tons of uranium are believed still bound up in the pipes that run through the plants at Oak Ridge and Portsmouth. He said this is an accumulation on the inside of the pipes that covers more than 20 years of plant operation.

The 80 tons of uranium that Fri said are still believed to be in the pipes include uranium that could be used only to make electricity as well as uranium rich enough in U-235 that it could be used to make bombs. Both types of uranium are produced in the same diffusion plants.

Fri said it is impossible to do anything more than estimate how much weapon-grade uranium is in the pipes. He said ERDA's best estimate is that 20 per cent of the uranium processed by the plants went into weapons. That would mean that 16 tons of weapons-grade uranium is in the pipes.

"It would takes weeks and even months of work to harden up that number," Fri said. "The 16 tons is the best we can do."

"Don't you think it's worth the time?" Dingell asked about the 16 tons, which could be used to make more than 1,000 atomic bombs the size of the one dropped on Hiroshima. "The substance of what we're discussing sends cold chills up my back."

Giller said that ERDA is not concerned about the missing 16 tons because the two diffusion plants are ringed by electrical fences and are under armed guard.

"The way those buildings are bounded and protected, everthing that goes in there either comes out the other side or stays inside," Giller said. "It would take a Mission Impossible scenario to get anything out of those buildings."