By H. Josef Hebert
Associated Press Writer
Thursday, Sept. 30, 1999; 10:26 p.m. EDT

WASHINGTON –– Despite strict rules to guard against unplanned nuclear chain reactions, U.S. nuclear fuel plants have had at least two incidents in which those safeguards failed, according to government and industry officials.

Still, these officials said, it is extremely rare for nuclear fuel to reach so-called "criticality," as apparently occurred at the fuel plant in Japan in which three workers were exposed to high levels of radiation and hundreds of people were evacuated.

The accident in Japan occurred at a fuel fabrication plant, similar to seven such plants operated by private companies in the United States.

President Clinton said "our thoughts and prayers are with the people in Japan" because of the nuclear accident. "You can only imagine how difficult this must be for them," said the president.

While the Japan incident continues to be investigated, it is believed the nuclear chain reaction occurred when workers placed more than six times the amount of uranium allowed into a vat of nitric acid, said an industry source, who spoke on condition of anonymity.

The acid normally is used to remove impurities from recycled uranium left over from fabricated fuel pellets.

"This is a serious nuclear accident," Energy Secretary Bill Richardson said in a telephone interview from Russia, where he is visiting nuclear facilities. "The good news is that it is not a widely contaminated area. It's a limited area."

Nevertheless, scientists at the Lawrence Livermore National Laboratory were monitoring atmospheric conditions to track the radioactive plume created when radiation rushed from the plant. Unlike nuclear reactors, fuel fabrication plants – both in Japan and in the United States – do not have protective containments.

No information on the plume was available from Livermore or the Energy Department late Thursday. Officials in Japan said radiation levels outside the plant had returned to normal by Friday morning there. As a result, the spread of significant radiation in the atmosphere became less likely as the early doses dissipated, an American official speculated.

Richardson said the United State and Russia were prepared to send a joint team of nuclear experts to help Japan deal with the problem. As of late Thursday, no request for such assistance had been received.

There have been a number of incidents of unplanned nuclear chain reactions, but most have involved very small amounts of material such as used in research environments.

In the United States there has only been one incident in which "criticality" had been achieved at a nuclear fuel processing plant. That occurred in 1964 at a plant, no longer in operation, near Charleston, R.I.

A worker poured a bottle of uranium solution into a container when it went critical as he leaned over the vat and was killed, said Michael Weber, an official of the Nuclear Regulatory Commission.

Eight years ago at another fuel fabrication plant operated by General Electric near Wilmington, N.C., workers struggled for several days to keep uranium in a water-filled tank from setting off a nuclear chain reaction. They averted criticality by pumping oxygen into the tank.

"They took a solution and introduced it into (what) ... was not a suitable tank," said Weber, violating NRC rules for safeguarding the nuclear material.

"There are very strict controls that we put on the plants," said Weber. These include specifications on how much uranium can be in a container, the shape of the container and the mass within a container. If any of these requirements are violated, criticality can occur, he said.

The uranium fuel cycle is spread across a number of facilities in the United States. First, uranium ore is converted to hexafloride gas, then it is shipped to another plant for enrichment, and then to one of seven fuel fabrication plants that turn the gaseous or liquid uranium into powder and then reactor fuel pellets.

Uranium powder left over from the molding of fuel pellets often is put into nitric acid for purification and recycling.

It is at this stage where it is believed workers in Japan placed as much as 16 kilograms of uranium into a tank of acid that should have held no more than 2.4 kilograms, according to industry sources who have been briefed on the incident.

According to the NRC, the seven U.S. nuclear fuel fabrication plants are: an ABB Combustion Engineering plant in Hematite, Mo.; a General Electric plant in Wilmington, N.C.; a Westinghouse plant in Columbia, S.C.; a Nuclear Fuel Services plant in Erwin, Tenn.; the Framatome Cogema Fuels plant in Lynchburg, Va.; the BWX Technolgies plant, also in Lynchburg, and Siemens Power Corp. plant in Richland, Wash.