WASHINGTON--The Nuclear Regulatory Commission (NRC) yesterday ordered the immediate shutdown of an atomic fuel fabrication plant in Tennessee after the plant reported the apparent loss of at least 20 pounds of weapons-grade uranium.

NRC spokesman Frank Ingram said yesterday Commission investigators have not yet determined whether the uranium is actually missing from the plant or has been lost somewhere in the plant's processing system.

Under licensing agreements with the Federal government, Nuclear Fuel Services Inc., the company which operates the plant, had to report any uranium loss of 19.8 pounds or more and shut down for an inventory within 72 hours.

However, an NRC spokesman said in this case the Commission intervened before the deadline expired and closed the plant immediately.

Ingram refused to say whether this action was connected with the amount or type of missing uranium or the circumstances of its disappearance.

He refused to say exactly how much uranium is missing. He said the information is classified.

A spokesman for Nuclear Fuel Services said yesterday the firm is investigating the apparent uranium loss.