WASHINGTON (AP) - 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 U.S. 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 do not have protective containments.