More than 33,000 nuclear plant mishaps - ranging from stuck valves to radioactive leaks - have been reported by utilities to the Nuclear Regulatory Commission in the 10 years since the partial meltdown at Three Mile Island, according to a study released Tuesday.

"A decade after the accident at Three Mile Island, the NRC and the nuclear industry have forgotten the lesson of TMI," said Joan Claybrook, president of Public Citizen, the advocacy group which prepared the report. "The plants pose a continuing danger to the public's health and safety."But NRC spokesman Joseph Fouchard, who characterized the study as "pinging the industry," said nuclear power plants are safer than they were a decade ago.

"There have been fewer challenges to the system, fewer automatic shutdowns, the amount of waste generated at these plants has gone down," Fouchard said.

The study, "Nuclear Power Safety: 1979-1989," charts an increase at the beginning of the decade in the number of reported nuclear plant mishaps. These are failures or breakdowns of plant systems and procedures that may pose a threat to public safety. The operating utility must report these incidents to the NRC by filing a "Licensee Event Report," or LER.

The NRC has a system for grading the severity of LERs but Public Citizen did not take this into account.

In 1979, there were 2,310 reported mishaps at the nation's 67 nuclear reactors, including the partial meltdown of TMI 2 in Middletown, Pa. The number increased to 3,804 in 1980, to 4,060 in 1981, and to 4,500 in 1982. By 1983, the number of reported incidents exceeded 5,000 at 82 operating plants, the group found.

In 1984, however, the NRC changed its system for reporting such events, and the total number of LERs dropped to 2,460 that year, according to Public Citizen. 1985, with 3,050 LERs, was described by the NRC as one of the worst years in the history of the nuclear power industry, marked by 645 emergency plant shutdowns and 38 fines totaling nearly $4 million levied against utilities.

Since then, the number of nuclear plant mishaps has apparently leveled off, although "even the NRC's data is incomplete and thus these figures probably do not reflect the entire sum of reportable mishaps each year," the report said.