For a decade, officials of a subsidiary of the nation's largest nuclear utility routinely falsified occupational health reports to the government and repeatedly overexposed workers at a Colorado uranium mill to radiation, according to a report written by the Colorado Bureau of Investigation.

From 1968 to 1978, the Cotter Corp., owned by Commonwealth Edison Co., frequently failed to conduct required tests to determine inhalation exposure of employes, said the report, which was made public Friday. In addition, the firm manipulated data by erasure or recalculation of figures and occasionally by resampling or disregarding air samples that showed unsafe conditions at its mill in Canon City, Colo., the document said.

Winstoh Duke, staff counsel for the Cotter Corp. in Golden, Colo., said today the frim needs more time to review the report and refused comment on its findings. Last year, Cotter executives denied that health records had been falsified or that the firm had violated other laws.

The report apparently will not result in prosecutions. CBI officials said late Friday that no felonies had been committed and misdemeanor violations of health department regulations indentified in the report are beyond the statute of limitations.

The number and frequency of overexposures of employes to inhalation of high concentrations of airborne uranium was "far in excess of norms for the industry," the report said. The corporation's failure to conduct proper tests and take corrective measures "greatly contributed to worker overexposures that went uncorrected and unreported," it said.

The report repeatedly cited evidence that "emphasis was placed on production at the exense of the health and safety program." The document did not specify how many employes had been overexposed or address the possibility of diseases such as cancer, leukemia and sterility that can be caused by radiation.

Herman J. Paas Jr., a former federal Nuclear Regulatory Commission employe who was an investigator in the 1979 plutonium-negligence lawsuit brought by the heirs of Karen Silkwood against the Kerr-McGee Corp., conducted an analysis of Cotter data for the CBI. His work showed that airborne radioactive dust at the Canon City mill at times exceeded limits by as much as 30 times.

The basic attitude of management was, "do your job, don't expose anybody [to charges of wrongdoing] and keep everything looking good on paper," said William B. Hayhurst, a Cotter employe until May 1979, who operated the air sampling program at the mill, which employed about 100 workers until an expansion to 300 last summer. The report said that although workers must be notified of an overexposure within 30 days, testing samples to determine possible hazards were evaluated as much as six months late.

Myles Fixman, long-time manager of the mill, one of about 20 nationwide where uranium is extracted from ore for use in nuclear reactors, once authorized overtime to ensure testing and related records were updated because of a pending government inspection. Important records which an employes said she saw in company files as late as last month turned up "conspicuously absent" after a request for them was made by investigators, the report noted.

Further, the Colorado Department of Health was negligent in its inspections and enforcement actions at the mill, according to the report. The department did not identify apparent violations during inspections and did not always include as violations all infractions cited in inspection reports in its endorsement letters to Cotter. Health officials also gave advance notice before inspections that allowed Cotter to "dress up" its operation, the report said.

For several years, the mill, located in a rugged sagebrush area of south central Colorado, has been a target of criticism from local residents. The mill which is the largest private employer in the area, has been blamed for radioactive pollution for at least 10 years. Water in several wells in an area where rural families support themselves with subsistence gardening has been declared unsafe.