After the near-diaster at Three Mile Island almost a year and a half ago, President Carter ordered the Federal Emergency Management Agency to assess the evacuation preparations of communities and state with nuclear power plants. The experts have completed their report, and what they found is no cause for complaceny.

For a number of reasons, local authorities are simply not ready to cope with mass evacuation, or the treatment of those exposed to radioactive emissions, in the event of a nuclear plant disaster.

The lack of preparation can be traced to the overblow claims of nuclear power advocates, who for years pooh-poohed the idea that the plants might be unsafe. Accidents could not happen, the public was told. Until two occurred within four years (the other was a serious fire at the Brown's Ferry plant in Alabama), public officials tended to believe this.

Local alerting and notification systems are deficient, the federal experts found, and the firemen, police and other emergency personnel who would be first on the scene have not been adequately trained.

On the other hand, the federal government has not moved to fill the vacuum created by local and state inaction, the report, the report states. As one example, the experts cite the question of potassium iodide, a chemical that protects individuals from ingestion of certain radioactive materials after a nuclear accident. The emergency agency recommends that the federal government stockpile the substance for distribution in an emergency.

Yet the Feds have no policy or plans for the purchase, storage, distribution or use of potassium iodide. And when the chemical was sent the Three Mile Island, it had been improperly packaged and could not be used.