The National Transportation Safety Board, reacting to the chemical disaster in Bhopal, India, yesterday prodded the Transportation Department to improve its method of evaluating the potential hazards of chemicals and to increase the minimum protections required when some chemicals are shipped.

In a letter to the DOT's Research and Special Projects Administration (RSPA), safety board Chairman Jim Burnett said the board's study "indicates that there is an urgent need to improve the manner in which toxic materials are classified and to raise the minimum levels of protection required by federal regulations in transporting these materials."

The board said the present classification method places more weight on a chemical's potential flammability than on its toxicity. The classification of a hazardous material determines the type of container in which it may be shipped.

The present classification system, the board said, was developed by industry and inherited by the DOT in 1967. Since then, the board said, "An overall objective assessment using current technology has not been made to determine the old system's continued adequacy for identifying fully the hazards posed to public safety . . . ."

Alan I. Roberts, associate director of the RSPA's Office of Hazardous Materials Regulation, said that "a number of points in the safety board's letter have merit," but that his agency has been working for more than a year on improving standards in a way that will lead "to a greater recognition of the toxic risks of materials as opposed to other risks, such as flammability."

That classification standard, he said, will be the basis for two important draft rules. One will be a new standard for cargo tanks, scheduled "in the near future;" the other will be a "complete restatement of U.S. hazard classification" to bring it into conformity with an international standard. That will be proposed "in several months," Roberts said.

The chemical involved in the India disaster, where more than 2,000 people died, is methyl isocyanate (MIC). Under existing federal regulations, the board said, a shipper "could elect to ship this material by rail in the least protected of DOT-specification tank cars."

The board said Union Carbide -- the only U.S. manufacturer of MIC -- controls its distribution and requires considerably higher standards than the minimum. However, the board said, "this is not true of other materials which pose similar toxic threats in the event of a transportation emergency."

Meanwhile, Union Carbide said yesterday that MIC shipments being returned from Brazil and France will go to its plant in Institute, W.Va., or to one in Woodbine, Ga.