Ralph Nader's Health Research Group rated one of the five nuclear power plants closed down yesterday as the safest and two others among the worst in the nation in terms of radiation exposure to their workers.

The Beaver Valley plant in Shippingport, Pa., near Pittsburgh, was number one in a survey of 57 of the nation's 72 operating nuclear power facilities, with ony 2.6 percent of its workers getting more than one-half a Rem of radiation in 1977. A Rem is a scientific measure of radiation levels on the human body.

The Surry I and II plants in Gravel Neck, Va., were rated third worst on the scale, having 49.7 percent of its employes exposed to more than half a Rem in 1977. The worst was Pilgrim I in Plymouth, Mass., with 52.3 percent in that category.

In a letter to President Carter on Monday, the Health Research Group director, Dr. Sidney M. Wolfe, said the government safety limit of 5 Rems per year had been proven too high in a recent British study of shipyard workers. He asked Carter to transfer jurisdiction of worker safety regulations from the Nuclear Regulatory Commission to the Occupational Safety and Health Administration and to lower the permitted radiation exposure to one-half a Rem per year.