Federal safety inspectors were turned away this week from one of the Army's most hazardous ammunition plants where workers have complained of air containing levels of cancer-causing chemical 700 times above federal safety limits.

The Oil, Chemical and Atomic Workers Union, which represents workers at the Radford, Va., Ammunition Depot, said yesterday that the Army and its civilian contractor, the Hercules Corp., withheld information from workers about the high levels of the carcinogen benzene at the plant for more than a year and a half.

Steve Wodka, international representative for the union, said tests in February 1977 by Hercules showed 700 parts per million of benzene in the air there. Federal safety standards limit benzene levels in any workplace to 1 part per million.

Wodka said workers at the Radford plant did not learn about the high benzene levels in the air until this month.

A spokesman for the Army's Material Development and Readiness Command (DARCOM), which is responsible for safety conditions at the plant, said yesterday he could not immediately comment on Wodka's allegation.

THe spokesman said that inspectors from the Labor Department's Occupational Safety and Health Administration were refused entry to the Radford plant on Monday because the Army claims safety jurisdiction for the plant.

The refusal to allow OSHA inspectors into the plant was the third such rejection by the Army this year and came despite a ruling by the office of the secretary of defense that OSHA should be allowed to inspect all military facilities operated by civilian contractors.

Federal safety inspectors from OSHA were turned away from the Radford plant after an explosion of 5,000 pounds of nitroglycerine there in January killed two workers and injured six others. They were refused entry again by the Army in May after a fire in the plant injured two more workers.

Wodka said workers at the Radford ammunition plant warned the management of the fire danger before the blaze broke out in late April.

According to federal and union safety experts the Radford plant is one of the most dangerous of the Army's 14 ammunition plants. The facility had one fatal explosion in 1971 and in 1974 it blew up again injuring about 100 persons.

OSHA officials said yesterday that the situation at Radford is part of a larger problem of a reluctance by federal agencies - primarily the military - to allow OSHA inspectors to look into worker safety problems on government facilities. A senior OSHA official said that the dispute may be costing the government as much as $5 billion annually in claims from accidents and property damage.

On Tuesday attorneys for OSHA obtained a federal warrant in U.S. District Court in Roanoke, Va., to be allowed to inspect the Radford plant. An OSHA spokesman said yesterday the warrant was not used becuase the regulatory agency feared it might not apply to a military installation such as Radford.

The Radford plant is owned by the Army but its operation is contracted to Hercules. The OSHA spokesman said that the agency is required to obtain federal inspection warrants in the case of a formal union complaint such as the one by the Radford workers. But he said the warrants apply to civilian plants and may not apply to those owned by the military.

Officials from OSHA and the Army were negotiating yesterday over the Radford situation. Sources said the Army had agreed to allow a one-time inspection of the plant by OSHA Firday.

A military spokesman for DARCOM said, however, that the Army maintained its position that it had jurisdiction over the inspection of Army ammunition plants contracted out to civilian operators. "The Army does not know of any ruling in writing that provides the authority for OSHA to inspect working conditions in Army contract ammunition plants," the spokesman said.