Radioactive water leaked out of Michigan's oldest nuclear power plant last week. Yesterday, state officials described the Consumers Power Co. leak as "a minor emergency" that posed no danger to residents or the nevironment.

The company said the water was confined to the building. The 17-year-old boiling water reactor is at Big Rock, 10 miles north of Charlesvoix.

The state's chief of environmental and occupational health, Lee Jager, said that the company didn't notify state and local officials of the Friday leak, but that they were informed by the federal Nuclear Regulatory Commission that evening.

"As far as we can determine," Jager said, "there was absolutely no release" of the radioactive coolant into the environment. "It is a minor emergency in terms of significance to the people who live in the area of the plant," the state official said. "There have been tens-if not hundreds-of similar incidents in the state of his magnitude."

Big Rock now is in "cold shutdown," the company said, with no danger of outside radiation leakage, and will remain shut indefinitely. Engineers from General Electric Co., which built the plant, are to confer today with the utility staff.

The reactor, which had not been at full capacity since Feb. 2, has been rated below average for safety by the NRC. Problems last year reduced its electric output by more than one fourth, NRC said.

On Wall Street yesterday, Standard & Poor's lowered its ratings on two securities of Metropolitan Edison Co., operator of the stricken reactor on Three Mile Island near Harrisburg, Pa.

S&P cited both the likely financial impact of the March 28 accident there and the uncertainties arising from a Pennsylvania Public Utilities Commission decision to suspend a $49 million rate increase previously awarded the company.

Met Ed is a subsidiary of General Public Utilities Corp., whose stock has lost about one third of its market value since the accident on the Susquehanna River Island.

Over the weekend in Harrisburg, a group of physicians and medical students organized an anti-nuclear drive, saying "the ultimate in preventive medicine is to eliminate nuclear power and nuclear weapons."