A nuclear utility considered so risky by investors that it can't raise the money it needs from banks and bonds will be allowed to set customer rates under the highest rate of return ever granted an electric or natural gas utility.

A Federal Energy Regulatory Commission judge has ruled that the Connecticut Yankee Atomic Power Co. is due at least a 17 percent rate of return on equity, compared with an average authorized rate of return for all electric utilities of 15 percent in 1981.

Connecticut Yankee's single asset is a nuclear generating plant, and thus it is viewed as a risky investment, according to Administrative Law Judge Jon G. Lotis, who granted the higher rate. The ruling must be affirmed by the commission.

Since the accident at Three Mile Island in 1979, "investors perceive that they have no security for their investment in a company whose sole asset is a nuclear generating unit," Lotis said.

Connecticut Yankee is located in Haddam, Conn., and owned by Northeast Utilities of Hartford and several investor-owned utility companies in Connecticut. Its customers are the utility companies that own it.

Even a debenture issue carrying a 17 percent interest rate failed to attract investors until the utilities that own the company guaranteed interest and principal payments, he said.

At that, the company was still able to sell only $50 million of its planned $75 million offering.

The rates that the utility was forced to pay prompted Lotis to set the authorized rate of return at the same level, he noted.

"Obviously what the judge is realizing is that equity, since it is higher risk than bonds, ought to carry a rate of return equal to bonds," said Gary Doughty, manager of nuclear information for Northeast Utilities.

Doughty said the increase would have a small impact on rates becauseConnecticut Yankee is a very low-cost producer.

The authorized rate of return acts as a ceiling on the actual rate of return, which generally lags behind. For instance, in 1981 the average actual rate of return was 12.5 percent for electric utilities. Rates paid by consumers are not specifically pegged to the authorized rate of return, but as it rises, they rise.

"In a general sense it's a very encouraging development," said Carl Goldstein, assistant vice president of the Atomic Industrial Forum. "As utilities realize better return on equity, they are able to borrow at lower rates and that will benefit consumers," he said. "It's the kind of relief we need and the kind of relief that will ultimately lower rates for customers."

Richard Udell of Public Citizen's Critical Mass project had a different view. "It appears as if FERC is going to enter into a conspiracy against the interests of American consumers to subsidize nuclear power," he said.

Udell said that the plant is one of 13 that the Nuclear Regulatory Commission identified in September as being in danger of shutting down within the next few years because of a brittle reactor vessel. That problem results from premature aging of the vessel that holds the reactor core caused by neutron bombardment. He also said it is one of 40 that have experienced some type of steam generating problems.

Pepco is authorized to have rates of return of 14 1/4 percent in D.C., 15 3/4 percent in Maryland and 13 1/2 percent in Virginia. Washington Gas Light is authorized to get a 15.9 percent rate of return in D.C. and 13 1/4 percent in Maryland. Vepco is allowed a 15 1/2 percent rate in Virginia