The Nuclear Regulatory Commission yesterday agreed to allow the reopening next week of the Oconee II nuclear power plant in South Caroline, one of the reactors shut in the wake of the Three Mile Island accident.

Another Oconee unit has completed changes ordered by the NRC and will not have to shut down, while a third that has made the required modifications will remain closed until refueling is completed in June.

Bill Lee, president of Duke Power Co., which owns the three plants, indicated that the company will treat the entire episode as though it had involved only routine shutdowns for maintenance.

The three units were among eight reactors built by Babcock & Wilcox Co. of Lynchburg, Va., the firm that constructed Three Mile island. The NRC last month ordered all the B&W plants to make alterations in machinery and procedure that would lessen the possibility of another major accident.

Under the agreement negotiated between the NRC and the utilities, one California plant closed immediately and one of the Oconee plants, as Lee put it, moved up its scheduled refueling date so as to close right away.

The other two Oconee plants were to close May 12 and May 19, respectively, if they had not made the necessary changes by those dates. Four other B&W plants, as well as Three Mile Island, were already shut for various reasons at the time the order was issued.

The second Oconee unit due to close, as it happened, was also suffering from a leaking steam condenser pipe. "We were just holding on by our teeth anyway and it got so bad," Lee told reporters with a grin, "that we just had to shut down on Friday." That is, on May 11, the day before the plant would have been required by the NRC to close.

In other words, one plant closed a little early for refueling and the other closed for pipe leak repair. "two minor outages for routine maintenance," according to Lee. The effect on billings to Duke Power's 1.2 million customers in the Carolinas, he said, will be "so small as not to be measurable."

In fact, a spokesman back at Duke headquarters in Charlotte, N.C., announced yesterday that the company hsd actually asked permission to reduce its June bills.

The Oconee plants "ran so well for the last three months that we didn't burn much coal, which is expensive, and our fuel bills were lower than normal," Dick Pierce told the Associated Press. "So we're refunding the extra charges."

If approved by the North Carolina Utilities Commission, Duke's 905,000 customers in that state would receive a credit of not quite two-tenths of 1 cent per kilowatt hour, or $1.93 on a usage of 1,000 Kilowatt hours.

The antinuclear People's Alliance of North Carolina branded the move "further public relations" and a meaningless gimmick.

In recommending that the Oconee plants be allowed to reopen, the NRC's reactor regulations director, Harold Denton, told the five commissioners that his office had insisted that Oconee operators pass tests on the new procedures with grades of 90 percent rather than the 70 percent proposed by the utility.

So far, he said, eight operators had passed, enough to allow continued operation of one Oconee plant, while several more are expected to be certified early next week. The same requirement will be made of all the other B&W plant operators, Denton said.