HARRISBURG, Pa.--Engineers at the Three Mile Island nuclear power plant substantially shrank the potentially explosive gas bubble in the plant's reactor yesterday. The bubble, once as large as 1800 cubic feet, was reportedly reduced to 50 cubic feet.

Plant and federal officials said the bubble is now a much safer size and the reactor's temperature has dropped significantly. Harold Denton, the Nuclear Regulatory Commission's (NRC) chief of operations said yesterday that only two out of 177 fuel cells in the reactor were still overheating.

No More Passing Gas

George Troffer, an official with Metropolitan Edison, the plant's operator, said yesterday the company has halted the radioactive gas emissions at the site.

The "dramatic decrease" in the size of the gas bubble "is cause for optimism," Denton said, adding that radiation levels in the vicinity of the reactor were low and confined to a small area.

The operators of the plant reduced the size of the bubble by letting the gas dissolve in the reactor's circulating coolant system, then removing them in a chamber outside the reactor.

Despite the improvement of the situation at the plant, local civil defense officials yesterday continued to review plans for a precautionary evacuation of 25,000 residents within a five-mile radius of the reactor.

Denton said, however, he believes conditions at the Three Mile Island have improved enough that an evacuation is unwarranted. Unofficial sources in Harrisburg said yesterday there are no definite evacuation plans in the foreseeable future.

Schools within a ten-mile radius of the plant were closed yesterday and some state government offices reported that up to one-third of their employees stayed home.

The NRC also began actions yesterday to avoid future cooling system breakdowns in other nuclear plants around the country. The commission ordered officials at seven nuclear plants to explain their plans to prevent accidents similar to the one at Three Mile Island last Wednesday.

All seven plants were designed by Babcock and Wilson, the firm which designed the Harrisburg reactor.