Energy Secretary James R. Schlesinger's contention that an administration bill would cut construction time for nuclear power plants almost in half was challenged yesterday in testimony before Congress.

"I think we need to recognize that this bill may not fully achieve the administration's stated projections," Nuclear Regulatory Commission Chairman Joseph M. Hendrie told the House Commerce subcommittee on energy and power, "particularly in reducing the overall time required to put a new power plant on line. Utility construction delays and financial difficulties are not easily susceptible to a legislative fix."

In testimony delivered by a deputy, Congressional Budget Office Director Alice M. Rivlin said she concluded that the legislation "will have a moderate effect in expediting" nuclear licensing and construction.

"Financing difficulties, reconsideration of electric power daemand, management problems during construction, Ddelivery of poor quality materials and components, will be relatively insensitive to regulatory reforms," Rivlin said. "Most likely, two thirds of the historical delays cannot be amcliorated by the proposed legislation."

In defense of the bill, Deputy Energy DSecretaary John F. O'Leary - while conceding that it might take time for the licensing reforms to take hold - insisted that it would reduce the time to license and construct a nuclear power plant to 6 1/2 years from the present 11 years. This is what Secretary Schlesinger said when he sent the legislation to Congress in March.

O'Leary defended what he called the two key provisions in the bill - approval of a plant site before issuance of a construction permit and standardization in plant design. O'Leary said that together the two provisions will save 4 1/2 years in construction time.

As O'Leary explained it, the legislation would allow electric companies to "bank" as many as 100 sites for future nuclear plants before they even decided to build. The banked sites would be approved by federal and state ragulatory agencies as sutiable locations for nuclear plants.

At the same time, the electric companies would agree on no more than 10 designs for nuclear plants, which could then win the same federal and state approval.

O'Leary conceded that the first plant licensed under the reform legislation might take as long as 10 1/2 years, because it would take four years to do the site banking and another year to license the sites. But the insisted that every plant tthereafter would take only 6 1/2 years to construct.

"It's true that the first time around the time is only rearranged, not shortened," O'Leary said. "But from then on, the licensing time is actually reduced."

O'Leary argued that the nation needs the legislation, even though electric companies right now are not ordering new nuclear power plants. He said one reason for the shortage of orders is that the nation keeps revising downward the demand for electricity.

"We still think the country will order another 200 nuclear power plants out to the end of the century," O'Leary said. "It may be we won't see that many, that the country will build more outlated oil-fired plants at great cost to the consumer and coal-fired plants that pollute the air, but right now we don't think so."

At present 69 nuclear power plants are licensed to operate, with another 89 under construction, 39 under construction permit review, nine ordered and seven publicly announced. If 200 new plants were ordered in the next 10 to 15 years, it would bring the total to 413 by the year 2000.