The Pentagon and NASA have agreed to pay for replacing a plant that produced a key rocket fuel element. The plant, in Henderson, Nev., was destroyed last month by fire and explosions.

NASA and the Pentagon also plan an addition to the Henderson plant of another company that makes the chemical, officials said Tuesday.It is estimated the new plant for Pacific Engineering and Production Co. and the addition for Kerr-McGee Chemical Co. will cost from $90 million to $95 million, said Lt. Col. Rick Oborn, a Pentagon spokesman. Company officials have been looking at Utah as a possible site for the new plant.

The Pacific Engineering facility was leveled by a powerful blast May 4 after a fire, believed fed by a broken underground gas line, started in a chemical mixing room. The blast killed two workers and injured more than 300 people. Damage was estimated at $74 million.

The two government agencies agreed Friday to guarantee each company annual purchases of 20 million pounds of ammonium perchlorate for the next five to seven years, Oborn said.

The sales will be "fully amortized over the five to seven years" to give Pacific Engineering sufficient money to build a new plant with an annual capacity of 30 million pounds to replace the destroyed plant, Oborn said.

"In effect, it's a surcharge," he said.

The money will allow Kerr-McGee to build an additional plant with a 20 million-pound annual capacity, Oborn said.

Ammonium perchlorate is an oxidizer used in the solid-fuel rocket motors on such missiles as the Air Force's MX and Minuteman ICBMs and the Navy's Trident submarine-launched missile, as well as the space shuttle and other NASA launch vehicles.

Oborn, in an interview with the Los Angeles Times, said Pacific Engineering and Kerr-McGee were unlikely to build the new ammonium perchlorate plants in Henderson, a suburb of Las Vegas. Pacific Engineering officials earlier had said that they were looking at possible sites in Texas, Utah or more remote regions of Nevada. Kerr-McGee declined to comment about its plans.

Oborn said it would take at least a year to build the new plants.

At the time of the fire, Pacific Engineering was the only producer of ammonium perchlorate for the Pentagon, which then had to shift to Kerr-McGee. Officials said the Pentagon had enough rocket motors on hand to fill its missile requirements for the next year, but they were worried about the long term.

The New York Times, quoting military and space agency officials, said impending shortages are expected to become severe next year and would remain a problem into the 1990s.

The loss of the plant cut in half the U.S. capacity of ammonium perchlorate, which constitutes 70 percent of the ingredients of solid fuel in rockets, the officials said.

Kerr-McGee's existing plant, which is down the road from the burned-out Pacific Engineering plant, has a capacity to produce about 40 million pounds of ammonium perchlorate, he said.

Oborn said the Pentagon and NASA this year requireslightly more than 50 million pounds of the chemical and project a 1989 requirement of 54 million pounds.

Oborn said the Pentagon anticipated the companies would meet "all government requirements by the middle of next year. We're looking at getting well by mid-1989."

An immediate shortage of ammonium perchlorate is therefore anticipated, he said, and NASA and the Pentagon will have to decide in the next few weeks which missile and space booster programs have first call on the key element.