Western Electrochemical Co. has begun gearing up its new plant to produce a critical rocket fuel ingredient in short supply since an explosion leveled a Nevada plant a year ago.

Western Electrochemical Co., formerly known as PEPCON, this week started up the electrical and water systems at the $73 million plant 15 miles west of Cedar City, said Fred D. Gibson, president of American Pacific Corp., the Las Vegas-based parent company.The plant is expected to begin manufacturing ammonium perchlorate, an oxidizing agent used in solid fuel for military rockets and space shuttle motors, by mid-June, he said. Earlier, officials had said they hoped to begin production by May 15.

"We expect to commence electrolytic production of sodium perchlorate (an intermediate chemical) in the last two weeks of May, and to produce ammonium perchlorate on or before June 9, 1989," Gibson said.

The new, 19-building complex replaces the old PEPCON plant near Henderson, Nev., which was flattened by explosions on May 4, 1988. The blasts killed two people and injured more than 300.

The loss left the nation's defense and space programs with only one domestic source of ammonium perchlorate.

Company officials have said the plant will have an initial capacity of 30 million pounds of ammonium perchlorate a year and can be expanded to 40 million pounds.