The Obama administration is providing seed money for two advanced nuclear reactor designs, part of its effort to keep nuclear power in the nation’s energy mix over the next several decades.

The Energy Department said it would provide up to $40 million each to two companies, X-energy and Southern Company, over about five years to help develop the alternative reactor designs. As a start, the department, which announced the investments last Friday, is giving each company $6 million this year.

Nuclear energy, which produces practically no greenhouse gas emissions, accounts for 19 percent of the electricity produced in the United States. But safety and radioactive waste handling have long been issues, and in recent years abundant, inexpensive natural gas has disrupted the economics of energy production, making some nuclear plants unprofitable. A few plants have been closed and only five new plants are under construction.

The Energy Department has worked to extend the operating lifetime of existing plants for 20 years beyond their original 40-year licenses. Even so, without further licensing extensions, many of the nation’s 99 operating plants will have to shut down by 2030.