Energy Secretary Steven Chu has announced that he will leave his post by the end of February or when a successor is approved and in office.

Chu, who announced his decision on Feb. 1, has led the Dept. of Energy since early 2009, a period during which several U.S. regions experienced the start of a natural-gas boom. At DOE, Chu, a Nobel Prize-winning physicist, oversaw energy-efficiency block grants and the growth of the Advanced Research Projects Agency—Energy (ARPA-E), which supports high-risk, high-reward technology development.

Chu also oversaw administration of the clean-energy loan guarantee program that came under fire when one beneficiary of the program—Solyndra, a solar company in Fremont, Calif.—went into bankruptcy.

In a letter to DOE employees announcing his resignation, Chu defended the loan program. "While critics try hard to discredit the program, the truth is that only 1% of the companies we funded went bankrupt. That 1% has gotten more attention than the 99% that have not," he said.

Environmental groups credit Chu with advancing the renewable-energy sector, doubling federal investment.

Chu further said that DOE has made gains in cleaning up contamination at the country's former nuclear-weapons plants but acknowledged "the environmental cleanup projects still have considerable technical and project management challenges."

Among possible candidates rumored to succeed Chu are former Sen. Byron Dorgan (D-N.D.), former governors Christine Gregoire (Wash.), Chet Culver (Iowa), Jennifer Granholm (Mich.) and Bill Ritter (Colo.)—all Democrats—and Ernest Moniz, MIT physics professor and former DOE undersecretary.