In particular he cited an effort focused on the waste treatment and immobilization plant at the sprawling Hanford complex in Washington state, saying DOE has been working with a restructured environmental management team  and has invited state officials "to join in our frank discussions." He added that "the DOE team is rebuilding trust that thad broken down over the last decade."

Senate Energy and Natural Resources Committee Chairman Ron Wyden (D-Ore.) said in a statement that Chu "did a lot of good in his time at the Dept. of Energy.” Wyden added, “As President Obama looks at potential successors, I hope he keeps in mind the importance of encouraging innovation in new energy technologies, safeguarding taxpayer dollars and continuing the manufacturing revival spurred by stable natural gas supplies.”

Rumored candidates to succeed Chu include 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 MIT Physics Professor and former DOE under secretary Ernest Moniz.

Before becoming DOE's chief, Chu led the department's Lawrence Berkeley National Lab and also taught at the University of California. In his letter, Chu said he had told President Obama shortly after the election that he and his wife "were eager to return to California." Chu and added that he wants to "return to an academic life of teaching and research, but will still work to advance the missions that we have been workign on together for the last four years."