WASHINGTON--The chairwoman of the U.S. Nuclear Regulatory Commission said Tuesday that she plans to step down in January, after a two-year run in which she sought to calm the agency amid clashes between her predecessor and the other commissioners.

Allison Macfarlane, appointed by President Barack Obama as NRC chairman in 2012 after a period of tumult on the commission, said in an email to staff Tuesday that she plans to leave the agency effective on Jan. 1 and join George Washington University as director of the school's Center for International Science and Technology Policy. Ms. Macfarlane, a Democrat, became chairman in July 2012 after the previous chairman, Gregory Jaczko, resigned after clashes with the agency's four other commissioners over a variety of issues, including Mr. Jaczko's leadership style and how he managed NRC's response to the 2011 Fukushima Daiichi nuclear disaster in Japan.

"I came to the Commission with the mission of righting the ship after a tumultuous period for the Commission, and ensuring that the agency implemented lessons learned from the tragic accident at Fukushima Daiichi, so that the American people can be confident that such an accident will never take place here," Ms. Macfarlane said in her email to NRC staff. "With these key objectives accomplished, I am now returning to academia."

Before her NRC post, Ms. Macfarlane was a professor at Virginia's George Mason University specializing in nuclear waste.

The NRC is an independent federal agency that regulates the nation's nuclear-power industry, which generates about 20% of the nation's electricity. The commission typically operates with five commissioners, including the chairman.

It wasn't immediately clear who would succeed Ms. Macfarlane as chairman or be tapped as the fifth commissioner. The White House may appoint one of the current four commissioners--two of whom just assumed their posts in September--as chairman, or it may appoint a new nominee as chairman. The nominee, which requires Senate confirmation, is expected to be a Democrat.

The White House didn't immediately respond to request for comment.

Whomever Mr. Obama appoints as chairman will likely receive scrutiny from Senate Majority Leader Harry Reid (D., Nev.), who opposes Yucca Mountain, a long-proposed nuclear-waste repository site in Nevada that Mr. Obama put on ice when he became president in 2009. The NRC has authority over its licensing process, which has been largely stalled the last five years. As majority leader of the Senate, Mr. Reid has ensured any chairman of the NRC doesn't support the project.

His influence could be weakened next year if the Senate flips to Republican control after November's elections, indicating Democrats could try to push through a new nomination in the lame-duck session of Congress between Election Day and next year.

Ms. Macfarlane, a geologist by training, wrote a 2006 book in which she raised technical concerns about the Yucca, while avoiding a clear pro-or-con position on the project.

Two other Democratic commissioners the Senate confirmed in September, Jeffrey Baran and Stephen Burns, have avoided public comments on Yucca Mountain in their confirmation hearings over the summer.

Write to Amy Harder at amy.harder@wsj.com

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