Federal law prohibits government employees from taking part in matters that they know could financially benefit them or anyone with whom the employee is negotiating or seeking employment. But according to an inspector general’s report on the case, Mr. Merrifield sought employment with not just Shaw but also General Electric and Westinghouse, both nuclear reactor makers, while still voting on two issues that affected them.

The conflict-of-interest case — which also included an allegation that Mr. Merrifield failed to disclose, upon departing the government, that he accepted travel reimbursements of $3,552.47 during his job hunt — was referred by the N.R.C. to the Justice Department for possible civil action and to the United States attorney’s office in Maryland for potential criminal action. Both offices declined to pursue it.

Mr. Mulley, who took part in the investigation, was outraged. “Even if the lawyers don’t want to go after him, the N.R.C. could make an example of him if they wanted to,” he said. “They could speak out in some way. But they don’t.”

In a statement last month, Mr. Merrifield said he told investigators and prosecutors that he did not believe, based on legal advice, that he had acted inappropriately, but that if he had been told a conflict existed, he would have recused himself. He added that when he was alerted to the disclosure oversight, he immediately filed the correct forms.

“Though the antinuclear community continues to try to raise these concerns,” Mr. Merrifield said, “I firmly believe that throughout my time as an N.R.C. commissioner, I acted in a fair and impartial manner and in the best interest of public health and safety.”

Other commissioners have also had close ties to the industry.

Environmental groups and industry monitors were angered, for example, when Mr. Obama nominated William D. Magwood, a former employee of Westinghouse Electric and more recently director of the Energy Department’s nuclear expansion program, to fill a vacant seat on the commission last year.

“Given his more than a dozen years promoting nuclear power, we do not believe Mr. Magwood has the independence from the nuclear power industry, nor the security oversight background, to regulate it,” said Danielle Brian, executive director of the Project on Government Oversight.