The idea was approved by the Republican Congress in 2005. Senator Lisa Murkowski of Alaska, now the ranking Republican on the Energy and Natural Resources Committee, has praised the Energy Department for issuing the first nuclear loan guarantee, for the Alvin W. Vogtle plant expansion, in Georgia. Senator Mitch McConnell of Kentucky, the Republican majority leader, supports loan guarantees as a step to build 100 new nuclear reactors.

One reason for all the financial support may be the way Congress does its accounting. The guarantees cost little or nothing to approve. “It’s not real money,” Mr. Wilmshurst said.

A federal loan guarantee is a little like a parent co-signing a child’s car loan; if the child makes the payments, the parent pays nothing. Under the 2005 law, borrowers pay a lump sum to the government to compensate the Treasury for the risk it is undertaking, and if the companies finish the projects and can pay back the loans, the government makes a profit.

The precise shape of new loan guarantees is uncertain, but when “scoring” the provisions for the purpose of calculating their expense, the White House says they cost nothing, and Congress assumes they cost 1 percent of the face value. But they are not without risk.

If the builders default, as happened on some nuclear construction projects in the 1980s, the taxpayer liabilities could run into the billions of dollars.

Officials at the Energy Department, which administers the loans, said they were confident that other developers would come forward and apply for the guarantees. Jonathan M. Silver, the executive director of the loan programs office, said, “There is a significant queue of nuclear power plants in house that we will and are working on.”

“They may just go forward under a different time frame,” he said, but he declined to estimate how many years it would be before the government could reach its goal of providing loan guarantees to six to eight reactor projects.