WASHINGTON -- As Washington considers closing a profusion of tax breaks in its quest to reduce record budget deficits, a new study suggests that some targets might produce far less cash than is commonly thought.

Tax preferences for retirement plans, such as 401(k) accounts, rank among the biggest money losers for the federal government, with official projections showing a loss of about $600 billion to federal coffers over the next five years. But a study set for release this week suggests that the accounts would have to be closed entirely, or contributions sharply limited, to realize much in the way of budget savings -- a move that could undermine some of the nation's most popular retirement tools.

"Given the questions about the future of Social Security, this is no time to be making bad policy decisions about private savings," said Brian Graff, executive director of the American Society of Pension Professionals and Actuaries, which conducted the study.

The ASPPA analysis highlights the murky math surrounding hundreds of tax breaks that collectively deprive the U.S. Treasury of more than $1 trillion each year.

The most expensive tax breaks from the government's standpoint are the tax-free treatment of employer- provided health benefits and the mortgage-interest deduction for homeowners. Those two provisions alone allow taxpayers to keep nearly $300 billion a year that would otherwise be collected in taxes.

Although that figure gauges the value of the policies to taxpayers, it says nothing about how much of that cash could be recovered through repeal or reform, tax experts say. For example, if the mortgage interest deduction were eliminated, people would probably shift their investments to other tax-preferred vehicles, thereby denying the taxman a portion of his expected rewards, said Ed Kleinbard, a former director of the congressional Joint Committee on Taxation.

The estimates are "an accurate measure of the extent to which the preference is being used by taxpayers," said Kleinbard, who teaches law at the University of Southern California. "But that's a completely different question than how much revenue could be raised if the preference were eliminated."

Tax preferences for retirement savings are particularly tricky, Kleinbard said, because they are not typically straight-forward tax breaks, whose entire value is realized each year. In the case of 401(k) savings accounts, for example, taxpayers merely defer taxation on contributions until they retire. In the meantime, they get to accumulate interest on their contributions tax-free.

To determine the loss to the government, the Joint Committee on Taxation and the Treasury look at taxes lost each year due to new 401(k) contributions, as well as the interest earned on existing contributions, and then subtract taxes paid on distributions from 401(k)s. The difference ranks 401(k) plans as the third most expensive provision on the administration's list of tax expenditures, projected to deprive the Treasury of an estimated $67 billion in 2012.