Rep. Al Ullman (D-Ore.), chairman of the House Ways and Means Committee, yesterday unveiled the details of his proposal for a new value-added tax, designed to replace a major portion of existing income and Social Security taxes.

The plan, which Ullman outlined broadly last month, would impose a 10 percent VAT -- a from of national sales tax that is levied at each stage of the manufacturing process -- on most goods and services sold in the United States.

They levy would bring in a staggering $130 billion a year in a new revenues, which Ullman would use to finance massive reductions in individual and corporate income taxes and in Social Security payroll taxes.

The result would be a major shift in the nation's tax structure toward taxing consumption rather than income and easing the taxation of savings and investment. Ullman contends both changes are needed to cure the nation's economic ills.

Neither Ullman nor staff aids offered any specific forecasts of the measure's impact on the economy.Even those supporting the VAT concept concede there would be major gainers and losers among various segments of the economy.

Ullman himself conceded there would be a "one-time" surge in the inflation rate because the VAT would be absorbed into consumer prices. But he insisted there would not be a disruption in the economy despite the huge shifts.

The 10 percent VAT would be based on the amount that each manufacturer or distributor adds to the value of a product or service. Each firm would pay the full VAT owed and claim a tax credit for that paid at earlier stages.

As in the case of a sales tax, the levy ultimately would be passed onto the consumer. However, under a VAT, the tax would become part of the sales price. As a result, consumers would not know what portion of the price is tax.

To ease the burden on the poor, Ullman would impose only a 5 percent VAT on retail sales of basic necessities such as food, housing and medical care. Staff experts said these amount to 40 percent of total consumption.

However, there would be only a handful of exemptions from the VAT: The tax would not be levied at all on farm poducts, commericial fishing, mass transit, public education, charities, exports and interest charges.

In exchange for enactment of the VAT, Ullman proposed a series of major reductions in income and Social Security taxes and a spate of new tax incentives that he said would spur savings and increase available capital.

The measures include:

A $52 billion cut in Social Security taxes that would reduce payroll tax rates for 1981 and later by 2.15 percentage-points both for employers and workers -- to 4.5 percent in 1981 from the 6.65 percent now scheduled.

$47 billion in income-tax reductions for individuals, with major rate cuts in the lower and middle brackets, an increase in the standard deduction, an expansion of the earned income tax credit for the poor and cash payments to retirees.

Ullman also would limit the maximum income tax rate, charged on nonwage income, to 50 percent, rather than the present 70 percent. And the minimum tax would drop to 10 percent, from 14 percent now.

22 billion in tax cuts for business, primarily by paring the top corporate income tax rate to 36 percent, from 46 percent now Ullman also would reduce further the capital gains tax paid by business.

$9 billion in new tax incentives to encourage investment and "capital formation" -- including deferral of taxes on savings new exclusions for stock income, and faster depreciation writeroffs for business.