The International Monetary Fund announced yesterday it finally is ready to use a special $9.9 billion "supplementary" loan fund approved 18 months ago to help nations with especially large balance of payments deficits.

The loan fund became effective last Friday after a $64 million contribution by the Austrian National Bank added enough to previous pledges to reach the minimum money pool needed to trigger the new operation.

The Imf/ said no nation has applied for a loan under the new program yet, but outside sources say they expect to see drawings soon by some hardpressed countries, such as Peru, Turkey or possibly Zaire.

The loan fund, known as the Witteveen Facility after its major advocate, former IMF managing diector H. Johannes Witteveen, was created in August, 1977, to aid countries which already had overstepped their credit limits.

The new money pool is being underwritten with loans from a combination of oil-exporting countries and major industrial nations. The U.S. pledge is $1.86 billion. Saudi Arabia's is $2.5 billion.

Austria's contribution last week raised the total money pool available to just over the $9.9 billion minimum needed to put the lending plan into effect. Before that, the pool was about $46 million short.

The loan fund is designed to help countries that already have exhausted their available credit as IMF members. The money is made available on condition that governments take steps to bolster their economic health.