Congress has urged the Carter administration to warn Switzerland it faces the loss of three batches of plutonium if the Swiss don't tighten up the conditions of a sale of machinery to Argentina to make heavy water.

In a letter to Energy Secretary Charles M. Duncan Jr., Rep. Clement Zablocki (D-Wis.), chairman of the House Foreign Affairs Committee, suggested that three Swiss requests to transfer plutonium and spent uranium fuel containing plutonium to other countries "be held in abeyance" until the Swiss change their position on the heavy water sale to Argentina.

In a reply to Zablocki, Energy Undersecretary John M. Deutch said no decision had been made to warn the Swiss on their plutonium requests but noted: "You may be assured that the degree of Swiss cooperation on the Argentine issue will be an important factor in our review of Swiss retransfer requests."

While any threats to deny the Swiss request may not be enough to cancel or even delay the heavy water sale, some people in the administration hope they'll force the Swiss to reconsider the terms of the deal they are on the verge of making with Argentina.

Administration sources said the Swiss are negotiating to supply the Argentines with at least $200 million worth of machinery to make heavy water without insisting on an Argentine pledge to never use the machinery to make atomic bombs.

Heavy water is used to slow down the escape of neutrons in a chain reaction, meaning that natural unranium can be used in a nuclear power plant instead of the enriched uranium that only a few of the world's countries can make. Natural uranium reactors moderataed with heavey water would generate electricity while producing plutonium in their spent fuel.

Switzerland has requested U.S. permission to transfer two batches of spent uranium fuel it owns to Britain and France for reprocessing. The uranium originally came from the United States, which stipulates in any uranium sale to a foreign country that it must approve any transfer of the fuel to a third country.

The Swiss have made a thrid request that is even more sensitive, asking U.S. permission to ship 154 pounds of plutonium to Italy to be fabricated into fuel for the Super-Phenix fast breeder reactor being built in France.

The United States has not granted any of the Swiss transfer requests. In urging that all three be held up indefinitely. Zablocki tied any action on the requests to a change in Swiss attitude about its pending heavy-water sale to Argentina.

"We find it hard to accept the fact that a close cooperating partner, such as Switizerland," Zablocki said in his letter to Energy Secretary Duncan "would acquiesce in an action which could so grievously affect the stability of our neighbors in South America and, ultimately, the security interests of the United States."

Heavy water is the name given a form of man-made water where the lighter hydrogen atoms are replaced by the heavier isotopes of hydrogen called deuterium. Chemically, heavy water resembles ordinary water, but physically it is quite different. It boils and freezes at a higher temperature.