In a sharp rebuff to President Carter, the Nuclear Regulatory Commission voted unanimously yesterday to disapprove the shipment of 38 tons of enriched uranium fuel to India.

In a 4-to-0 vote, the commissioners said they could not by law approve the exports because India has refused to sign the treaty prohibiting the spread of nuclear weapons, has refused to open up its nuclear facilities to international inspection and has not ruled out the possibility that it will explode further nuclear weapons. It tested one underground six years ago.

"Since Indira Gandhi has again become prime minister of India, no progress has been made" toward agreement on any of these three issues, NRC Chairman John Ahearne said yesterday after the vote, "and Prime Minister Gandhi has not ruled out the option of so-called peaceful nuclear experiments should this be considered to be in India's interests."

The NRC vote sends the issue to the White House, where President Carter has already indicated he will approve the shipments by executive order. Once he does so, the Congress has 60 days to approve a joint resolution against it. If it does not, the shipments may then go.

Early indications are that there will be stiff opposition on Capitol Hill. An aide to Sen. Alan Cranston (D-Calif.) said he expected the senator to lead a fight against the shipments if Carter approves them and the issue is put to Congress.

"We see lots of opposition to shipping uranium to India," the aide said. "In our opinion, it's going to be very messy."

Before the NRC vote yesterday, Carter administration sources said the White House would approve the shipments because of the situations in Iran and Afghanistan that have destabilized much of south Asia. Sources said the shipments would be approved to strengthen U.S. ties to India.

"It is an unfortunate accident of history that these license applications have come at a time when the international situation is thought to require a serious compromise off our long-standing security objective of preventing the spread of nuclear weapons," NRC Commissioner Victor Gilinsky wrote in a separate opinion yesterday.

"It would be even more unfortunate, however, if the decision to except India from this central provision of the Nuclear Non-Proliferation Act were made without a full understanding of the price we may be forced to pay," Gilinsky said.

The 38 tons of uranium requested by India represent a two-year supply of fuel for its nuclear power station at Tarapur, north of Bombay. India already has 50 tons of U.S.-supplied fuel in inventory for the station, which means the additional shipments would give India nearly a five-year supply.

India has been receiving U.S. uranium for Tarapur for more than 10 years, a fact that Gilinsky noted with some concern in the opinion he wrote yesterday.

"India has made it clear that if there is any halt or even lapse of fuel shipments to Tarapur," Gilinsky wrote, "it will consider itself free of the contractual obligation of the original agreement for cooperation [with the United States] and at liberty to reprocess as it sees fit the 200 tons of spent fuel it already holds hostage.

"It has not excluded making explosive use of the more than one ton of plutonium that can be reprocessed and separated from this U.S.-supplied fuel."

The nuclear facilities that India has refused to open up to inspections include a research reactor and two reprocessing plants at Trombay. Plutonium for India's first nuclear explosion was extracted from spent fuel taken from the research reactor. The research reactor has operated for more than 10 years and has presumably made enough plutonium in that time for at least three more bombs.

An agreement signed more than 10 years ago stated that India would not reprocess any of the U.S.-supplied fuel without U.S. consent and would not extract any plutonium from this fuel under any conditions.