When Indian Prime Minister Morarji Desai sits down this afternoon with members of the Senate Foreign Relations Committee and House International Relations Committee, he will discuss the U.S. Nuclear Regulatory Commission.

Why the Nuclear Regulatory Commission? Because by a 2-to-2 tie vote last month, the NRC held up the shipment of 17,000 pounds of uranium destined for India's biggest nuclear power station. President Carter overturned that decision and approved the shipment but because the NRC denied the export the Congress is now in the act. Congress can block the Indian shipment if it is not satisified with Desai's explanation of how the uranium will be used.

Desai's unusual appearance before Congress to appeal the NRC vote is one measure of the rapid rise of the Nuclear Regulatory Commission. In its 3 1/2 years of existence, the NRC has managed to become one of the busiest and most controversial regulatory bodies in the federal government.

The NRC employs 2,800 persons and spends $331 million a year. It regulates the generation of 49 million kilowatts of nuclear electricity and is supervising construction for 98 million more kilowatts. It licenses nuclear exports to most of the countries of the world, including the Soviet Union and the People's Republic of China.

Congress has legislated the NRC to respond to some of the most serious challenges of the day. Is nuclear power safe and can it be made safer Can radioactive wastes be safely disposed of for periods up to 100,000 years? Will the NRC guarantee that U.S. uranium will never turn up as plutonium in another nation's atomic bombs?

Its role in nuclear exports is clearly the NRC's most controversial. The export to India is the only one the NRC has denied, but it has delayed shipments to South Africa, Brazil, Spain, the Philippines and Yugoslavia.

The NRC is sitting on a shipment of uranium that Canada said it would use to power its first nuclear ice-breaker on grounds that Canada's government has not yet decided whether the icebreaker will be nuclear-powered. It has delayed shipment of the highly enriched uranium (the same kind used in weapons) to Iran for use in a research reactor because the Iranian government has yet authorized construction of the reactor.

Ahead of the NRC are votes on whether to license exports to Mexico South Korea, Taiwan, Israel, Bolivia, Greece, Malaysia and Bangladesh. Soon to come before it will be another uranium fuel load for the Indian power station at Tarapur. Spain has filed a request for the export of a complete nuclear power station.

Half these countries have not signed the nonproliferation treaty prohibiting the spread of nuclear weapons. A few like Brazil and South Africa have hinted they plan to extract plutonium from spent uranium and use it as a nuclear fuel. The Carter adminstration is against the use of plutonium because it can be used in atomic bombs.

"There will be some exports proposed or applied for," NRC Chairman Joseph Hendrie said in a recent interview, "that I think will have to be turned down." Why? "Well, if Pottsylvania comes to us for 200 kilos of plutonium and won't tell us what they're going to do with it, you know it's inconceivable that we would turn against a negative recommendation." Would Hendrie elaborate? "Of course not. It would be improper to speak in advance on matters like that."

Though it voted control over nuclear exports to the NRC when it passed the nonproliferation act, Congress still seems divided on the NRC's role in the nuclear export process.

Critics like Sen. James A. McClure (R-Idaho) feel the NRC usurps White House control of foreign policy, but others like Sen. Gary Hart (D-Colo.), chairman of the subcommittee on nuclear regulation, feel differently: "Why shouldn't they have a role? It's the only involved agency with the expertise in the field. You can't separate the international activities of nuclear power from the domestic."

The commission itself is divided on its export powers.In voting to grant the Indian license, NRC Commissioner Richard Kennedy said he did not think the NRC was given a mandate "to make foreign policy." And while he talks of denying future licenses, Chairman Hendrie tends to agree.

"The question arose to what extent that [the Indian vote] was an instrusion into the president's foreign policy prerogatives," Hendrie said. "I feel you get into an area where these things get to be foreign policy matters."

Hendrie and Kennedy voted in favor of the Indian export. Commissioners Victor Gilinsky and Peter Bradford voted against it, bringing about the situation the license is in today. Gilinsky has served on the NRC the longest, Bradford the least. Why did they vote against it?

"Peter and I read the law [the nonproliferation act] that said certain criteria had to be met and they were not met," Gilinsky said in an interview. "Basically, there have to be safeguards on that fuel shipment, on previous fuel shipments and on the reactor and a no-explosives pledge and whether or not reprocessing would be done [to extract the plutonium] only with U.S. approval. And we said, taking a look at what we know, you can't say with confidence that these conditions will be met."

Outsiders insist that the 2-to-2 vote on the Indian export reflects a philosophical division inside the NRC, that Hendrie and Kennedy tend to vote in favor of nuclear power and that Gilinsky and Bradford tend to vote against it.

"There's nobody on this commission who's antinuclear," Bradford said. "I don't know how big the group is in the country who feel flatly that nuclear power is irredeemably a bad idea but Victor and I just don't fit that category . . ."

Of the 192 votes the commission has taken since last July, just seven have not been unanimous. Five of the seven involved a single dissent. Only two were standoffs that involved two dissenting votes. One was the Indian export license; the other came over whether two domestic nuclear licenses should be reviewed.

All four commissioners point out that they often have heated debates before agreeing on some decisions. All four claim they've changed their minds during debates.

"There isn't anything exotic or special about it," Hendrie said. "We're all agreed to accept the responsibility of coming together and bumping heads until we grind out common grounds that can be majority positions."

The months ahead may see changes in NRC voting patterns, if only because tougher decisions will come to a vote. No fewer than 90 nuclear power plants under construction will come up for operating license permits. All 90 are being contested by intervenors.

Nuclear waste disposal decisions will soon be coming to the NRC, in part because spent fuel is piling up and in part because the Department of Energy will soon be reaching decisions on how to store spent fuel wastes. The NRC will be voting on whether to license any new waste disposal methods.

However tough the decisions are, they'll be made a little easier by the imminent addition of a fifth member to the five-seat commission. This week, the Senate subcommittee on nuclear regulation will hold a confirmation hearing on President Carter's nomination of John F. Ahearne to fill the fifth seat.

Two frequent critics of the NRC are all in favor of Ahearne, who is on the staff of Energy Secretary James R. Schlesinger and helped to draft nuclear licensing legislation and spent-fuel policy that the NRC will regulate in the years ahead.

"I'm heartened Ahearne is coming on board," said Rep. Morris K. Udall (D-Ariz.), chairman of the House subcommittee on energy and the environment. "He brings the makings of balance to the commission."

"I've watched him work and his philosophy and temperament make him suitable," said Rep. John D. Dingell (D-Mich.), chairman of the subcommittee on energy and power. "It's about time they willed that seat."