Prospects for a Sino-American nuclear cooperation agreement improved significantly in the wake of Chinese Premier Zhao Ziyang's unequivocal statement: ''We do not engage in nuclear proliferation ourselves, nor do we help other countries develop nuclear weapons.''
China has implied as much previously, but Western diplomats here say Mr. Zhao's remark, made at President Reagan's state banquet in his honor Tuesday, is the clearest Chinese statement to date on this subject.
Cooperation for the peaceful uses of nuclear energy is one promising area of the burgeoning Sino-American relationship. But it has been hampered by American suspicions - indignantly denied by the Chinese - that Peking has been helping such countries as Argentina, Pakistan, and even South Africa develop nuclear weapons.
Zhao's speech said nothing about these suspicions, but it was obviously aimed at refuting them as categorically as possible by making clear what had only been implied before. China is one of the five members of the select nuclear weapons club, having exploded its first atom bomb in 1964. (The other club members are the United States, the Soviet Union, Britain, and France.)
China's nuclear weapons and rocket development program continues apace, with no help from any other power. But in the field of nuclear energy for generating electricity, China has so far lagged behind the world's major industrialized powers, and even some other developing nations. China is only now building its first 300,000 kilowatt nuclear power plant at Qinshan in the greater Shanghai area. Another larger plant with 900,000 kilowatts of generating capacity is to be built, probably with French and British cooperation, at Daya Bay near Hong Kong.
China's ambitious modernization program requires the quadrupling of electric power supply by the end of this century. But central and south China are short of coal and oil. Chinese experts note that the Daya Bay plant will need only 25 tons of nuclear fuel per year, whereas it would require 3 to 4 million tons of coal, all of which would have to be transported long distances on the country's overloaded, inadequate railway system. The prospects for nuclear power, therefore, are bright. Because China's own technology in this field is still in its infancy, American, British, French, West German, and Japanese manufacturers are all vying with one another to supply the required machinery, technology, and parts.
Framatome, the French company considered most likely to win contracts to supply the Daya Bay project, uses technology that originally was licensed from Westinghouse. China has been discussing contracts with General Electric and other American companies as well.
Yet the Americans compete at a disadvantage because of the restrictions placed on them by the nuclear Non-Proliferation Act of 1978. This act was passed by Congress during the Carter administration at a time when Washington was highly concerned about the diversion of nuclear materials intended for peaceful purposes to the making of weapons. Companies may not export technology to countries that do not meet the act's requirements.
For over a year, Washington and Peking have been discussing a nuclear cooperation agreement that would satisfy the provisions of the 1978 act. After three intensive rounds of talks, they are reported to be close to agreement. But the remaining points at issue are said to be delicate and complicated.
In Mao Tse-tung's day, China opposed the whole concept of nonproliferation as showing the arrogance of the superpowers. Zhao's Washington speech shows how far China has moved from those days, even though it continues to oppose the Non-Proliferation Treaty itself as discriminatory.
The speech does not affect the Sino-American talks per se, but it will certainly help the general atmosphere surrounding them.
Both sides hope to reach an agreement in time for President Reagan's visit here in April.