London — * Only one-eighth of all electricity in the United States is generated by nuclear plants.
* Memories of Three Mile Island, films such as ''The China Syndrome,'' detailed regulations, and inflation have lengthened the time between the first blueprints and opening ceremony for a nuclear plant to an average of 12 to 16 years.
* Falling demand for energy since 1978 led to 18 proposed new plants being cancelled in 1982 alone.
But this picture of slowdown does not tell the whole story worldwide, according to a new report and estimates emerging on this side of the Atlantic.
Advocates of nuclear-generated electricity as one replacement for expensive oil agree their cause has suffered sharp setbacks. Many worry about hazards. Start-up costs are very high. But they say as fast-breeder reactors are developed, the nuclear-power industry will continue to do much better than their critics predict.
''Look at the US and you conclude that nuclear power is dead,'' says Prof. Wolf Hafele of West Germany's Nuclear Research Institute in Julich. ''But that's a wrong conclusion. Growth in nuclear-generated power has been steady since 1970 , and it will continue despite difficulties.''
Part of the reason for the slowdown in the US is extreme environmental concern, and part is its system of strict siting and engineering regulations.
West Europeans also face complex regulations, and Western industrial nations have had to lower their forecasts of nuclear capacity on line by 1990. However, European regulations are not as demanding as those in America. And in the East bloc, the Soviet Union, which is pushing ahead with nuclear plants, tends to subordinate environmental concerns to the national need for more power in the populous western areas of the country.
While the antinuclear lobby remains strong - especially when it comes to weapons - in West Germany and elsewhere, Professor Hafele and other Europeans point to these developments:
* Electricity generated by nuclear plants in the major noncommunist industrial countries rose by 8.3 percent last year, according to figures just released by the Nuclear Energy Agency (NEA) of the Organization of Economic Cooperation and Development (OECD) in Paris.
The share of electrical power now generated by nuclear fuel by member states of the OECD is now 14.8 percent. It is expected to rise to 24 percent by 1990 and 29 percent by the year 2000. A new NEA report says that ''despite problems in various countries and cancellations of reactors, the contribution from nuclear power is steadily growing.''
* Five OECD states now generate more than one-quarter of their electricity from nuclear fuel. These are: Finland (40.3 percent), France (38.7 percent), Sweden (38.6 percent), Belgium (30.2 percent), and Switzerland (28.2 percent).
The figures, also from the OECD, show that 229 operable nuclear reactors were located in the OECD countries last year, 15 more than in 1981. Total installed capacity was 147 gigawatts (1 gigawatt equals 1 million kilowatts). Under construction were 149 plants with a capacity of 151 gigawatts, and 27 more were on order.
* Worldwide, the decade of oil price rises (1971-1981) saw nuclear electricity production shoot up by almost 600 percent, gaining a 5 percent share of total energy requirements, according to the 21-nation International Energy Agency (IEA) based in Paris.
Oil's share in total energy demand dropped from 50.9 to 45.5 percent, a new IEA report shows, while coal demand jumped by 22.4 percent, and natural gas by 12.9 percent. Hydroelectricity production increased by 26.9 percent.
* Professor Hafele, speaking at a two-day energy seminar at the University of Surrey in Guildford, Britain, said that by the year 2030, nuclear power could be providing one-quarter of world energy requirements.
He said that despite public alarm, the safety of nuclear plants was a spent issue. The plants, he asserted, were ''absolutely safe'' when used to generate electricity. At the same, time he stressed that more innovative work had to be done to reduce pollution generally.
* In the communist bloc, the Soviet Union leads the way in giving priority to nuclear power plants. The Kremlin sees nuclear power as a key answer to the problem of having 80 percent of its energy resources in the eastern half of the USSR, while 75 percent of the population lives in the west. (About 40 percent of Soviet rail freight consists of energy traveling from east to west.)
In a lecture to the International Atomic Energy Agency (IAEA), Boris Semenyov , a deputy director of the IAEA and head of its department of nuclear energy and safety, said that 6.5 percent of Soviet electricity came from nuclear plants in 1981. By 1985, the amount is scheuled to reach about 14 percent in the USSR as a whole, and 24 percent in the western region of the nation.
Mr. Semenyov conceded that capital-investment costs of nuclear plants are twice as much as for plants using organic fuel, but operating costs were ''competitive.'' In Eastern Europe, he said, four pressurized water reactors were operating in East Germany, four in Bulgaria, and two in Czechoslovakia. Also, Hungary has just commissioned its first reactor, and Poland and Cuba are now building reactors.
The pros and cons of nuclear-power plants are currently under scrutiny in West Germany. And in Britain, the spotlight is on a long public hearing being held about plans to build a $1.8 billion US-designed pressurized water reactor at Sizewell, on the Suffolk coast.
The state-owned Central Electricity Generating Board (CEGB) and its chairman Sir Walter Marshall are firm advocates of nuclear power for Britain's future. Some 80 percent of electricity here comes from coal, which is vulnerable to strikes by the coal miners' union. The CEGB already has excess generating capacity thanks to falling demand, but claims the US model will be 25 percent cheaper to build than Britain's own advanced gas reactors.
Critics reply the Sizewell reactor will replace only 3 to 4 percent of the coal Britain now uses. They point out that estimates of whether Sizewell will actually save any money in the next few decades depend on controversial forecasts of demand and fossil fuel prices over the next half century.