Prime Minister Margaret Thatcher's government is planning a major expansion of Britain's nuclear energy program designed to produce as much as half the country's electricity through nuclear power by the 1990s. Although no formal decisions or announcements are expected for several weeks, both Thatcher and her energy secretary, David Howell, have made clear their commitment to nuclear energy expansion in public speeches and private discussions.

At last week's Conservative Party convention at Blackpool, Howell said he favored greatly expanding Britain's nuclear generating capacity and was "taking urgent steps" to help gear up the country's nuclear construction industry to build many more reactors.

The government is seriously considering proposals that most of the new reactors be pressurized water-cooled reactors like the Three Mile Island reactor near Harrisburg, Pa. The widely publicized nuclear accident there earlier this year raised new questions about nuclear safety around the world.

Proponents of pressurized water reactors have argued that they can be built more quickly and cheaply, and operated more efficiently, than Britain's advanced gas-cooled reactors, which could not suffer an accident like that at Three Mile Island. Howell's predecessor as energy secretary in the last Labor government, Anthony Wedgewood Benn, warned before leaving office that American and British businessmen were strongly lobbying the government on behalf of the pressurized water-cooled reactor, which Benn said was made suspect by the Three Mile Island accident.

Britain's nuclear energy program was the first in operation in Europe after World War II and is still the largest in the total amount of power generated by nuclear reactors -- more than 8,000 megawatts, or about 13 percent of Britain's electricity.

But it soon will fall behind France, which is well into Europe's biggest nuclear energy expansion and expects 60 percent of its electricity to be nuclear by the end of the 1980s. On one of her first trips abroad after becoming prime minister last May Thatcher toured a nuclear plant in France and brought back to her Cabinet an enthusiastic report on the French nuclear program.

Similarly ambitious nuclear energy expansion plans in West Germany and Sweden have been hampered by anti-nuclear environmentalists through protest demonstrations, court suits and political action.

Swedish voters, who are almost evenly divided on nuclear power, according to public opinion polls, will decide in a refrendum next March whether that country's nuclear energy output should be doubled, largely by using reactors already built but which have not been allowed to become operational. The Danish government also has agreed to hold a referendum before deciding whether to start a nuclear energy program there.

In Britain, as in France, environmentalists have not yet succeeded in building an effective antinuclear lobby, despite scattered protests and two serious reactor accidents over the years. Howell recently began an extensive publicity campaign to convince any wavering Britons that nuclear power is safer, cheaper and more reliable than electricity generated with coal or oil.

Although Britain now gets enough oil and gas from the North Sea to meet all its energy needs, it prefers to sell more than half of it to other nations to gain more revenue. It currently expects its North Sea reserves to have mostly run down by the end of the century.

Britain now has 11 major nuclear reactors generating electricity with three more under construction, according to government energy officials, as well as a number of research and other specialized reactors supplying small amounts of power. It also has a prototype fast breeder reactor that produces plutonium.