When the French Navy finishes renovating the submarine ``Lieutenant'' sometime this year, it plans no fancy christening or public announcement. But the event will be important. The prospect of the Lieutenant's improved nuclear capacity hangs over the Geneva nuclear arms negotiations. Soviet leader Mikhail Gorbachev demands that the French, as well as the British, stop their nuclear modernization programs. Until the issue is resolved, arms control experts doubt that an agreement between the superpowers can be reached about their intermediate nuclear forces based on the Continent.
The Soviets look likely to step up their pressure. Soviet Foreign Minister Eduard Shevardnadze will almost certainly reemphasize the Soviet determination to negotiate a reduction of European nuclear potential separately with the British and French when he visits London next month. His visit was announced Monday.
The British response is predictable. Like the French, they refuse any such negotiations. ``Although this modernization worries the Soviets,'' says Dominique Moisi of the French Institute of International Affairs, ``it is critical to the credibility of our nuclear force.''
The Lieutenant will add significant new punch to the French nuclear force. It will be the second French sub to carry multiple-warhead missiles, and its 96 warheads will equal a third of the total French nuclear submarine arsenal. The new weapons will increase the range of France's sea-launched nuclear strike force from 1,900 miles to 2,800 miles, enabling the French to reach more distant targets in the Soviet Union from relatively safe international waters. The Lieutenant will carry upgraded equipment to reduce noise levels, making it harder to detect.
Mr. Moisi and other defense experts have long considered the land and air components of France's nuclear force vulnerable. A preemptive Soviet strike, they say, would probably destroy the ground-launched missiles on the Plateau d'Albion in southern France and the nation's fleet of Mirage nuclear bombers.
Britain faces a similar problem. Its nuclear force currently depends on German-based Tornado strike aircraft carrying tactical nuclear weapons and four 1960s-vintage submarines. The Conservative government of Prime Minister Margaret Thatcher argues that these arms soon will not be able to penetrate Soviet defenses.
To upgrade their forces, both France and Britain are counting on new submarines armed with modern, multiple-warhead missiles. France's plan is to refit its five older submarines with noise reducers and multiple launchers similar to those on Lieutenant, while finishing two new-generation subs before the end of the decade. In all, French officials say their submarine force will be comprised of 500 nuclear warheads by 1992.
The British are building four new submarines, each about twice the size of their current ones. The submarines will carry US-built and maintained Trident missiles fitted with British-made multiple reentry vehicles. Britain plans to equal France's strength by the early 1990s.
Such firepower worries the Soviets. Since France and Britain are US allies, the Kremlin has long said their forces should be included in the overall count of US nuclear forces. When the French and British forces were miniscule in comparison to superpower nuclear potential, the Kremlin argument could be dismissed as a propaganda ploy. No longer.
By ``deploying around 500 independent, targetable warheads each -- an increase from 2.7 percent of the Western submarine strike force to 26 percent, Britain and France might possess over a quarter of the West's most secure second-strike nuclear capacity,'' says Eric Grove, an independent defense consultant based in London. ``The Soviet Union would find this hard to ignore.''
The French and British argue that their forces cannot be reduced and remain credible. Modernization, they add, is necessary to maintain minimum deterrent. Only if Soviet and US systems are reduced significantly first would either of the two European countries be willing to talk about arms control, they contend.
Both France and Britain reject the idea of a winnable nuclear war. Although nuclear modernization means they can become more selective in targeting, their nuclear strategy is to deter any aggressor with the threat of a massive strike against its cities.
``Our new submarines will be able to kill 50 million people in a half an hour,'' says Dominique David, a director of the French Institute of Military Strategy. ``We believe that's enough to dissuade any adversary.''
This strategy could be undercut by the successful deployment of a space-based strategic defense shield. For that reason, many British and French officials are wary of President Reagan's Strategic Defense Initiative (SDI, or ``star wars''). While Britain finally signed an agreement to participate in the US research program after negotiations, the French remain adamant in their refusal. Conservative Prime Minister Jacques Chirac talked positively last week about the SDI program -- but even he has ruled out a government-to-government pact.
There is also domestic political opposition to British plans for nuclear modernization. In France, all major political parties, including the Communists, support nuclear weapons. But in Britain, the opposition Labour Party has called for unilateral nuclear disarmament, while the Social Democratic-Liberal alliance opposes Trident as too expensive.
Moscow is trying to exploit this opposition. Just last week, the Kremlin said that the Soviets might reduce their missiles by the same number as Britain.
``The Soviet Union is trying to appeal to those people in Britain who want to eliminate nuclear technology,'' says Robert Elliot of the London-based International Institute for Strategic Studies.
Washington is also concerned. Traditionally, US strategists have viewed the independent European nuclear forces as a valuable supplement to the defense of the overall Western alliance.
But what happens when the modernized British and French forces become more powerful? So far, the US has insisted that it cannot negotiate for the British and French. Their enlarged forces may strain this position, analysts and diplomats say, producing uncertain results.
``The British and French factor could provide greater incentive for the Soviets to strike a deal,'' says one US diplomat. ``Or it could undercut our position at Geneva.''