Now that the beasts and gods of the superpowers' atomic arsenals have become familiar to the world -- the Poseidons and Titans, Bisons and Bears -- along comes another fearsome array: the Hades and Plutons and Chevalines of France and Britain.

The French and British nuclear arsenals have moved to center stage as potential stumbling blocks in U.S.-Soviet arms talks. Although relatively small, they are growing dramatically. And they are controlled by governments that do not view nuclear disarmament as inevitably desirable, the speeches of President Reagan and Soviet leader Mikhail Gorbachev notwithstanding.

"When people start talking about a world in which the nuclear umbrella would not exist, you get an awful lot of disagreement over whether that's a worthy objective," said one British official. "There are very few Europeans who could contemplate a world of no nuclear weapons -- certainly without a very radical change in what is happening in the conventional balance, the chemical balance and political relations in general."

Because superpower arms talks in Geneva appear deadlocked on the issue of longer-range nuclear missiles and space weapons, most hopes for a treaty breakthrough have focused on intermediate-range forces (INF). But Paul H. Nitze, special adviser to the president on arms issues, underscored the obstacles the European arsenals pose when he said yesterday that a recent Soviet proposal to limit intermediate-range nuclear missiles contained "unacceptable conditions" by demanding restrictions on the British and French arsenals.

Both nations are in the process of "modernizing" their nuclear forces, which means fielding greater numbers of increasingly accurate warheads. Both describe their programs as responses to the Soviet Union's construction more than a decade ago of missile defenses around Moscow. France is even developing a "stealth" warhead to evade future Soviet defenses.

By the mid-1990s, the combined force of the two nations will total more than 2,000 warheads, according to current projections, more than double their current arsenals. The Soviet Union and United States have about 50,000 warheads, while China, the fifth acknowledged member of the nuclear club, is believed to have about 300.

The French and British modernization programs, controversial in Britain and less so in France, have regained world attention because of an apparent flexibility in Soviet bargaining position. The Soviets formerly insisted that all western forces be essentially lumped together, but Gorbachev recently said that he would accept parity with U.S. intermediate-range missiles in Europe as long as France and Britain agree to a "freeze."

That apparent compromise has caused consternation in Paris and London, where officials fear it could increase the pressure on them to scrap or trim their modernization plans.

Reagan told Gorbachev that the United States could not negotiate on behalf of other nations, a slightly less emphatic response than the flat rejection the British sought. While some State Department officials hint that perhaps the British should be more explicit about the conditions that might lead them to disarm, Defense Department officials -- generally the bureaucrats least open to arms control compromise -- have embraced the French and British cause with enthusiasm, according to U.S. officials.

"We would not as Americans presume to make decisions on behalf of our allies," said Richard N. Perle, assistant secretary of defense, restating what has long been U.S. policy.

Perle added that the Soviet offer "is in fact an effort to assure there is no agreement," but that the Soviets perceive the British and French freeze as "a favorable issue in terms of international relations."

British and French officials said they do not know whether Gorbachev's "freeze" would prevent any modernization or only cap their arsenals at current levels. "We have not sought to find out; we don't want to engage in discussion on that," a French official said.

By either definition, a freeze would disrupt current plans. The French, who spend about one-third of their procurement budget on nuclear weapons, are planning to outfit six submarines with 96 missiles carrying six warheads each, replacing their single-warhead weapons; to replace the land-based Pluton with Hades, which unlike its predecessor will be able to reach targets beyond West Germany in eastern Europe; to buy a force of Mirage 2000 jets equipped with short-range nuclear missiles, and to improve its ground- and space-based nuclear command capabilities.

In addition, the French are developing a neutron warhead, which may be deployed, and a mobile land-based missile similar to the proposed U.S. Midgetman.

"We are not aiming at nuclear superiority," one French official joked, before adding seriously, "The French don't like the zero option."

The British navy is also building a new submarine force: four boats equipped with 16 Trident missiles that will probably each carry eight, separately targeted warheads. Nuclear bombs will be carried by new Tornado jets.

Although officials in all three countries acknowledge that nuclear war plans would be coordinated, particularly between Britain and the United States, they also insist that France and Britain should keep an "independent deterrent."

Opposition politicians in Britain argue that Trident is not worth the cost, but the Conservative government is committed.