Can Britain afford nuclear deterrent?

Can Britain afford to pay its part in the defense of Western Europe and still retain an independent nuclear deterrent into the next century? British defense planners, already under pressure to upgrade the nation's conventional contribution to NATO, are beginning to eye costs for the future nuclear option with increasing concern.

In a new defense white paper released by Defense Secretary Michael Heseltine, British military spending in the coming year is forecast at a record (STR)17 billion ($23.6 billion). This will enable Britain to improve its nonnuclear contribution to NATO.

But the worrying figure buried in the defense estimates is the predicted cost of replacing the aging Polaris nuclear deterrent with another seaborne system: Trident missiles supplied by the United States.

The overall cost of Trident has already risen to (STR)9 billion ($12.5 billion) - twice the estimate of three years ago and still rising. This huge chunk of the defense budget suggests that Britain's contribution to the defense of Europe may soon be threatened by the money-hungry Trident program.

Britain began depending on sea-launched nuclear missiles in the 1960s, following a decision in 1962 to opt for the American Polaris missile.

Ever since, British-built Polaris boats have sustained the nation's independent nuclear deterrent.

By the late 1970s, however, it was clear that Polaris would soon have to be replaced, despite efforts to ''stretch'' it by development of a multiwarhead system.

Soon after Prime Minister Margaret Thatcher came to office in 1979, her government decided to begin phasing in the advanced Trident system using a new generation of British-built submarines.

The project was controversial even then. It is estimated that the Trident system, which uses long-range multiwarhead missiles, will increase the nuclear strength of the Royal Navy's deterrent force by a factor of 14.

Even loyal Conservatives began wondering whether Britain needed such a massive increase in its strategic nuclear capacity. Some suggested that an effort should be made to ''stretch'' Polaris still further or to replace it with cruise-type missiles. But Mrs. Thatcher decided otherwise. She seems committed to the construction of three or four Trident submarines, each with a full consignment of long-range missiles.

The government's opponents, however, think otherwise. The Labour opposition, suggesting a parallel between President Ronald Reagan's enthusiasm for the MX missile and Mrs. Thatcher's commitment to Trident, want Britain to abandon nuclear weapons altogether.

The third major party - the Liberal-Social Democratic alliance - is also opposed to Trident, arguing that spending so much on a weapon Britain does not need unnecessarily restricts the amount for improving Britain's commitment to NATO's conventional defenses.

Defense Secretary Heseltine argued that every effort is being made to avoid administrative waste in the defense establishment. He has ambitious plans to upgrade British air defenses and to keep a tidy number of Royal Navy frigates and destroyers on the high seas.

The one area of defense spending he appears to regard as sacrosanct is the Trident program. In terms of doctrine, Heseltine is under discreet attack even from some members of the ruling Conservative Party. They think retention of a sea-based deterrent runs counter to NATO's hopes of easing back on the nuclear option and depending more on conventional forces. In Whitehall, however, it is pointed out that Mrs. Thatcher is convinced that Trident is vital if Britain is to retain a fail-safe national defense and at the same time carry clout in international affairs.

A weak spot in the prime minister's calculations is that the really heavy costs of the Trident program are not yet beginning to ''bite.'' That will not happen for a year or two. When it does, the government will have to strike a balance between spending on conventional arms and the cost of keeping Britain in the strategic nuclear club.