Choosing a new leader for NATO will test unity of Western alliance

For months, even years, the imminent departure of Joseph Luns - secretary-general of NATO for the past 12 years - has been forecast by friends and foes alike. Now the talk has turned from gossip to serious business.

But few here, including Mr. Luns, who is 71, believe the secretary-general will leave his long-cherished job before the end of this year.

United States-Soviet arms-reduction talks have reached a crucial stage in Geneva, and any change in leadership of the North Atlantic Treaty Organization could be used by Moscow - as well as the peace movement - to signal slackening unity within the alliance.

Moscow would hope to forestall the deployment of 572 new US nuclear missiles in five West European countries, set to begin at the end of this year if the Geneva talks fail.

The prime candidate to replace Luns is former British Foreign Secretary Lord Carrington, who resigned last year for failing to predict the invasion of the Falkland Islands by Argentine troops. Since then, he has headed British General Electric - a job he is said to dislike.

Also in the running are Henri Simonet, a former Belgian foreign minister who is now a local politician; Gaston Thorn, president of the European Commission; and the current Belgian foreign minister, Leo Tindemans.

At NATO headquarters here, it is clear the time has come to breathe new life into NATO's top civilian job, if only to help polish the organization's image in the eyes of the general public. Sources say Luns has increasingly alienated Western Europe's so-called ''successor generation'' - the under-40 generation which is moving into positions of responsibility - with his unabashed conservatism on almost all issues.

Luns, who is given high marks for helping the alliance reach agreement on numerous occasions, has denied that he will be quitting soon. Asked if he would be retiring this year, he replied, ''No, not this year. Certainly not this year.''

But some officials report that Luns has told NATO ambassadors that he will leave, emphasizing that no date has been fixed. It could be early next year, according to some sources.

Diplomats say that Lord Carrington - a highly respected statesman - would like the job and is well suited for it. British Prime Minister Margaret Thatcher , moreover, is said to be pushing hard for Carrington's appointment.

But some long-term observers of NATO say Carrington is too ''strong'' to be accepted by NATO governments. ''No NATO government wants to be lectured by the secretary-general,'' an observer said.

Whatever happens, the selection process could be long and difficult. There is no set procedure for picking a new NATO secretary-general. Officials explain that a prospective candidate must not only receive the blessing of all NATO governments but also survive an elimination process during which each government reviews available candidates and rejects them one-by-one until one is left.

The tasks facing the new secretary-general will be formidable. He will have to mediate between those in the alliance, including the Reagan administration, who want to spend more on defense and those, including many West European governments, who will find it politically and economically impossible to boost defense expenditures.

He will have to walk a tightrope in the lingering conflict between two of NATO's key members - Greece and Turkey. And he will have to help the alliance define a common approach to East-West relations, with many West European governments continuing to demand ''normalcy'' and hard-line Reaganites pushing to ''punish'' the Soviet bloc by restricting trade and minimizing contacts on other levels.