THE NUCLEAR AGE; A novel by Tim O'Brien. Knopf. 312 pp. $16.95.

A LARGE SECTION of Tim O'Brien's anti-extinction of human life novel is set in the late '60s among anti-Vietnam war radicals, one of whom complains to a reluctant compatriot, "We're tired of jump-starting your conscience." It's a vivid phrase, jump-starting a conscience, and no doubt states one of O'Brien's aims in writing this imperfect but very lively novel about a man in 1995 digging a hole he plans to crawl into.

As he showed in Going After Cacciato, his prize-winning 1978 novel about a soldier's life in Vietnam, O'Brien knows something about lethal surrealism overwhelming real life. In The Nuclear Age he has his work cut out for him again as he takes for his theme a man's unshakeable conviction that the human race is tying to murder itself.

"Am I crazy?" William Cowling asks as he constructs a bomb shelter in the back yard of his Montana home. Cowling is far from being any kind of right-wing loony, but his wife thinks he's lost his marbles and plans on leaving him, and his 12- year-old daughter is alternately bemused and terrified. "The critical dynamic of our age . . . was . . . escalation," however, and Cowling can think of no other sane reaction to the fact that "the bombs are real."

O'Brien has come up with a marvelous character in William Cowling, who as a boy in the '50s reacts to the Cold War by converting his family's basement ping-pong table into a bomb shelter, lining its top with "lead" pencils and charcoal briquettes. His normal, level-headed parents ridicule him cheerfully, and by the time he's in high school Cowling regards himself as a "slightly warped ding-a-ling" who still sees flashes in the night. The 1962 Cuban missile criss confirms his worst fears about the future of the planet.

Cowling develops an interest in geology because, "Rocks lasted. Rocks could be trusted." When his parents take their neurotic loner of a son to a psychotherapist, William discovers that the shrink, Chuck Adamson, is just as frightened by the fragility of human life as he is. William advises his therapist to take up rock collecting as therapy. Adamson -- among whose first words to William are, "Try not to bore me" -- is grateful for the advice and they become friends.

ADAMSON is a memorable creation, as are several other of O'Brien's misfits here, most notably Sarah Strouch, the pom-pom girl of the '50s who becomes a revolutionary in the '60s because the radical underground offers a life whose passionate intensity provides a kick as satisfying as the one she got from being a cheerleader. Sarah "wanted to be wanted and soon would be." Unlike a number of other recent chroniclers of the anti-war movement, O'Brien is good at showing the mixed and sometimes wacky motives of the most radical protestors without knocking their cause. One of the likable things about this novel is that it was written by an honest man.

It would be good then to report that The Nuclear Age is as coherent as it is fair-minded, that its ideas come together with devastating effect, that it is the Catch-22 for the age of nuclear anxiety. Regrettably none of this is the case. O'Brien has miscalculated in making his central characters 1960s anti-war radicals, whose daily concerns and even broad aims during those years are so peripheral to his point in 1995 (or now) that the impact of his main message keeps getting lost. O'Brien wants to show the lunacy of the nuclear age, but for most of the action of the story his characters live back in the napalm era, a connected but still very different matter -- a lesser matter, it could even be argued.

Flawed as it is, The Nuclear Age is still in many ways wonderful novel. For one thing, it is repeatedly eloquent in stating the obvious central question of our time: "Why don't we stand on our heads and filibuster by scream? Nuclear war! Nuclear war! Why such dignity? . . . Why do we blush at our own future?" And (meanwhile) the book is also enormously entertaining. Here is William Cowling at the age of 49 in 1995 looking back at the year 1976: "There were fireworks and tall ships. Amnesia was epidemic. Gerald Ford: My life was like his presidency; it happened, I'm almost certain."