Lord Walter of Cronkite could not get through an hour on the nuclear age without some sermonizing. "Hiroshima Plus 40 Years . . . and Still Counting," the "CBS Reports" program at 8 tonight on Channel 9, includes Dire Pronouncements from Uncle Walter and a closing-act Warning to All Humanity. It might seem superficially objectionable, but then you have to remember, only Walter Cronkite could really bring this kind of thing off, and bring it off he does.

The report is a solid and straightforward hour, a little short on imagination, but very heavy on tony expertise; two former presidents, Richard Nixon and Jimmy Carter, are heard from, as well as Caspar Weinberger and three previous secretaries of defense: Robert McNamara, Harold Brown and James Schlesinger. Finally, though, it is Walter Cronkite who gives this broadcast the weight of authority and hortatory impact.

He is unique among working broadcast journalists, to state the obvious. NBC News has no one of his generation in his league, and ABC News has only David Brinkley, who unlike Walter is easily bored.

Keyed to the Aug. 6 anniversary of the bombing of Hiroshima, the program begins with engrossing recollections of the event by, among others, Paul Tibbets, the pilot and group commander of the Enola Gay -- a man who says he has "no regrets" and looks as though he means it. It is noted that if the war in Europe had not ended when it did, there might well have been twin bombing missions that August day, with the other using Germany as a nuclear target.

There is news footage of one crew autographing a bomb. And physicist Harold Agnew stands and talks matter-of-factly next to models of "Little Boy," the Hiroshima bomb, and "Fat Man," dropped on Nagasaki on Aug. 9, 1945. Throughout the program, Agnew keeps resurfacing, always strolling among bombs and warheads and pointing out various features, like Betty Furness extolling the storage compartments and ice makers of Westinghouse refrigerators back in the TV '50s.

Weinberger is the object of imprudent editing at about this point in the program. He is talking about the simple strategic necessity of using the bomb, but the producers cut to shots of burned Japanese children, and the effect is to make Weinberger look callous. If it's unintentional, it's sloppy; if not, it's the definitive cheap shot.

From there on, Cronkite surveys the global crises of intervening years in which nuclear weapons might have been, or indeed very nearly were, used. They were mulled as a possibility during the Berlin airlift of 1948. And Eisenhower let it be known to the Russians and the Chinese during the Korean war, Nixon says, "that we were prepared to use nuclear weapons if necessary to bring the war to a conclusion."

Later, the possibility of using nuclear weapons popped into the mind of John F. Kennedy during the Cuban missile crisis of 1962. Audio tape of an actual White House meeting on the subject, with the voices of Kennedy and Gen. Maxwell Taylor distinguishable, is played. McNamara recalls leaving the White House after one apocalyptic powwow "wondering if I'd ever see another Saturday night."

Near the program's end, Cronkite asks, "What have we learned?" in 40 years of life on the nuclear brink. Although the program has focused on the nuclear capability of the superpowers, Schlesinger notes that a more lethal threat is probably posed by nuclear proliferation in Third World countries. Nixon worries that "some nut like Qaddafi" will get the bomb and observes oh-so-reassuringly of the Soviet Union, "The men in the Kremlin are not madmen and they are not fools . . . They want to win without war."

Agnew suggests that above-ground nuclear testing be resumed to this extent: Every five years, the leaders of nuclear-armed nations should be gathered to observe the blinding and nightmarish spectacle of an actual high-megaton detonation, preferably, he says, "in their underwear." They should be reminded of the nature of the weapon they brandish.

Back for a wrap-up, Cronkite stands in present-day Hiroshima and observes little paper lanterns floated silently down the Ota River as an annual commemoration of the planet Earth's first nuclear victims. The imagery is so poetically mordant that it's a pity the producers didn't give it a few more seconds of screen time (it is reprised under the closing credits).

Walter Pincus and Bob Blum are the producers in question, under executive producer Burton Benjamin, a member of the CBS News old guard. Their sobering distillation of the first four decades of the nuclear age is gimmick-free and unhurried, though never to the point of video arthritis. Offhand, there seems no reason why a jazzy ground breaker like CBS News' imminent "West 57th" magazine show can't coexist with the more traditional hour-long documentaries like this one. Assuming network brass will continue to find time for both on the schedule, a very unsafe assumption to make.

Of course the title of tonight's program is unwieldy and clunky, but the perfect title for a report about how human beings have managed to survive 40 years under the bomb was already used by Thornton Wilder. It was, "The Skin of Our Teeth."