Young Americans seem to have found themselves a grandfather figure: Adm. Hyman G. Rickover.

When he spoke here to a packed house of 4,000 students at Virginia Polytechnic Institute, they couldn't hear him in the back. Rickover glanced at his introducer and cracked:

"Here's the dean of the college, and he's never taken the trouble to find out if anybody can be heard . . ."

A mighty roar of approval drowned him out.

The famously abrasive 82-year-old father of the nuclear Navy, let go just this January after six decades of "controversy and genius," as his biographers put it, has taken to the lecture circuit, reaching across the middle generation, the parental generation, the establishment he has so often excoriated, to the restless young. He was in his element at Virginia Tech.

The very moment he arrived at Roanoke airport Thursday morning, with people still coming off the plane, he started his characteristic needling tactics, teasing a woman on the reception committee: "Aren't you Miss Roanoke?" and asking for her barrette, even trying to get a student to steal it for him. Roaming the campus, where he had been invited for a day of individual conferences and a lecture, he stopped a woman jogger to chat and, according to a photographer accompanying him, criticized another woman for chewing gum ("you look like a cow"), pulled off a wad of her gum and popped it into his own mouth.

Student leaders were so nervous about not being able to fill the hall that they mustered the Corps of Cadets, 380 strong, to sit in a block down front, in dress uniform. They needn't have bothered. The admiral was box office. His formal speech, on the need for a humanistic technology, went quickly.

"It troubles me that we are pressured by technology to alter our lives, without attempting to control it," he said, "as if technology were an irrepressible force of nature to which we must submit. Not everything hailed as progress contributes to happiness; the new is not always better, nor the old always outdated."

People confuse science, or "pure thought," with technology, which is "action," he noted, and while science need not be humanistic, technology must be "subject to the law of the cosmos and the law of man." He also spoke of the "irretrievable damage" done by those who use technology "without giving thought to its effect on our environment." For example, he said he once refused to reduce radiation shielding in nuclear submarines, though a superior wanted to risk crew health to save on weight, arguing that "if we raised radiation exposure, we might find the resulting mutations to be beneficial . . ."

Rickover talked about his life in the Navy from 1918 to 1982, of his building the pioneer nuclear sub, the Nautilus, in 1954 (perhaps as much as 15 years before it would have happened without his singleminded, driving leadership, his unauthorized biographers say) and the trailblazing reactor plant at Shippingport, Pa. The nuclear Navy, he added, now has 121 subs, 39 with ballistic missiles, four carriers, nine cruisers and 164 reactors.

The range of nuclear subs, he said, "is a secret--not from me, but from you. I'm afraid some of you might yak about it to your girlfriends."

He described the old diesel subs: 120-degree heat, bedbugs, cockroaches and dripping water, baths rare, bunks used in shifts and so on.

The question period showed his debating style best. What one piece of advice would he have for Ronald Reagan? "Run for reelection." (Cheers, roars, whoops and rebel yells plus a hiss or two.)

Why don't we put all our missiles in subs and not bother with land silos? "Each one of you has one vote, that's worth one 240-millionth, and it's your fault if you don't agree with the uses of weapons."

How to proceed with the SALT talks? "The United States has always wanted to limit arms . . . but we can't get the other outfit to stop. I wanted 'em to put me in charge of that but they wouldn't do it."

What is the destructive capability of a nuclear sub? "If I were you, I'd get the hell out of the way."

During the day Rickover met with several small groups of students and faculty. An airport press conference had been canceled at his insistence, in line with his lifelong distrust of the media. Press coverage of the day's meetings was not allowed.

However, it was learned that at one session the admiral took on a nuclear engineering professor who apparently had irritated him with questions. At another, a student, Rik Van Hemmen, from Holland, undertook to defend American education after Rickover trotted out his familiar contention that the Russian system is better than ours.

"I hardly got two words out," Van Hemmen said, "before he attacked me, said I was stupid, and how old was I, 22? And I had no experience and what did I know about Russia?"

Joe Wiencko, president of the Student Engineers' Council, said Rickover seemed to be playing mind games with everyone he met, distracting them from serious conversation. And Gene Knight, at 34 a former member of the nuclear Navy working his way through school as a reactor laboratory research engineer, noted that when one young man from a local newspaper admitted to working a 40-hour week, the admiral took him to task for wasting his time, "yet he was doing exactly what Rickover kept recommending: He was learning a trade."

Bear in mind that this was anything but a hostile audience. These students are children of the nuclear age.

Wiencko: "We're the first generation to have to deal with nuclear power plants and warheads. They were already in place when we came along. It's too late now for questions about their existence. That was what the last generation should have asked."

Ann Digges, Student Engineers'Council member: "We are sort of ambivalent about the talk of an arms freeze. Of course, we want to hear about it, but on the other hand the money and the best job offers come from nuclear companies, defense companies."

Van Hemmen: "It's a problem we have to face. But nuclear power is many different things. It's like there's gasoline and there's napalm. We recognize reactors as simply tools if used properly."

Knight: "Some people tend to think of nuclear power; we think of nuclear utilization."

Scott Richardson of the Cadet Corps: "Three Mile Island wasn't that bad. The radiation in a granite building is higher than what was released at TMI."

Emily McLaughlin, nuclear reactor trainee: "It's too bad that the admiral's statements are used by different sides to make their point. He's been striving for excellence all his life, but I think he's almost self-defeating if he berates you so much. You've got to show respect to command it."

Several students said they had seen one of Rickover's aides kicking him under the table when he started fulminating.

One unloved feature of the admiral's long career was the "Interview": the personal confrontation he had with every officer candidate for his nuclear Navy. Horrifying tales of abuse and eccentric behavior have been told around the Navy for years, and the charge has been made that he kept useful people out of that service. It appears that Admiral Rickover has brought the Interview to the campus.