NEW YORK — Harold M. Agnew, the last surviving major figure to have been present at the birth of the nuclear age, died Sunday at his home in Solana Beach, Calif. He was 92.
He had recently been given a diagnosis of chronic lymphocytic leukemia, his family said.
Dr. Agnew helped build the world’s first reactor and atomic bombs, flew on the first atomic strike against Japan, filmed the mushroom cloud, helped perfect the hydrogen bomb, and led the Los Alamos National Laboratory at the height of the Cold War.
A physicist, he was no giant of discovery, but was ingenious technically and wielded great influence for decades as a presidential adviser and a gregarious hawk, as restless and unpredictable as the tumultuous age he helped define.
“We did pretty good,” he remarked during a 1992 flight. “We brought a quick end to a devastating war and maintained the peace and eventually saw democracy prevail. That’s something you can hang your hat on.”
Charlie McMillan — the current director of Los Alamos, the birthplace of the bomb in the mountains of New Mexico — called Dr. Agnew “a national treasure,” saying the United States “will be forever in Harold’s debt.”
Harold Melvin Agnew was born in Denver, the only child of a stonecutter of Scotch-Irish heritage. A natural athlete, he pitched his softball team to a Denver championship. He graduated from Denver University with a chemistry degree and won a scholarship to Yale.
But the secret wartime effort to build an atomic bomb intruded on his studies. In 1942, he was assigned to Enrico Fermi, the Italian Nobel laureate who was helping to lead the project at the University of Chicago. Dr. Agnew did what he called “grunt work,” making scientific measurements and getting a hefty dose of radiation.
Redirected because of the health danger, he helped stack tons of graphite bricks and uranium into a neat pile at a university squash court.
On a blustery Chicago day, Dec. 2, 1942, Dr. Agnew and a few dozen other people gathered to see if the pile could sustain a chain reaction. Recording pens jumped as atoms split in two. The success meant that, in theory, the human race now had the means to illuminate cities or level them. He was 21.
Dr. Agnew arrived at Los Alamos in March 1943 with his wife, Beverly. The couple shared a bunkhouse with another couple and a man who cooked nothing but Chinese food, even for breakfast. Amid the tall pines and deep canyons, Dr. Agnew helped build and run a particle accelerator whose data helped demonstrate the merits of various bomb designs.
When the world’s first nuclear blast lit up the New Mexico desert before dawn on July 16, 1945, Dr. Agnew was already far away, preparing for the bombing of Hiroshima. He was “aching” for the run to Japan, he recalled, because so many friends had died in the war. He autographed the bomb.
On Aug. 6, he boarded a B-29 that accompanied the Enola Gay, which was carrying the bomb code-named “Little Boy.” Dr. Agnew and two other scientists measured the size of the shock wave and thus the bomb’s power.
Afterward, he and his colleagues took turns peering out a small window at the mushroom cloud and the ground damage. Dr. Agnew, filming the devastation with a 16-millimeter Bell & Howell movie camera, was the only person to witness the whole undertaking, from reactor to weapon to Hiroshima.
After the war, he studied with Fermi at Chicago and received his doctorate in physics in 1949. Returning to Los Alamos, he joined the hunt for a technical edge over the Soviet Union, which had exploded an atomic bomb in 1949.
The first hydrogen bomb, tested successfully by the United States in 1952, weighed 65 tons. Dr. Agnew helped perfect lighter H-bombs that were deliverable over long distances.
In the 1960s, serving as scientific adviser to the supreme allied commander in Europe, he led the development of protective devices for nuclear arms that kept them under tight presidential control.
Returning to Los Alamos, he was named head of the weapons division, which developed the warhead for the Minuteman intercontinental ballistic missile, among other arms. In 1970 he became director of the lab, with a staff of about 7,000.
Under his leadership, Los Alamos developed an underground nuclear test containment program and trained the first class of International Atomic Energy Agency inspectors, according to the lab.
Meanwhile, political radicals at the University of California, which ran Los Alamos, labeled Dr. Agnew a war criminal and tried him in absentia.
In 1978, he advised President Carter against seeking a comprehensive ban on nuclear testing. He argued that such an East-West stoppage would have ended the development of new arms and, more important, reduced confidence in the potency of the existing arsenal.
Upon retiring from Los Alamos, Dr. Agnew sought to encourage the growth of civilian nuclear power. In March 1979 he became president of General Atomics, a San Diego-based developer of reactors.
From 1982 until 1989, he served as a science adviser to the White House under President Reagan. After the Cold War, in 1991, he participated in the first meeting between US bomb makers and their Russian counterparts, seeking ways to reduce arsenals. In 1992, he urged the United States to buy bomb-grade uranium from scrapped Soviet nuclear arms to bolster the shaky Russian economy and reduce the risk of nuclear war, accident, and theft.
In August of that year, the White House announced a plan to buy at least 500 metric tons of the material in a deal worth several billion dollars.
The Russian bomb uranium was diluted into fuel for nuclear reactors that make electricity, turning a major danger into a peaceful bonanza.
Dr. Agnew leaves a daughter, Nancy E. Chapman; a son, John; four grandchildren; and three great-grandchildren.
He also left a towering reputation at Los Alamos. Late in life, Dr. Agnew noted with pride that cars could occasionally be seen there bearing bright orange bumper stickers reading, “Harold, come back.”Material from the Associated Press was used in this obituary.