On July 16, 1945, a Queens College math student who quit to join the Army two years earlier was involved in a top-secret project in the deserts of New Mexico when what was then history's largest man-made explosion shook the Earth.

"The mountains for miles around were illuminated brighter than the noonday sun. A gigantic fireball rose high into the sky," the former student, Alan G. Vorwald, wrote one month later, recalling eyewitness descriptions provided by colleagues. "The nuclear reaction had been a success."

Vorwald, a computer engineer at Los Alamos, N.M., when Manhattan Project scientists there tested the world's first nuclear weapon, and later a math teacher and principal in the Plainview-Old Bethpage school district, died Jan. 12 at his South Setauket home. He was 90.

In the August 1945 letter to his parents, Vorwald expressed the urgency with which Manhattan Project scientists had pressed to develop the atomic bomb.

They feared losing out to Germany, which had been close to developing its own bomb before its defeat that May. And they saw the bomb as a way to spare war-weary American troops from the carnage of an assault on Japan.

"Most everybody working here had confidence that the bomb would work," said Vorwald, who met Nobel laureates Niels Bohr and Enrico Fermi while at Los Alamos. "So much depended on it."

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He received an honorable discharge in 1946, having attained the rank of Tech 4.

He completed a degree in education at Queens College in 1947, and later launched a 35-year career culminating in his rise to principal at Jamaica Avenue Elementary School in Plainview. He received a doctorate in education from Nova University in 1975. Vorwald retired in 1983, the family said.

Fellow Plainview administrator Marvin Witte of South Carolina described Vorwald as an understanding and compassionate educator. "He was very sensitive, and had a great feeling for people."

Survivors include his sons Brian of Stony Brook, and Gary of South Setauket; daughter Lynn Federman of Huntington; and sister Doris L. Vorwald of South Setauket. His wife, Sarah, whom he married in 1950, died in 2003.

Interment with military honors took place Jan. 16 at Pinelawn Memorial Park, Pinelawn.