Ralph E. Lapp, 87, a nuclear physicist who helped develop the atomic bomb as part of the Manhattan Project and who wrote and lectured about the hazards of nuclear fallout, died Sept. 7 at Inova Mount Vernon Hospital of pulmonary failure following routine surgery. He was an Alexandria resident.

Dr. Lapp's involvement with the Manhattan Project was almost accidental. In December 1942, while Enrico Fermi and his team of scientists were at work in a squash court under the stands at the University of Chicago's Stagg Field, Dr. Lapp was in the stadium's press box studying cosmic rays.

According to his son Dr. Christopher Lapp, he was lugging heavy pieces of lead to the press box when he discovered a ramp that led to the activities taking place below ground. Once he got past a security guard, he introduced himself as a nuclear physicist and was put to work.

Although he was deeply involved in the development of the atomic bomb, he was one of 77 atomic scientists who signed a petition July 17, 1945 -- less than a month before the attack on Hiroshima -- urging President Harry S. Truman not to order a surprise bombing of the Japanese.

The petition acknowledged that "atomic bombs may very well be an effective method of warfare" but added that a nuclear attack on Japan "could not be justified" unless the Japanese were first "given an opportunity to surrender."

Anticipating the coming Cold War, Dr. Lapp and his fellow scientists warned that if the proliferation of nuclear weapons continued unchecked, "the cities of the United States as well as the cities of other nations will be in continuous danger of sudden annihilation."

Dr. Lapp told a United Press International reporter in 1995 that he distributed copies of the petition to scientists at Los Alamos but nothing happened. "Los Alamos was on a collision course with history," he said.

Controversy flared 50 years later, however, when the petition was scheduled for display at the National Air and Space Museum, along with the Enola Gay, the plane that dropped the atomic bomb on Hiroshima. Veterans' groups protested.

A few years earlier, Dr. Lapp had written: "If the memory of things is to deter, where is that memory? Hiroshima has been taken out of the American conscience -- eviscerated, extirpated."

Dr. Lapp was one of the first nuclear scientists to enter Japan at the end of the war. In 1978, two historians turned up the first evidence that Japan had tried to develop an atomic bomb during World War II, but Dr. Lapp told The Washington Post that he and his colleagues had seen no signs of nuclear bomb development anywhere in the devastated country.

One of Dr. Lapp's first efforts to dramatize the danger of nuclear fallout was his 1958 book, "The Voyage of the Lucky Dragon," which told the story of 23 Japanese fishermen who were caught in the fallout of a 1954 U.S. hydrogen bomb test. He also was an early opponent of atmospheric nuclear weapons-testing and gave frequent testimony before various governmental bodies. His position later became U.S. policy.

Ralph Eugene Lapp was born in Buffalo. He was a graduate of the University of Chicago, where he also received a doctorate in physics in 1946.

He began his career in high-energy physics research under the Nobel Prize-winning physicist Arthur H. Compton, before working on the Manhattan Project.

After the war, Dr. Lapp served as science adviser and executive director of research and development at the War Department. In 1949, he joined the Office of Naval Research as head of the nuclear physics branch. He also worked for the Atomic Energy Commission as assistant director of Argonne National Laboratory, the nuclear research laboratory operated for the government by the University of Chicago. In 1950, he left government to form his own consulting firm, Nuclear Science Services, in Washington.

In addition to his book about the Japanese fishermen, Dr. Lapp co-wrote "Nuclear Radiation Physics" (1948), which has gone through several editions and is still used as a reference, and "My Life With Radiation: Hiroshima Plus Fifty Years" (1995). He wrote 22 books in all on nuclear safety and radiation, for professionals and the general public, as well as numerous articles.

During the 1950s and 1960s, he toured the country lecturing on radiation, the nuclear arms race, civil defense and the peaceful uses of nuclear energy.

In 1954, citing evidence that the United States had exploded four hydrogen bombs in the Pacific, Dr. Lapp warned that the world was dangerously close to the maximum safe level of radioactivity. He often clashed with the Atomic Energy Commission over what he considered the commission's reluctance to give the American people straight answers about the dangers of fallout. A Post book reviewer in 1956 called him "a one-man atomic truth squad and nuclear lie detector."

Dr. Lapp believed that nuclear energy could be put to safe and peaceful uses and that the dangers of radiation -- if not fallout -- were often overstated. He also debated Ralph Nader, Jane Fonda and other public figures who opposed using nuclear energy.

Dr. Lapp was a co-founder and senior staff member of the Washington firm Quadri-Science Inc., a nuclear energy consulting firm that operated from 1964 to 1978. In 1983, he founded a similar company, Lapp Inc. It stayed in business until 2002.

Survivors include his wife of 48 years, Jeannette "Jerry" Lapp of Alexandria; and two sons, Dr. Christopher Lapp of Alexandria and Nicholas D. Lapp of New York.

Dr. Ralph E. Lapp worked on the Manhattan Project but opposed dropping the atomic bomb.