WHEN THE NATION'S worst nuclear accident shocked and terrified the public at Three Mile Island in 1979, radiation scientist Sydney Wynne Porter Jr. was on the job.

He was one of the first radiation safety experts called to the scene near Harrisburg on that early morning of March 28, and he and his team worked for months, often putting in 18-hour days, assessing the damage from a core meltdown in one of the reactors, and supervising the response.

His company, Porter Consultants Inc., founded in 1974 to provide radiation safety and health information to industry, ran the TMI radiological environmental monitoring program until 1982.

Sydney Porter, past president of the Delaware Valley Society for Radiation Safety, an adjunct professor at Drexel University's Graduate School of Environmental Studies, among many other positions in the field over the years, died April 23. He was 78 and lived in Ardmore and St. Michaels, Md.

Last year, the Delaware Valley organization presented him with its Meritorious Achievement Award. In presenting the award, Kent Lambert, radiation-safety officer of Drexel, said Syd "is an extraordinary health physicist who has been a mentor to many and whose career is characterized by leadership, initiative and passion for the profession."

Syd was a science-history buff and worked with the Mutter Museum in Philadelphia to preserve and restore a piezoelectric sensor donated by radiation pioneer Marie Curie, whose husband, Pierre, invented it in 1880. The device measures pressure, acceleration and other forces.

He arranged with Eagle Pitcher, a Kansas company, and Teledyne Isotope of New Jersey to create quartz crystal lamina to replace the decayed and ruined originals. He conducted a radiological survey of the sensor, of Curie's correspondence and of areas of the museum where he found and corrected radioactive contamination.

Robert Hicks, museum director, said Syd's work on the exhibit "put the entire museum staff at ease." He did the work gratis.

A native of Baltimore, Syd graduated in 1954 from St. John's College, in Annapolis, where he was a star in lacrosse. He completed graduate studies in physical chemistry at Johns Hopkins and in radiochemistry at New York University. In the late '50s and early '60s, Syd developed radiation-safety programs for the Navy's early nuclear submarines, including the USS Nautilus, as coordinator of health physics for General Dynamics' Electric Boat Co.

In 1963, he became head of the Radiological Safety Department at the Armed Forces Radiobiology Research Institute at the National Naval Center in Bethesda, Md. He was a member of the Department of Defense's reactor inspector general's team and was awarded the department's Antarctica Service Medal for his work at the McMurdo Station research center in Antarctica.

He was a co-founder of Radiation Management Corp. in Philadelphia in 1969. When it folded in 1974, he founded his Porter Consultants.

Syd was a devoted patron of the arts and encouraged family and friends to attend the Bach Choir Festival in Bethlehem, Pa., every May. He served as board member and guarantor of the choir for many years.

He was an ardent sailor and deep-sea diver.

He is survived by his daughter, Dawn; two brothers, Temple and Geoffrey; a sister, Marti Porter Andrews; his former wife, Lynn Kony Porter; his companion, Barbara Opper; and two grandchildren.

Services: Memorial services: 10:30 a.m. June 11 at Christ Church, St. Michaels, and 10:30 a.m. June 14 at St. Christopher's Church, 226 Righters Mill Road, Gladwyne.

Donations may be made to the Bach Choir of Bethlehem, 40 Heckewelder Place, Bethlehem, Pa. 18018.