Emily Caplis Slocum, 79, an illustrator and draftswoman whose work included the design drafting of nuclear weapons, died of congestive heart failure Jan. 23 at her home in Washington.

Mrs. Slocum, a resident of the Washington area for 25 years, retired in 1984 as a heraldic artist at the Institute of Heraldry at the Defense Department. She designed seals for the armed services and agencies, flags and other military insignia.

Mrs. Slocum was born in Philadelphia. She attended the University of Denver and the University of Michigan.

Early in her career, she was an engineering draftswoman for the Interior Department's Bureau of Reclamation in Denver. She later did geological drafting for the U.S. Geological Survey, True Oil Co. and Geophoto Co.

She did nuclear weapons design work at the Hanford and Los Alamos National laboratories, turning the rough sketches of scientists into engineering draft plans, and then was chief nuclear designer for the Atomic Energy Commission in Casper, Wyo.

Before moving to the Washington area, Mrs. Slocum was a medical illustrator at Fitzsimmons Army Hospital, where she illustrated books, reports and teaching materials for Army physicians. After she retired, she did freelance illustrations and cartoons, contributing to books on heart disease and other publications.

She was a member of St. Joseph's Catholic Church in Washington.

Her husband, Theodore Doremus Slocum, died in 1963.

Survivors include a daughter, Sheila Slocum Hollis of Washington, and a granddaughter.