Harold C. Tellock, 64, a retired nuclear engineer who has lived in the Washington area since 1956, died Thursday in his Potomac home. He had a heart ailment.

Mr. Tellock was asistant director of the reactor engineering division at Argonne National Laboratory near Chicago for seven years before coming to Washington. He was a product manager with Allis-Chalmers in Bethesda until 1972. He spent five years with the Bechtel Corporation before retiring earlier this year.

He helped found Nutec, a Rockville firm that designs safety programs for nuclear reactors, and was a vice president at the time of his death.

He was a member of the American Nuclear Society and the Society of Automotive Engineers. He also belonged to the American Legion and the National Wildlife Federation.

Mr. Tellock was a native of Wisconsin and studied automotive and mechanical engineering at the Armour Institute of Technology in Chicago. He worked for the Tucker Corporation in Chicago in the late 1940s where he helped develop an air-cooled engine for automobiles.

He served with the Army in Italy during World War II.

He is survived by his wife, Helen, of the home in Potomac; a son, Paul of Frederick, md.; a daughter, Barbara Thorp, of Raleigh, N.C., and one grandchild.