John J. Morabito, 51, a nuclear energy specialist who worked on the development of two of this country's early nuclear projects, died of cancer Tuesday at Suburban Hospital in Bethesda.

A mechanical engineer, Mr. Morabito was known as a specialist in the field of liquid metal-cooled breeder reactors. In 1954, he became Fermi project engineer and manager of the design division of Atomic Power Development Associates for the construction of the Enrico Fermi Fast Breeder Reactor in Detroit.

In 1965, he joined the old Atomic Energy Commission, where he was project manager for the world's largest test reactor for the development of fuels and materials for the liquid metal fast breeder reactor program the Fast Flux Test Facility.

Mr. Morabito was responsible for the project, considered to be of central importance to this country's efforts in breeder reactor development through the design phase and a major part of its construction.

He later became responsible for fast reactor component development for the Energy Research and Development Administration (ERDA) and its auccessor agency, Energy. He was chief of the engineered components branch in the Division of reactor Research and Technology at his death.

Mr. Morabito wrote numerous papers on his field and was a member of the American Nuclear Society and the American Society of Mechanical Enginerrs.

He was born in Brooklyn, N.Y. and earned a bachelor's degrees in mechanical engineering from the Pratt Institute there in 1950.

Survivors include his wife, Paula, and two children, Anne Mary, and John Paul, all of the home in Rockville, and a sister, Connie of Brooklyn.

The family suggests that expressions of sympathy be in the form of contributions to the Montgomery County unit of the American Cancer Society in Silver Spring. CAPTION: Picture, JOHN J. MORABITO