The early laboratory had several specialist divisions: Chemistry (initially headed by Egon Bretscher, later by Robert Spence), General Physics (Herbert Skinner), Nuclear Physics (initially headed by Otto Frisch, later E. Bretscher), Reactor Physics (John Dunworth), Theoretical Physics (Klaus Fuchs, later Brian Flowers), Isotopes (Henry Seligmann) and Engineering (Harold Tongue, later Robert Jackson). Directors after Cockcroft included Basil Schonland, Arthur Vick and Walter Marshall.
Such was the interest in nuclear power and the priority devoted to it in those days that the first reactor, GLEEP, was operating by 15 August 1947. GLEEP (Graphite Low Energy Experimental Pile) was a low energy (3 kilowatt) graphite-moderated air-cooled reactor. The first reactor in Western Europe, it was remarkably long-lived, operating until 1990.
One of the most significant experiments to occur at AERE was the ZETA nuclear fusion experiment. An early attempt to build a large-scale nuclear fusion reactor, the project was started in 1954, and the first successes were achieved in 1957. In 1958 the project was shut down, as it was believed that no further progress could be made with the kind of design that ZETA represented. (see Timeline of nuclear fusion).