End of an Era for the Los Alamos Critical Experiments Facility: History of critical assemblies and experiments (1946–2004)

Abstract

The Los Alamos Critical Experiments Facility (LACEF) was the last operational, general-purpose, critical-mass laboratory in the United States. The long history of remote operations and large-scale critical-mass experiments at LACEF began in 1948, and it effectively ended in July 8th, 2004, when the last critical experiment was performed on the Planet critical assembly. The experimental activities at the Pajarito Site began in April 1946 as a way to obtain subcritical measurements for weapons safety guidance. A year later, the first Kiva (a concrete-reinforced building) was constructed, and 18 months afterward the first remote critical operation was reported with the Topsy critical assembly. In the early years, the Pajarito Site primarily supported the weapons program; later, for almost 17 years, the neutronics of the Rover nuclear-propulsion program dominated activities at Pajarito Site. More recently, Pajarito Site added some new dimensions to its operations in order to support emergency response, the Nuclear Criticality Safety Program, and radiation-detection development. The long history of critical-assembly measurements and operations is documented in hundreds of peer-reviewed technical papers, laboratory reports, personal files, and video sessions with some of the pioneers. It is the intent of this paper to capture, in one single document, a summary and the highlights of the glorious days of this facility. In essence, this paper is a summary of the programs conducted in the last 58 years and of the numerous critical assemblies and reactors that operated at LACEF. It also provides a list of references to the reader who might want to learn more about this facility’s rich history.