TA-18, Pajarito Site, a collection of general purpose laboratories capable of subcritical, delayed and super-prompt critical operations at Los Alamos National Laboratory. Courtesy/LANL

Courtesy/Bradbury Science Museum

By RICHARD (DICK) MALENFANT
Highlands Ranch, Colo.
Formerly of Los Alamos

On November 10, 2015, Secretary of the Interior Sally Jewel and Secretary of Energy Ernest Moniz signed an MOA for creating and managing the Manhattan Project National Historical Park. 

Congress passed the National Defense Authorization Act of 2015, which included provisions authorizing the Park to be located at three sites: Oak Ridge, Tenn., Hanford, Wash.; and Los Alamos, NM. Specific locations at the three sites include the X-10 Graphite Reactor, buildings 9731 and 9204-3 at the Y-12 complex, and the K-25 building site at Oak Ridge; the B-Reactor and the Hanford High School at Hanford; and the Gun Site (TA-8) and Pajarito Site (TA-18) at Los Alamos.

I worked at Pajarito Site much of my time at Los Alamos between January 1961 and November 1996. As such I have a particular interest and association with TA-18. The buildings open at TA-18 include TA-18-29[1] (the Pond Cabin); TA-18-2 (the Battleship Bunker); and TA-18-1 (the Slotin Building). 

Emilio Segre used the Pond Cabin and a Forest Service Building at the site to make the definitive measurements that demonstrated that reactor produced plutonium could not be used in a gun-type weapon because of pre-detonation. The Battleship Bunker was used in July 1945 for the definitive measurements of the implosion method that was employed for Fat Man[2]. TA-18-1, the Slotin Building, was the location of the May 23, 1946 accident that was fatal to Louis Slotin. Following that accident, Raemer Schreiber penned a memo to Darol Froman, Deputy Director of the laboratory, with recommendations to minimize the probability of such accidents in the future.

Those recommendations form the basis of procedures for the conduct of critical experiments to this day. Froman also prohibited future experiments until they could be done remotely. This resulted in the construction of a remotely operated assembly facility (designated Kiva I) ¼ mile away from the control room. The first experiment under remote control was conducted on April 13, 1947. Following the recommendation by Raemer Schreiber, the two-man operating crew was augmented by a third man whose task was to observe and to call attention to possible errors but not participate. 

Unlike some other facilities that employed dedicated technicians for controlling the machines, the operators at Pajarito Site included the scientists that designed the experiments and analyzed the results.  In many instances, they also designed the machines to conduct the experiments. The classic general-purpose vertical-lift critical assembly machine, Comet, was designed by Jano Haley; hence the name! A variant of Comet employing some components of the original stand is still in use today. 

TA-18 was remote from the main laboratory, which led to a great camaraderie similar to a university campus and collaboration on the design, execution, and analysis of experiments.

Pajarito Site was used for offices, machine shops, control rooms, and the fixtures for the conduct of experiments under remote control until the facility was relocated to the Nevada Test Site in 2000. Prior to the move, initial approaches to critical were made using all fissionable species as bare metal and compounds of solids, liquids, or gases[3]. Although there were incidents involving accidental criticality, spills, or contamination; there were no injuries resulting from experiments.

TA-18 was recognized for 1,500,000 man-hours (25 years) from August 1948 to August 1973 without a disabling injury.  Some of the work was described by Hugh Paxton in LA-7121-H, Thirty Years at Pajarito Canyon Site, March 1978 and the follow-on LA-7121-H, Rev., Thirty Five Years at Pajarito Canyon Site, May 1981.

Significant work at the site included the Keepin-Wimett delayed neutron data, the Hansen-Roach 16-group cross-sections, the verification of safe storage of interacting arrays of nuclear material, the construction and operation of the Fast-Burst Reactors including the Lady Godiva (a bare near-sphere of highly enriched uranium) and the still operational Godiva IV,  the replication and operation at critical of Little Boy to measure the leakage and spectra of neutrons and gamma-rays to reconstruct the doses at Hiroshima, and the construction and operation of propulsion prototype rocket reactors for the Rover Program.

Staff at Pajarito Site also initiated training for criticality safety specialists using a hands-on approach to a critical configuration. In 1993 the American Nuclear Society awarded the Pajarito Canyon Site the Nuclear Historic Landmark Award. It is unfortunate that the site has been cleared of all structures built after the Manhattan Project except Kiva I.

The site is located inside of the 38 square mile Los Alamos Restricted Area and it is still off-limits to uncleared personnel except for occasional guided visits. I was able to participate in one such visit on April 5, 2019; and I look forward to the time when the site can be opened so the legacy of the Manhattan Project and Pajarito Site will not be forgotten.

[1] The terminology is Technical Area (TA) 18 followed by the building number.

[2] See, for example, Critical Assembly; Hoddeson, Henriksen, Meade, & Westfall; Cambridge; 1993.

[3] For example, experiments were conducted with solutions of the nitrate and fluoride and a driven reactor of UF₆ as a gas.