Criticality Experiments with Fast ²³⁵U and ²³⁹Pu Metal and Hydride Systems During the Manhattan Project

Abstract

Criticality experiments with 235 U (metal and hydride) and 239 Pu (metal) were performed during the Manhattan Project. Results from these experiments provided necessary information for the success of the Manhattan Project. These experiments have been previously described in compilations made after the Manhattan Project, 1-3 but those works are either lacking in technical details or are not publicly available. This work aims to provide detailed information while showcasing the enduring impact of these experiments 75 years after they were performed. Furthermore, we use modern computational methods embodied in the MCNP6 code and ENDF data to analyze and interpret these historic measurements. The world's first four criticality accidents are also discussed, as lessons learned from these helped shape the field of criticality experiments.

Nuclear Science for the Manhattan Project and Comparison to Today’s ENDF Data

“…For brevity this paper describes just bare critical mass comparisons, but the focus at Los Alamos in 1943 quickly became tamped (reflected) critical masses (as described in Hutchinson's paper 22 in this issue). Tamped critical assemblies typically require a little less than half the amount of 235 U or 239 Pu compared to the bare assemblies, and therefore they use the available precious special nuclear material more efficiently.…”

“…A two-pronged approach was taken, one being to measure the fundamental "differential" cross sections and neutron energy and scattering angle spectra, the other being to measure integral quantities. This paper focuses on the former approach, while other papers in this issue by Hutchinson, Kimpland, Myers et al [22][23][24] and Ref. 25 focus on the latter approach: integral critical assembly measurements.…”

“…But it should be remembered that for most of the Manhattan Project, substantial amounts of enriched 235 U and plutonium were simply not available in Los Alamos -they only started to arrive in late 1944 and 1945. Metal "25" c spheres started to become available in October 1944, but the larger spheres were not available until April 1945 as described by Hutchinson; 22 plutonium "49" spheres c Los Alamos shorthand used the last number of the isotope's Z followed by the last number of its A value, so 235 U is "25", and is also sometimes written u5; 239 Pu is "49" and also pu9, etc. were only available after February/March1945.…”

“…Therefore many fascinating topics are not covered here or are only covered briefly, including: tamped critical masses (see this issue, Ref. 22 ); fission delayed-neutron measurements; spontaneous fission properties; (alpha,n) reactions that meant impurities would be a concern and plutonium purification methods had to be developed (see this issue, Ref. 32 ); lower energy neutron resonance and thermal fission measurements; fission product radiochemical measurements (see this issue, Ref.…”

Foreword: Manhattan Project Nuclear Science & Technology Developments at Los Alamos

“…Papers on the first fast critical assemblies (Hutchinson et al 6 ) and pulsed and solution assembly experiments (Kimpland et al 7 ) provide details about how critical masses were determined and how they influenced subsequent research across the world on nuclear criticality and criticality safety. The Los Alamos "water boiler" assembly was the world's third reactor to become operational (in 1944, after Chicago's CP-1 and Oak Ridge's X-10 piles), the first to use a solution, and the first to use enriched uranium fuel.…”