Neutronics Calculation Advances at Los Alamos: Manhattan Project to Monte Carlo

Abstract

The history and advances of neutronics calculations at Los Alamos during the Manhattan Project through the present is reviewed. We briefly summarize early simpler, and more approximate neutronics methods. We then motivate the need to better predict neutronics behavior through consideration of theoretical equations, models and algorithms, experimental measurements, and available computing capabilities and their limitations. These, coupled with increasing post-war defense needs, and the invention of electronic computing led to the creation of Monte Carlo neutronics transport. As a part of the history, we note the crucial role that the scientific comradery between the great Los Alamos scientists played in the process. We focus heavily on these early developments and the subsequent successes of Monte Carlo and its applications to problems of national defense at Los Alamos. We cover the early methods, algorithms, and computers, electronic and women pioneers, that enabled Monte Carlo to spread to all areas of science.

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

“…It was, because these cross sections then became available for use use in accurate simulation codes. 113 Their use extended well beyond addressing the original critical mass question. The rapidly-improving neutronics simulation capability opened up the ability to rapidly explore a wealth of physics questions, to advance the technologies in both defense and civilian nuclear power arenas.…”

“…The best value today from a ENDF/B-VIII.0 and MCNP6 Monte Carlo transport calculation, 46.4±1.7 kg, is shown as a horizontal line in Figure 23. The figure also shows an accurately-calculated value of 45 kg obtained in 1952 by Bengt Carlson 112 using the Serber-Wilson neutron diffusion method 113 with three neutron energy groups (an approach that was developed by the US and the British during the Manhattan Project, 1944Project, -1945. Note that Carlson and Wilson both had connections with the Montreal/Chalk River Manhattan Project laboratories as described in this issue by Andrews, Andrews and Mason; 114 Carlson came to Los Alamos in 1945, where he spent the rest of his career.…”

Thirty Minutes Before the Dawn

“…8 The Theoretical Division under Hans Bethe's leadership played a central role in advancing the basic science studied during the Manhattan Project. These include shock hydrodynamics (Morgan and Archer, 9 this issue) and neutronics (Sood et al,10 this issue). Bethe and Feynman, both future Nobel laureates, developed an important equation for predicting the expected nuclear fission efficiency, as described by Lestone.…”