Checking in Otto (Robert) Frisch's 'What Little I Remember', I see that the Los Alamos casualty happened in this way. The theorists already had a very good idea as to what constituted a critical mass, but this needed to be tested. The way they devised to do that was to drop a plug of U-235 near to a doughnut of the same, on a piece of string. Neither the plug, nor the doughnut in themselves constituted a critical mass, but their masses together did. The built-in fail-safe was that if the operator (who was measuring the radiation levels as the masses approached each other) collapsed, the plug would fall straight through the hole in the doughnut, under the effect of gravity, so quickly that an explosion would not occur. One operator's pencil became entangled in the string - so much for theory - unfortunately for him, but fortunately for the rest of Los Alamos the entanglement didn't leave the plug and doughnut perfectly aligned.