In this paper, I present a criticality study on the Louis Slotin “demon core” criticality accident, which happened in Los Alamos Scientific Laboratory (LANL) on 21 May 1946. For the numerical reconstruction of the nuclear system, I used the Monte Carlo Continuous Energy Burnup Code (MCB) developed at the AGH University, Krakow, Poland (Akademia Górniczo-Hutnicza w Krakowie).
I present the influence of the environment on the system criticality in the laboratory at the time of the accident. I consider locations, geometry and material composition of the elements forming the nuclear system: the plutonium core, the beryllium reflector, the human body.
The numerical approach consists of 3 steps. Firstly, the isotopic composition of the core is estimated using the criterion of 10 cents excess reactivity achieved during the accident. Secondly, the effective neutron multiplication factor [keff] in the function of the Be hemisphere angle above the Pu core is shown. Lastly, the influence of each system component on keff is calculated. Additionally, the influence of the position of Slotin’s hand on the criticality and the neutron spectrum in the core is presented.
The study fills the gap in the numerical reconstruction of early criticality accidents and thus helps to preserve critical nuclear knowledge for future generations.
[Keywords: criticality, accident, Louis Slotin, MCB, Monte Carlo]
…4.4. The hand effect: During the accident, the left hand of Louis Slotin was placed directly on the top Be hemisphere, which suggests its large influence on the criticality due to its moderation capabilities. The effect was investigated in a series of calculations considering an increase of the distance between the hand and the top reflector in 10 intervals of 0.5 cm for the reference calculation case (Figure 10). The palm of Louis Slotin was modelled as a 1⁄4 of spherical shell covering the top reflector, while the thumb was modelled as a 1⁄4 of opening in the top Be reflector. The dimensions of the hand, due to its complicated geometrical shape, were normalized to its volume of 362 cm3, calculated using the mass and the density shown in Table 2. Figure 11 presents the effective neutron multiplication factor keff in the function of the distance from the top Be reflector. The influence of the hand is quite important and equals 127 ± 6 pcm for ENDF/B-VII.1 and 138 ± 6 pcm for JEFF3.1 nuclear data libraries. The influence of the whole body of Louis Slotin on the criticality is about 210 pcm (Table 7). Therefore, the hand effect corresponds to about 60–65% of excess reactivity due to the body of Louis Slotin in the considered interval of 5 cm. The results suggest that the placement of the hand was the key factor leading to the prompt critical state of the assembly. The prompt criticality would not have been achieved if the hand had been placed a bit further from the top Be hemisphere.