Serpent 2 simulations of the historic Haigerloch B8 nuclear reactor of 1945

We use Serpent 2 as a contemporary Monte Carlo tool to simulate the effective neutron multiplication factor keff of the historic Haigerloch B8 nuclear reactor designed by Heisenberg and Wirtz in February to April 1945. The device parameters are either measured in the Haigerloch replica, taken from the historic reports or adapted from literature. We obtain keff = 0.952 which coincides with previously calculated results reported in the literature, but which is larger than the historically reported measured value of keff = 0.85. In addition, we tune the design parameters in order to reveal the most important device characteristics. We find an optimized reactor set-up that, under the condition to use as little uranium and heavy water as possible, is characterized by an increase of the vessel diameter by 35 % which would have brought the reactor to criticality with the available amount of uranium fuel. We show that the bottleneck would have been the lack of a sufficient amount of heavy water which would have been almost two-and-a-half times the then available quantity to reach criticality.