TRIPOLI-4® Neutron Multiplication Calculations for the Subcritical Experiments of the BeRP Ball Reflected by Tungsten

International audience TRIPOLI-4®** is a general-purpose Monte Carlo radiation transport code developed by the Service d'Études des Réacteurs et de Mathématiques Appliquées (SERMA) at CEA-Saclay. It uses continuous-energy nuclear data to simulate neutron, photon, electron and positron transport in fields like nuclear criticality safety, radiation protection and shielding, fission reactor physics, and nuclear instrumentation [1]. Neutron multiplication calculations of subcritical systems are important for several application areas including criticality safety, reactor ex-core monitoring, and nonproliferation of nuclear materials. The subcritical states are required in different nuclear fuel cycle steps including the fuel loading of shipping casks, storage pools, and nuclear reactor cores. One of the TRIPOLI-4® simulation modes can perform the subcritical analysis and calculate multiplication factor in fixed-source problems in which neutron-multiplicative reactions are produced. Neutron multiplication calculations of TRIPOLI-4® shielding mode for (n, 2n) reactions were previously validated against KANT experiments using a 14 MeV fusion neutron source [2]. In this work, the fixed-source criticality mode of TRIPOLI-4® code is used to calculate the neutron multiplication of subcritical systems with fissile materials to consider neutrons generated from (n,f) reactions of successive fission generations. The BeRP (Beryllium Reflected Plutonium) ball reflected by tungsten subcritical experiments were performed by DOE and LANL teams and published in the 2016 edition of ICSBEP handbook [3]. The reactivity range of BeRP/W experiments is from keff = 0.78 to keff = 0.94 using seven different thicknesses of tungsten as reflectors. The He-3 tubes inside polyethylene and wrapped in Cd were used to measure the neutron counting rates and neutron leakage multiplication. The TRIPOLI-4® modeling of the BeRP/W experiments and the calculated parameters including MT (total multiplication), ML (leakage multiplication), and other associated terms will be evaluated and presented in the full paper. The impact of the initial neutron source distribution in space and energy will be considered. Additional reflector materials modifying the leaking neutron energy spectra of related subcritical systems are also interesting to investigate [4]. ** TRIPOLI-4® is a registered trademark of CEA. The author thanks EDF for partial financial support.[1] E. Brun et al., Tripoli-4®, CEA, EDF and AREVA reference Monte Carlo code, Annals of Nuclear Energy 82, 151-160, (2015).[2] Y.-K. Lee, Analysis of the KANT Experiment on Beryllium Using TRIPOLI-4 Monte Carlo Code, Fusion Eng. and Design, 86, (2011) 2246.[3] B. Richard and J. Hutchinson, NEA/NSC/DOC(95)03/IX, FUND-NCERC-PU-HE3-MULT-002, ICSBEP, (2016). [4] J. Hutchinson et al., Validation of statistical uncertainties in subcritical benchmark measurements: Part I and II, Annals of Nuclear Energy 125, 55-62 and 342-359, (2019).