Development of an Event Tracking Feature in OpenMC for Neutron Noise Analysis

¹ École Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland
² Argonne National Laboratory, 9700 S. Cass Ave, Lemont, IL 60439, United States
³ United Neux, Copenhagen, Denmark
⁴ Paul Scherrer Institut (PSI), 5232 Villigen, Switzerland

^* This email address is being protected from spambots. You need JavaScript enabled to view it.

Published online: 6 November 2025

Abstract

We present the development and implementation of a neutron event-tracking capability in OpenMC, an open-source, community-driven Monte Carlo radiation transport code. This feature can be utilized for explicit neutron noise simulations, enabling the estimation of integral kinetic parameters such as the prompt decay constant, which are important for accurate predictions of reactor transient behaviors and validation of computational tools. Experimentally, the prompt neutron decay constant (α) can be measured non-invasively during steady-state reactor operation via neutron noise analysis techniques such as the Rossi-alpha method. Accurately simulating these neutron noise measurements requires explicit tracking of individual neutron interactions, preserving detailed timing information essential for subsequent statistical analysis. Previously, such capabilities were primarily restricted to export-controlled software like MCNPX PoliMi or MCNP 6.3. Our open-source implementation in OpenMC offers customizable filters that discriminate and record specific neutron interactions, based on parameters such as reaction type, energy, material, and geometry. Recorded events are stored externally in either OpenMC’s native HDF5 format or the MCPL format for versatility. We demonstrate the application of this feature by employing the Rossi-alpha method for noise analysis on simulated neutron event data. Benchmarking simulations using the Godiva I geometry and a model of the CROCUS zero-power reactor, employing ENDF/B.VII.0 nuclear data, confirm the reliability of our approach through comparisons with Serpent 2 results. This development significantly expands the accessibility of neutron noise simulation capabilities. The eventtracking feature could also see use in other detection applications, such as time-of-flight experiments or radiation imaging.