Simulation of Time-Dependent Neutron Populations for Reactor Physics Applications Using the Geant4 Monte Carlo Toolkit

Abstract

<p>When the material or geometry of a reactor varies with time, the neutron flux will respond in the form of a reactor transient. These transients can occur during normal operations when control rods are moved or the reactor is refuelled (CANDU). During a reactor accident, the transient response is especially important because the reactor properties vary quickly with large amplitudes. Therefore, better understanding these conditions allows for improved identification, prevention and mitigation of reactor transients. However, current nuclear simulation codes are generally limited in their ability to model transient behaviour.</p> <p>The NStable code was created to model time-dependent neutron populations in multiplying mediums using the Geant4 Monte Carlo toolkit. The neutron population is allowed to evolve in time, but is periodically renormalized so that the total number of neutrons is constrained within a manageable range. This ensures that the simulation is viable even in highly sub- or supercritical environments. Since Geant4 was not intrinsically designed to track a neutron population over "long" time periods (up to 10 s), the population renormalization mechanisms needed to be created and integrated with Geant4. Additionally, nuclear reactor analysis functionality was added to calculate important quantities such as k_eff.</p> <p>The NStable code was validated using three established nuclear simulation codes: MCNP 5, DRAGON 3.06J, and TART 2005. The validation cases compared spatial distributions and criticality estimates for either homogeneous spheres (uranium-235 or a uranium-heavy water mixture) or the standard CANDU 6 lattice cell. For all three systems, the criticality estimates in NStable agreed with the appropriate validation code within 10 mk (TART for the spheres and DRAGON for the CANDU 6 lattice). Finally, the NStable code was also used to simulate a temperature transient in a UHW sphere where the temperature linear increased by 700 K over 50 ms. In response to the increasing temperature, k_eff decreased by 100 mk over the same period. In the future, transient modelling in NStable should be investigated further to reproduce actual experimental results, and to couple NStable with a thermohydraulics code to simulate a full transient response.</p>