Multi-Physics Approach to the Modeling and Analysis of Molten Salt Reactors

In this book, a multi-physics approach to the modeling and analysis of nuclear reactor core behavior is presented, and applied to study the dynamics of Molten Salt Reactors (MSR). The Multi-Physics Modeling (MPM) approach is implemented in a unified simulation environment, which results able to catch the synergy between the different phenomena involved in the reactor core behavior, whose modeling would otherwise require either the adoption of existing simulation tools with drastic modifications of their structure and possible loss of significant information or the development of purpose-made numerical codes for the specific analyzed situation. The book is organized into four chapters, which are conceived so as to be read independently of each other. Here, the rationale of the R&D activities performed at the Politecnico di Milano by the authors, and giving rise to the book contents, is briefly outlined. In the last years, MSRs have been the subject of a growing and renewed interest from the scientific community in the framework of the Generation IV International Forum. Due to the availability of an advanced conceptual design, the Molten Salt Breeder Reactor (MSBR) developed at the Oak Ridge National Laboratory during the 1960s is usually considered in the literature as reference system for benchmark analyses and validation purposes. Coherently, the MSBR has been chosen as reference configuration for the analyses presented in the book. In such kind of Circulating Fuel Reactor (CFR), like the other MSRs, the coupling between neutronics and thermo-hydrodynamics is a key issue. This feature cannot be neglected in order to perform an adequate description of the reactor dynamic behavior, which shows peculiar aspects with respect to solid-fuelled conventional nuclear power plants. Relevant differences in terms of safety, fuel cycle and technology also distinguish the MSRs from the other nuclear reactors (Chapter 1). The developed MPM methodology and the adopted models for neutronics and thermo-hydrodynamics required a deep investigation for what concerns the assessment and the extension of the simulation environment, represented in the specific case by the finite element COMSOL Multiphysics® software (chosen thanks to its flexible and modular numerical structure), but the same methodology can be applied to other multi-physics platforms. As far as thermo-hydrodynamics is concerned, a generalized approach was developed (Chapter 2) and exploited for the assessment of COMSOL simulations (Chapter 3), making use of a dedicated finite volume computational fluid dynamics code (FLUENT®), in order to better appreciate the differences in numerical approaches to turbulence. The generalized approach was built in order to carefully take into account the molten salt mixture specificities (i.e., a fuel that operates also as coolant), the reactor core power conditions and the heat transfer in graphite. In this context, a Nusselt number correlation was developed, which takes into account the effect of internal heat generation on fluid heat transfer characteristics. As far as neutronics is concerned, a module for the "reactor physics" was built in the COMSOL environment of simulation, and allowed to extend the potentialities of this computing platform. Numerical results were assessed by means of: (i) a code-to-code comparison with dedicated neutron transport codes, in the case of static fuel; (ii) a comparison with simplified neutron kinetics models, representative of the zero-power dynamics, in the case of circulating fuel. After the assessment of the COMSOL capabilities to cope with the adopted models for neutronics and thermo-hydrodynamics, the MPM approach was applied to study the dynamic behavior of a single-channel representative of the average conditions of the MSBR core (Chapter 4). For this case study, the MPM approach resulted particularly suitable because of the strong non-linear coupling between the fuel motion and neutron dynamics, which requires a careful description of the time-space distribution of the physical quantities. Several different transients were analyzed, such as those driven by: reactivity variations due to control rod movements; fuel mass flow rate variations due to the change of the primary pump working conditions; presence of periodic perturbations, due to local precipitation of fissile solid compounds within the molten salt mixture. The analyses gave significant information on the MSBR dynamic behavior and highlighted the several advantages and potentialities offered by the proposed MPM approach (for instance, the "modularity", namely the possibility to include other physical phenomena and couplings). These potentialities are of more general interest in the prospect of studying the design configuration, the dynamics and the control strategy of next generation MSRs, as well as of other nuclear reactor types.