title: Coupling of Subchannel Analysis Tools with Advanced Multiscale Core Simulations
Creators: name: Davies, Sebastian
Publisher: name: University of Liverpool Repository
Year: 2024

description: Nuclear reactors could allow to answer the energy demands and achieve low CO2 emissions in the country while nuclear simulation software can improve the efficiency without affecting the safety of nuclear reactors and therefore, the UK government is currently investing resources in the next generation of PWR and new nuclear simulation software. In nuclear reactors, the physical phenomena such as power production, heat and mass transfer, and fuel behaviour are coupled in between, although in nuclear simulation software, these physical phenomena have often been simulated independently. Several state-of-the-art multiscale and multi-physics software developments, including NURESIM and CASL, are being created, which include improved nuclear codes and coupling software environments. These cannot answer the demands of academia, the industry, and the nuclear regulator in the UK. NURESIM does not generally include the most advanced neutron transport methods, only providing improved coupled reactor physics. CASL generally requires thousands of processor/hours to deliver a solution which cannot be covered by the available computational clusters or workstations, providing full coupled reactor physics in all the reactor core. 

A multiscale and multi-physics software development is being created for the UK, which currently includes a nodal code, a transport code, a subchannel code, and a customized coupling software environment. It will eventually answer the demands of academia, the industry, and the nuclear regulator in the UK. It includes the most advanced neutron transport methods, providing simplified, improved, and full coupled reactor physics. It requires few processor/hours to deliver a solution which can be covered by the available computational clusters or workstations. It provides improved and full coupled reactor physics only in the hottest fuel assemblies with boundary conditions obtained providing simplified coupled reactor physics in all the reactor core. Validation and verification are fundamental for it to become state-of-the-art software. In this PhD project, the multiscale and multi-physics software development has been created along with its associated acknowledgements, validations, and verifications. These acknowledgements, validations, and verifications have outlined or proven several outcomes. 

Initially, an acknowledgement of the neutronics, thermal hydraulics, coupled reactor physics, SCALE-POLARIS code system lattice module, LOTUS, and Open MC transport codes, DYN3D nodal code, and CTF subchannel code were performed through their description. This has allowed an understanding of the theory used in the nuclear codes and of the nuclear codes used in later work. Then, validations and verifications of the accuracy and methodology available in CTF and FLOCAL (module of DYN3D) to provide thermal hydraulics at the fuel rod level were performed through the PSBT benchmark, previously covered by other partners using CTF, and through the FLOCAL developer benchmark not covered before. These have proven that CTF provides high accuracy in 1x1 and 5x5 bundles when compared to experimental data and other thermal hydraulics codes and a wide range of crossflow and turbulent mixing methods in a 2x1 bundle when compared to FLOCAL. 

Later, a one-way DYN3D and CTF coupling and the associated verification of the inner coupling iterations within an outer iteration were performed through the KAIST benchmark, previously tested by other partners using other neutronics codes, and through coupling scripts. These have proven that the DYN3D and CTF coupling provides improved feedback in 17x17 fuel assemblies compared to DYN3D using 1 processor within computational times of 20 minutes compared to 2 minutes. Then, a two-ways DYN3D and CTF coupling and the associated verification of the outer coupling iterations and convergence were performed through modified and created modules within DYN3D and a customized coupling software environment, and through the modified KAIST benchmark. These have proven that the DYN3D and CTF coupling provides improved coupled reactor physics in 17x17 fuel assemblies and 51x51 mini cores compared to DYN3D using 1 processor within computational times ranging from 1 to 10 hours compared to 2 to 20 minutes. 

Finally, a multi ways coupling between LOTUS, CTF and DYN3D and its associated verification were performed through the customized coupling software environment, and through the customized benchmark. These have proven that the LOTUS and CTF coupling with DYN3D provides full coupled reactor physics in a 3x3 quarter core with reflectors composed of 17x17 fuel assemblies or a 34x34 quarter core without reflectors compared to a DYN3D and CTF coupling with DYN3D applying parallelization across 36 processors within computational times ranging from 3 to 24 hours compared to 1 to 8 hours.