Coupled Thermal-hydraulics And Neutron-physics Analysis Of Scwr Core With Mixed Spectrum

Among the six GEN-IV reactor concepts recommended by the Gen-IV International Forum, supercritical water-cooled reactor (SCWR) is the only reactor type with water as coolant. Due to its economical advantage, technology and experience continuity, SCWR has achieved significant interests of nuclear industries and research institutions. In the last few years, extensive R&D activities have been launched covering various aspects of SCWR development. A large number of pre-conceptual designs of SCWR core have been proposed, including cores with thermal spectrum and fast spectrum, respectively.The main objectives of this thesis are: (i) to develop numerical tools for the design analysis of SCWR cores, (ii) to propose a conceptual design of SCWR core, and (iii) to investigate the core performance and optimize the core design parameters based on the new simulation tool. For the analysis of SCWR core, a coupled simulation tool of neutron-physics with thermal-hydraulics (sub-channel analysis) is selected. The sub-channel code COBRA-SC is developed and coupled with neutronic code. Verification and validation is performed to assess the accuracy of the codes used. At first, new fuel assembly (FA) designs, i.e. two row fuel assembly and multi-layer fuel assembly are proposed and optimized separately. For the core design, a new SCWR concept is proposed based on mixed neutron energy spectrum (SCWR-M). The core consists of two zones with different neutron spectrums, one with thermal spectrum and the other with fast spectrum. It combines the merits of both thermal and fast SCWR cores, and at the same time minimizes their shortcomings.The thesis consists of several steps: The first step is devoted to the development and validation of the numerical codes for their application to SCWR fuel assemblies and cores. (i) The new sub-channel code COBRA-SC is developed, in which the moderator channel and several new heat transfer correlations are introduced. (ii) The discontinuity factor and pin-power reconstruction model are implemented to the SKETCH-N code. Preliminary validation is performed based on comparison with MCNP results. (iii) The coupled procedure between COBRA-SC and SKETCH-N is realized by the internal data exchange. This procedure can be applied to steady-state calculation of SCWR with burn-up characters.In the second step, fuel assemblies for the thermal spectrum zone are proposed and optimized, using the developed analysis tools. (i) Two row fuel assemblies are proposed to achieve a uniform moderation. (ii) The geometric parameters of the fuel assembly are optimized, such as fuel rod diameter, pitch to diameter ratio (P/D) and clearance of the wall. Two designs are recommended as the reference designs for the thermal spectrum zone: the fuel rod diameter is 7~8mm, the P/D should be 1.15~1.20.In the third step, attention is paid to the design and optimization of fuel assemblies in the fast spectrum zone. (i) At first, the multi-layer fuel rod structure is proposed. The feasibility of the multi-layer fuel assembly is studied. (ii) By comparing different axial arrangements of multi-layer fuel assembly, an optimized structure with 11 layers is identified and recommended. (iii) Geometric parameters are optimized based on systematic sensitivity studies: the fuel rod diameter is 7.5~8mm, the P/D should be 1.15~1.20.In the forth step, the neutron-physical and thermal-hydraulic performance of the SCWR-M core is investigated. (i) By selecting and matching the fuel assemblies of the thermal and fast zone, three SCWR-M core arrangements are proposed. (ii) Based on the developed numerical tools and the results obtained, measures are proposed to improve the neutronic and thermal-hydraulic behavior of the core. (iii) At the end, one SCWR-M design (design 2) is selected as the reference design for future applications due to its better thermal-hydraulic and neutronic performance.The numerical tools developed in this thesis can be applied to future activities in the design of SCWR cores. The results achieved provide valuable information for future SCWR R&D in China.