Yasushi Uehara

Evaluation Methods for Corrosion Damage of Components in Cooling Systems of Nuclear Power Plants by Coupling Analysis of Corrosion and Flow Dynamics (III) : Evaluation of Wall Thinning Rate with the Coupled Model of Static Electrochemical Analysis and Dynamic Double Oxide Layer Analysis

Flow accelerated corrosion (FAC) is divided into two processes: a corrosion (chemical) process and a flow dynamics (physical) process. The former is the essential process to cause FAC and the latter is the accelerating process to enhance FAC occurrence. The chemical process in the surface boundary layer is analyzed to evaluate FAC rate. Contributions of flow dynamics on wall thinning rate due to FAC are expressed as a function of mass transfer coefficient but not that of flow velocity. FAC evaluation procedures were divided into 5 steps as follows. (1) Flow pattern and temperature in each elemental volume along the flow path were obtained with 1D computational flow dynamics (CFD) codes, (2) corrosive conditions, e.g., oxygen concentration and electrochemical corrosion potential (ECP) along the flow path were calculated with a hydrazine oxygen reaction code, (3) precise flow patterns and mass transfer coefficients at the structure surface were calculated with 3D CFD codes, (4) danger zones were evaluated by coupling major FAC parameters, and then, (5) wall thinning rates were calculated with the coupled model of static electrochemical analysis and dynamic double oxide layer analysis at the identified danger zone. Anodic and cathodic current densities and ECPs were calculated with the static electrochemistry model and ferrous ion release rate determined by the anodic current density was used as input for the dynamic double oxide layer model. Thickness of the oxide film and its characteristics determined by the dynamic double oxide layer model were used for the electrochemistry model to determine the resistances of cathodic current from the bulk to the surface and anodic current from the surface to the bulk. Two models were coupled to determine local corrosion rate and ECP for various corrosive conditions. The calculated results of the coupled models had good agreement with the measured ones.

Evaluation Methods for Corrosion Damage of Components in Cooling Systems of Nuclear Power Plants by Coupling Analysis of Corrosion and Flow Dynamics (II): Evaluation of Corrosive Conditions in PWR Secondary Cooling System

Flow accelerated corrosion (FAC) is divided into two processes: a corrosion (chemical)process and a flow dynamics (physical) process. The former is the essential process to cause FAC and the latter is the accelerating process to enhance FAC occurrence. The chemical process in the surface boundary layer can be analyzed to evaluate FAC rate. In this paper, corrosive conditions along the flow path of the PWR secondary cooling system were evaluated. To do this, flow velocity and temperature in each elemental volume along the flow path were obtained with 1D computational flow dynamics (CFD) codes, distribution of oxygen concentration along the flow path was calculated with a oxygen hydrazine reaction code, and then electrochemical corrosion potential (ECP) was evaluated by using the Evans diagram. In the proposed calculation procedures for corrosive conditions, the oxygen hydrazine reactions were divided into bulk and surface reactions and the oxidation reaction of hydrazine on the surface was considered to obtain ECP under hydrazine coexisting conditions. Calculations of precise flow patterns and mass transfer coefficients at the structure surface made with 3D CFD codes and calculations of wall thinning rates made with the coupled model of static electrochemical analysis and dynamic double oxide layer analysis agreed with the calculations of corrosive conditions to evaluate FAC rate.

Next Generation Safety Analysis Methods for SFRs—(8) Analyses of Eutectics Between Fuel and Steel in Metal Fuel With FPMD Code VASP

There are two main objectives in this study. One is to estimate atomic diffusion coefficients in eutectic reaction between metal fuel and cladding materials in order to establish the atomic diffusion model for the COMPASS code. The other is to estimate their material properties such as Young’s modulus in high temperature up to near melting points in core disruptive accidents (CDAs) in Sodium-cooled Fast Reactors (SFRs). We used the first principle molecular dynamics (FPMD) code VASP to realize the two objectives. We tried to understand the initiation mechanism of eutectics based on change of electronic state energy accompanied by change of Kohn-Sham energy, including phonon effect. In this project [1], three methods, phase diagram calculation (CALPHAD), classical molecular dynamics (CMD), and FPMD, are employed to understand the mechanism of eutectics and to introduce dynamic characteristics in eutectic phenomena into the COMPASS code.Copyright

Evaluation of Wall Thinning Rate Due to Flow Accelerated Corrosion With the Coupled Models of Electrochemical Analysis and Double Oxide Layer Analysis

Systematic approaches for evaluating flow accelerated corrosion (FAC) are desired before discussing application of countermeasures for FAC. Future FAC occurrence should be evaluated to identify locations where a higher possibility of FAC occurrence exists, and then, wall thinning rate at the identified FAC occurrence zone should be evaluated to obtain the preparation time for applying countermeasures. Wall thinning rates were calculated with the coupled models of static electrochemical analysis and dynamic double oxide layer analysis. Anodic current density and electrochemical corrosion potential (ECP) were calculated with the electrochemistry model based on an Evans diagram and ferrous ion release rate determined by the anodic current density was applied as input for the double oxide layer model. The thickness of oxide layer was calculated with the double oxide layer model. The dependences of mass transfer coefficients, oxygen concentrations ([O2 ]), pH and temperature on wall thinning rates were calculated with the coupled model. It was confirmed that the calculated results of the coupled models resulted good agreement with the measured ones. The effects of candidates for countermeasures, e.g., optimization of N2 H4 injection point into the feed water system, on FAC mitigation was demonstrated as a result of applying the model.Copyright

Next Generation Safety Analysis Methods for SFRs—(5) Structural Mechanics Models of COMPASS Code and Verification Analyses

A five-year research project started in FY2005 (Japanese Fiscal Year, hereafter) to develop a code based on the Moving Particle Semi-implicit (MPS) method for detailed analysis of core disruptive accidents (CDAs) in sodium-cooled fast reactors (SFRs). The code is named COMPASS (Computer Code with Moving Particle Semi-implicit for Reactor Safety Analysis). CDAs have been almost exclusively analyzed with SIMMER-III [2], which is a two-dimensional multi-component multi-phase Eulerian fluid-dynamics code, coupled with fuel pin model and neutronics model. The COMPASS has been developed to play a role complementary to SIMMER-III in temporal and spatial scale viewpoint; COMPASS for mesoscopic using a small window cut off from SIMMER-III for macroscopic. We presented the project’s outline and the verification analyses of elastic structural mechanics module of the COMPASS in ICONE16 [1]. The COMPASS solves physical phenomena in CDAs coupling fluid dynamics and structural dynamics with phase changes, that is vaporization/condensation and melting/ freezing. The phase changes are based on nonequilibrium heat transfer-limited model and all “phase change paths” considered in SIMMER-III are implemented [20]. In FY2007, the elastoplastic model including thermal expansion and fracture are formulated in terms of MPS method and implemented in the COMPASS, where the model adopts the von Mises type yield condition and the maximum principal stress as fracture condition. To cope with large computing time, “stiffness reduction approximation” was developed and successfully implemented in the COMPASS besides parallelization effort. Verification problems are set to be suitable for analyses of SCARABEE tests, EAGLE tests and hypothetical CDAs in real plants so that they are suggesting issues to be solved by improving the models and calculation algorithms. The main objective of SCARABEE-N in-pile tests was to study the consequences of a hypothetical total instantaneous blockage (TIB) at the entrance of a liquid-metal reactor subassembly at full power [21]. The main objectives of the EAGLE program consisting of in-pile tests using IGR (Impulse Graphite Reactor) and out-of-pile tests at NNC/RK are; 1) to demonstrate effectiveness of special design concepts to eliminate the re-criticality issue, and 2) to acquire basic information on early-phase relocation of molten-core materials toward cold regions surrounding the core, which would be applicable to various core design concepts [22, 23]. In this paper, the formulations and the results of functional verification of elastoplastic models in CDA conditions will be presented.Copyright

Next Generation Safety Analysis Methods for SFRs—(6) SCARABEE BE+3 Analysis With SIMMER-III and COMPASS Codes Featuring Duct-Wall Failure

Governing key phenomena in core disruptive accidents (CDAs) in sodium-cooled fast reactors (SFRs) are supposed to be (1) fuel pin failure and disruption, (2) molten pool boiling, (3) melt freezing and blockage formation, (4) duct wall failure, (5) low-energy disruptive core motion, (6) debris-bed coolability, and (7) metal-fuel pin failure with eutectics between fuel and steel [1]. Although the systematic assessment program for SIMMER-III [4–7] has provided a technological basis that SIMMER-III is practically applicable to integral reactor safety analyses, further model development and validation efforts should be made to make future reactor calculations more reliable and rational. For mechanistic model development, a mesoscopic approach with the COMPASS code [1, 2, 3] is expected to advance the understanding of these key phenomena during event progression in CDAs. The COMPASS code has been developed since FY2005 (Japanese Fiscal Year, hereafter) to play a complementary role to SIMMER-III. In this paper, the overall analysis of SCARABEE-BE+3 test with the SIMMER-III and those with COMPASS, focusing the duct wall failure in a small temporal and spatial window cut from the SIMMER-III analysis results of the test, are described.Copyright