[en] Highlights: • It is required to evaluate re-evolved iodine from sump water after LOCA. • pH evaluation based on Gibbs free energy minimization. • Program was developed to evaluate chemical equilibrium and pH solutions. • Predictions are in good agreement with experimental data. - Abstract: Radioactive iodine, which is released into the atmosphere of the containment building, is absorbed into the containment spray water and dissolved to be ionized. This iodine-rich water is then transported to the in-containment refueling water storage tank (IRWST) in APR1400 nuclear power plants. When the pH of the water is below 7, the dissolved iodine converts to molecular iodine and re-evolves from the water and returns to the atmosphere. A series of studies have been conducted in order to evaluate the iodine re-evolution from the IRWST. This study consists of two parts: the pH evaluation method and the evaluation of the iodine re-evolution. This paper presents the first part, i.e. the pH evaluation method. The equilibrium concentrations of various chemicals in a solution are determined at the minimum Gibbs’ free energy. This method is useful for complex reactant problems rather than equilibrium constants method because the latter method requires numerous equilibrium constants and there might be missing equilibrium constants associated with the solution. The calculated pH values of solutions are compared with the experimental measurements in order to validate this method and the thermodynamic data of the chemicals incorporated into the program. The estimated values for solutions are in good agreement with the experimental measurements within a difference of less than 3.3%.

[en] Highlights: • It is required to evaluate re-evolved iodine from sump water after LOCA. • Transport of iodine and chemicals influencing pH were analyzed using CFD. • Chemical conditions of the iodine-rich region suppress iodine re-evolution. • The current evaluation method for I_2 re-evolution is excessively conservative. - Abstract: Radioactive iodine that is released during a postulated loss of coolant accident is dissolved into the containment spray water and transported into the in-containment refueling water storage tank (IRWST). The re-evolution of iodine from the water is a safety concern. In this study, three-dimensional computational fluid dynamics (CFD) analyses are conducted in order to analyze the transport of chemical species including iodine in the IRWST and to calculate the amount of iodine that re-evolves from the IRWST water. The CFD analyses demonstrate that the pH of water is high where the iodine concentration is high. Considering that the creation rate of molecular iodine declines as the pH increases, it can be understood that the iodine re-evolution is not so strong in practical situations because the chemical conditions of the iodine-rich region suppress the re-evolution of the iodine. In addition, four different methods for evaluating the amount of re-evolved iodine are examined. The amount of re-evolved iodine calculated using the total-volume-average values, which are currently used for safety analyses, appear to be significantly higher than those determined using other methods. The amount of re-evolved iodine estimated using a realistic method with a conservative assumption of volatilization appears to be approximately one thousandth of that evaluated using the current method. This implies that the current method is very conservative.

[en] Air-water two-phase flow through a narrow vertical rectangular channel has drawn an increased research interest in recent decades owing to its numerous applications such as in high heat-flux compact heat exchangers, plate type research reactors, high performance micro-electronics and space applications. In the narrow rectangular flow channel, the two-phase flow characteristics is different from those in conventional channels, largely because the bubbles are being sandwiched between the two surfaces. From this point of view, the authors did the adiabatic air-water flow experiment in vertical upward and downward direction in narrow rectangular channel with the aim of (1) measuring void fraction and differential pressure, (2) plotting the flow regime maps for both the flow directions based on visual observation and the void fraction, and (3) analysis and comparison of the present results with the previous studies. Falling film flow is absent in vertical upward flow direction and is observed in only vertical downward flow direction at low superficial liquid and gas velocities.

[en] The adequacy of external cooling by SFP cooling system and of internal pool water circulation by natural convection are analyzed with SFP filled to full storage capacity about 20 years storage. In order to take into account the interaction between the racks, which is not considered in convectional Computational Fluid Dynamics(CFD) methods, which enables to analyze the global behavior of the cooling pool including particularities of all racks. I compared the SFP licensing report of Hanul 2 with realistic CFD analysis models and assessed the impact of model difference. CFD results of the SFP licensing report in Hanul 2 are compared with the realistic CFD analysis models. Maximum local water temperature of licensing model was evaluated to be higher than the maximum temperature of realistic model. Therefore, licensing model is more conservative than realistic model. If using a realistic model in the thermal evaluation of SFPs, we can achieve more heat margin

[en] The relatively high temperature, differential pressure between the inside and outside of the fuel rod, and hoop stress will lead to permanent deformation of the cladding during dry storage period. Although extensive efforts over the last several decades have been devoted toward proving the technical basis for the dry storage of spent fuel assemblies, these efforts have done mainly for the fuel assemblies with average burnup less than 45 GWd/MTU. In accordance with US NRC ISG-11 Rev.3, high burnup fuel with average burnup exceeding 45 GWd/MTU may have comparatively thin cladding walls from in-reactor formation of oxide and hydride, therefore, the maximum thickness of cladding oxide and hydride layer should be specified for evaluating the structural integrity of the cladding during dry storage. EPRI has also reported a newly developed a hydride reorientation model for irradiated Zirconium alloy cladding materials, but it has not been implemented by coupled analysis method with fuel rod performance code. This paper presents modified creep modeling incorporating the effect of hydrogen on cladding creep rate using data from Bouffioux et al. as well as the evaluation results of the creep and hydride reorientation analysis for the high burnup spent fuel during 40 year dry storage by FALCON code. The creep and hydride reorientation study using the advanced model with fuel performance code for the spent fuel in dry storage was investigated in this paper, which demonstrates that spent fuel in dry storage can be reliably analyzed in terms of the true physical conditions of the cladding. This paper focuses on evaluating the cladding temperature, rod internal pressure, hoop stress and strain and radial hydride fraction of the spent fuels having assembly-average burnup of 60 GWd/MTU during the dry storage.

[en] The fuel temperature should also generally be maintained below 570 .deg. C for the short-term off-normal and accident conditions. The decay heat removal system is designed by passive or an active cooling system for the dry storage of the spent fuel. A modular storage system was proposed as part of the effort. A modular storage system is a canister-based spent nuclear fuel dry storage system that consists with three or seven canisters. In this study, thermal performance evaluation carried out with computational fluid dynamics(CFD) code to estimate temperature of concrete overpack, canister structure and air according to NUREG-2152 as additional guidance on the use of CFD. Thermal analyses of ventilation system have been carried out for the determination of the optimum inlet and outlet duct height. The computational fluid dynamics code ANSYS CFX 17 was used for the thermal analysis of modular storage system. In this study, thermal performance evaluation carried out with ANSYS CFX, commercial CFD code, to estimate temperature of concrete overpack, canister structure and air. The purpose of this assessment is to derive the area of inlet and outlet for storing a number of canisters. As a result of thermal analysis, the hydraulic diameter has a significant impact on the heat transfer. When the hydraulic diameter are increased, the heat transfer increases.

[en] The purpose of this study is to investigate the results of potential fuel failure on the criticality for a transport cask. Criticality evaluations for fuel failure scenarios in which the geometric structure or conditions of spent fuel assemblies and fuel rods are changed due to beyond design basis accidents are performed for KN-18 transport cask. KENO-VI was used to evaluate the criticality for those conditions of the cask. Critical evaluations of the KN-18 transport cask were performed using the KENO-VI code. In case of normal condition, we compare MCNP6 code with the result for modeling verification and confirm that the criticality is within the statistical error range. Criticality evaluations were performed for three types of fuel failure scenarios; loss of a single fuel rod, loss of multiple fuel rods, and loss of rod cladding.

[en] The purpose of this study is to investigate the results of potential fuel failures on the external radiation dose rates for a transport cask. The dose rates may be changed by fuel failures which are considered very improbable. Shielding evaluations for fuel failure scenarios in which the geometric structure or conditions of spent fuel assemblies and fuel rods are changed due to beyond design basis accidents are evaluated for KN- 18 transport cask. MAVRIC was used to evaluate the dose rates for the conditions of the cask. In this study, the shielding evaluations of KN-18 cask for spent fuel transportation were carried out for normal conditions and various fuel failure scenarios. It is expected that the shielding evaluations using MAVRIC for normal and these fuel failure scenarios can be used for the development of a new cask for future transportation or storage purposes.

[en] A good understanding of the behavior of iodine is required to evaluate the safety and emergency procedures after a LOCA. The quantity of re-evolved iodine is related to pH level, temperature, and iodine concentration of water pool. In the calculation of pH for water pool, sequence calculations must consider this variable if any aqueous iodine is present, even if it is initially present in stable forms. The present study consists of two parts: the pH evaluation and the evaluation of the iodine re-evolution. The current paper focuses on the pH evaluation method, the development of a user-defined function (UDF) and the iodine re-evolution from the water pool. CFD that incorporates the UDF was used in this study to calculate the local pH level in the transient condition. The amount of re-evolved iodine was calculated based on the iodine concentration, temperature, and pH. The transportation and resulting distribution of the iodine concentration, temperature, and pH were calculated using transient analyses with CFD. The quantity of reevolved iodine was obtained with several assumptions. The quantitative evaluation of re-evolved iodine during a LOCA in a commercial nuclear power plants is done in two stages. The first stage is to calculate the pH in the water pool, and the second stage is to calculate the quantity of re-evolved iodine. Evaporated iodine, from the water pool water to the containment atmosphere, can be estimated from characteristic iodine behaviors and pH calculations. The 3D CFD analysis results show that the pH reached 7.0 after 149.5 minutes. Near the spillway, the change in averaged pH was faster than the change in wholevolume averaged pH. Evaluating the amount of reevolved iodine were examined using four different methods. As a result of our evaluation of iodine reevolution, the initial molecular iodine concentration of a water pool has a significant impact on the amount of gaseous iodine, more so than the pH or temperature, due to the locally similar distributions of TSP and iodine

[en] The objective of a thermal evaluation of a spent fuel storage system is to ensure that a decay heat removal system is capable of a reliable operation so that the temperatures of the spent fuel and storage system components remain within the allowable limits under normal, off-normal, and accident conditions. The SFP temperature should be maintained under the saturation temperature. At the top of racks, the location is approximately 7.8 m below the water surface. Therefore, acceptance criteria for local maximum temperature of SFP is the local saturation temperature that is 115℃. From the CFD analysis results of Case A and Case B those are concluded that the local water temperature remains below saturation temperature. The local maximum temperature of Case B is higher than Case A about 1.4℃. Thus, the analytical method using un uniformed heat generation rate based on peaking factor has more conservativeness than uniformed heat generation rate in the conservative views of the safety.