High neutron flux in core of a nuclear reactor can affect the material of Reactor Pressure Vessel (RPV). The neutron radiation has a detrimental impact on the mechanical properties of the RPV material such as hardening (or embrittlement) while neutrons are absorbed by the material. A major concern in embrittled RPVs is propagation of critical flaw which may cause through-wall cracks. Some transients leading to overcooling of RPV intensify the propagation of the cracks and known as Pressurized Thermal Shock (PTS). Such situation could be created in case of Emergency Core Cooling System (ECCS) actuation which leads to injection of cold water into the cold leg of the primary loop in some accidents, e.g. Loss Of Coolant Accident (LOCA). The investigation of PTS is performed throughout three steps including Probabilistic Safety Assessment (PSA); thermal-hydraulics and structural mechanics i.e. fracture mechanics. The final goal of the thermal-hydraulics analysis step is the prediction of the imposed temperature gradient on the downcomer as a result of single-phase and two-phase phenomena. Depending on the leak size, its location and the operation condition of the plant after LOCA, water in the cold leg can be in single-phase or two-phase condition. Two-phase PTS will occur when steam and water are in the cold leg during the injection of ECCS water. The ECCS water enters the hot steam flow environment in the cold leg and two phase stratified flow propagates in the cold leg by means of density difference between water and steam. Condensation of steam and mass transfer between two phases is the only heat source in this zone and the accurate modeling of DCC plays a significant role to predict temperature profile in downcomer. The interfacial heat transfer coefficient is defined according to eddy contact time with both steam and water at interface which is a function of turbulence characteristics. The high gradient of velocity generates too high turbulence when differential eddy viscosity models are used and some modifications should be considered in turbulence models at the interface. Implementation of turbulence damping function in turbulence eddy frequency transport equation is one of this modification. Although the new source function improves velocity profile of smooth stratified flow, but significant deviations reveal when the vertical motion of the interface is considerable. Also, the value of turbulence kinetic energy decreases substantially by employment of damping function without the other modification. The reduction of turbulence kinetic energy at the interface changes the value of heat transfer coefficient. In this paper, the other source function of turbulence is proposed by consideration of different boundary condition at the interface and the effect of turbulence characteristics on condensation rate is demonstrated. The results show that implementation of damping function, without any special treatment of turbulence kinetic energy, leads to a considerable overestimation of condensation rate which would be improved with employment of proposed turbulence source function at the interface. then, the new turbulence model is used for stratified flow zone in PTS scenario in VVER-1000 RPV to find the effect of interfacial heat transfer coefficient on temperature profile in the cold leg. For this purpose, the plant response to LOCA is simulated by RELAP system code until the injection of ECCS water. The results of RELAP simulation before injection point of ECCS is considered as input of CFD part. CFX code is employed for mixing and stratification zone of cold leg needing 3D nodalization for prediction of temperature profile.
Application of new turbulence modeling in stratified flow of PTS
D’Auria F.
2017-01-01
Abstract
High neutron flux in core of a nuclear reactor can affect the material of Reactor Pressure Vessel (RPV). The neutron radiation has a detrimental impact on the mechanical properties of the RPV material such as hardening (or embrittlement) while neutrons are absorbed by the material. A major concern in embrittled RPVs is propagation of critical flaw which may cause through-wall cracks. Some transients leading to overcooling of RPV intensify the propagation of the cracks and known as Pressurized Thermal Shock (PTS). Such situation could be created in case of Emergency Core Cooling System (ECCS) actuation which leads to injection of cold water into the cold leg of the primary loop in some accidents, e.g. Loss Of Coolant Accident (LOCA). The investigation of PTS is performed throughout three steps including Probabilistic Safety Assessment (PSA); thermal-hydraulics and structural mechanics i.e. fracture mechanics. The final goal of the thermal-hydraulics analysis step is the prediction of the imposed temperature gradient on the downcomer as a result of single-phase and two-phase phenomena. Depending on the leak size, its location and the operation condition of the plant after LOCA, water in the cold leg can be in single-phase or two-phase condition. Two-phase PTS will occur when steam and water are in the cold leg during the injection of ECCS water. The ECCS water enters the hot steam flow environment in the cold leg and two phase stratified flow propagates in the cold leg by means of density difference between water and steam. Condensation of steam and mass transfer between two phases is the only heat source in this zone and the accurate modeling of DCC plays a significant role to predict temperature profile in downcomer. The interfacial heat transfer coefficient is defined according to eddy contact time with both steam and water at interface which is a function of turbulence characteristics. The high gradient of velocity generates too high turbulence when differential eddy viscosity models are used and some modifications should be considered in turbulence models at the interface. Implementation of turbulence damping function in turbulence eddy frequency transport equation is one of this modification. Although the new source function improves velocity profile of smooth stratified flow, but significant deviations reveal when the vertical motion of the interface is considerable. Also, the value of turbulence kinetic energy decreases substantially by employment of damping function without the other modification. The reduction of turbulence kinetic energy at the interface changes the value of heat transfer coefficient. In this paper, the other source function of turbulence is proposed by consideration of different boundary condition at the interface and the effect of turbulence characteristics on condensation rate is demonstrated. The results show that implementation of damping function, without any special treatment of turbulence kinetic energy, leads to a considerable overestimation of condensation rate which would be improved with employment of proposed turbulence source function at the interface. then, the new turbulence model is used for stratified flow zone in PTS scenario in VVER-1000 RPV to find the effect of interfacial heat transfer coefficient on temperature profile in the cold leg. For this purpose, the plant response to LOCA is simulated by RELAP system code until the injection of ECCS water. The results of RELAP simulation before injection point of ECCS is considered as input of CFD part. CFX code is employed for mixing and stratification zone of cold leg needing 3D nodalization for prediction of temperature profile.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
