Modified Evaporation Model for ATHLET System Code in a Passive Containment Cooling System for Nuclear Safety

Influence of Passive Containment Cooling System on containment thermal hydraulic transients during a postulated severe accident

Based on the outside cooling of the Passive Containment Cooling System (PCCS), a simplified containment mathematical model was built and was numerically solved. Air circulation driven by natural forces for a steady condition along with the coupled thermal-hydraulic phenomena was then obtained and analyzed. Variations of humid air temperature and velocity, water film sub-cooling and heat fluxes along the flow direction were discussed and driving forces were compared to obtain the ranked important phenomena of the PCCS outside cooling. The conclusions are significant to reasonably understand the PCCS outside transport phenomena.

The heat transfer characteristics of passive containment cooling system (PCS) are very important to protect the nuclear reactor from early release in some severe accident scenarios. In this paper, based on the natural convection and the VOF (Volume of Fluid) multiphase model, the two-phase flow and heat transfer characteristics of the water film of PCS at postulated accident was numerically investigated. To economize the calculation resource, a two dimensional axis symmetry cooling system was included in the simulation model owing to the symmetry of the system. The transportation of mass and energy during the phase change at film interface was established by adding source terms to the mass and energy equations with User Defined Function (UDF). Because of the lower heat capacity of the cooling gas, the air flow rate and flow pattern of natural convection were the governing factor of the heat transfer through containment. Generally, the heat transfer rate is very lower because the heat and flow attached layer at air baffle and water film are very thick due to the flat plate air baffle, but stronger vortex produced by air baffle will reduce the heat transfer because of damage of the water film. The dimensions and the shape of the air baffle have important effect on air flow rate and flow pattern of the natural convection, and the optimal option was chosen based on the simulation results.

To evaluate the heat removal capability of a water wall type cooling system, which is one passive containment cooling system (PCCS), the thermal hydraulic behavior in the suppression pool (S/P) and the outer pool (O/P, flat plate water wall) have been investigated experimentally. The following results were obtained. (1) A thermal stratification boundary, which separates the pools into the upper high temperature and lower low temperature regions, was formed just below the vent tube outlet. (2) Convection heat transfer characteristics in the S/P and O/P along the primary containment vessel (PCV) wall had no significant differences and were those of natural convection. Correlation of the natural convection heat transfer up to the Ra number of 2×1014 was obtained. (3) Vertical variations of local condensation heat transfer coefficients under a noncondensable gas presence were within ±10% of the average value for the 4.7 m heat transfer length. An experimental correlation for the average condensation heat tran...

Passive Containment Cooling System (PCCS) response with Drywell Gas Recirculation System (DGRS) activated during a severe accident scenario with release of non-condensable gas

A condensation experiment in the presence of noncondensable gas in a vertical tube of the passive containment cooling system of the CP-1300 is performed. The experimental results show that the heat transfer coefficients (HTCs) increase as the inlet air mass fraction decreases and the inlet saturated steam temperature decreases. However, the dependence of the inlet mixture Reynolds number on the HTC is small for the operating range. An empirical correlation is developed, and its predictions are compared with experimental data to show good agreement with the standard deviation of 22.3%. The experimental HTCs are also compared with the predictions from the default and the alternative models used in RELAP5/MOD3.2. The experimental apparatus is modeled with two wall-film condensation models in RELAP5/MOD3.2 and the present model, and simulations are performed for several subtests to be compared with the experimental results. Overall, the simulation results show that the default model of RELAP5/MOD3.2 underpredicts the HTCs, and the alternative model overpredicts them, while the present model predicts them well throughout the condensing tube.

To evaluate the system pressure response of a water wall type containment cooling system, which is one of the passive safety systems, the evaporation and condensation behaviors in a suppression chamber have been experimentally examined. In the system, the suppression pool water evaporates from the pool surface, passing into the wetwell due to pool temperature rise, while steam in the wetwell condenses on the steel containment vessel wall due to the heat release through the wall. The wetwell is a gas phase region in the suppression chamber and its pressure, which is expressed as the sum of the noncondensable gas pressure and saturated steam pressure, is strongly affected by the evaporation heat transfer from the suppression pool surface and condensation heat transfer on the containment vessel wall. Based on the measured temperature profiles near the heat transfer surface and the wetwell pressure using two apparatuses, evaporation and condensation heat transfer coefficients were evaluated. The following results were obtained. (1) Both heat transfer coefficients increased as the ratio of the steam partial pressure to the total pressure increased. (2) Comparison of the results from two types of test apparatuses confirmed that the size of the heat transfer surface did not affect the heat transfer characteristics within these tests. (3) The heat transfer coefficients were expressed by the ratio of the steam to noncondensable gas logarithmic mean concentration, which considered the steam and gas concentration gradient from the heat transfer surface to the wetwell bulk.

RELAP5 MOD3.1.1 is being used to simulate Loss of Coolant Accidents (LOCA) for the Simplified Boiling Water Reactor (SBWR) being proposed by General Electric (GE). One of the major components associated with the SBWR is the Passive Containment Cooling System (PCCS) which provides the long-term heat sink to reject decay heat. The RELAP5 MOD3.1.1 code is being assessed for its ability to represent accurately the PCCS. Data from the Phase 1, Step 1 Heat Transfer Tests performed at Toshiba`s Gravity-Driven Integral Full-Height Test for Passive Heat Removal (GIRAFFE) facility will be used for assessing the ability of RELAP5 to model condensation in the presence of noncondensables. The RELAP5 MOD3.1.1 condensation model uses the University of California at Berkeley (UCB) correlation developed by Vierow and Schrock. The RELAP5 code uses this heat transfer coefficient with the gas velocity effect multiplier being limited to 2. This heat transfer option was used to analyze the condensation heat transfer in the GIRAFFE PCCS heat exchanger tubes in the Phase 1, Step 1 Heat Transfer Tests which were at a pressure of 3 bar and had a range of nitrogen partial pressure fractions from 0.0 to 0.10. The results of a set of RELAP5 calculations al these conditions were compared with the GIRAFFE data. The effects of PCCS cell nodings on the heat transfer process were also studied. The UCB correlation, as implemented in RELAP5, predicted the heat transfer to {+-}5% of the data with a three-node model. The three-node model has a large cell in the entrance region which smeared out the entrance effects on the heat transfer, which tend to overpredict the condensation. Hence, the UCB correlation predicts condensation heat transfer in the presence of noncondensable gases with only a coarse mesh. The cell length term in the condensation heat transfer correlation implemented in the code must be removed to allow for accurate calculations with smaller cell sizes.

An experimental study of air–steam condensation on the exterior surface of a vertical tube under natural convection conditions

The Passive Containment Cooling System is the key means of removing core residual heat during severe LOCA accidents. Based on a certain type of nuclear power plant, a passive containment residual heat removal exchanger were designed to be row-tube type and researched in this article. The major design parameters and dynamic response characteristics of this exchanger were given. The analysis showed that the number of pipe are closely related with the fluid flow condition in the heat exchanger tube, the pressure in the containment changes linearly with the steam quality under LOCA, and this heat exchanger can ensure the integrity of the containment under severe accident conditions.

Scaling analyses for the open flashing-driven natural circulation system

Experimental study of steam and steam–air mixture condensation over vertical chrome-plated tube and polished tube exterior surface

To evaluate the system pressure response of a water wall type containment cooling system, which is one of the passive safety systems, the evaporation and condensation behaviors in a suppression chamber have been experimentally examined. In the system, the suppression pool water evaporates from the pool surface, passing into the wetwell due to pool temperature rise, while steam in the wetwell condenses on the steel containment vessel wall due to the heat release through the wall. The wetwell is a gas phase region in the suppression chamber and its pressure, which is expressed as the sum of the noncondensable gas pressure and saturated steam pressure, is strongly affected by the evaporation heat transfer from the suppression pool surface and condensation heat transfer on the containment vessel wall. Based on the measured temperature profiles near the heat transfer surface and the wetwell pressure using two apparatuses, evaporation and condensation heat transfer coefficients were evaluated. The following results were obtained. 1. Both heat transfer coefficients increased as the ratio of the steam partial pressure to the total pressure increased.2. Comparison of the results from two types of test apparatuses confirmed that the size of the heat transfer surface did not affect the heat transfer characteristics within these tests.3. The heat transfer coefficients were expressed by the ratio of the steam to noncondensable gas logarithmic mean concentration, which considered the steam and gas concentration gradient from the heat transfer surface to the wetwell bulk.

An efficient method for passive safety systems reliability assessment

A detailed analysis of liquid film evaporation and condensation has become increasingly important, especially in water reactor designs.For example, the prediction of liquid film evaporation is critical to the design of Passive Containment Cooling Systems (PCCS), which are important passive safety facilities in modern Pressurized Water Reactors (PWR).To keep the maximum design values of pressure and temperature under control, a water/air counter-current flow is often employed within the external channel of the PCCS.It is thus essential to analyse the evaporation mass flow rate and heat transfer of the water film for the overall performance of PCCS.On the other hand, hydrogen might be released into the reactor containment during an incident due to cladding oxidation at elevated temperatures.Given the high flammability of gaseous hydrogen, it is paramount to know its local distribution in the containment building while considering wall condensation, which will increase the non-condensable gas concentration on the walls.Another example is evaluating the containment loads in the Light Water Reactor (LWR) plants and predicting their condensation behaviour in the presence of non-condensable gases.With the advantages of Computational Fluid Dynamics (CFD) and recent model developments, the numerical analysis of evaporation and condensation of liquid films has become more convenient than expensive experiments for real-world applications.In this paper, the numerical model of Eulerian Wall Film (EWF) implemented within Ansys Fluent® has been validated based on two canonical cases.The first one refers to the experiments performed by Hu et al. [1], where the heat transfer during the water film evaporation on a vertical plate has been studied.The average heat flux and water film evaporative ratio match the available experimental data well.The second case relates to the experiments done by Ambrosini et al. [2], also known as the "CONAN" case, where the wall film condensation in the presence of non-condensable substances has been studied under different steam mass fraction and velocity conditions.The surface heat flux and condensate mass flow rate also match the experimental measurements.The present numerical workflow using EWF could be applied to more complex devices for liquid film evaporation and condensation simulations.