Heat transfer coefficients for the pressure-regulated hybrid control rod according to heat loads and operating pressure: (a) heat transfer coefficients for evaporation, (b) heat transfer coefficients for condensation. Heat transfer coefficients of the self-printed hybrid guide rod according to specified conditions: (a) evaporation heat transfer coefficients, (b) condensation heat transfer coefficients Fig. 3-23 Maximum heat transfer rates for the hybrid control rod according to neutron absorber diameters and operating pressure.
Research background and motivation
Review on passive safety systems
Nam et al.5 used a multipod heat pipe (MPHP) design in a passive containment cooling system for the APR-1400, representative commercial pressurized water reactor in Korea. Hejzlar et al.9 attempted to use the sodium heat pipe as a device for removing heat from decay under accident conditions. 8] Heat pipe system for heat transfer between RPV and steam supply system Hu et al.
Concept of passive in-core cooling system based on hybrid control rod
Objectives and Scope
The evaporation and condensation heat transfer coefficients of the pressure-controlled hybrid control rod are shown in Figure 2-5. Heat transfer coefficients of the pressure-controlled hybrid control rod according to heat load and operating pressure. The higher temperature difference between sleeve and condenser could improve the heat transfer rate of the hybrid control rods.
PEFORMANCE ANALYSIS OF PRESSURIZED HYBRID CONTROL ROD
Literatures on thermosyphon heat pipe
Nucleation pool boiling in the way of nucleation, growth and removal of bubbles in the surface cavities of the heater is the heat transfer mechanism of the heat pipes. Boo et al.41 investigated the heat transfer characteristics of concentric ring thermosyphons by varying the diameter ratio between the inner and outer tubes and the filling ratio of the working fluid. Faghri et al.42 conducted a study on the flooding-based operating limit and heat transfer coefficients of concentric ring thermosyphons.
Characteristics of hybrid control rod
Therefore, quantitative observation of the effects of non-condensable gas on the performance of thermosyphons was not carried out. In integrated systems, the inclusion of a neutron absorber differentiates the geometry point of the hybrid control rod from general thermosyphons. Significantly high operating pressures achieved by non-condensable gas charging and water vapor accumulation are the main characteristics of the hybrid control rod.
Pressure control mechanism of hybrid control rod
For passive control of hybrid control rod operating pressure, non-condensable gas will be charged. Thus, control of the volume occupied by the non-condensable gas in the condenser section is an issue in hybrid control rod pressure control. In this strategy, non-condensable gas is charged to the test section at a target pressure.
Experimental setup and procedure
- Test section
- Experimental apparatus
- Test procedure and experimental uncertainty
The thinner liquid film thickness of the self-printed hybrid control rod could be confirmed by comparison of condensation heat transfer coefficients. The two-phase heat transfer of the heat pipes could be divided into several representative mechanisms. Imura's correlation was chosen as the evaporative heat transfer model of the hybrid control rod due to good agreement with experimental data.
Result and discussion
- Heat transfer characteristics
- Maximum heat transfer rate
MODELING OF PRESSURIZED HYBRID CONTROL ROD
Analysis of hybrid control rod using MARS code
- Analysis models and conditions
- Simulation results
- Limitation in prediction capability of MARS code
To evaluate the predictive ability of the current system security code, the experiments were carried out with self-printed hybrid control rod simulated. The initial volume of the working fluid was modeled at collapsed water level and the initial pressure for the test section was given. The decrease in the liquid fraction at adiabatic section explains the steam accumulation with increase in operating pressure and transport of the working fluid to the condenser section.
The oscillation of the liquid fraction at the evaporator and the adiabatic section means the oscillating flow pattern or the effect of countercurrent. Regarding the condensation heat transfer, the predicted temperature distributions of the condenser wall showed a large deviation from the experimentally measured temperature data, as shown in Figure. Therefore, the reason for the limited predictability of the standard model for condensation in the hybrid control rod will be discussed later. .
The heat flux that the wall temperature excursion observed during the simulation was determined as a maximum heat transfer rate of the test section. The reverse flow boundary, which limits liquid return from the condenser section to the evaporator section, was noted as a critical phenomenon of the test conditions. Previous studies developing condensation heat transfer models within heat pipes assumed non-condensable gas stratification by steam flow.
In the case of flooding-based maximum heat transfer rate of the hybrid control rod, MARS code used Wallis, Kutateladze or Bankoff models to predict the CCFL.
Development of models on performance of hybrid control rod
- Evaporation heat transfer
- Condensation heat transfer
- Maximum heat transfer rate
Integrals of the local heat transfer coefficients throughout the total condenser length give a total condensation heat transfer coefficient. Thus, the effect of entrained droplet on condensation heat transfer was expressed as an exponential function of density ratio which is second term of the proposed model. The validity of the assumptions about non-condensable gas and entrainment on condensation heat transfer will be discussed in Section 3.4.
Ignorance of the surface tension in the force balance can affect the predictability of the model. -14), and it showed deviations from the measured maximum heat transfer rates for the experimental plants. The previously proposed models were used to predict the measured operating limits of the hybrid control rod.
The superficial velocity of the liquid film must thus be defined by the cross-sectional area of adiabatic section. The previous studies that developed the models on maximum heat transfer rate through changing the Kutateladze number suggested different Ck in Eq. Thus, the vapor to liquid density ratio was chosen as a variable for Ck.
As a result, the maximum heat transfer rate of the hybrid control rod caused by flooding, which has different cross-sectional areas by sections, is proposed as Eq.
Validation of models
- Validation of condensation heat transfer model
- Validation of flooding limit model
When the internal pressure of the test section increases with the heat load on the evaporator section, the non-condensable gas will be compressed. The calculated effective condensation heat transfer lengths according to heat loads are plotted in Figure. The rest of the theoretical background of the proposed condensation heat transfer model is heat transfer degradation due to entrainment.
The reason for weighting the entrainment effect exponentially can be explained by observing the instability of liquid films exerted by differences in densities and velocities of the phases. Then, the entrainment rate is proportional to the growth rate of the ripples multiplied by the fluid density as Eq. In this case, the fastest growth rate of the ripple (real part of the exponent) is Eq.
3-19, the gas Weber numbers (Weg) were lower than 6.0 during the experiments with the pressurized hybrid control rod. Therefore, it can be concluded that the assumed thermal-hydraulic phenomena at the condenser section of the hybrid control rod are valid. The predictions using the existing correlations were performed using the cross-sectional area of the evaporator section.
23 showed that the experimentally measured performance limits of the hybrid joystick were in agreement with the predictions of the model proposed in this study.
Guideline for Hybrid Control Rod Design
As a result, the predicted temperatures at the heat source–sink interfaces will be compared with the target temperatures, and the design of the hybrid control rod will be confirmed with iterative calculation to meet the functional requirements of the target nuclear plant. It is expected that the design of the hybrid control rods in accordance with the requirements of the application system will be possible using the proposed guideline. The guideline does not include the effect of the length ratio of evaporator, adiabatic and condenser sections, assuming that the length ratio effect is negligible.
Therefore, hybrid control rod design using the proposed guideline is recommended to be performed within the suggested ranges.
APPLICATION STUDY OF HYBRID CONTROL ROD
- Issues on spent fuel dry storage cask
- Concept of UCAN based on hybrid control rod
- Experimental setup and procedures
- Results and discussion
- Effects of hybrid control rod and heat sink
- Thermal analysis
The test results showed that the hybrid control rod could improve the thermal margin of the dish by providing additional heat transfer path. The effectiveness of decay heat removal through the hybrid control rod according to the condenser design was also demonstrated in this study. The integration of the hybrid control rod on dry storage casks for spent fuel has several advantages compared to previously developed casks.
If the stored energy inside the heat sink of the hybrid control rod is recycled, an electrical generation system or power plant can be constructed in the basement of the dry spent fuel storage container. At that time, the hybrid control rod acts as a heat transfer medium, and the heat sink on the dish plays a role as a heat source. The heat loads showing similar temperature distributions with hybrid control rod installed and heat sink installed dish design for quantitative evaluation of the efficiency of the UCAN design.
Three designs were studied to observe the effect of the hybrid control rod and heat sink design on the thermal limit of the barrel. The effect of the hybrid control rod on transient phase temperature evolutions and steady state temperature distributions were measured. However, the axial temperature gradients of UCAN with air condenser and normal barrel were higher than UCAN with water condenser.
The application of the hybrid control rod to spent fuel dry storage vessel will ensure the integrity of the spent fuel in terms of issues related to the thermal stress and temperature induced material embrittlement.
CONCLUSIONS AND RECOMMENDATIONS
Thermal performances of pressurized hybrid control rod
Prospect of PINCs on nuclear safety
Recommendations
Kaminaga, F.; Okamoto, Y.; & Suzuki, T.; Correlation study of wave heat transfer in a closed two-phase thermosyphon. El-Genk, M.S.; & Saber, H.H.; Heat transfer correlations for liquid film in closed gravity assisted thermosyphon evaporator. Hashimoto, H.; & Kaminaga, F.; Heat transfer characteristics in a closed two-phase thermosyphon condenser: effect of draft on heat transfer deterioration.
Rosler, S.; Takuma, M.; Groll, M.; & Maezawa, S.; Limitation of heat transfer in a vertical annular closed two-phase thermosyphon with small filling rates. Lin, T.F.; Lin, W.T.; Tsay, Y.L.; & Wu, J.C.; Experimental investigation of geyser boiling in an annular two-phase closed thermosiphon. Boo, J.H.; & Park, S.Y.; An experimental study on the thermal efficiency of a concentric annular heat pipe.
Faghri, A.; Chen, M.M.; & Morgan, M.; Heat transfer characteristics in conventional and concentric closed two-phase annular thermosyphons. Faghri, A.; & Thomas, S.; Performance Characteristics of a Concentric Annular Heat Pipe: Part I- Experimental Prediction and Capillary Boundary Analysis. Young, W.; Kim, Y.G.; & Lee, J.; Transient characteristics of a heat pipe based hydraulic temperature control technique.
Kim, K.M.; & Bang, I.C.; Heat Transfer Characteristics and Operating Limit of Pressurized Hybrid Heat Pipes for Small Modular Reactors.
