[en] A generalized two-dimensional analytical model and a computer code POSTCHF for both dispersed flow and inverted annular flow during post-CHF region have been developed. The model was based on the basic two-phase conservation equations and related constitutional correlations. The basic idea and mathematical model are presented. It was concluded from the calculation that the initial droplet size in dispersed flow has strong influence on wall temperature, but the initial film thickness in inverted annular flow has been found less influencing. The Tatterson annular droplet model and the turbulent coefficient C_v = 0.55-0.62 were used with which the results were agreed well with the experimental data

[en] Highlights: • A frequency-domain model for a SCWR-M reactor core was developed. • Stability maps for SCWR-M core were constructed. • Sensitivities of several parameters were studied for system stability boundaries. • A time-domain model was developed and applied to nonlinear stability analysis. • A more reasonable system decay ratio was redefined. • Subcritical bifurcation phenomenon in SCWR-M core was studied. - Abstract: The supercritical water reactor (SCWR) is one of the most prominent Generation IV reactors due to its high efficiencies. However, the stability issues, which are mainly caused by the great changes of thermodynamic properties and transport properties of supercritical water near the pseudo-critical temperature, are a challenge to the system safety and must be studied carefully. This paper is focused on 1-D dynamic stability analysis of mixed-spectrum SCWR (SCWR-M) reactor core. To this end, a frequency-domain model has been developed for linear stability analysis, and marginal stability boundaries under both the fixed inlet flow boundary conditions and the fixed external pressures boundary conditions are generated, which indicate that the system normal operational condition is in stable regions. Parametric sensitivity studies in frequency domain have been carried out. Increasing the wall thermal conductivity and mass flows can increase system stability. The system is more stable if the thermal zone has a lower power fraction. System with the designed non-uniform axial power distribution is also more stable than with the uniform distribution. A time-domain model has also been developed for nonlinear analysis, and the system marginal stability boundaries calculated by this method is consistent with those by frequency domain method. The existence of transitional stable region has been observed. A more reasonable definition for system logarithmic decay ratio has been achieved. The SCWR-M core has a subcritical bifurcation characteristic under fixed external pressures boundary conditions, thus its dynamic behaviors are not only related to systematic parameters, but also to the amplitudes of perturbations.

[en] Nearly twenty years, a lot of resources have been devoted to understand the mechanism of the phenomena in nuclear severe accidents, to find the consequences resulted by these phenomena and to establish the countermeasures for safety. The research scales and the current research status are introduced. (authors)

[en] A steam explosion may occur due to high-speed transient heat transfer with the interaction of high-temperature melt and coolant. This phenomenon always induces severe accidents in some industry areas. Remarkable progress in describing the pre-mixing and the expansion stages of steam explosion has been achieved in the past years. However, doubt still remains on some basic mechanisms related to the particle movement and heat transfer. An evaporation drag model to explain the pressure distribution surrounding a high-temperature particle due to evaporation was developed for the resistance calculation by one of the authors [1]. For verifying and further developing the model, a small-scale visualization experiment has been carried out for high-temperature particles falling into coolant pool. The whole process of the particle falling down was recorded by a high-speed video camera, and the image records determined the moving speed of high-temperature particles. By analysis of the experimental data, some important factors, which affect evaporation rate, film growing rate and resistance surrounding the particles, were found and analyzed. The basic assumption of the evaporation drag model was validated. This study lays an experimental and theoretical fundament for further investigation of resistance and high-speed transient heat transfer in steam explosion. (authors)

[en] The buoyancy term of turbulence model is studied, and modification of the buoyancy model has been carried out. After that, several cases of supercritical flows with heat transfer are calculated and the results are compared with DNS and experiment data in the open literatures. The results show that the modified model can predict the buoyancy term correctly, and thus the calculated wall temperature agrees well with the experimental data and DNS data. In contrast, the wall temperature obtained by the traditional models is much higher than the experimental and DNS data. (authors)

[en] Highlights: • Sensitivity analysis is performed on the reflood model of RELAP5. • The selected influential models are discussed and modified. • The modifications are assessed by FEBA experiment and better predictions are obtained. - Abstract: Reflooding is an important and complex process to the safety of nuclear reactor during loss of coolant accident (LOCA). Accurate prediction of the reflooding behavior is one of the challenge tasks for the current system code development. RELAP5 as a widely used system code has the capability to simulate this process but with limited accuracy, especially for low inlet flow rate reflooding conditions. Through the preliminary assessment with six FEBA (Flooding Experiments with Blocked Arrays) tests, it is observed that the peak cladding temperature (PCT) is generally underestimated and bundle quench is predicted too early compared to the experiment data. In this paper, the improvement of constitutive models related to reflooding is carried out based on single parametric sensitivity analysis. Film boiling heat transfer model and interfacial friction model of dispersed flow are selected as the most influential models to the results of interests. Then studies and discussions are specifically focused on these sensitive models and proper modifications are recommended. These proposed improvements are implemented in RELAP5 code and assessed against FEBA experiment. Better agreement between calculations and measured data for both cladding temperature and quench time is obtained.

[en] A two-dimensional two-fluid model is developed to predict the liquid and vopour parameters as well as the surface temperature of a heated tube. Aphysical model with two regions represents inverted-annular film boiling (IAFB) based on author's visual observations. The analytical model is consisted of a set of differential conservation equations together with appropriate closure correlations and solved numerically. Successful comparisons are made between model predictions and Stewart's experimental data. Generally, the model predicts correctly the dependence of wall temperature on liquid subcooling, mass flow rate, pressure and heat flux input. The two dimensional effectness isn't very significant. The initial vapour film thickness doesn't influence the wall temperature predictions very much

[en] When the plant is modeled detailedly for high precision, it is hard to achieve real-time calculation for one single RELAP5 in a large-scale simulation. To improve the speed and ensure the precision of simulation at the same time, coupling methods for parallel running RELAPSim codes were proposed in this study. Explicit coupling method via coupling boundaries was realized based on a data-exchange and procedure-control environment. Compromise of synchronization frequency was well considered to improve the precision of simulation and guarantee the real-time simulation at the same time. The coupling methods were assessed using both single-phase flow models and two-phase flow models and good agreements were obtained between the splitting–coupling models and the integrated model. The mitigation of SGTR was performed as an integral application of the coupling models. A large-scope NPP simulator was developed adopting six splitting–coupling models of RELAPSim and other simulation codes. The coupling models could improve the speed of simulation significantly and make it possible for real-time calculation. In this paper, the coupling of the models in the engineering simulator is taken as an example to expound the coupling methods, i.e., coupling between parallel running RELAPSim codes, and coupling between RELAPSim code and other types of simulation codes. However, the coupling methods are also referable in other simulator, for example, a simulator employing ATHLETE instead of RELAP5, other logic code instead of SIMULINK. It is believed the coupling method is commonly used for NPP simulator regardless of the specific codes chosen in this paper.

[en] China is coming to a high speed developing stage for nuclear power generation. Since 2005, a middle-long-term plan to develop nuclear power plant was decided by Chinese Government. The plan states that 40 GWe of nuclear power generation will be in operation by 2020. To promote the high development of nuclear power generation and increase the national production ability, China wishes to unify the technique way and aims to become self-sufficient in reactor design and construction, as well as other aspects of the fuel cycle. This year, the U.S.-based Westinghouse's AP1000 technique has been decided for this purpose. A framework contract had been signed in February 2007 to buy four third-generation pressurized water reactors from the Westinghouse Electric Co., which also included technologies transfers to China. In China, there are four universities which have nuclear departments with long history. Since 2004, with the increase of the demand for the development of nuclear power generation, many universities in China began to set up the department of nuclear engineering. Currently, the annual graduation is about 500 nuclear engineering students, above 80 graduate students, and the number is still increasing year on year. In addition, the adoption of new college students pre-service training, internal job transfer training, above 2000 new staff each year for nuclear industry. With the continuing rapidly growing demand and the development of nuclear power generation, this form of training in recent years China is the only way to solve the shortage of nuclear professional personnel. (author)

[en] Background: In nuclear plant vapor explosion analysis, fuel drop diameter is an important parameter which could significantly influence the evaluation of explosion pressure. Purpose: Decrease the uncertainty of vapor explosion calculation caused by fuel drop diameter. Methods: A simulation model of typical vapor explosion was built using MC3D to take sensitive analysis of fuel drop diameter. Results: The calculation relates to fuel drop energy, fuel drop fragmentation rate and vapor explosion pressure. The effect of fuel drop diameter in vapor explosion is analyzed based on theoretical analysis and the calculation. Conclusions: The results show that the vapor explosion pressure is very sensitive to fuel drop diameter, which is mainly caused by the fuel drop energy and the fuel drop fragmentation rate. (authors)