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[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] The complex pipe network characters can not directly presented in single phase flow, gas-liquid two phase flow pressure drop and void rate change model. Apply fluid network theory and computer numerical simulation technology to phase flow pipe networks carried out simulate and compute. Simulate result shows that flow resistance distribution is non-linear in two phase pipe network
[en] Available experimental data of forced convective heat transfer of supercritical water in vertical upward channels are extensively collected to establish a wide-ranged data bank in this paper. Two groups of dimensionless number are introduced to reflect special influence on heat transfer which are induced by the drastic variation of supercritical water properties and related secondary effects. Based on sensitivity analysis and multi-collinearity evaluation of the dimensionless numbers, two types of wide- ranged heat transfer correlations are regressed through principal component analysis. Further, the two correlations developed are assessed using the wide-ranged data bank in comparison with various other correlations. It is concluded that the two correlations developed are superior to others in applicable range, error and accuracy aspects. (authors)
[en] Highlights: • A frequency-domain model for supercritical flow stability analysis was developed. • The stability maps for parallel-channel heat exchanging system were constructed. • The sensitivities of several parameters were studied for system stability boundary. • A time-domain model was developed and applied to nonlinear stability analysis. • The importance of the second eigenvalue has been studied. - Abstract: The flow in the core of supercritical water reactors (SCWRs) experiences drastic change in its thermodynamic properties and transport properties near the pseudo-critical temperature, thus the core flow may be susceptible to density wave oscillation instability, which is a challenge to the system safety and must be studied carefully. This work studies the stability characteristics of parallel-channel systems with heat exchanging, the prototype of which is originated from the thermal-spectrum zone assemblies of a newly designed mixed-spectrum SCWR (SCWR-M). A frequency-domain model has been developed for linear stability analysis, and marginal stability boundaries under several conditions are generated, which indicate that the system normal operational condition is in an absolute stable region. Decreasing the wall thermal conductivity can improve system stability while increasing mass flow is beneficial for the system stability. The system is not very sensitive to the axial power distributions. A one-dimensional time-domain model has also been developed for nonlinear analysis, and several transients with mass flow perturbations are calculated. The system marginal stability boundaries calculated by using frequency-domain and time-domain methods are in good agreement with each other. The existence of transitional stable region has been observed. A special case of parallel-channel systems with heat exchanging has been studied and achieved the conclusion that the second eigenvalue should be considered when studying the stability characteristics of complicated systems by using frequency-domain methods
[en] The reliability evaluation of passive safety system plays an important part in probabilistic safety assessment (PSA) of nuclear power plant applying passive safety design, which depends quantitatively on reliabilities of passive safety system. According to the object of reliability assessment of passive safety system, relevant parameters are identified. Then passive system behavior during accident scenarios are studied. A practical example of this method is given for the case of reliability assessment of AP1000 passive heat removal system in loss of normal feedwater accident. Key and design parameters of PRHRS are identified and functional failure criteria are established. Parameter combinations acquired by Latin hyper~ cube sampling (LHS) in possible parametric ranges are input and calculations of uncertainty propagation through RELAP5/MOD3 code are carried out. Based on the calculations, sensitivity assessment on PRHRS functional criteria and reliability evaluation of the system are presented, which might provide further PSA with PRHR system reliability. (authors)
[en] Fluid-to-fluid modeling of critical heat flux (CHF) is to simulate the CHF behaviors for water by employing low cost modeling fluid, and the flow scaling factor is the key to apply the technique to fuel bundles. The CHF experiments in 4x4 rod bundles have been carried out in Freon-12 loop in equivalent nuclear reactor water conditions (P=10.0-16.0 MPa, G=488.0-2100.0 kg/m2 s, Xcr=-0.20-0.30). The models in fluid-to-fluid modeling of CHF is verified by the CHF data for Freon-12 obtained in the experiment and the CHF correlation for water obtained by Nuclear Power Institute of China (NPIC) in the same 4x4 rod bundles. It has been found that the S.Y. Ahmad Compensation Distortion model, the Lu Zhongqi model, the Groeneveld model and Stevens-Kirby model overpredict the bundles CHF values for water. Then an empirical correlation of flow scaling factor is proposed. Comparison of the CHF data in two kinds of test sections for Freon-12, in which the distance of the last grid away the end of heated length is different, shows that the spacer grid, which is located at 20 mm away from the end of the heated length, has evidently influenced on the CHF value in the 4x4 rod bundles for Freon-12. This is different from that for water, and the need for further work is required
[en] The dynamical behavior of the subcooled boiling two-phase system was introduced and discussed. With the introduction of fractal concept, an analysis of the fractal feature of pressure wave signals from nonlinear dynamics point of view, was carried out. Meanwhile, the pseudo phase diagrams of typical time series of sound pressure were given. Finally, through dynamic clustering and on the basis of calculating correlation dimension and Hurst exponent of pressure wave time series on different subcooling conditions, the recognition of developing regime of the two-phase system was delivered, which might provide a promising approach of recognition and diagnosis of a boiling system
[en] Conclusion: IVR is one of important severe accident management strategies of CAP1400. The purpose of IVR-ERVC experiments is to obtain CHF at RPV lower head and research its relevant mechanism. IVR-ERVC experiment facility was designed and built with a series of improvements. Insights achieved in IVR-ERVC experiments contribute to IVR evaluation, design improvement and safety review of CAP1400.
[en] Applying theory of bifurcation and its DERPAR algorithm, the static bifurcation solution diagram of a typical two-phase natural circulation system based on homogenous model is calculated. With the solution diagram, the static bifurcation mechanism due to the nonlinear variation characteristics of buoyancy and loss with the changing of heating power (quality) is discussed and revealed. Moreover, the static instability phenomenon in two-phase natural circulation, corresponding to the Ledinegg instability in forced one, and its criterion is hereby delivered. And the instability margin, heat transport capability and thermal siphon hysteresis phenomenon is accordingly defined. Furthermore, effect of such parameters as system pressure, subcooling , loss and geometric boundary condition on running stability and transport capability of heat load is also presented. Finally, it is emphatically pointed out that the strategy of this analysis using simple model may lead to an outline understanding of the process in startup , varying regime, transient, accident and trip conditions, which deserves to be studied intensively through experiment and finer two-phase flow model
[en] An experiment on boiling in rectangular narrow space is presented. Under different narrow space sizes and heat conditions, the experiment is recorded with camera and data acquisition system. Based on experiment results, some conclusions are drew: In narrow space, bubble sizes are constrained by the narrow space and pressure inside bubble block contributes a lot to bubble movement; narrow space dose enhance boiling heat transfer, the superheat is smaller under heated symmetrically than that under heated asymmetrically