[en] Fuel assembly bowing may induce the reloading or unloading difficulties, and the incomplete insertion of control rod. The fretting between fuel assemblies could result in the component damage and the leakage of nuclear fuels. These phenomena directly affect the safe operation and economic benefits of nuclear power plants. This paper summarizes the typical mechanic problems, such as bowing, fretting, and integrality analysis in PWR fuel assembly research and development. The key factors, resolving methods and prospect are also proposed. (authors)

[en] The analytical model of finite plate stiffened by blocking mass under a point force is established. The structural response is derived by a combination of the modal method and the traveling wave method, and used for studying effect of blocking mass on energy transmission from the exciting plate to the receiving plate taking mean square velocity as evaluation index. The effect by the parameters of the blocking mass such as the mass ratio on energy transmission is discussed. It is shown that in low frequencies range, blocking mass can effectively impede vibration energy transmission when the rigidity of the blocking mass is considered and will enhance vibration energy transmission when its rigidity is neglected. And it is also found that in higher frequencies range the attenuation effect gets better with the increase of the mass ratio in the higher frequencies. (authors)

[en] Fluidelastic instability causes the large amplitude of the heat transfer tube, which results in its wear out. It is the key mechanism of the fluid-induced vibration of the team generator tube bundles subject to two-phase flow. In order to predict more accurately the critical flow of instability, the motion equation of single tube is firstly established utilizing the unsteady fluid force coefficients of two-phase flow which obtained by fitting experimental result data. After the nondimensionalize and Galerkin discretization of the analytical model, the critical flow velocity of each void fraction is calculated by solving the system of equations. Numerical results show that the numerical critical fluid force of instability agrees well with experimental results, which proving that the analytical model utilizing fitting parameters of unsteady fluid force of two-phase flow is available for the predicting of the critical velocity of instability. (authors)

[en] Air valves are common devices in nuclear power plants to control air flux and pressure. To ensure the structural safety, the air valve of the emergency negative pressure system of the nuclear power plant is analyzed by finite element method (FEM). Based on the calculation of the inherent frequency of the air valve structure, the stress and the deformation of the air valve structure under earthquake are obtained. The analysis results show that the stress and the deformation satisfy the requirements of relative seismic codes, indicating that the air valve structure is safe and reliable under earthquake. (authors)

[en] As discussing the relation between power flow difference and vibration level difference, the simplification of power flow difference expression of vibration isolation devices leads to an expression which shares the same characteristics with power flow difference by using only vibration response, for example, vibration displacement, and by the comparison of the new expression and the one of vibration level difference, the article puts forward a new calculating method of vibration level difference with the consideration of weight factor. The FEM results show that in the not very high frequency region, both the simplification and approximation of power flow difference expression are reasonable and the vibration difference curve is more similar to the curve of power flow difference when mixed with a coefficient on behalf of force factor. (authors)

[en] As an important engineering structure, the pipe system is widely used in the primaryloop of the reactor. To solve the vibration and safety problems in pipeline system, a type of metal rubber hanger is designed. The mechanical characteristics and the effectiveness of the metal rubber hanger is validated through experiments. In the experiments, the vibration of the pipe and the vibration transmission in the pipe system are mitigated. (authors)

[en] The Leak Before Break (LBB) concept, as important feature of the third generation nuclear power technology, is widely used in the high-energy piping design in nuclear industry. However, for some historical reasons, the application of LBB concept in China lagged behind compared with that in the developed countries. Up to now, there are no credible codes developed by China which can be applied to an actual project, and the relevant design mainly relies on foreign companies. Hence, the development of an approved LBB design code has great theoretical and practical importance for China. In this paper, the background of the fracture mechanics analysis, Crack Opening Displacement (COD) calculation and leak rate calculation in LBB design are briefly introduced firstly. and then the R and D situation of the key code in NPIC is also presented, and some examples from the approved codes and published papers are used to verify the self-developed code. The calculation results show that the accuracy of the code is in well consistence with the examples. This code can be used in the practical engineering after refinement and validation. (authors)

[en] Based on the design loads of the pressurizer in nuclear power plants, this paper investigates the crack propagation induced by the fatigue and the stress corrosion of the design overlay weld structure in the pressurizer nozzle safety end. The crack propagation depth is obtained, and the safety assessment is carried out. At the end of the cycle, the maximum circumferential crack growth depth and axial crack growth depth in dissimilar metal weld zone are 0.4 × 10^-3 mm and 23.6 × 10^-3 mm, respectively, while the maximum circumferential crack growth depth and axial crack growth depth in stainless steel weld zone are 12.4 × 10^-3 mm and 0 mm. The results show that the crack propagation meets the design requirement. This analysis can provide a reference for the overlay weld structure design and evaluation of the pressurizer nozzle safety end. (authors)

[en] With the extension of operating life of nuclear power plants, the vibration fatigue of the small branch pipes becomes more and more prominent, which poses a threat to the safety and economic of nuclear power plants. Therefore, to ensure the safe operation of nuclear power plants, it is necessary to reconstruct the constraint of the sensitive pipes. In this paper, a branch pipe of a nuclear power plant was taken as the research object, and experimental and numerical studies were performed to obtain the modal characteristics of the pipe. Spectrum analysis was conducted to calculate the vibration response of the pipe in the operation condition. The calculation results showed that the fatigue stress of the branch pipe exceeds the allowable alternating stress of fatigue, that is, the branch pipe is a sensitive pipe. According to the numerical analysis, not only the sensitive pipe was accurately judged, but also the theoretical basis was provided for the reconstruction and optimization of the constrained structure of subsequent branch pipe. (authors)

[en] During the core meltdown severe accident in the nuclear power plants, the strategy of In-Vessel Retention (IVR) to contain the melt in the Reactor Pressure Vessel (RPV) is a key mitigation measure. During the implementation of IVR strategy, the RPV lower head is likely to fail due to excessive creep deformation under the combined action of extremely high temperature loads and mechanical loads. Therefore, it is necessary to perform the analysis of the creep deformation of the RPV lower head, to ensure the structural integrity of the RPV under the condition of melt retention. In this paper, under the assumption of IVR, the finite element method is used to perform the thermal-structural coupling analysis of the RPV lower head. The temperature and stress fields of the vessel wall, and the plasticity and creep deformation of the lower head are calculated. Combining the plasticity and creep rupture criteria, the failure was analyzed. Results show that the deformation of the structure will be greatly increased when creep is considered. During the IVR strategy under severe accident, the main failure mode of the RPV lower head is creep failure instead of plastic failure. Further analysis implies that the internal pressure has a significant influence on the creep deformation and failure time. This paper provides the method for the creep and failure analysis of the RPV lower head under severe accident. (authors)