[en] The invention is a method and apparatus for calculating and predicting the impending violation of a design limit if no corrective action is taken. Various mathematical indices can be devised, such as the departure from nucleate boiling ratio (DNBR), which indicate the proximity of the limiting conditions for the design limit in question, i.e., the occurrence of departure from nucleate boiling (DNB) and fuel clad overheating. A critical value of the index can be defined to be that value at which design limit violation occurs

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[en] Instability of the process of nucleus formation in metastable medium and model of essentially non-linear relaxation autooscillations arisinq in steep dependence of the rate of nucleus formation on temperature are investigated. Equation of heat balance with account of medium heating by an external source and heat absorption by growing nuclei as well as a kinetic equation for nucleus distribution function by dimensions with account of their continuous generation and removal from the system are used to describe the process of nucleus homogeneous generation. These equations are reduced to the single integrodifferential equation for relative superheating of medium having the only stationary solution. The mentioned solution is instable at certain values of parameters, and autooscillating regime is establnshed in the system. Characteristics of this regime are studied for the case when the dependence of the formation rate on superheating temperature is very steep and autooscillations have breaking character. Similar autooscillations can be stated undeK liquid cooling in metastable phase, i. e. under homogeneous crystallization

[en] Highlights: •Improved RPI wall boiling model was used to study DNB phenomena. •DNB phenomena and CHF value were obtained by CFD method. •Maximum deviation between predicted and experimental CHF was less than 15.0%. •Mean absolute relative error of predicted CHF was 7.1%. -- Abstract: Two-fluid model coupled with improved RPI wall boiling model (CHF model) was employed to investigate the departure from nuclear boiling (DNB) phenomena in vertical pipes under extremely high heat flux by using FLUENT 14.5 code. The distributions of temperature before and after DNB and the critical heat fluxes (CHF) at corresponding DNB points were obtained. The maximum deviation of predicted CHFs with experimental data was less than 15.0%. The results proved that two-fluid model coupled with CHF model was qualified to predict CHF accurately. This work can be referred when one predicts CHF in complicated geometry by CFD methodology.

[en] The feasibility of implementing a post-DNB fuel design limit (FDL) for Condition II (moderate frequency) events is investigated. The FDL is defined, a comprehensive separate effects investigation is performed, and limits are specified for post-DNB operation. Limiting transients are evaluated using the FDL, and power benefits are determined. A research program required for implementation of the new FDL is formulated. Finally, recommendations are made regarding implementation of the post-DNB FDL in plant licensing

[en] In smooth heat transfer tubes, spirals are inserted in contact with the inner tube surface. The inserted spirals have a different pitch and cross section which is either circular, elliptic or polygonal. This equipment caused additional turbulence of the flowing steam-water mixture in the region of departure nucleate boiling and shifts the point of departure nucleate boiling towards the region of higher steam dryness where temperature pulsations on the tube wall are no longer dangerous. (J.P.)

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[en] In the DNB (Departure from Nucleate Boiling) region of steam generator evaporator, the heat transfer tubes are subjected to oscillating thermal condition with high frequency and the resulting high cycle fatigue damage in these tubes is usually evaluated on the basis of the alternating stress intensity and the mean stress intensity during the thermal cycle. Due to the three-dimensional nature of the thermal loading, a three-dimensional stress analysis is required to rigorously determine the stress intensity. However, in practical design, simplified stress analyses with proven validity are preferred because of the analytical complexity involved in the three-dimensional analysis. This paper presents a systematic numerical comparison of the simplified and three-dimensional analyses and recommends an appropriate simplified model for practical applications. (orig.)