Borut Mavko

Application of the fault tree analysis for assessment of power system reliability

A new method for power system reliability analysis using the fault tree analysis approach is developed. The method is based on fault trees generated for each load point of the power system. The fault trees are related to disruption of energy delivery from generators to the specific load points. Quantitative evaluation of the fault trees, which represents a standpoint for assessment of reliability of power delivery, enables identification of the most important elements in the power system. The algorithm of the computer code, which facilitates the application of the method, has been applied to the IEEE test system. The power system reliability was assessed and the main contributors to power system reliability have been identified, both qualitatively and quantitatively.

A dynamic fault tree

Abstract The fault tree analysis is a widely used method for evaluation of systems reliability and nuclear power plants safety. This paper presents a new method, which represents extension of the classic fault tree with the time requirements. The dynamic fault tree offers a range of risk informed applications. The results show that application of dynamic fault tree may reduce the system unavailability, e.g. by the proper arrangement of outages of safety equipment. The findings suggest that dynamic fault tree is a useful tool to expand and upgrade the existing models and knowledge obtained from probabilistic safety assessment with additional and time dependent information to further reduce the plant risk.

Genetic algorithm optimisation of the maintenance scheduling of generating units in a power system

A new method for optimisation of the maintenance scheduling of generating units in a power system is developed. Maintenance is scheduled to minimise the risk through minimisation of the yearly value of the loss of load expectation (LOLE) taken as a measure of the power system reliability. The proposed method uses genetic algorithm to obtain the best solution resulting in a minimal value of the annual LOLE value for the power system in the analysed period. The operational constraints for generating units are included in the method. The proposed algorithm was tested on a Macedonian power system and the obtained results were compared with the results received from the approximate methodology. The results show the improved reliability of a power system with the maintenance schedule obtained by the new method compared to the results from the approximate methodology.

DNS of Turbulent Heat Transfer in Channel Flow With Heat Conduction in the Solid Wall

The Direct Numerical Simulation (DNS) of the fully developed velocity and temperature fields in the two-dimensional turbulent channel flow coupled with the unsteady conduction in the heated walls was carried out. Simulations were performed at constant friction Reynolds number 150 and Prandtl numbers between 0.71 and 7 considering the fluid temperature as a passive scalar. The obtained statistical quantities like root-mean-square temperature fluctuations and turbulent heat fluxes were verified with existing DNS studies obtained with ideal thermal boundary conditions. Results of the present study were compared to the findings of Polyakov (1974), who made a similar study with linearization of the fluid equations in the viscous sublayer that allowed analytical approach and results of Kasagi et al. (1989), who performed similar calculation with deterministic near-wall turbulence model and numerical approach. The present DNS results pointed to the main weakness of the previous studies, which underestimated the values of the wall temperature fluctuations for the limiting cases of the ideal-isoflux boundary conditions. With the results of the present DNS it can be decided, which behavior has to be expected in a real fluid-solid system and which one of the limiting boundary conditions is valid for calculation, or whether more expensive conjugate heat transfer calculation is required. @DOI: 10.1115/1.1389060#

Application of first and second order reliability methods in the safety assessment of cracked steam generator tubing

Abstract The First- and Second Order Reliability Methods (FORM and SORM) have been applied in the safety assessment of steam generator tubes with through-wall axial stress corrosion cracks. The underlying probabilistic fracture mechanics model takes into account the scatter in tube geometry, material properties and stable crack propagation. Also, the effect of the maintenance strategy has been considered. A realistic numerical example has been given to compare the failure probabilities calculated by FORM and SORM to those obtained by different versions of Monte Carlo simulations. The relative errors of the numerical methods employed have been analysed, which has shown that FORM performs in an acceptable and SORM in an excellent manner. Some changes in failure surface properties, caused by different maintenance strategies, are indicated and a sensitivity analysis of influencing parameters is made. The results obtained demonstrate the applicability of FORM and SORM in the safety assessment of stress corrosion cracked steam generator tubing.

Review of quantitative accuracy assessments with fast Fourier transform based method (FFTBM)

Abstract In the past 10 years various methodologies were proposed to evaluate the uncertainty of BE code predictions. One common step to all methodologies is the use of experimental and plant data for the nodalization development and qualification. When thermal–hydraulic computer codes are used for simulation the questions raised are: ‘How long improvements should be added to the model, how much simplification can be introduced and how to conduct an objective comparison?’ The proposed fast Fourier transform based method (FFTBM) assists in answering these questions. The method is easy to understand, convenient to use, user independent and it clearly indicates when simulation needs to be improved. The FFTBM shows the measurement–prediction discrepancies—accuracy quantification—in the frequency domain. The acceptability factor for code calculation was determined based on several hundreds of code calculations. The FFTBM method has been applied to various international standard problems, standard problem exercises and other experiment simulations that are presented in the paper. The result shows that the quantitative comparison between thermal–hydraulic code results and experimental measurements with qualitative evaluation may assist the decision whether or not the simulation needs to be improved.

Component reliability assessment using quantitative and qualitative data

Abstract The estimation of a component failure rate depends on the availability of plant specific numerical data. The purpose of this study was development of a new method that explicitly includes numerical and linguistic information into the assessment of a specific failure rate. The basis of the method is the Bayesian updating approach. A prior distribution is selected from a generic database, whereas likelihood is assessed using the principles of fuzzy set theory. The influence of component operating conditions on component failure rate is modeled using a fuzzy inference system. Results of fuzzy reasoning are then used for building an appropriate likelihood function for the Bayesian inference. The method was applied on a high voltage transformer. Results show that with the proposed method, one can estimate the specific failure rate and analyze possible measures to improve component reliability. The method can be used for specific applications including components for which there is not enough numerical data for specific evaluation.

Modelling of low-pressure subcooled flow boiling using the RELAP5 code

Abstract A new model for upward vertical subcooled flow boiling at low pressure has been proposed. The model considers the most relevant closure relationships of one-dimensional thermal-hydraulic codes that are important for accurate prediction of vapour contents in the channel: wall evaporation model, condensation model, flow regime transition criterion and drift-flux model. The new model was incorporated in the current version of the RELAP5 code, MOD3.2.2 Gamma. The modified code was validated against a number of published low-pressure subcooled boiling experiments, and in contrast to the current code, shows good agreement with experimental data. The presented analysis also leads to a better understanding of the basic mechanisms of subcooled flow boiling at low pressure.

Evaluating Code Uncertainty - I: Using the CSAU Method for Uncertainty Analysis of a Two-Loop PWR SBLOCA

When best-estimate calculations are performed, the uncertainties need to be quantified. Worldwide, various methods have been proposed for this quantification. Rather than proposing a new uncertainty methodology, a contribution is made to the existing code scaling, applicability, and uncertainty (CSAU) method. A small-break loss-of-coolant accident with the break in the cold leg of a Westinghouse-type two-loop pressurized water reactor was selected for the analysis, and the CSAU methodology was used for uncertainty quantification. The uncertainty was quantified for the RELAP5/MOD3.2 thermal-hydraulic computer code. Some tools suggested by the uncertainty methodology based on accuracy extrapolation (UMAE) method were successfully applied to improve the CSAU methodology, particularly for nodalization qualification. A critical scenario with core uncovery was selected for the analysis, which showed that when uncertainty is added to the peak cladding temperature, the safety margin is sufficient. The tools developed by the UMAE method showed that the structure of the CSAU method is universal because it does not prescribe tools for the analysis.

Propagation of stress corrosion cracks in steam generator tubes

Abstract A model suitable to describe the propagation of stress corrosion cracks in steam generator tubes made of Inconel-600 is proposed in this paper. It concentrates on axial cracks located in the tube expansion transition zones which are assumed to be through-wall. The residual stress field is therefore considered as the major contributing factor driving short cracks while operational stresses dominate the growth of longer cracks. An estimate of residual hoop stresses is obtained using a non-linear finite element simulation of the tube to tube-sheet rolling process. Scatter of the residual stresses due to the stochastic variations of the dominant influencing parameters was studied. The crack propagation model utilizes linear-elastic fracture mechanics theory. In particular, both crack tips are modelled to propagate with different velocities due to the highly asymmetric stress field. Provisions are also made to account for the reactor coolant temperature and chemical composition effects. The model performance is demonstrated by a numerical example considering the crack propagation data from D4 steam generators during a 15 month operational cycle as recorded by subsequent non-destructive tube examinations.