Next generation high temperature gas reactors : a failure methodology for the design of nuclear graphite components

Hindley, Michael Philip (2015-03)

Thesis (PhD)--Stellenbosch University, 2015.


ENGLISH ABSTRACT: This thesis presents a failure evaluation methodology for nuclear graphite components used in high-temperature gas reactors. The failure methodology is aimed at predicting the failure of real parts based on the mechanical testing results of material specimens. The method is a statistical failure methodology for calculating the probability of failure of graphite components, and has been developed and implemented numerically in conjunction with a finite element analysis. Therefore, it can be used on any geometry and load configuration that can be modelled using finite element analysis. The methodology is demonstrated by mechanical testing of NBG-18 nuclear grade graphite specimens with varying geometries under various loading conditions. Some tests were developed as an extension of the material characterisation, specifically engineered to assess the effect of stress concentrations on the failure of NBG-18 components. Two relevant statistical distribution functions, a normal distribution and a twoparameter Weibull distribution are fitted to the experimental material strength data for NBG-18 nuclear graphite. Furthermore, the experimental data are normalised for ease of comparison and combined into one representative data set. The combined data set passes a goodness-of-fit test which implies the mechanism of failure is similar between data sets. A three-parameter Weibull fit to the tensile strength data is only used in order to predict the failure of independent problems according to the statistical failure methodology. The analysis of the experimental results and a discussion of the accuracy of the failure prediction methodology are presented. The data is analysed at median failure load prediction as well as at lower probabilities of failure. This methodology is based on the existence of a “link volume”, a volume of material in a weakest link methodology defined in terms of two grouping criteria. The process for approximating the optimal size of a link volume required for the weakest link failure calculation in NBG-18 nuclear graphite is demonstrated. The influence of the two grouping criteria on the failure load prediction is evaluated. A detailed evaluation of the failure prediction for each test case is performed for all proposed link volumes. From the investigation, recommended link volumes for NBG-18 are given for an accurate or conservative failure prediction. Furthermore, failure prediction of a full-sized specimen test is designed to simulate the failure condition which would be encountered if the reactor is evaluated independently. Three specimens are tested and evaluated against the predicted failure. Failure of the full-size component is predicted realistically but conservatively. The predicted failure using link volume values for the test rig design is 20% conservative. The methodology is based on the Weibull weakest link method which is inherently volume dependent. Consequently, the conservatism shows that the methodology has volume dependency as experienced in the classic Weibull theory but to a far lesser extent.

AFRIKAANSE OPSOMMING: Hierdie tesis beskryf ‘n metode wat gebruik kan word om falings in kern grafiet komponente te voorspel. Hierdie komponente word in hoë temperatuur gas reaktore gebruik. Die falings metodologie beoog om die falings van regte komponente te voorspel wat gebaseer is op meganiese toets resultate van materiaal monsters. Dit is ‘n statistiese falings metodologie wat die waarskynlikheid van faling vir grafiet komponente bereken. Die metode is numeries ontwikkel en geïmplementeer deur middel van die eindige element metode, dus kan die metodologie toegepas word op enige geometrie en belastingsgeval wat dan gemodelleer kan word deur gebruik te maak van eindige element metodes. Die metodologie word gedemonstreer deur gebruik te maak van NBG-18 kern grafiet toets monsters. Sommige van hierdie toetse is ontwikkel as ‘n uitbreiding van die materiaal karakterisering wat spesifiek ontwerp is om die effek van die spannings konsentrasies op die faling van die NBG-18 komponente te evalueer. Twee relevante statistiese verspreiding funksies word gekoppel aan die eksperimentele sterkte data van die NBG-18 kern grafiet, naamlik ‘n normale verspreiding en ‘n twee-parameter Weibull verspreiding. Die data stelle word ook genormaliseer vir gemak van vergelyking en gekombineer in een verteenwoordigende data stel. Die gekombineerde data stel slaag ‘n korrelasie toets wat impliseer dat die meganisme van faling soortgelyk is tussen die data stelle. ‘n Drie-parameter Weibull korrelasie op die trek toets monsters word gebruik vir die statistiese falings metodologie. Die analise van die eksperimentele resultate sowel as ‘n bespreking van die akkuraatheid van die faling voorspelling metodologie word voorgelê. Die data word geanaliseer by gemiddelde faling voorspelling asook by laer voorspellings van falings. Hierdie metode is gebaseer op die bestaan van ‘n “ketting volume” wat die volume van ‘n materiaal wat gebruik word in die swakste ketting voorstel en koppel aan die metodologie. ‘n Metode vir die benadering van die ketting volume word voorgestel en daaropeenvolgend gebruik om die ketting volume te bereken vir NBG-18. ‘n Gedetailleerde evaluasie van die falings voorspelling vir elke toets geval word uitgevoer vir die voorgestelde ketting volumes. Gebaseer op hierdie ondersoek is voorgestelde ketting volumes vir NBG-18 gegee vir beide akkurate en konserwatiewe falings voorspellings. Verder was ‘n volgrootte strukturele toets ontwikkel om dieselfde falings omstandighede te simuleer wat verwag is gedurende normale werking van die reaktor. Drie monsters word getoets en geëvalueer teen die voorspelde faling vir beide die berekende ketting volume groottes. Faling van die volgrootte komponente word realisties asook konserwatief voorspel. Die voorpselling is 20% konserwatief. Die metodologie is gebaseer op die Weibull metode wat inherent volume afhanklik is; gevolglik dui die konserwatisme aan dat die metodologie oor volume afhanklikheid beskik soos ondervind word in die klassieke Weibull teorie, maar tot ‘n baie kleiner mate.

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