Reliability Analysis Of Nuclear Waste Storage Containers Based On Time-dependency Of Fiber Concrete Strength

The service period of concrete nuclear waste storage containers for keeping low-medium level radioactive waste is 300 years, During this period, the storage containers bore sustainability thermal stresses which generated by decay heat. Long-term effect of temperature-load coupling caused the performance degradation and reduction in strength of concrete material. The nuclear waste storage containers of concrete being used are the scientific payoffs in 1989, which made of ordinary concrete. The long-term effect of temperature stress to the containers were not fully considered at design time, which cause cracks appear on the surface of concrete waste containers that has been put into use. Polypropylene-basalt fiber reinforce concrete was used to improve the material properties of storage containers. The intensity attenuation of fiber concrete that caused by creep property and the time-varying mechanics characteristic under the action of thermal-mechanical coupling were considered and the reliability during service were analysed. The research content of this paper is as follows:The fiber dosage and length factors were considered to make nine kinds concrete specimen for different mix proportion, the slump test, compressive strength test and permeability resistance test were analysed to analysis the enhancement mechanism of polypropylene fiber and basalt fiber in working performance, mechanical property, durability performance, and determine material compos that on the basis of influence significance for fiber on concrete.Based on the uniaxial compression creep test of concrete, the characteristics creep curve was analyzed, and come to the conclusion that burgers model was applicable to describe the creep characteristics of ordinary and fiber concrete. Using the creep curve to back analysis the parameter of burgers model with the Least-square method, A one-dimensional constitutive equation of ordinary and fiber concrete were established. The theoretical methods of solving time-varying strength through creep constitutive equation were analysed systematic. Creep compliance was introduced to establish the relationship between long-term elastic modulus and instantaneous elastic modulus, and then, the theoretical formulas of time-varying compressive and tensile strength were derived. The results of time-varying strength analysis show that compressive and tensile strength of fiber concrete are greater than that of ordinary concrete in every time, and the difference were continue to reduce in 300 years.The working position of concrete nuclear waste storage containers was simulate, nuclear decay heat was replace by electro-heating system and heating-thermometry measuring device were used to obtain the actual temperature distribution inside the container. The MATLAB was used to programming the BP neural network, back analysis the thermal parameters of concrete based on the measured temperature and evaluate the results of the analysis by posterior deviation test. The initial conditions and boundary conditions were to be sure according to the actual work condition of nuclear waste storage containers, the variation of temperature with time of three-dimensional models were analyzed, and then the temperature stress inside the storage containers are obtained. By analyzing the time relationship of radial stress, hoop stress, axial stress and the first and the third principal stress, the biggest stress of concrete waste storage containers were got during the service period, and provide mechanical reference for reliability analysis.Considering the influence of temperature on the strength of concrete and its creep properties, amended the time-varying model of concrete strength in The condition of the atmosphere, get comprehensive model which reaction quantitative the influence of atmospheric environment、temperature effect and load duration on concrete. Considering the uncertainty for the strength and thermal parameters of concrete, get the failure probability of fiber concrete nuclear waste storage containers which through the Monte-Carlo simulation.