Influence of stresses on deformation process under the irradiation creep and swelling

Approaches for describing the deformation of structural elements made from the material, in which radiation creep and swelling strains develop simultaneously, are discussed. The technique for description of irradiation swelling strains, which is used for calculational analysis of stress-strain state arising in structural elements under the joint action of irradiation and thermal-stress fields, is regarded. A complete system of equations of the boundary –initial value problem is presented, in which elastic and thermal strains, strains of radiation creep and swelling are taken into account. Numerical modelling was carried out using the specialized software FEM Creep, in which the boundary value problem is solved by the Finite Element Method, and the initial one is integrated in time by the difference predictor-corrector method. Two forms are given for the equation of state describing the radiation swelling strains: first is for the components of the strain tensor as well as second is prepared for their rates. The hypothesis about the linear correspondence of the received radiation dose and the deformation time, during which radiation swelling strains develop, are analyzed. A number of questions that require answers when using equations with a complex stress state in which the radiation swelling strains are directly depend on stresses, are discussed. Based on the processing of experimental data on the swelling of tubes made of steel 316Ti in the temperature range of 450-460 °С, a form of the equation for the radiation swelling strain rate is proposed, and the constants included in it are determined. Using the example of numerical modelling of the deformation of tubes were made of steel 316Ti and loaded by inner pressure, the applicability of the classical approach for the analysis of the stress-strain state in the presence of radiation swelling strains is shown.

Study of the Effect of Doping ZrO2 Ceramics with MgO to Increase the Resistance to Polymorphic Transformations under the Action of Irradiation

The purpose of this study is to assess the effect of doping ZrO2 ceramics with MgO on radiation swelling and polymorphic transformations, as a result of irradiation with heavy ions. Interest in these types of materials is due to the great prospects for their use as structural materials for new-generation reactors. The study established the dependences of the phase composition formation and changes in the structural parameters following a change in the concentration of MgO. It has been established that the main mechanism for changing the structural properties of ceramics is the displacement of the cubic c-ZrO2 phase by the Zr0.9Mg0.1O2 substitution phase, which leads to an increase in the stability of ceramic properties to irradiation. It has been determined that an increase in MgO concentration leads to the formation of an impurity phase Zr0.9Mg0.1O2 due to the type of substitution, resulting in changes to the structural parameters of ceramics. During studies of changes in the strength properties of irradiated ceramics, it was found that the formation of a phase in the Zr0.9Mg0.1O2 structure leads to an increase in the resistance to cracking and embrittlement of the surface layers of ceramics.

STUDY OF THE RADIATION SITUATION IN MOSCOW BY INVESTIGATING ELASTOPLASTIC BODIES IN A NEUTRON FLUX TAKING INTO ACCOUNT THERMAL EFFECTS

This article discusses the study of the background radiation level in Moscow and its impact on the environment. To identify the radiological impact on living conditions, a study of the effects of fast neutrons on infrastructure and biological objects was made. Large-scale research organizations carrying out technological development, scientific and materials research using nuclear materials in the city of Moscow and the region are considered. A system of environment radiation monitoring, covering an area of more than 1091 km², was examined in detail. A radioecological survey conducted in the Troitsky and Novomoskovsky administrative districts was analyzed to identify and describe the radiation situation. An analysis of background radiation in Moscow shows that the values of the controlled parameters of radioactive pollution of environmental objects are within the long-term fluctuations of the technogenic background of the capital. In this case, various thermal effects arise caused by radiation exposure to solids. The study of infrastructure and biological objects is carried out based on the determination of effects, because of which the elastic and mainly plastic characteristics of substances exposed to neutron irradiation and thermal effects change. Here, the main essential factors are radiation hardening of the material (increase in yield stress) and radiation swelling (increase in volumetric strain).

Homogenization of Cr-Ni Austenitic Steel Studying: Liquation and Microhardness Heterogeneity Equalization

Currently, a new generation of pressurized water reactor for nuclear power plants with an extended service life (up to 60 years) and a guarantee of their complete safety are being designed in Russia. Analysis of the reactor internal elements performance showed, that designed service life cannot be guaranteed if the reactor’s internal parts would be made from currently used stainless steel (18-10 alloy type). Instead of the used steel, to ensure operability, new austenitic stainless steel (16-25 alloy type), with increased resistance to radiation swelling, is being developed for production of forged ring blanks for core baffle. The use of new steel requires revision of the existing metallurgical production technology stages. Therefore in this paper diffusion experiment was carried out to determine the required duration of homogenization. The results are presented in terms of different duration of the high-temperature exposure effect on the liquation heterogeneity equalization. The relation between duration of homogenization and microhardness is also shown.

Improving the Methods for Estimating Radiation Swelling and Progressive Dimensional Changes of the Elements of VVER-1000 Internals

The problem in the computer prediction of radiation swelling in the elements of reactor internals when justifying VVER-1000 safe operation is quite relevant. In this regard, the approaches of the current Standard Program (PM-T.0.03.333-15) for assessing the technical condition and lifetime extension of VVER-1000 internals were considered. The paper presents various models of radiation swelling for 08Х18Н10Т steel of which elements of internals are made. A comparative analysis of the prediction results for various models has been performed. It is shown that the approaches of the current Standard Program, which take into account factors such as the stress-strain state and radiation creep of the material, are the most relevant. The influence of various fuel campaigns and their sequence on the nature of distribution and the maximum value of radiation swelling, as well as on the stress state in the baffle after long-term operation has been investigated. It is determined that the calculated stationary temperature distribution in the baffle may differ significantly depending on the input data on heat release and accumulated fluence, therefore, it must be calculated for each fuel campaign, and the calculation of the input data averaged for all fuel campaigns may lead to significant errors in the results. From the point of view of the reliability, it is also important to take into account the sequence of fuel campaigns, since, depending on the radiation swelling kinetics, the resulting baffle form can significantly change. The consequences of a possible decrease in cooling efficiency on the external surface of the baffle in the case of reducing the gap between the inner cavity and the baffle are considered, which may cause a local temperature increase in the baffle, but does not lead to a noticeable change in the radiation swelling. However, the radial displacement of the baffle due to thermal expansion increases significantly, which can cause an increase in stresses in the contact area, both in the wall of the cavity and in the baffle. According to the results of the computer analysis, recommendations were formulated to improve the approaches to estimating radiation swelling and progressive dimensional changes in the elements of VVER-1000 internals.

Molecular Dynamics Simulations of the Excess Vacancy Evolution in V and V-4Ti

Titanium additions suppress the radiation swelling in vanadium alloys, but the mechanism of the suppression is debatable. It is claimed that interactions of Ti atoms with vacancies in vanadium are crucial in the suppression. In this paper we study clustering of excess vacancies in pure V and V-4Ti by molecular dynamics simulations at 700 K based on our interatomic potentials constructed earlier for the Ti-V system. For pure V the process results in the formation of a Frank sessile dislocation while the clustering in V-4Ti ends with the formation of the Ti-vacancies complexes with a specific structure. This allows us to discuss a possible mechanism of the swelling reduction in V caused by Ti alloying.