HTGR safety is secured by a system of barriers limiting the emission of fission products from the core into the surrounding environment during normal operation and postulated anticipated accidents. An experimental-computational analysis of two fundamentally important barriers -fuel kernels and their coating, whose function is to contain radionuclides and to protect workers and the environment, is examined. The function of the barriers and the requirements which they must satisfy are examined for HTGR fuel particles. The results of post-reactor studies are analyzed. Mathematical models and computational codes simulating the behavior of fuel particles are analyzed. Probabilistic-statistical models and the GOLT code are being developed to evaluate the behavior of fuel particles under irradiation. Together with other models, this code is used for comparative test calculations of the behavior of particle fuel under normal irradiation conditions (<1300°C). The first results of such calculations are discussed.A system of barriers which limits the emission of radionuclides from the core into the surrounding environment during normal operation and postulated anticipated accidents secures the safety of HTGR. Two basic concepts for HTGR are currently being developed: spherical fill in the core and prismatic fill with a core assembled from graphite blocks. In both cases, fuel particles are the main fuel elements.There are five fundamentally important barriers in HTGR which confine radionuclides and provide protection for workers and the environment: fuel kernels, coatings on the fuel kernels, matrix graphite of spherical fuel elements, fuel compacts, graphite fuel blocks, airtight coolant loop which includes the reactor vessel, a connecting vessel, and a turbomachine vessel, and an outer protective shell of the reactor with a special ventilation system. Of these barriers, the multilayer coating of the fuel particles ( Fig. 1) is decisive and the silicon carbide layer, in turn, plays the main role.The capability of coatings to contain radionuclides depends on the fuel quality, which is taken to mean the minimum statistical variance of the prescribed characteristics of a large mass of fuel particles, for example, the number of particles in the core of GT-MGR, which is currently being designed [1], is ~10 10 , as well as the maximum possible stability of the coatings as the fuel burns up. The main damaging factors for the coatings of fuel particles are high fuel temperature, fast-neutron fluence, irradiation intensity, power density, and burnup as well as the chemical action of the fission products, increase of the internal pressure of CO and gaseous fission products, and other factors. The role of most of these factors can change because of a change in the fuel structure, including a change in the kernel size and composition and in the coatings of the fuel particles.

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The coated particles (CP) performance computer code GOLT (Russian abbreviation of Gas-Cooled Fuel) is under development at the A. A. Bochvar All-Russia Research Institute of Inorganic Materials. The main goal of the code is supporting development of fuel for the Gas-Turbine Modular Helium Reactor (GT-MHR). The first version GOLT-v1 has capable to calculate temperature distribution along particle radius, fuel kernel swelling, development of internal pressure under coating due to formation of gaseous fission products and CO, development of stresses and deformation in each coating layer. For TRISO-type particles special probabilistic failure model was developed. According to the failure model integrated probability of silicon carbide failure depends on probability of each dense pyrocarbon layer failure. Probabilistic version GOLT-v2 takes into account possibility of gap formation between buffer and inner dense pyrocarbon layer or between kernel and buffer that influences on maximal fuel temperature and stresses distribution in coating. More detail model of buffer performance at irradiation was developed and included in the code. List of probable coating failure mechanisms was extended. The ability of coating failure due to Kernel-Coating Mechanical Interaction (KCMI) as well as model of failure due to kernel migration was added. Thermo-dynamical code ASTRA is used in some tasks as supporting tool for calculating internal pressure and chemical interaction between SiC coating and fission products and CO. The version GOLT-v3 has accumulated all capabilities of previous versions and included Monte-Carlo analysis for estimation of fraction of failed particles with account of statistical dispersion of structural, materials and operating parameters. In the paper short description of capabilities of last versions of the code is presented. Main attention is putted to results of development version GOLT-v2a for evaluation fuel performance during accidents.

Interest in high-temperature gas-cooled reactors has increased substantially in recent years. Our country is developing, jointly with the USA, the GT-MGR reactor facility, where a fuel particle is the basic fuel element. Particle-based fuel is one of the most promising variants of nuclear fuel for not only HTGR but also VVER [1]. The main feature of HTGR particle-based fuel is the possibility of attaining high coolant (helium) temperature up to 1000°C at the exit from the core with a high degree of confinement of fission products, which is achieved by, first and foremost, maintaining the airtightness of the protective coatings. As a result, theoretical-computational modeling of the behavior and prediction of the fraction of particles whose coating has sustained through damage during irradiation plays a role. Computational modeling makes it possible to work out a structure, choose the main parameters, predict the serviceability of fuel particles, and develop the optimal plan for reactor studies at lower cost.A fuel particle consists of a fuel kernel surrounded by coating layers consisting of pyrocarbon and silicon carbide (Fig. 1).Tens of computational codes have been developed to simulate the behavior and predict the serviceability of a fuel particle [2][3][4][5][6]. In most of these codes, it is assumed that a fuel particle possesses a perfectly symmetric shape, which greatly simplifies the problem. Correspondingly, one-dimensional codes are fast, which makes it possible to take account of a larger number of physical and chemical processes occurring as fuel burns as well as the statistical character of different parameters affecting the lifetime of the fuel.However, codes in this class cannot take account of possible deviations of the shape of real fuel particles from an ideal sphere. Shape distortions can arise during the fabrication of the particles as well as during reactor operation. Figure 2 shows a radiophotograph of fuel particles with typical shape deviation in the form of faceting arising at the time the coatings are deposited.Under irradiation, fission products accumulate in the fuel particles. Some fission products, for example, palladium, chemically interact with the silicon carbide coating. The chemical interaction between silicon carbide and the fission products and CO, which is formed as fuel burns up, can be local, which is often observed in studies of irradiated fuel.Aside from local corrosion, phenomena such as partial separation of the coating, crack formation in individual layers of the coating, and displacement of the kernel relative to the center of a fuel particle (amoeba effect) can also be observed. The development of models and computer codes for describing such phenomena and evaluating their effect on the serviceability of fuel particles has started only in the last few years. Examples of such codes are PARFUME [2, 3] and ATLAS [4], which employ the finite-element method.The objective of the present work was to evaluate the effect of different types of asphericity on the strength of th...