Formation of multifunctional barriers to increase the radiochemical resistance of the protective coatings of HTGR fuel elements

Abstract

The radiation-size changes of pyrocarbon protective coatings on HTGR microfuel elements are analyzed. It is shown that there is a relationship between the microstructural inner pyrolytic layers and the formation of cracks in these layers as the irradiation dose accumulates. The effect of cracks in the inner pyrocarbon layers on the damage to the silicon carbide layer is examined. It is determined that incorporating into the inner pyrocarbon layer or forming on the inner pyrocarbon-silicon carbide interface compositions, for example, silicon carbide-carbon, Ti 3 SiC 2 , ZrC, TiC, and nitrides of Zr, Ti, and Al creates an obstacle to interior cracks, increasing the radiation-chemical resistance of the carbide layer and the microfuel as a whole.The structure of the microfuel of high-temperature gas-cooled reactors has evolved from particles with a single-layer protective coating consisting of pyrocarbon with a laminar structure through a bilayer pyrocarbon coating with different density to a fuel microsphere with three layers made of, respectively, inner high-density isotropic pyrocarbon and silicon carbide and an outer layer of high-density isotropic pyrocarbon ( Fig. 1) [1].The specifications for the microsphere fuel for HTGR take account of the following essential characteristics: thickness, density, anisotropy of the pyrocarbon coating, contamination of the coatings by fissile materials, defects in the silicon carbide layer, and others [2-6].The operational parameters (temperature, degree of fuel burnup, fluence of fast neutrons, and others) which have been obtained on the experimental stand satisfied the technical requirements for uranium dioxide microspherical fuel based on multilayer coatings (see Fig. 1, position I) and even exceeded the requirements in individual cases (see Fig. 1, positions II and III).In connection with new problems arising in the development of fourth-generation nuclear-energy systems, the requirements for the operational parameters (nuclear fuel burnup, irradiation temperature, hast-neutron fluence, energy release from a fuel microsphere, and others) are increasing substantially. Probably the biggest problem will be due to the realization of deep burnup of fuel at high irradiation temperature -up to 20% h.a. and higher than 1100°C under stationary operating conditions of a very-high-temperature gas-cooled reactor (VHTR, Japan).One of the variants under consideration variant for improving the performance of multilayer coatings is replacing the silicon carbide load-bearing layer by a zirconium carbide layer. However, such a replacement will make it necessary to solve a multitude of materials-engineering problems, develop methods for monitoring the properties of carbide-zirconium coatings and rejecting microfuel based on them, and determine the capability of zirconium carbide to confine solid fission products at elevated irradiation temperatures taking account of the high neutron fluence for VHTR and GFR (see Fig. 1, positions IV, V).Laboratory investigations performed in the USA on micros...

Pyrocarbon Coating on Granular Al2O3 for HTGR-Type Power Reactor

“…Kurbakov [23] investigated the effect of cracks in the inner pyrocarbon layers on damage to silicon carbide layers. As a result, it was determined that the introduction of pyrolytic carbon into the inner layer or the formation of "pyrocarbon-silicon carbide" compositions ("silicon carbide-carbon", Ti 3 SiC 2 , ZrC, TiC, and nitrides of Zr, Ti, Al) on the inner interface leads to internal cracks.…”