Possibility of Hydrogen Confinement by Helium Bubbles with Different Pressure

Abstract

The problem of helium and hydrogen in reactors is due to their adverse effect on the radiation resistance of structural materials. In addition, a new phenomenon has been discovered in the last few years -radiation swelling at relatively low temperatures (about 300°C) of in-reactor equipment in thermal reactors, thought to be caused by the accumulation of hydrogen in very small pores which are stabilized by helium [1,2]. This is why the mechanisms leading to the development of helium porosity and the behavior of the subsequently embedded hydrogen in ferrite-martensite ÉP-900 (16Kh12MVSFBAR) and austenitic Kh18N10T steels are of interest.The samples were subjected to standard heat treatment: ÉP-900 steel -normalization at 1100°C, 40 min, cooling in air plus tempering at 720°C, 3 h, cooling in air; Kh18N10T steel -austenization at 1100°C, 40 min, cooling in air. Some samples were irradiated with 40 keV He + up to dose 5·10 20 m -2 in the range 20-650°C in order to produce different defect structure and helium porosity with different gas pressure in the bubbles. Next, 25 keV hydrogen ions were embedded up to dose 5·10 20 m -2 with the same projected travel distance as for helium ions in unirradiated and irradiated samples at room temperature. The microstructure of the samples, thinned on the unirradiated side, was studied with a JEM-2000EX transmission electron microscope (Japan). The hydrogen content in the steel was determined by reductive melting in vacuum, using a model RH-402 high-sensitivity gas analyzer manufactured by the Leco Company (USA).The microstructure formed in the steel by irradiation with helium ions at different temperatures is presented in Figs. 1 and 2. The parameters of the bubbles formed are presented in Table 1.Bubbles did not form in ÉP-900 steel at 300°C -helium is present in the lattice in the form of various complexes [3][4][5][6]. In studying the microstructures of samples irradiated at 420°C, bubbles also were not observed, although the presence of a characteristic contrast under conditions of overfocusing of an image [7] attests to the possible presence of bubbles. Bubbles, 0.5-1.5 nm in size with volume density ~10 25 m -3 , which are well resolved in a microscope, formed after helium was embedded at 500°C. As the irradiation temperature increases to 630°C, the size of the bubbles formed increases sharply as their density decreases (see Fig. 2, Table 1). A characteristic feature of bubbles, formed under high-temperature irradiation, is their faceting and nonuniform distribution with predominant arrangement along dislocations (shown in Fig. 2 by arrows).At ion-irradiation temperatures 420 and 500°C, the characteristic features of the microstructure of Kh18N10T steel are the uniform distribution of helium bubbles in the matrix and the spherical shape of the bubbles (see Fig. 1a). For hightemperature irradiation (630°C), large faceted bubbles (see Fig. 1b, c) are formed, and in contrast to ÉP-900 ferrite-martensite steel they are distributed uniformly in the matrix. In addition, bubbles a...