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11Наноматериалы и нанотехнологии в аэрокосмической отрасли
УДК 620.9
I. Vetvicka
Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Czech Republic, Prague
CREEP TEXTURES OF VVER CLADDING TUBES MADE OF Zr1Nb ALLOY EXAMINED BY NEUTRON DIFFRACTION SUPPLEMENTED BY METALLOGRAPHIC RESEARCH OF HYDRIDES
In nuclear power engineering it is possible to see the tendency to increase the burn-up of nuclear fuel, which leads to more effective fuel utilization and reduces operating costs. However, together with increasing burn-up, the demands on the resistance of the cladding tubes in which the nuclear fuel is encapsulated are increasing. Niobium-alloyed zirconium meets the increasing requirements best. ZrNb alloys have been produced in Russian federation for many years and they have started to be produced also in the USA and France.
At elevated temperatures during reactor operation the Zr alloy shows some creep and the cladding tube undergoes changes. The goal of this work was to study creep-caused texture changes in ZrlNb alloy (also denoted as E-110) by neutron diffraction. Tubes composed of Zr and 1 wt % Nb, contain 400 ppm O2 and 10-50 ppm H2, too. Tubes were made in USSR between the years 1980-1985 for fuel rod construction for water-water nuclear power reactors (VVER) and were stored in UJP PRAHA.
The outer diameter of tubes prior to deformation was 9,16 mm and their wall thickness was 0,70 mm. Five tubes (samples D, E, F, G, H) of initial length of 100 mm
were exposed to 350-850 oC and constant tensile stress of 5-200 MPa in axial direction (AD). Samples were extended by 36-48 %. As technical-grade argon was used as inert atmosphere, traces of water vapour (residual air humidity) caused oxidation and hydridation of the alloy.
The experiment was carried out in UFM AVCR [1]. The texture was analyzed by KSN-2 difractometer, thermal neutrons were produced by LVR-15 reactor in Nuclear Research Institute in Rez u Prahy. The results were plotted as inverse pole figures (IPF, see figure) calculated by Harris method [2; 3]. The hydrides were documented on metallographic polished sections in UJP PRAHA:
Решетневскце чтения
Temperatures corresponding to VVER operation temperature or higher, combined with constant tensile stress, cause creep leading to increased formation of zirconium crystallites, which rotate their bases in a direction perpendicular to tangential direction (TD). This represents conditions favourable for hydride precipitation in the radial direction (RD), which dramatically reduce tube wall resistance to rupture. However, this effect was observed during experiments with open tubes without internal overpressure. Under real operation conditions in a reactor, the overpressure of gasses inside the fuel rod contributes to the deformation of cladding tubes. Results of Rogozyanov et al. [4] suggest that in standard operation conditions of VVER, the effect of axial tensile stress slightly dominate over the internal overpressure effects. The final deformation is approx. 80 x smaller than the deformation resulting from performed experiments and resulting texture changes will have only small influence on the orientation of hydrides. Big unfavourable changes in hydride orientation can be expected only in emergency conditions.
In cladding tubes exposed to 700 °C and constant tensile stress of 10 MPa for 184 h (sample G), a new (not yet described in literature) texture appeared: the highest pole density (p') in TD was found for 101l pyramid followed by p' for 1010 prism (see figure). This texture
can be explained by {112 1} twinning, as only this twin can face both 1011 and 1010 planes perpendicular to TD. Moreover, only in this sample the 1120 prism shows the highest p' in AD of all of the observed planes in this sample, while the 1010 prism has the lowest p' compared to all samples. High p' of the 1120 prism and the occurrence of {1121} twinning prove recrystallisation. Reversible phase transformation of a significant amount of a-Zr ^ p-Zr probably contributes to the formation of this new texture.
Inverse pole figures (IPF) showing pole densities of crystallographic planes of chosen samples C (undeformed), E and G (after the creep test) of Zr1Nb cladding tubes in axial, radial and tangential directions (AD, RD and TD). The three highest pole density values are printed in bold, the highest pole density is underlined with a solid line, the second highest with a dashed line (a). The position of measured planes in IPF (b). The correlation of hydride precipitation (black scale-like objects) to the orientation of hexagonal zirconium crystallites in the cladding tube wall. AD, RD and TD are marked (c).
Reference
1. Sklenicka V., Kucharova K. Priprava realizace programu creepovych zkousek povlakove trubky paliva pro lehkovodni reaktory : technicka zprava UFM AV CR № 70430. Brno, 2009. P. 10.
И. Ветвика
Факультет ядерной и прикладной физики, Чешский технический университет, Чешская республика, Прага
ИССЛЕДОВАНИЕ ДЕФОРМАЦИИ ПОЛЗУЧЕСТИ СТРУКТУР ЗАЩИТНЫХ ТРУБ ИЗ СПЛАВА Zr1Nb ДИФРАКЦИЕЙ НЕЙТРОНОВ, ДОПОЛНЕННОЙ МЕТАЛЛОГРАФИЧЕСКИМ ИЗУЧЕНИЕМ ГИДРИДОВ
В ядерной энергетике наблюдается тенденция усиления выгорания ядерного топлива, что приводит к более эффективной утилизации отработанного топлива и сокращению расходов. Однако вместе с усилением выгорания топлива повышаются требования к надежности защитных труб реактора, в которых располагаются капсулы с топливом. Цирконий, легированный ниобием, наилучшим образом удовлетворяет этим требованиям. Сплавы многие годы производились в Российской Федерации, также их производство началось в США и Франции.
© УеМска I., 2012
УДК 539.21:537.86
С. С. Аплеснин, О. Н. Бандурина
Сибирский государственный аэрокосмический университет имени академика М. Ф. Решетнева, Россия, Красноярск
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