Creep textures of water-water nuclear power reactors cladding tubes made of Zr1Nb alloy examined by neutron diffraction supplemented by metallographic research of hydrides

Ivan Větvička

Czech Technical University in Prague, Faculty of Nuclear Sciences and Physical Engineering, Department of Solid State Engineering, Trojanova 13, 120 00 Praha 2, Czech Republic

nunatak@centrum.cz

Introduction

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.

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 Zr1Nb alloy (also denoted as E-110) by neutron diffraction.

Samples and methods

Tubes composed of Zr and 1 wt % Nb, contain 400 wppm O₂ and 10 – 50 wppm H_2, too. Tubes were made 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 %. The atmosphere composed of argon, 10 wppm H₂O, 5 wppm O₂ and 1 wppm CO₂ was used. Traces of water vapour and oxygen caused hydridation and oxidation of the alloy. The experiment was carried out in ÚFM AVČR [1]. The texture was analyzed by KSN-2 difractometer, thermal neutrons were produced by LVR-15 reactor in Nuclear Research Institute in Řež u Prahy. The results were plotted as inverse pole figures calculated by Harris method [2, 3]. The hydrides were documented on metallographic polished sections in UJP PRAHA.

Changes of texture and reorientation of hydrides

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. Distinctive unfavourable changes in hydride orientation can be expected only during accident of cask/container accompained by temperature rise and break of the cask/container.

The orientation of hydrides did not changed continuously: hydride orientation is usually random and directional alignment was found only in extremely deformed tubes, where the pole density (p´) of plane (100) in AD exceeded the boundary which lies in the interval 6,9 – 8,6.

New texture

In cladding tubes exposed to 700 ^oC 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 (101) pyramid followed by p´ for (100) prism. This texture can be explained by {111} twinning, as only this twin can face both (101) and (100) planes perpendicular to TD. Moreover, only in this sample the (110) prism shows the highest p´ in AD of all of the observed planes in this sample, while the (100) prism has the lowest p´ compared to all samples. High p´ of the (110) prism and the occurrence of {111} twinning prove recrystallisation. Reversible phase transformation of a significant amount of α‑Zr ↔ β‑Zr probably contributes to the formation of this new texture.

Literature

[1] Sklenicka V., Kucharova K., Priprava realizace programu creepovych zkousek povlakove trubky paliva pro lehkovodni reaktory. Technicka zprava UFM AV CR, no. 704309, Brno 2009, 10 p.

[2] Harris G.B., Quantitative measurement of prefered orientation in rolled uranium bars. Philospophical Magazine Series 7, vol. 43, 1952, no. 336, p. 113–125

[3] Kruzelova M., Vratislav S., Dlouha M., Study of zirconium based alloys by neutron diffraction. Materials Structure 18, 2011, no. 3, p. 184–187

[4] Rogozyanov A. Ya., Smirnov A. V., Kanashov B. A., Polenok V. S., Nuzhdov A. A., Use of the Irradiation-Thermal Creep Model of Zr-1% Nb Alloy Cladding Tubes to Describe Dimensional Changes of VVER Fuel Rods. Journal of ASTM International 2, 2005, no. 3, p. 651–665