Oxidation and nitridization of Zr1%Nb Z. Hózer, M. Kunstár, L. Matus, N. Vér presented by L. Matus 11th International QUENCH Workshop Karlsruhe, 25-27 October, 2005, Germany
1. Introduction • In the frame of our CLAIR (Oxidation and nitridization of Zr CLaddings in AIR) program we are dealing with the high temperature behaviour of VVER cladding material Zr1%Nb in air, oxygen and nitrogen. For comparison some tests were performed by Zircaloy-4. • The experiments were made up to low extent of reactions, because we wanted to remain • - in the parabolic range of reaction rate, • - at low embrittlement to have possibility for ring rupture tests.
2. Experimental • A resistance heated vertical tube furnace was applied with quartz inlet tube for getting closed area to get definite atmosphere at experiments. • Both air and oxygen were dried with adsorber. • In case of nitrogen special attention was paid to remove any oxidative component. At the bottom of high temperature stage Zr turnings were placed in to result sufficiently pure nitrogen. In this case double walled quartz tube was applied with vacuum or nitrogen filled to avoid oxygen diffusion through the high temperature quartz.
With a small device it was possible to place the sample into the upper cold part of quartz inlet tube and lowering it to the hot stage of furnace after appropriate gas flushing and furnace temperature. It made also possible to interrupt reaction by pulling back samples to cold top. • The samples were weighted and geometrically measured before and after tests.
This slide shows the Zr turnings after some hours nitrogen streaming through. The lower dark stage formed because of oxidant species. The upper gold coloured part indicate that the escaping nitrogen was pure enough.
3. Resultsa. Air oxidation of Zr1%Nb • Some years ago we studied the air oxidation of Zr1%Nb separate effect test for CODEX-AIT. • The present results agree well with the earlier as can be seen in the next plot. • Andreeva-Andrievskaya et. al. got similar results for the same alloy. • The results in both laboratory showed that the Arrhenius type plot is not a straight line, the reaction rate increase with reduced rate at higher temperature.
Air oxidation of Zr1%Nb, weight gain vs. square root of time Using a new cladding material the air oxidation showed a delay in start of process. It is certainly coming from its original surface oxide layer.
Growth of test samples during air oxidation It is well known that about ECR~10% is the allowed oxidation for cladding. At that value ca. 3% increase of cladding geometries has been found.
b. Oxygen oxidation of Zr1%Nb • Weight gain of some zirconium alloy and pure zirconium in O2 oxidation can be seen in figure left.
Similar to the air oxidation a delay appeared in start of oxidation process. The time of delay was also about 40 s, similar to air. At Zircaloy-4 no delay was observed.
Growth of test samples during O2 oxidation Linear growths vs. ECR are similar as at air oxidation. Moreover the length and diameter were growing on the same extent.
C. Reaction of N2 with Zr1%Nb In figure left the mass gain vs. square root of time is shown. The parabolic law is valid and no delay in start of reactions has been found.
Reaction rates of N2 with Zr and its alloys, summery On left our results for Zr1%Nb and one point for Zircaloy-4 together with others from literature for pure Zr can be seen. At atmospheric pressure N2 all values are on the same line. Under them are given points from Dravnieks, who used lower N2 pressures.
Growth of test samples during N2 reactions It seems so that at higher temperature the deformation with ECR is going back. Certainly with rising temperature the nitrogen is distributed more in the Zr material.
4. Summary • Reactions of Zr1%Nb VVER cladding material has been studied in air, oxygen and nitrogen at high temperatures (800-1200 oC) and low extent of scaling. • The parabolic law is valid in the ECR < 10 % range. • The new cladding type shows a delay in start of air and oxygen atmosphere, but not in case of nitrogen. • As comparison the Zircaloy-4 showed no delay. • Geometrical parameters were growing approximately linear with the extent of reactions.
View of samples Zr1%Nb in air 880 oC, 212 s ECR = 7,32% Zr1%Nb in O2 880 oC, 227 s ECR = 8,28% Zircaloy-4 in O2 880 oC, 160 s ECR = 2,31% Zr1%Nb in N2 980 oC, 8 h ECR = 1,21% Zircaloy-4 in N2 1080 oC, 4 h ECR = 1,55%