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A Study on Corrosion Mechanism of Fbr Structural Material in Small Sodium Leak Under Insulator

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A Study on Corrosion Mechanism of Fbr Structural Material in Small Sodium Leak Under Insulator FR0200443 A STUDY ON CORROSION MECHANISM OF FBR STRUCTURAL MATERIAL IN SMALL SODIUM LEAK UNDER INSULATOR T. FURUKAWA (1), D. PI AT (2), S. ROSANVALLON (2) and C. LATGE (2) (1) Japan Nuclear Cycle Development Institute, O-arai Engineering Center (JNC-OEC) 4002 Narita-cho, O-arai-machi, Higashi-lbaraki-gun, Ibaraki, 311-1393, JAPAN (2) Commissariat a I'Energie Atomique, Centre de Cadarache (CEA-Cadarache) 13108 Saint Paul Lez Durance Cedex, FRANCE Key words: Corrosion, Sodium leak, Structural material ABSTRACT was thermodynamically stable, and no direct In order to clarify the corrosion mechanism influence on the steel was observed. of FBR structural material by small sodium Based on these results, a number of cor- leak under mineral wool insulator in the sec- rosion tests were further carried out at OEC ondary or ancillary circuits of Fast Breeder (JNC) in order to obtain the corrosion behav- Reactors, sodium leak test has been carried ior below the melting point (about 873K) of out in the FUTUNA loop at Cadarache (CEA). the main product, Na4Fe03. It was concluded The mock-up was a 316L stainless steel pipe that the weight loss at 873K or below could 12-inch in diameter covered with the insula- be predicted by the time dependence proper- tor. The test lasted for 240 hours at a leak ties based on the diffusion law. rate of 0.1cc/min at 793K in atmosphere. The corrosion of the structural material has INTRODUCTION been extensively observed under the periph- As a coolant of Fast Breeder Reactors ery of the massive leakage products. The cor- (FBRs), sodium reacts with oxygen and mois- rosion mechanism has been estimated based ture in air at elevated temperature. And it is on the results of material analyses and ther- known that structural material is damaged by modynamic data. It was found that the main its by-products. Therefore, it is essential to corrosion mechanism could be similar to that clarify the corrosion phenomena prevailing prevailing in Na2O+Na environment, namely under the sodium leak condition to estimate the 'NaFe double oxidization type corrosion'. the structure integrity. Na-Si complex oxide, which was identified in Research on the corrosion mechanism of the outer region of the corrosion product structural materials in leakage sodium has formed from the reaction between the main been actively carried out since 1980's. elements of the insulator and leakage sodium, Newman [1], [2] has conducted corrosion tests oooo in various sodium compounds on the assump- between Europe and Japan. tions of the sodium-water reaction in the steam generator and of the sodium leak in the sec- EXPERIMENT ondary circuits. In addition, corrosion formula The testing mock-up is shown in figure 1. has been proposed in the latter. Knights et The material of the 12-inch diameter pipe was al. [3] have performed experiments under cor- made of type 316L stainless steel. On the rosive environments generated by the sodium- lower side of the orifice, the analyzing plate water reaction, and suggested one corrosion for X-ray examination which consisted of the mechanism. Aoto [4] has proposed two cor- same material, was fitted. The heater was rosion mechanisms based on thermodynamic installed inside the pipe. The insulator of 250 study to explain the corrosion behavior ob- mm thickness was directly placed around the served in sodium leak tests corresponding to pipe corresponding to that of European FBRs. the secondary circuit condition and the sodium Its chemical composition is given in table 1. leak incident which occurred in Japanese pro- The mock-up was set up in a sodium leak test totype Fast Breeder Reactor 'Monju' in 1995. facility called FUTUNA-2 (figure 2). The test Two corrosion models based on the findings lasted for 240 hours at a leak rate of 0.1 cc/ have been put forward [5], [6]. min at 793K in atmosphere (table 2). In order to describe the corrosion behav- After the test, measurement of the pipe ior of the structural materials under small so- thickness, X-ray analyses of the leakage prod- dium leak in the secondary or ancillary circuits ucts and microstructural observations of the of European FBRs, in connection with the leak pipe were performed. before break (LBB) research, Commissariat a I'Energie Atomique (CEA) has conducted RESULTS studies since 1989 [7]. Until now, it has been TEMPERATURE BEHAVIOR confirmed that leakage sodium reacts with the The temperature behaviors in the vicinity insulator directly covering the sodium pipe, of the orifice throughout the test period are and that sodium leak can continue in certain illustrated in figure 3. conditions to proceed as massive leakage According to the output of the thermo- product is formed. The corrosion of the struc- couple set up in the orifice, the temperature tural material has been observed under the rise which was apparently resulted from the periphery of the products to a great extent. interaction between sodium and oxygen, had Based on the investigation, they have estab- been observed starting from leak initiation (To) lished electrochemical model for the phenom- ena. Insulator Orifice (<t> 6mm) In this paper, the corrosion mechanism of Sodium le, the materials under the insulator is proposed detectors based on the results of more detailed obser- vations and thermodynamic estimation. Sub- sequently, the corrosion formulas in the envi- Heater Analyzing Plate (316L-SS) ronment are presented. 316L-SS Pipe (£ 324mm) This study has been carried out at CEA- Cadarache under the collaboration on FBRs Fig. 1 Apparatus of testing mock-up OGOO Table 1 Chemical Composition of Insulator dium and the insulator, was seen from T.+38 (mass%) hours to T0+40 hours. SiO2 CaO AI2O3 MgO 42% 37% 10.7 4% POST-TEST OBSERVATION Table 2 Test Condition The photographs of the pipe after the test Temperature 793K are shown in figure 4. Although discoloration Leak Rate 0.1cc/min owing to high temperature heating was ob- Leak Time 240h served on the surface of the insulator, leak- age sodium was not observed (figure 4(a)). Green product was deposited on the pipe re- Ventilation moved from the insulator, with a dimension of Test cell about 400 mm wide and 90 mm high (figure 4(b)). From this finding, it was considered that most of the leakage sodium was held within the product. The cross-section of the product is shown in figure 4(C). Semi-lustrous white Heater product was observed inside the green prod- uct, and black one at the interface of the white layer and the pipe. ) Sodium storage tank Fig. 2 Overview of the test facility for 14 hours. The temperature eventually kept at818K. The temperatures of the three thermo- couples mounted on the pipe at a distance of 50 mm from the orifice, sharply fluctuated in the range between 793K and 973K before About 100mm from orifice these thermocouples failed. In this region, grooving corrosion with luster was observed 800 (ret. next chapter). The temperatures of other 700 thermocouples installed on the pipe were 1100 50mm from orifice maintained at approximately 818K during the 1000 900 In insulator, 75mmH test period, excluding T0+38 hours to T0+40 800 hours. Based on the data from the thermo- 700 couple placed in the insulator, the tempera- 600 48 96 144 192 240 ture fluctuation, which seemed to be due to Passage Time / h the chemical reaction between leakage so- Fig.3 Temperature behavior OGOO Table 3 X-ray Results The result of X-ray analyses is summarized Sampling Positions XRD Results in table 3. Green product near Na4SiO4, NaAIO2 The green product was identified mainly surface Black product on as Na4Si04, which was formed by chemical NaFeO2, NaOH surface reaction between leakage sodium and the White product near Na2O2, Na2O, NaOH, main element (SiO2) of the insulator. surface (SiO2) 2Na2O[s] + SiO2[sl] = Na4Si04[sl] = "342-9 kJ/mo1 The white product consisted of Na2O, which existed at low oxygen pressure, and stable compounds (Na2O2 and NaOH) at at- mospheric pressure. However, Na-Si-0 com- plex oxide was not identified here. It seemed that Na2O2 and NaOH were formed during storage because the sample was kept in the glove box in the presence of approximately Fig. 4 The photograph of the mock-up 200ppm oxygen for 10 days. Photographs of the pipe following water cleaning and of the pipe thickness are shown in figure 5 and 6. Three differential phenom- ena prevailed on the surface. The color of the inner region located under the white prod- uct was brown with partial gray, and the cor- rosion depth was less than 0.7 mm. The outer region located under the green product was blackish brown, and the depth was the same as that of the inner region. Concentric groov- ing corrosion of maximum depth 2.1 mm was Fig. 5 The appearance of the pipe after observed between the two regions. The width water cleaning was about 20mm, and the appearance was silver metallic with luster. There was an insig- I 2-2.5 D 2.5-3 D 3-3.5 m 3.5-4 • 4-4.5 D 4.5-5 -130 nificant amount of corrosion product on the Thickness (mm) -90 spot. Only NaFeO2 and NaOH were identi- -50 fied in the product by X-ray analysis. In short, -10 it was considered that the chemical reaction 30 in Na-Fe-0 system occurred in the area. 70 The results of optical microscopy of the 110 cross section and scanning electron micros- T- -i- CM CM CO CO -<t copy of the surface are shown in figure 7 and Xaxis/mm Fig.
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