Experimental study on sodium insulation interaction and its effect on structural material

CH.S.S.S. AVINASH*1, V. SNEHALATHA1, E. HEMANTH RAO1, M. NANIBABU2, S. R. POLAKI3, SANJAY KUMAR DAS1, D. PONRAJU1, S. ATHMALINGAM1 and B.VENKATRAMAN1

1Safety, Quality and Resource Management Group, IGCAR, Kalpakkam, India

2Metallurgy and Materials Group, IGCAR, Kalpakkam, India

3Materials Science Group, IGCAR, Kalpakkam, India

* Email contact of corresponding author: chavin@igcar.gov.in

Abstract

Liquid sodium, owing to its high heat transfer properties and excellent compatibility with structural materials is the most preferred coolant for Liquid Metal cooled Fast Breeder Reactors (LMFBR). Apart from its favorable properties, sodium also poses a concern due to high chemical reactivity with air. Accidental sodium leaks from secondary circuit may result in fire due to sodium reaction with oxygen and moisture in ambient air. Usually, leak incidents with leak rates above 100 g/h are detected by redundant and diverse leak sensors. However, if the leak origin is a pin hole or a hairline crack, sodium leak can go undetected and may not lead to fire. In fact, sodium reacts with thermal insulation mounted over the piping and oxygen available within leak vicinity. The solid reaction products can settle near the leak path and eventually plug the leak path, even before the leak detectors respond. The structural material at the leak zone can undergo localized degradation under the influence of sodium, oxides and other reaction products of sodium. An experimental study was carried out at IGCAR to investigate sodium interaction with thermal insulation and effects of reaction products on the structural material. Sodium leak experiments were performed with rock wool insulation and specimens of SS-316 LN at a temperature of 300oC in air. Subsequently, post-test analysis of the samples by using X-ray Diffraction (XRD) revealed presence of silicates and aluminates of sodium in the outer layers and complex ternary and quaternary oxides of sodium, iron, chromium and nickel in the inner layers. Scanning Electron Microscopy (SEM) analysis of the specimen revealed deposition of oxides on the exposed surface. Elemental analysis of the samples carried out by Energy Dispersive Spectroscopy (EDS) indicated leaching of major alloying elements like chromium, nickel and iron to a depth of about ~ 25µm from the exposed surface. Room temperature Fatigue crack growth studies on the exposed specimen shown considerable reduction in crack initiation fracture toughness.

1. INTRODUCTION

Sodium cooled Fast Reactors (SFRs) are one of the most promising Gen IV concepts for rapid expansion of power and efficient burning of minor actinides [1]. Though sodium has excellent heat transfer properties and excellent compatibility with structural materials, it also poses a concern due to high chemical reactivity [2]. In case of an accidental leak, it undergoes vigorous exothermic reaction with oxygen and moisture content in atmosphere. Usually, the primary sodium capacities are double enveloped with inert gas filling or enclosed in inert gas filled cells. However, the secondary sodium systems are single walled and few leaks are envisaged in the life span of the reactors. Despite of defence in depth in design, stringent quality control during manufacturing, erection and operation, sodium leaks [3] are still considered in the design safety analysis. Austenitic stainless steel (ASS) is the preferred material for sodium systems owing to its corrosive resistance and high temperature creep strength. However, its low thermal conductivity coupled with high coefficient of thermal expansion leads to high thermal stresses. In such conditions of high thermo mechanical stresses, even minor flaws such as a surface notch or a part through crack in the piping/ component can develop into a through wall defect and can act as leak initiator. In general, all the secondary sodium pipe lines and vessels are provided with surface heaters and thermal insulation enveloped by aluminium clad along with surface leak detectors. During an accidental leak, sodium is exposed to the outer surface of the pipe and interacts with chemical constituents of thermal insulation and other gases at the leak vicinity. Depending on temperature of the sodium, oxygen availability near leak zone, there could be local sodium fire or sodium oxidation without fire. If the leak path is a torturous one such as a hair line crack or a pin hole, sodium leak rate will be less and leaked sodium may not travel up to leak detector, resulting in deposition of the reaction products near the vicinity of leak zone [4, 5, 6]. These solid reaction products may hinder sodium leak resulting in plugging of the leak path before leak detector responds. Build-up of reaction products at the leak point will lead to localized corrosion of the structural material [7]. The presence of oxides and hydroxides of sodium coupled with high temperature near the leak vicinity leads to de-alloying, inter granular attack and ferritization of ASS [8, 9, 10]. This will result in loss of strength, which in combination with high thermo mechanical stress would lead to changes in micro structure [11] and faster growth of the defect causing leak. Towards understanding this phenomenon, an experimental study has been taken up at Safety Engineering Division, IGCAR to investigate the sodium interaction with thermal insulation and its effect on structural material. The main objectives are to determine chemical composition of the solid reaction products in the leak vicinity and also to figure out degradation of the structural material in terms of loss of alloy composition and crack growth resistance.

2.0 Materials and Methods

A test setup was designed and installed for performing experiments with sodium at 350 oC and rock wool insulation. Quarter sized Compact Tension (CT) specimens of SS 316 LN conforming to ASTM E647 standard were embedded within the rock wool insulation and subjected to sodium leak. Post-test samples were collected from CT specimen surface as well as different depths of insulation and subjected to X-Ray Diffraction (XRD) analysis to determine composition of the reaction products. Surface morphology and elemental analysis of the exposed specimen were characterized through Scanning Electron Microscopy (SEM) - Energy Dispersive Spectroscopy (EDS). Cross sectional analysis of the CT sample was done to determine compositional change from exposed edge to middle surface towards assessment of material loss from the sample due to corrosion. After XRD and SEM characterization, fatigue crack growth studies at room temperature were carried out on exposed CT and compared with that of unexposed ones.

X-ray analysis was carried out using “INEL X-Ray diffractometer-Equinox 2000”, at 35 kV and 20 mA. The machine is of powder diffraction type with unique real time acquisition mode over 110° 2θ. X-ray generator has a capacity of 3.5 kW with standard sealed X-ray tube of Co Kα (λ=1.7879 Å) radiation. The optics is of monochromatic type. Curved position sensitive, Argon gas X-ray detector with a curvature radius of 180 mm is used as the detector. Acquisition is in asymmetric mode with the goniometric table having no moving parts. Diffraction of the specimens is carried out on rotating sample holders. The diffraction patterns were analysed using Qual-2X software with COD1703 ino data base which basically follows the Hanawault search process. The software matches the given spectra with restraints applied and the products were identified by this peak match process.

Supra 55, carl Zeiss, Germany make Field Emission gun SEM was employed to evaluate the morphology and elemental analysis of the rock wool insulation samples as well as stainless steels specimens before and after sodium interaction. The morphology was imaged using the secondary electron detector with electron beam energy of 5 keV. Cross sectional SEM of the exposed CT sample was carried out using Energy Dispersive Spectroscopy (EDS) (INCA X-act, Oxford, UK) under 10 keV electron beam.

2.1 Thermal insulation specifications

Rockwool of density 96 kg/m3 conforming to IS 8183 standard, which is used as thermal insulation for sodium piping, was chosen in the experiments. Rockwool is an inorganic fibrous material made from basalt rock with phenol based organic binder. Its thermal conductivity is ~ 0.03 W/mK. Representative sample of insulation was analysed for its initial constituent elements by means of Energy Dispersive X-ray Fluorescence (ED-XRF) technique and the elemental composition obtained is given in Table 1.

TABLE.1 ELEMENTAL COMPOSITION OF THERMAL INSULATION

Analyte %	96 g/cc
Avg.	Stdev
Mg	4.13	0.12
Al	10.23	0.03
Si	11.56	0.14
S	0.27	0.01
K	0.52	0.02
Ca	21.33	0.04
Ti	0.54	0.01
V	0.03	0.001
Cr	0.009	0.002
Mn	0.21	0.001
Fe	1.12	0.04
Ni	0.01	0.01
Cu	0.07	0.03
Zn	0.012	0.008
Zr	0.057	0.009

2.2 Compact Tension specimen

Single edge notched CT specimens of SS316LN with 25 mm length, 25 mm width and 4 mm thickness were exposed to sodium insulation interaction. Composition of the specimen is given in Table 2. The specimen has an initial notch of 15 mm length, 0.2 mm width and a notch radius of 0.02 mm (Fig.1).

TABLE 2 CHEMICAL COMPOSITION (MASS %) OF SS316LN BASE METAL

C	Mn	Cr	Mo	Ni	Si	S	P	N	Fe
0.025-0.028	1.7-1.74	17.53-17.57	2.49-2.54	12.15-12.36	0.2-0.22	0.0041-0.0055	0.013-0.018	0.05-0.07	Bal

FIG.1 Edge notched compact tension specimen used in the studies

2.3 Experimental setup

The experimental setup consists of a stainless-steel cylindrical test chamber of 75 litres capacity, stainless steel specimen holder cup and auxiliary systems (Fig. 2). The chamber is provided with pressure gauges, data logger and toughened glass viewing windows. Magnesium oxide (MgO) insulated, SS sheathed and ungrounded K-type (Chromel - Alumel) thermocouples were employed for measuring temperatures at various locations within the holder cup and cylindrical enclosure. K-type extension cables are used to extend the thermocouples up to the measuring instrument terminals.

FIG.2 Test Chamber, schematic of the arrangement of materials within the holder cup

3.0 Experimental procedure

Initially about 30 g thermal insulation materials (Rockwool) along with the two CT specimens are loaded in a holder cup. Freshly cut pieces of sodium of ~ 10 g were placed on the specimens and covered with insulation. Thermocouples were positioned in the holder cup and insulation at various depths including the specimen-insulation interface and temperature data was acquired through a 60 channel HP data logger. Oxygen concentration inside the chamber was maintained at 21 Vol%. The specimens were heated to 450 oC. The sodium insulation interaction was imaged though video camera. After the experiment, the CT specimens and insulation at various zones of interaction were retrieved for characterization of the reaction products.

4.0 Results and discussion

When the specimen temperature reached ~ 320 oC, sodium burning was noticed and the test chamber was filled with combustion aerosol (Fig. 3). The same was also confirmed by the sudden rise in temperature at the specimen-insulation interface as shown in Fig. 4. The maximum temperature recorded during the experiment was 628 oC. The maximum temperature of the test chamber was ~ 235oC. After the sodium fire event, the CT specimen and the insulation were maintained at 350oC for span of 48 hours to simulate the realistic scenario of undetected sodium leak from a hot sodium pipeline. Subsequently, the heaters were switched off and the solid interaction products were retrieved at room temperature for further characterization. Few samples of sodium-insulation reaction products and the CT specimen are shown in Fig. 5.

FIG.3 Test Chamber during sodium- insulation interaction experiment

FIG.5 A. Debris collected from layers of insulation away from the SS sample, B. Debri recovered from sides of holder cup, C.SS tensile and CT sample post interaction FIG. 4 Temperature data at various locations of the test setup during the experiment

Initiation of sodium leak C A 4.1 XRD analysis of the exposed insulation and CT specimen

Samples retrieved from the specimen and insulation at various depths, were subjected to XRD analysis. XRD pattern of insulation collected at 2 cm above SS316LN sample did not show any corrosion product. The sample retrieved at 1 cm above the CT specimen indicated presence of sodium silicate, sodium aluminates and magnesium oxide which are products of sodium reaction with major constituents of rock wool insulation. Fig. 6 shows the identified phases of reaction products. XRD pattern of un-interacted insulation revealed a broad hump like feature indicating the amorphous nature of the raw rock wool insulation sample, which is attributed to its fine fibrous morphology. The reaction products in the vicinity of the material specimen, was observed to form a brownish compound (as shown in Fig.7) adhering to the CT specimen. XRD analysis of this compound revealed presence of ternary oxides of sodium, iron, molybdenum and nickel (shown in Fig. 8), indicating change in the alloy composition at the surface of the CT specimen.

XRD analysis of the reaction products from CT specimen surface indicated several peaks corresponding to sodium-dioxo-ferrate, binary oxides of chromium and sodium, oxides of chromium and oxides of nickel (shown in Fig. 9) indicating leaching of major alloying elements chromium and nickel due to sodium oxide/ hydroxide.

FIG .8 Phases identified from the reaction products in the vicinity of CT specimen FIG. 6 Phases identified in sodium thermal insulation interaction layer away from the CT sample FIG.7 Reaction product adhered to the SS sample

FIG.9 Phases identified on the surface of the reacted SS sample

4.2 Scanning electron microscopy analysis

FIG. 10 SEM micrograph of the raw thermal insulation FIG. 11 SEM micrograph of the rock wool thermal insulation after reaction with sodium SEM analysis of unexposed rock wool sample indicated randomly oriented micron sized fibrous structure with visible globules of binder as shown in Fig. 10. The fibres are long and measure about few mm in length with diameters ranging from 6 to 10 micron. Sodium exposed samples indicated, a crystalline morphology suggesting formation of reaction products as confirmed by the XRD results discussed above. SEM micrograph of sodium exposed insulation sample is given in Fig. 11. SEM analysis of surface of the exposed CT specimen has shown presence of brittle structure consisting of distinct grain boundaries.

FIG.12 SEM micrograph of the outer surface of interacted SS sample	4.3 Elemental analysis of CT Specimen by EDS

FIG.13 Variation of elemental composition across the thickness of the exposed CT sample obtained from EDS analysis Cross sectional SEM of the exposed CT sample was carried out using Energy Dispersive Spectroscopy (EDS) (INCA X-act, Oxford, UK) under 10 keV electron beam. The line scan spectrum was taken across the cross section of the sample from the exposed surface to mid-section to obtain change in composition due to exposure to the reaction products. The EDS plot reveals that alloying elements like nickel, iron, chromium & molybdenum leached upto the depths of 8 µm, 30 µm, 25 µm & 25 µm, respectively. Whereas, sodium was found to be penetrated upto a depth of 25µm as Fig.13)

4.4 Effect of sodium-insulation reaction products on crack propagation in CT specimen

Post SEM and XRD characterisation, the CT specimen were also subjected to fatigue loading for assessment of crack growth in the exposed specimens. The specimen was loaded in crack opening mode-I, on a servo hydraulic actuator (MTS make, loading capacity: 50 kN) as shown in Fig.14. Online crack length measurements were done using Direct Current Potential Drop (DCPD) technique. Initially fatigue pre-cracking was carried out starting with a maximum load of 0.9 kN at load ratio, ‘R’ of 0.1. Test data showed that initial number of cycles required for notch to develop into crack were more for the exposed specimen when compared to unexposed ones. This indicates some amount of hardening near the notch which could be due to presence of oxide deposits (interaction products). However, the threshold fracture toughness required for crack growth in case of exposed sample is ~ 7.49 Mpa and is lesser than that of unexposed sample which is ~ 8.64 Mpa. This shows that certainly the reaction product deposits will result in early crack initiation and growth, which is of concern for the realistic scenarios where through thickness defect can lead to faster crack growth. However, in case of undetected leaks it is beneficial as increased crack results in more leak which in turn increases their chances of detectability.

FIG.14 CT sample loaded in a servo hydraulic actuator for FCG studies 5.0 SEM CHARACTERIZATION OF FRACTURE SURFACES OF THE CT SPECIMENS

FIG.15 Surface morphology of the exposed specimen near the notch in the plane of crack, after rupture Post crack growth studies, both exposed and unexposed specimens were ruptured under monotonic loading and morphology of the ruptured surfaces were examined under SEM. Preliminary observations of the SEM micrographs of the crack growth plane of the ruptured sample indicated river like markings up to few microns in the initial crack growth phase near to the notch (Fig.15). In the later phase of the crack growth, many micro voids were observed with the surface appearing dimpled and rough which is characteristic of a typical ductile rupture. Fractured surface of exposed SS16LN CT specimen indicated a distinct boundary between the notch and crack initiation (Fig.16), where as there is a gradual transition from notch to crack observed in unexposed specimen. Both exposed and unexposed specimens exhibited a dimpled rupture towards the end of rupture.

6.0 CONCLUSION

An experimental study has been carried out to investigate sodium interaction with thermal insulation simulating sodium leaks from the secondary sodium pipelines of SFRs. Compact tensile specimens were exposed to the interaction zone to study influence of the reaction products on crack growth behavior. The reaction products at the CT specimen surface and at various depths of insulation from the leak zone were characterized by XRD and SEM. XRD of the samples revealed presence of silicates and aluminates of sodium in the outer layers and ternary oxides of sodium, iron, chromium and nickel in the inner layers. Elemental analysis of the samples carried out by EDS indicated leaching of major alloying elements like chromium, nickel and iron to depths ranging from ~10 µm to 25 µm from the exposed surface. Fatigue crack growth studies carried out on the exposed specimen indicated a reduction of ~13 % in threshold fracture toughness compare to that of unexposed ones. Future experiments are planned to be carried out as a function of exposure time, sodium temperature and composition of material specimen, for establishing the rate of dealloying, leaching saturation limits and also to understand the influence of exposure time on crack growth in Paris regime. These studies will unfold the knowledge on thickness of de-alloyed layer, modified Paris constants, threshold fracture toughness which will aid as inputs for damage tolerant design of sodium piping of existing and new plants.

FIG.16 Surface morphology of the unexposed specimen near the notch in the plane of crack, after rupture

ACKNOWLEDGEMENTS

Authors would like to thank Dr. B.Venkatraman Director, IGCAR for his support and encouragement for carrying out this experimental work. Authors also acknowledge Shri Vetrivendan and Shri Paulson, MMG, IGCAR for supporting XRD and SEM characterization.

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