2017 Water Reactor Fuel Performance Meeting September 10 (Sun) ~ 14 (Thu), 2017 Ramada Plaza Jeju Jeju Island, Korea

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1 WELD DEVELOPMENT OF FE-CR-AL THIN-WALL CLADDING FOR LWR ACCIDENT TOLERANT FUEL Jian Gan 1, Nathan Jerred 1,2, Emmanuel Perez 1, DC Haggard 2, Haiming Wen 1,3 1 Idaho National Laboratory: 1625 PO Box, Idaho Falls, Idaho, ID 83415, Jian.Gan@INL.gov 2 Center for Space Nuclear Research: 995 University Blvd, Idaho Falls, ID 83401, Nathan. Jerred@INL.gov 3 Idaho State University: 995 University Blvd, Idaho Falls, ID 83401, Wenhaim@ISU.edu ABSTRACT: The development of new cladding materials to replace Zr-based cladding for light water reactors (LWRs) has been actively pursued with improved high-temperature oxidation resistance to significantly suppress hydrogen production from an accident such as a loss of coolant event. FeCrAl alloy with a typical composition of approximately Fe-15Cr-5Al has been selected as a candidate material because of its improved high-temperature oxidation resistance over the present LWR zircaloy cladding. As a result of higher thermal neutron absorption for FeCrAl alloy, the cladding wall thickness needs to be reduced by approximately half compared to that of Zircaloy cladding to be comparable to the current LWR fuel performance under the normal operating condition. New weld techniques for joining the endplug to thin-walled cladding need to be developed. Both laser beam weld and pressure resistance weld were employed. This paper summarizes the recent progress on the laser weld development for FeCrAl cladding with 350 µm wall thickness. The results of microstructural characterization and mechanical property testing of the weld for the cladding-endplug set are discussed. KEYWORDS: Laser beam weld, pressure resistance weld, FeCrAl, thin-walled cladding, characterization. I. INTRODUCTION The Fukushima Daiishi reactor incident as a result of devastating tsunami in Japan in 2011 raised a serious concern about the performance of Zircaloy cladding under the loss of coolant accident (LOCA) conditions. It indicates that innovative new cladding materials need to be developed for light water reactor (LWR) with improved resistance to accident conditions. Development of accident tolerant fuel (ATF) for LWR requires the development of robust cladding materials with high strength at high temperatures, good corrosion resistance and radiation tolerance under normal operation, and excellent resistance to oxidation in steam at very high temperature (> 1000 C). One class of materials that has improved properties over the current LWR Zircaloy cladding under high temperature steam condition is a Fe-based stainless steel with higher Cr and Al additions (FeCrAl alloy). The Al (~5 wt.%) in FeCrAl alloy forms Al 2 O 3 at the alloy surface which is much more protective than Cr 2 O 3 in conventional stainless steel. This material has been considered as a candidate for the accident tolerant fuel (ATF) for LWRs. The LWR-ATF program includes weld development for thin wall cladding made from MA 956 oxide dispersion strengthened (ODS) alloy (an ODS FeCrAl alloy), commercial FeCrAl alloy (Kanthal-D) and experimental FeCrAl alloys developed by Oak Ridge National Laboratory (ORNL). The details on ORNL FeCrAl alloy development can be found in the reference 1. The weld technique development in this project was focused on laser beam welding (LBW) and pressure resistance welding (PRW). The former is a high energy intensity fusion-based weld while the latter is a solid state weld without melting. It appears that the laser weld has advantages over the PRW when the cladding wall thickness is significantly reduced compared to the current LWR cladding. Through the course of the laser weld development for thin-wall cladding, it was learned that the optimized weld parameters are sensitive to cladding wall thickness and endplug configuration. This is mainly due to the alteration in heat penetration and energy distribution. Effort was given to investigate the physics based laser parameters (energy per laser pulse, energy profile, pulse rate etc.) rather than the machine dependent parameters (laser voltage setting, etc.). This is important to allow the established LBW parameters transferable to other laser weld systems. 1

2 For laser weld, the use of thin plate coupon samples (355 μm plate on top of a recessed ~ 800 μm thick plate) is effective to test a broad range of laser weld parameters with significantly reduced machining cost and material consumption. Since the ORNL FeCrAl experimental alloy is quite limited in quantity for the project, the use of commercial FeCrAl (Kanthal-D) to study LBW is also critical before proceeding to join the cladding-endplug set fabricated from the ORNL experimental FeCrAl alloy. For the welded endplug-cladding sets only those samples with high quality, as determined by visual inspection and X-ray 3D CT evaluation, were used for tensile testing and the follow on microstructural characterization. The logical steps are (1) use the coupon samples to identify the optimal condition for laser weld, (2) use the identified optimal condition to join the emulated cladding-endplug set fabricated from commercial FeCrAl alloy, and if satisfied, (3) proceed to join the emulated cladding-endplug set fabricated from ORNL experimental FeCrAl alloy. Table 1 lists the weld technique, materials and experimental procedure for thin-wall cladding weld development. Weld characterization and testing include visual inspection, optical microscopy of cross sectioned sample, X-ray 3D CT inspection, scanning electron microcopy (SEM), electron backscattered diffraction (EBSD), microhardness test, tensile testing and hydraulic pressure burst testing (HPBT). The nominal compositions of the materials are listed in Table 2. Note that although most results reported in this work are based on the endplug and thin-wall cladding made from a commercial FeCrAl Kanthal-D, this material is less likely to be considered for actual ATF cladding since the high Cr (> 15 wt%) is known to develop α precipitates in Fe-based alloy under irradiation and result in material embrittlement. TABLE 1. Technique, materials and experimental procedure for thin-walled cladding weld development Technique Material (FeCrAl) Sample Form Objectives LBW ORNL FeCrAl. Thin plate coupon set LBW parameter study Kanthal-D Emulated cladding-endplug Use optimized LBW parameters ORNL FeCrAl. Emulated cladding-endplug Use optimized condition for candidate material TABLE 2. Composition of various FeCrAl alloys in wt%. FeCrAl Alloy Fe Cr Al Ti Si Y 2 O 3 Other ODS-FeCrAl (MA 956) Bal C_0.01 Kanthal-D Bal C_0.08 ORNL-2 nd Gen. FeCrAl (C35MN5-C) Bal C_0.003, Y_0.032 Mo_1.99, Nb_0.97 II. EXPERIMENTAL RESULTS AND DISCUSSION The laser welding process can be complex with many parameters involved (power, pulse width, pulse rate, beam travel speed and % overlap). The quality of the weld is sensitive to the dimensional change such as cladding wall thickness and the condition at contact interface. The laser weld equipment used for this project is a LaserStar Compact System (Model: ) with a maximum pulse energy of 80 joules (400V, 20 ms) and up to 20 Hz pulse rate as shown in Figure 1. The laser weld is conducted in an inert environment using an argon cover gas. Its maximum peak power can reach 10 kw. Figure 2 shows a sample configuration for laser weld of thin coupon at 355 µm thickness to investigate weld parameters before applying those parameters to the laser weld of endplug and thin-walled cladding samples. The details on LBW parametric evaluation of coupon samples can be found in a previous report 2. Figure 3 shows the general sample configuration for emulated thin-walled cladding and endplug, and the setup for laser weld of thin walled cladding where the local environment is controlled with flowing argon gas. Before the laser welding, the endplug and thin-walled cladding are pressed together. During the welding process, the sample set rotates with a controlled rate to reach a steady linear speed of laser beam along the joining line on the sample. 2

3 Fig. 1. Laser weld system (left) consisting of a solid state Nd-YAG pulsed laser operating at λ=1064 nm. The laser power measurement tools (right) are used to check the laser power condition. Fig. 2. Schematic illustrates thin plate samples configuration for the laser weld of FeCrAl coupon sample. It is designed to form a joining at the recessed corner with both vertical and horizontal weld. Fig. 3. Left: Illustration of a set of emulated endplug and thin walled cladding sample. Left: before (top) and after the joining (bottom). Right: The setup of laser beam weld of the thin walled cladding. The pictures of the LBW bonded sample set 32 are shown in Figure 4. Samples with threads are for tensile test and samples without threads are for hydraulic pressure burst test. All welds were made with 20 ms pulse width, 2.0 Hz pulse rate, ~ 18.5 Joule pulse energy, average power of 37 W, peak power of 1.68 kw and a travel speed of 0.3 mm/s. All six samples went through the post-weld-heat-treatment at 700 C for 2 hours in an argon box filled with ultrahigh purity argon. The nondestructive X-ray CT scans of the weld zone are shown in Figure 5. No defects or micro cracks were found in the bonded region. The material loss from ablation appears minor under these weld conditions. The power-time profile of laser pulses before and after the laser weld were recorded to ensure the stability of the laser weld system. The laser power meter has a 10 µsec response time allowing for instantaneous measurement of power. It has multiple measurement modes to allow for a sampling rate from 20 khz up to 625 khz. It allows for the capture of the temporal pulse shape, which in turn allows to determining pulse width achieved. The software records and graphs the power output of pulses over time, which allows to determining the repetition rate achieved as well as visualize the power stability of the laser system. The detailed laser pulse analysis helps to optimize the LBW process 3. 3

4 2017 Water Reactor Fuel Performance Meeting Fig. 4. Photographs of samples 32-1 and 32-2 for tensile test and samples 32-3 to 32-6 for hydraulic pressure burst test. All six sets of samples had post-weld-heat-treatment at 700 C for 2 hours in flowing argon plus furnace cool for stress release. Fig. 5. X-ray 3D CT analysis of LBW sample set 32-1, 2, 5 and 6 revealing good bond with only minor ablation loss. Tensile tests of the surrogate endplug-cladding LBW samples were carried out using an INSTRON system (Frame: 5967) at room temperature with an extension rate of 10 µm per minute. The load cell has a capacity of 10 kn. A customer designed tensile fixture was used to adapt the welded sample to the INSTRON machine. The sample loading configuration and stress-strain curve of the test are shown in Figure 6. Failure of sample 32-1 and 32-2 occurred in the tube region of each sample as shown in Figure 7. Note that the nominal yield stress and tensile strength for Kanthal-D are 470 MPA and 630 MPa, respectively. These tensile test results demonstrated good metallurgical bonds from the LBW conditions. Fig. 6. Loading configuration in an INSTRON 5967 system (left) and the stress-strain curve (right) for tensile testing of Kanthal-D FeCrAl samples 32-1 and 32-2 joined using LBW. 4

5 Fig. 7. Pictures of LBW sample 32-1 and 32-2 after tensile testing (left) along with maximum stress and strain data (right). It clearly shows the failure occurred at tube section outside the weld zone. The hydraulic pressure burst tests (HPBT) were conducted on the laser welded commercial FeCrAl alloy (Kanthal-D) thin-walled cladding-endplug sample 32-3 through The HPBT system is an Additel 928, model number ADT928-N with a pressure range of 0 to 15,000 psi. Pressure is recorded via a calibrated transducer using the LabView UI software. The HPBTs were conducted at both room temperature and 180 C with two sets tested for each condition for data repeatability. All four samples demonstrated good bonding strength at the weld with failure occurred at an estimated hoop stress higher than the nominal yield stress. The hoop stress for thin-walled tube was estimated using a simple equation: σ h = (PD)/(2t), where σ h is hoop stress, D is the cladding outside diameter and t is cladding wall thickness. Fig. 8. The HPBT setup (left), a close up view of sample connected to the pressurized system (middle) and the high temperature test setup (right). Figure 9 shows the pressurization profile of HPBT. Sample 32-3 was tested at room temperature (~25 C) with a working fluid of mineral oil with a test procedure: (1) pressurize to 2500 psi, and hold for 10 min, (2) pressurize to 5000 psi, and hold for 5 min, (3) pressurize to 6000 psi, and hold for 2 min, (4) pressurize to 7000 psi, and hold for 2 min. The failure occurred at 7578 psi (hoops stress ~ 737 MPa) when pressurizing to 8000 psi. Similar procedure was applied to sample 32-4 and the failure occurred at 7900 psi (hoop stress ~ 775 MPa). For the high temperature HPBT at 180 C for sample 32-5, it was initially pressurized to 1000 psi at room temperature, then increase temperature to 180 C, pressurizes to 2500psi and hold for 10 min, pressurizes to 5000 psi and hold for 5 min, pressurizes to 6000 psi and hold for 2 min, pressurize to 7000 psi and hold for 2 min. The failure occurred at 7437 psi (hoop stress ~ 728 MPa) when pressurizing to 7500 psi. Similar schedule was applied to sample 32-6 and the failure occurred at 7311 psi (hoop stress ~ 718 MPa). 5

6 Fig. 9. Pressure vs. time for HPBT of LBW sample 32-3 (red) and 32-4 (blue) conducted at room temperature (left) and sample 32-5 (red) and 32-6 (blue) tested at 180 C (right). Figure 10 shows the pictures of samples 32-3 through 32-6 failed after the HPBT. One thing to note is that, although all four samples had roughly similar bursting pressures, the failure mode seems to be related to the testing temperature. Room temperature samples (32-3 & 32-4) did show bursting pressures on the higher range but showed failure in the weld (32-3 did show tube failure in addition to weld failure). Note that sample 32-4, although failure occurred very close to the weld zone, is the sample failed at the highest burst pressure among the 4 sets tested where, tests conducted at ~180 C showed yielding and strict tube failure. It is speculated that the stress concentration is higher at weld zone than in the tube section for room temperature test, therefore leading to the failure in the weld region before tube deforms. Samples 32-5 & 32-6 both showed similar failure modes, being axially and in the tube section. They both also showed significant signs of yielding prior to failure. Overall, all four burst samples failed between ~ psi, which is nearly 3 times of the design internal pressure of 2500 psi for typical condition of PWR cladding. Fig. 10. Pictures of the LBW sample after hydraulic pressure burst test at room temperature (32-3 and 32-4) and at 180 C (32-5 and 32-6). 6

7 III. CONCLUSIONS The parametric studies for laser weld development with the detailed laser power measurement at very high sampling rate were carried out. A post weld heat treatment has been established and through tensile testing it was found to be more beneficial compared to non-heat treated samples with the mode of failure transferring from the weld region to that of the tube. It indicates that a post weld heat treatment at 700 C for 2 hours in an argon glovebox followed by furnace cool in argon atmosphere is proven to be effective for stress release for LBW of Kanthal-D FeCrAl alloy. The penetration depth of a weld appears to be related more to pulse energy than any other input parameter or measured parameter. When welding parameters were changed but laser pulse energy was maintained the penetration depth appeared to also remain unchanged. The level of surface material loss by ablation at joining zone and weld roughness appears to be related to the measured peak power of the laser beam. The surface ablation increases with increasing peak power. The optimized LBW parameters have delivered a weld with limited surface ablation loss and good weld penetration and bond strength. Repetitive tensile tests have shown consistent results with failure and extensive deformation occurring in the tube region. The hydraulic pressure burst testing at both room temperature and high temperature has shown favorable results for the laser welding parameters. ACKNOWLEDGMENTS This work is funded by the United States Department of Energy (DOE), Office of Nuclear Energy (NE) under Fuel Cycle Research & Development (FCRD) Program (FCRD-FUEL and NTRD-FUEL ). REFERENCES 1. Y. Yamamoto, Y. Yang, K.G. Field, K. Terrani, B.A. Pint, and L.L. Snead, Letter Report Documenting Progress of Second Generation ATF FeCrAl Alloy Fabrication, ORNL/LTR-2014/219 Report, (2014). 2. J. Gan, N. Jerred, E. Perez, DC Haggard, C. Nichol, Report on the Development of Weld Techniques for Thin Walled Tubing, INL/LTD Report, (2015). 3. J. Gan, N. Jerred, E. Perez, DC Haggard, H. Wen, Status Report on Thin-Walled Cladding Weld Development and Test, INL/LTD Report, (2016). 7