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Effect of impact frequency on the wear behavior of spring-supported tubes in room and high temperature distilled water
Wear 259 (2005) 329–336
Case study
Effect of impact frequency on the wear behavior of spring-supported tubes in room and high temperature distilled water Young-Ho Lee ∗ , Hyung-Kyu Kim, Youn-Ho Jung Korea Atomic Energy Research Institute, 150 Deokjin-dong, Yuseong-gu, Daejeon 305-353, Republic of Korea Received 1 August 2004; received in revised form 16 December 2004; accepted 18 January 2005 Available online 10 May 2005
Abstract Sliding and impact/sliding wear tests have been performed in order to evaluate the wear properties of nuclear fuel rods in room and high temperature distilled water. In this study, tests have been conducted at a normal load of 10 N, sliding amplitudes of 10–100 m and four impacting frequencies of 0, 5, 10 and 30 Hz by using a spacer grid spring of fuel assemblies. The results indicated that the wear volume of the fuel rods is rapidly increased with an increasing impact frequency and slip amplitude. This is mainly concerned with the behavior of the generated wear particles on the worn surfaces, which is affected by the impacting frequency of the spring specimen. Based on the observations of the worn area, the wear particles in the room temperature water remained in the contact surfaces below 10 Hz of an impact frequency even though the water fluctuation at the contact surfaces due to the impact motion was expected to easily remove the wear particles. However, the wear volume was decreased by increasing the test temperature. This seems to be closely related to the changed properties of the spring stiffness in the high temperature water and the formation of the load-bearing layers rather than the changed material properties on the worn surface. Based on the present results, the detailed mechanisms of the wear particle behavior according to the test conditions were investigated and the relationship between the spring characteristics and the wear behaviors in high temperature distilled water is discussed. © 2005 Elsevier B.V. All rights reserved. Keywords: Impact/sliding wear; Nuclear fuel rod; Spring characteristics; Frequency effect; Wear particles
1. Introduction Fretting wear generally occurs due to the relatively small displacement between the contacting parts. The extent of the surface damage depends on the slip amplitude, operating environment, contact geometry and contact shape. This kind of degradation atmosphere is easily produced in operating nuclear power plants. In the nuclear core, primary coolant rapidly passes around the fuel rod in order to remove the excess heat generated from the nuclear reaction. Consequently, this gives rise to a flow-induced vibration (FIV) and fretting damage is frequently reported between the nuclear fuel rods and their supporting structures (springs and dimples). However, if the contact conditions such as the ∗
Corresponding author. Tel.: +82 42 868 8761; fax: +82 42 863 0565. E-mail address: [email protected] (Y.-H. Lee).
0043-1648/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2005.01.019
contact force and slip amplitude between the fuel rod and the spring are frequently changed and time dependent, it is difficult to estimate the wear damage of the fuel rod using the test results of variable effects such as an applied normal load, slip amplitude, number of cycles, etc. This is because the fretting mechanisms related to wear particle behavior on a worn surface could be changed with the contact condition [1]. Fretting-related degradations due to a FIV in nuclear power plants have been concentrated on the fuel rods, steam generator tubes and control rods. In the case of the nuclear fuel rod, the springs (and dimples) secure the fuel rods during an initial operating period and some of the contact loads are exerted between the contacting surfaces. At this time, the wear mode due to a FIV is governed by the sliding motion. After an operation, however, the spacer grid materials loosen up and a small clearance opens up because the spacer grid
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materials are degraded due to the neutron irradiation effect and high temperature environment in the nuclear core. So, it is reasonable to assume that the wear mode could be changed from a sliding wear (existence of a contact force) to an impact/sliding wear (existence of a gap). In order to evaluate the wear behavior of the nuclear fuel rod, the impact/sliding wear behavior should be examined in actual operating conditions even though it is difficult to define the exact impacting mode. The previous studies for an impact/sliding wear in steam generator tubes, conducted by Fisher et al. [2], Gu¯erout et al. [3] and Ko [4], have demonstrated that the wear resistance and mechanism were determined by the applied work-rate which was expressed as follows: ˙ = 1 F dS W (1) t ˙ is work-rate; F is applied normal force; S is sliding where W distance. However, each test variable effect comprising the work-rate such as the normal load, slip amplitude (distance), frequency, etc. is still unclear. When the impact load and slipping interact between the contacting materials, the wear mechanism is determined by the mechanical characteristics of the contacting materials. In the steam generator tube, the applied loads result in a severe plastic deformation on the worn surface or in the subsurface and the wear rate was determined by the characteristics of the plastic deformation layers in the subsurface [5]. But a supporting structure (spring of grid spacer) in a nuclear fuel assembly could be elastically deformed by a contacting load. This results in different wear behaviors at the same work-rate because the extent of the normal load and slip amplitude are quite different. Besides, the spring stiffness which affects the contact geometry could be varied with an increasing temperature. However, there has been little effort made to examine the impact/sliding wear behavior of a nuclear fuel rod with a consideration of a spring characteristic variation due to the impacting frequency, temperature, etc. Therefore, the objective of this study is to examine the effect of the impact frequency on the wear behavior of nuclear fuel rods for the spacer grid spring in room and high temperature distilled water. Also, the relationship between the formation of the wear particle layers on the worn surfaces and the changed mechanical behavior of the spring under an impact loading at 300 ◦ C water was examined in detail.
Table 1 Chemical composition of the tested zirconium alloy (wt.%) Nb Sn Fe Cr T.E. O Zr
1.0 1.0 0.11 – – – Bal.
Table 2 Mechanical properties of the tested zirconium alloy Temperature (◦ C)
Yield strength (MPa)
Tensile strength (MPa)
Elongation (%)
25 300
565 324
807 519
17 14.8
a straight rod. The springs are fabricated by pressing and punching the same alloy with a thickness of 0.46 mm. The schematic feature of this spring is shown in Fig. 1. 2.2. Impact/sliding wear tester A high temperature and pressure impact/sliding wear tester was developed for the present works in order to simulate the fretting wear phenomena due to a FIV between the fuel rod and spring in a nuclear power plant environment (above 320 ◦ C, 15 MPa). Fig. 2 shows the schematic diagram of specimen array in an autoclave of this tester. This test facility mainly consists of an autoclave and a sliding and impacting axis. The fuel rod specimen is attached to the sliding axis, which is oscillated up to 100 m by means of an eccentric cylinder, lever and movable hinge, etc. The spring specimen is also attached to the impact axis, which is arranged at 90◦ to the sliding axis. The impact axis reciprocates with displacements of 0–360 m and frequencies of 0–50 Hz using an eccentric cylinder and a dc servomotor, respectively. A two-axis load cell for a high temperature and pressure environment was equipped between the spring specimen holder and the end of the impact axis in the autoclave. The displacement range of the impacting and sliding motion was also monitored using a high temperature and pressure displacement transducer (KAMAN® ) [6]. During the reciprocating motion of the sliding axis, the sliding and impact/sliding conditions could be
2. Experimental procedure 2.1. Specimen In this experiment, the tube and spring specimens are made of a zirconium alloy, which is applied to fuel cladding materials. The chemical compositions and mechanical properties are shown in Tables 1 and 2. The fuel rod specimen, 9.5 mm in diameter, 0.6 mm in thickness and 50 mm long, is cut from
Fig. 1. Schematic shape of the spring specimen used in this study.
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Fig. 2. Schematic diagram of the high temperature sliding and impact/sliding wear tester.
archived by controlling the servomotor speed of the impact axis; 0 rpm is a sliding wear and 1800 rpm is impact/sliding wear with 30 Hz. 2.3. Test condition and wear measurements The fuel rod specimen equipped at the sliding axis oscillates with a peak-to-peak amplitude of 10, 30, 50, 80 and 100 m at a frequency of 30 Hz. The normal load of 10 N in the sliding wear was applied by using the impact axis. During the impact/sliding wear test, the maximum impact load was set to 10 N with a reciprocating displacement of 360 m at a frequency of 5, 10 and 30 Hz. All the wear tests were carried out up to 105 cycles in distilled water and the high temperature condition was set to a temperature of 300 ◦ C and a pressure of about 8.6 MPa. After the tests, the wear volume and worn area of the fuel rod specimen were measured by using a surface roughness tester and an optical microscope. 2.4. Worn area analysis The worn area of the fuel rod was examined using an optical microscope to analyze the wear particle behavior on the worn surfaces and to calculate the size (width and length) of the wear scar by using commercial software [7]. Also, a scanning electron microscopy (SEM) was used to evaluate the wear mechanism at each test condition. Prior to the SEM observation, the tested specimens were acoustically cleaned in acetone for 10 min and dried in air.
Fig. 3. Variation of the wear volume with an increasing slip amplitude: (a) 25 ◦ C and (b) 300 ◦ C.
3. Results and discussion 3.1. Effect of slip amplitude and impact frequency After the sliding and impact/sliding wear tests for each test condition, the average wear volume was evaluated as shown in Fig. 3. In this figure, the results of the slip amplitude of 10 and 30 m were omitted because the wear volume and wear scar were not detectable using a surface profilometer. With an increasing slip amplitude, the wear volume increased in both the room and high temperature water. This behavior is similar to the previous studies that dealt with the spring shape effects [8,9]. To sum up, the formation and removal of the wear particles between the contact surfaces were accelerated with an increasing slip amplitude. Especially, the critical slip amplitude where the wear volume was not detectable, below a specific slip amplitude, appeared and it was determined by the spring shapes and test variables (slip amplitude, applied load, environment, etc.). From the result of the sliding wear, the critical slip amplitude is about 50 m in both the room and
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Fig. 4. The relationship between the slip amplitude and the maximum wear depth at each test condition: (a) 25 ◦ C and (b) 300 ◦ C. Note that the test result at 300 ◦ C is very different when compared with the volume data as shown in Fig. 3.
high temperature water. Even though it is difficult to define the critical slip amplitude in a high temperature condition due to the relatively smaller wear volume, the increasing trend of the wear volume was quite similar to the room temperature condition. So, it is necessary to examine the worn surface in order to evaluate the correlation between the critical slip amplitude and the wear mechanism in the room and high temperature water. In the case of the zirconium alloys used as fuel rods, however, the reliability and life-time are evaluated and estimated by the extent of the wall thickness decrease regardless of the extent of the wear volume. This is because the tube wall opening due to the excessive wear and fission products release to the coolant is directly connected to a severe accident in nuclear power plants. Fig. 4 shows the variation of the maximum wear depth in both the room and high temperature water with an increasing slip amplitude. It is apparent that the variation of the maximum wear depth, especially in the high temperature water condition, is very different from that of the wear
Fig. 5. Results of the evaluated average wear depth and protruded height at each test condition.
volume as shown in Fig. 3. The higher wear depth at 100 m of a slip amplitude appears at 30 Hz of an impact frequency in the room temperature water, but at 10 Hz in the high temperature water. From Figs. 3 and 4, the wear behavior at an impact frequency of 30 Hz in 300 ◦ C water is mainly concerned with the worn area expansion rather than the wear depth increase. However, it is possible to consider that this is due to an abnormal wear in a specific region which could generate a localized severe wear depth if the variation of the impact frequency and the test environment affects the mechanical properties, contact condition and wear debris behavior. In order to remove the abnormal effect, a variation of the average wear depth and protruded height are shown in Fig. 5 to compare the relative wear behavior. The protruded height could be regarded as the average thickness of the wear particle layer that adheres to the worn surface of the fuel rod. In the room temperature water, the average wear depth increased with an increasing slip amplitude and impact frequency, but the average height was not affected by the slip amplitude and decreased by increasing the impact frequency. This result indicates that the wear particles on the worn surface were dominantly removed by
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explain the above results by using the accumulation of the wear debris on the worn surface according to the slip amplitude. If a spring specimen that has a relatively thin thickness and long contact region could be easily deformed in a high temperature, the debris behavior on the worn surface could be expected to change with a dynamic response of a spring according to the impact frequency. However, it is difficult to consider the frequency response of a supporting spring in a high temperature water. Instead of this, the deformation properties of the tested spring in a high temperature water were observed. 3.2. Worn area observation To evaluate the wear mechanism on the worn surface with the variation of the slip amplitude and the impact frequency in the room temperature water, the worn surfaces were observed using an optical microscope and the results are shown in Fig. 7. In this figure, dark colors indicated the accumulated wear particles on the worn surface even though the wear tests were performed in a water environment. In the results of the sliding wear, the worn area was covered with wear particles regardless of the slip amplitude. For the impact frequencies of 5 and 10 Hz, wear particle layers were well-developed
Fig. 6. Effect of the impact frequency on the wear volume.
the impacting motion rather than the sliding one. However, in the high temperature water, the average wear depth and protruded height did not vary with an increasing impact frequency. When impact/sliding wear damage occurs in a water environment, it is expected that the wear volume will be increased because the wear particles are easily removed by the water fluctuations during a non-contacting time. Fig. 6 shows the variation of the wear volume with the impact frequency at each test condition. It is apparent that the wear volume in the room temperature water was rapidly increased with the impact frequency. However, in the high temperature water, the wear volume did not show a linear relation with the impact frequency. In the slip amplitudes of 80 and 100 m, a lower wear volume appeared at the impact frequency of 5 Hz. These small wear volumes compared with the sliding wear (i.e. impact frequency of 0 Hz) seems to be related to the formation of the wear particle layer and the variation of the spring characteristics. When the slip amplitude between the contact surfaces is small, the formation of the wear particle layer which restricts an excess wear is more dominant and could act as a load-bearing layer [10]. However, it is not enough to
Fig. 7. Variation of the wear scar with an increasing impact frequency and slip amplitude in the room temperature distilled water.
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at the slip amplitude of 50 m, but these layers were easily removed with an increasing slip amplitude. For the impact frequency of 30 Hz, the wear particle layer disappeared regardless of the slip amplitude. So, the generated wear particle layers in the room temperature water were rapidly fractured and disappeared by increasing the slip amplitude and the impact frequency. The expansion of the worn area in Fig. 7 is mainly due to the third body abrasion between the contacting surfaces when the wear particle is ejected from the worn surface. This result is in good agreement with the critical slip amplitude as shown in Fig. 3a. In the high temperature water, however, the results of the worn surface observation were quite different when compared with the room temperature results as shown in Fig. 8. In the sliding wear (Fig. 8a), well-developed wear particle layers also appeared, but the accumulation of the wear particles is concentrated at the worn area boundaries. So, the lower wear volume in a high temperature water is associated with these barriers which restrains any further wear process. But, the removal of these layers due to a crack initiation and fracture seems to be accelerated by an increasing slip amplitude. When an impact loading is applied, it is expected that wear particle layers will rapidly develop if the debris removal is restricted. For the impact frequency of 5 Hz, it is apparent that the formation of uniform wear particle layers is similar to the results of the sliding wear (Fig. 8b). But, with an increasing impact frequency, cracks on the wear particle layers were easily observed (Fig. 8c) and evidence of the removed particle layers is easily detectable (Fig. 8d). Therefore, the impact load could accelerate the crack formation in the generated particle layers and it is assumed that it will accelerate the fracture and removal of the wear particle layers with an increasing slip amplitude.
However, it is not enough to explain the lower wear volume in the high temperature water condition which was entirely due to the wear particle layers on the worn surface when compared with room temperature results. Because the wear particle layers could not accommodate the severe strain and deformation between the contact surfaces under an impacting/sliding wear. The spring characteristics could be changed with an increasing temperature and this could also vary the wear behavior between the contact surfaces. 3.3. The variation of the spring characteristic In the results of the sliding and impact/sliding wear tests, the wear scars had common elliptical shapes and their size (length and width) was depended on the test conditions. In the room temperature water, the length and width of the wear scar showed a good linearity with the increasing slip amplitude and impact frequency, but very scattered results at 300 ◦ C in water as shown in Fig. 9. In this figure, the slope, the ratio of the length to width, in the high temperature water condition has smaller value when compared with that in the room temperature condition. This means that the wear damages were less dominant in the direction of the length (i.e. slip direction) rather than the width direction. If the contact region of a spring specimen which is relatively long and thin could be easily deformed in a high temperature, the actual slip displacements between the contact surfaces were expected to have lower values when compared with the slip amplitudes applied at each test condition. In order to verify the possibility of a spring deformation, the load-displacement (P–δ) curves of the spring specimens were obtained and their results are shown in Fig. 10. With an increasing temperature, the slope of the P–δ curve (i.e. spring stiffness) was gradually decreased
Fig. 8. SEM results of the worn surface observation in a high temperature water.
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and the stiffness value at 300 ◦ C was decreased to about half of the one at room temperature. This result means that the spring specimen in a high temperature must accommodate more strain and deformation under the same applied normal load. If the spring specimen does not deform in a high temperature during a sliding and an impact/sliding wear, the wear scar after the high temperature tests should have a large size when compared with the room temperature results. But, the results were the opposite and it is thought that the deformed contact region of the spring specimen under a contact load results in a decrease of the actual slip displacement. Therefore, the wear volume of the fuel rods in a high temperature was decreased when compared with the results of the room temperature water condition.
4. Conclusions Sliding and impact/sliding wear tests were performed by using nuclear fuel rods for an evaluation of the spacer grid springs in room and high temperature distilled water. From these experimental results, the following conclusions are drawn.
Fig. 9. Results of the wear scar analysis using an optical microscope. The relationship between the width and the length of the wear scar could be changed by the test environment.
(1) With an increasing slip amplitude, the wear volume increased in both the room and high temperature water. In addition, the wear volume was rapidly increased with the impact frequency in the room temperature water. However, in the high temperature water, the wear volume did not show a linear relation with the impact frequency. (2) In the room temperature water, the formation of the loadbearing layer on the worn surfaces is closely related to the critical slip amplitude. (3) In the high temperature water, the impact load could accelerate the crack formation in the generated particle layers and it is assumed that it will accelerate the fracture and removal of the wear particle layers with an increasing slip amplitude. (4) The deformed contact region of the spring specimen under a contact load results in the decrease of the actual slip displacement. Therefore, the wear volume of the fuel rods in a high temperature was decreased when compared with the results of the room temperature water condition.
Acknowledgements
Fig. 10. Effect of the temperature on the load-displacement curve of the spring specimen.
This study has been carried out under the Nuclear R&D Program by Ministry of Science and Technology in Korea. The authors thank the research student, Mr. Ju-Sun Song from the Graduate School of Choongnam National University for conducting the wear volume measurements.
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References [1] Y.-H. Lee, H.-K. Kim, K.-H. Yoon, H.-S. Kang, K.-N. Song, J.-W. Ha, S.-S. Kim, Wear mechanism of tube fretting affected by support springs, KSTLE Int. J. 3 (2002) 68–73. [2] N.J. Fisher, A.B. chow, M.K. Weckwerth, Experimental fretting-wear studies of steam generator materials, J. Press. Vessel Technol. 117 (1995) 312–320. [3] F.M. Gu¯erout, N.J. Fisher, D.A. Grandison, M.K. Weckwerth, Effect of temperature on steam generator fretting-wear, in: ASME PVP, vol. 328, Flow-Induced Vibration, 1996, pp. 233–246. [4] P.L. Ko, Wear of power plant components due to impact and sliding, Appl. Mech. Rev. 50 (1997) 387–411.
[5] Y.-H. Lee, I.-S. Kim, The effect of subsurface deformation on the wear behavior of steam generator tube materials, Wear 253 (2002) 438–447. [6] www.kamaninstrumentation.com. [7] Matrox Inc., Ver. 2.0, User’s manual for Matrox Inspector® , USA, 1997. [8] Y.-H. Lee, H.-K. Kim, Y.-H. Jung, Relationship between spring shapes and the ratio of wear volume to the worn area in nuclear fuel fretting, KSTLE Int. J. 4 (2003) 31–36. [9] H.-K. Kim, Y.-H. Lee, Influence of contact shape and supporting condition on tube fretting wear, Wear 255 (2003) 1183–1197. [10] J. Jiang, F.H. Stott, M.M. Stack, The role of triboparticulates in dry sliding wear, Tribol. Int. 31 (1988) 245–256.
Case study
Effect of impact frequency on the wear behavior of spring-supported tubes in room and high temperature distilled water Young-Ho Lee ∗ , Hyung-Kyu Kim, Youn-Ho Jung Korea Atomic Energy Research Institute, 150 Deokjin-dong, Yuseong-gu, Daejeon 305-353, Republic of Korea Received 1 August 2004; received in revised form 16 December 2004; accepted 18 January 2005 Available online 10 May 2005
Abstract Sliding and impact/sliding wear tests have been performed in order to evaluate the wear properties of nuclear fuel rods in room and high temperature distilled water. In this study, tests have been conducted at a normal load of 10 N, sliding amplitudes of 10–100 m and four impacting frequencies of 0, 5, 10 and 30 Hz by using a spacer grid spring of fuel assemblies. The results indicated that the wear volume of the fuel rods is rapidly increased with an increasing impact frequency and slip amplitude. This is mainly concerned with the behavior of the generated wear particles on the worn surfaces, which is affected by the impacting frequency of the spring specimen. Based on the observations of the worn area, the wear particles in the room temperature water remained in the contact surfaces below 10 Hz of an impact frequency even though the water fluctuation at the contact surfaces due to the impact motion was expected to easily remove the wear particles. However, the wear volume was decreased by increasing the test temperature. This seems to be closely related to the changed properties of the spring stiffness in the high temperature water and the formation of the load-bearing layers rather than the changed material properties on the worn surface. Based on the present results, the detailed mechanisms of the wear particle behavior according to the test conditions were investigated and the relationship between the spring characteristics and the wear behaviors in high temperature distilled water is discussed. © 2005 Elsevier B.V. All rights reserved. Keywords: Impact/sliding wear; Nuclear fuel rod; Spring characteristics; Frequency effect; Wear particles
1. Introduction Fretting wear generally occurs due to the relatively small displacement between the contacting parts. The extent of the surface damage depends on the slip amplitude, operating environment, contact geometry and contact shape. This kind of degradation atmosphere is easily produced in operating nuclear power plants. In the nuclear core, primary coolant rapidly passes around the fuel rod in order to remove the excess heat generated from the nuclear reaction. Consequently, this gives rise to a flow-induced vibration (FIV) and fretting damage is frequently reported between the nuclear fuel rods and their supporting structures (springs and dimples). However, if the contact conditions such as the ∗
Corresponding author. Tel.: +82 42 868 8761; fax: +82 42 863 0565. E-mail address: [email protected] (Y.-H. Lee).
0043-1648/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2005.01.019
contact force and slip amplitude between the fuel rod and the spring are frequently changed and time dependent, it is difficult to estimate the wear damage of the fuel rod using the test results of variable effects such as an applied normal load, slip amplitude, number of cycles, etc. This is because the fretting mechanisms related to wear particle behavior on a worn surface could be changed with the contact condition [1]. Fretting-related degradations due to a FIV in nuclear power plants have been concentrated on the fuel rods, steam generator tubes and control rods. In the case of the nuclear fuel rod, the springs (and dimples) secure the fuel rods during an initial operating period and some of the contact loads are exerted between the contacting surfaces. At this time, the wear mode due to a FIV is governed by the sliding motion. After an operation, however, the spacer grid materials loosen up and a small clearance opens up because the spacer grid
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materials are degraded due to the neutron irradiation effect and high temperature environment in the nuclear core. So, it is reasonable to assume that the wear mode could be changed from a sliding wear (existence of a contact force) to an impact/sliding wear (existence of a gap). In order to evaluate the wear behavior of the nuclear fuel rod, the impact/sliding wear behavior should be examined in actual operating conditions even though it is difficult to define the exact impacting mode. The previous studies for an impact/sliding wear in steam generator tubes, conducted by Fisher et al. [2], Gu¯erout et al. [3] and Ko [4], have demonstrated that the wear resistance and mechanism were determined by the applied work-rate which was expressed as follows: ˙ = 1 F dS W (1) t ˙ is work-rate; F is applied normal force; S is sliding where W distance. However, each test variable effect comprising the work-rate such as the normal load, slip amplitude (distance), frequency, etc. is still unclear. When the impact load and slipping interact between the contacting materials, the wear mechanism is determined by the mechanical characteristics of the contacting materials. In the steam generator tube, the applied loads result in a severe plastic deformation on the worn surface or in the subsurface and the wear rate was determined by the characteristics of the plastic deformation layers in the subsurface [5]. But a supporting structure (spring of grid spacer) in a nuclear fuel assembly could be elastically deformed by a contacting load. This results in different wear behaviors at the same work-rate because the extent of the normal load and slip amplitude are quite different. Besides, the spring stiffness which affects the contact geometry could be varied with an increasing temperature. However, there has been little effort made to examine the impact/sliding wear behavior of a nuclear fuel rod with a consideration of a spring characteristic variation due to the impacting frequency, temperature, etc. Therefore, the objective of this study is to examine the effect of the impact frequency on the wear behavior of nuclear fuel rods for the spacer grid spring in room and high temperature distilled water. Also, the relationship between the formation of the wear particle layers on the worn surfaces and the changed mechanical behavior of the spring under an impact loading at 300 ◦ C water was examined in detail.
Table 1 Chemical composition of the tested zirconium alloy (wt.%) Nb Sn Fe Cr T.E. O Zr
1.0 1.0 0.11 – – – Bal.
Table 2 Mechanical properties of the tested zirconium alloy Temperature (◦ C)
Yield strength (MPa)
Tensile strength (MPa)
Elongation (%)
25 300
565 324
807 519
17 14.8
a straight rod. The springs are fabricated by pressing and punching the same alloy with a thickness of 0.46 mm. The schematic feature of this spring is shown in Fig. 1. 2.2. Impact/sliding wear tester A high temperature and pressure impact/sliding wear tester was developed for the present works in order to simulate the fretting wear phenomena due to a FIV between the fuel rod and spring in a nuclear power plant environment (above 320 ◦ C, 15 MPa). Fig. 2 shows the schematic diagram of specimen array in an autoclave of this tester. This test facility mainly consists of an autoclave and a sliding and impacting axis. The fuel rod specimen is attached to the sliding axis, which is oscillated up to 100 m by means of an eccentric cylinder, lever and movable hinge, etc. The spring specimen is also attached to the impact axis, which is arranged at 90◦ to the sliding axis. The impact axis reciprocates with displacements of 0–360 m and frequencies of 0–50 Hz using an eccentric cylinder and a dc servomotor, respectively. A two-axis load cell for a high temperature and pressure environment was equipped between the spring specimen holder and the end of the impact axis in the autoclave. The displacement range of the impacting and sliding motion was also monitored using a high temperature and pressure displacement transducer (KAMAN® ) [6]. During the reciprocating motion of the sliding axis, the sliding and impact/sliding conditions could be
2. Experimental procedure 2.1. Specimen In this experiment, the tube and spring specimens are made of a zirconium alloy, which is applied to fuel cladding materials. The chemical compositions and mechanical properties are shown in Tables 1 and 2. The fuel rod specimen, 9.5 mm in diameter, 0.6 mm in thickness and 50 mm long, is cut from
Fig. 1. Schematic shape of the spring specimen used in this study.
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Fig. 2. Schematic diagram of the high temperature sliding and impact/sliding wear tester.
archived by controlling the servomotor speed of the impact axis; 0 rpm is a sliding wear and 1800 rpm is impact/sliding wear with 30 Hz. 2.3. Test condition and wear measurements The fuel rod specimen equipped at the sliding axis oscillates with a peak-to-peak amplitude of 10, 30, 50, 80 and 100 m at a frequency of 30 Hz. The normal load of 10 N in the sliding wear was applied by using the impact axis. During the impact/sliding wear test, the maximum impact load was set to 10 N with a reciprocating displacement of 360 m at a frequency of 5, 10 and 30 Hz. All the wear tests were carried out up to 105 cycles in distilled water and the high temperature condition was set to a temperature of 300 ◦ C and a pressure of about 8.6 MPa. After the tests, the wear volume and worn area of the fuel rod specimen were measured by using a surface roughness tester and an optical microscope. 2.4. Worn area analysis The worn area of the fuel rod was examined using an optical microscope to analyze the wear particle behavior on the worn surfaces and to calculate the size (width and length) of the wear scar by using commercial software [7]. Also, a scanning electron microscopy (SEM) was used to evaluate the wear mechanism at each test condition. Prior to the SEM observation, the tested specimens were acoustically cleaned in acetone for 10 min and dried in air.
Fig. 3. Variation of the wear volume with an increasing slip amplitude: (a) 25 ◦ C and (b) 300 ◦ C.
3. Results and discussion 3.1. Effect of slip amplitude and impact frequency After the sliding and impact/sliding wear tests for each test condition, the average wear volume was evaluated as shown in Fig. 3. In this figure, the results of the slip amplitude of 10 and 30 m were omitted because the wear volume and wear scar were not detectable using a surface profilometer. With an increasing slip amplitude, the wear volume increased in both the room and high temperature water. This behavior is similar to the previous studies that dealt with the spring shape effects [8,9]. To sum up, the formation and removal of the wear particles between the contact surfaces were accelerated with an increasing slip amplitude. Especially, the critical slip amplitude where the wear volume was not detectable, below a specific slip amplitude, appeared and it was determined by the spring shapes and test variables (slip amplitude, applied load, environment, etc.). From the result of the sliding wear, the critical slip amplitude is about 50 m in both the room and
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Fig. 4. The relationship between the slip amplitude and the maximum wear depth at each test condition: (a) 25 ◦ C and (b) 300 ◦ C. Note that the test result at 300 ◦ C is very different when compared with the volume data as shown in Fig. 3.
high temperature water. Even though it is difficult to define the critical slip amplitude in a high temperature condition due to the relatively smaller wear volume, the increasing trend of the wear volume was quite similar to the room temperature condition. So, it is necessary to examine the worn surface in order to evaluate the correlation between the critical slip amplitude and the wear mechanism in the room and high temperature water. In the case of the zirconium alloys used as fuel rods, however, the reliability and life-time are evaluated and estimated by the extent of the wall thickness decrease regardless of the extent of the wear volume. This is because the tube wall opening due to the excessive wear and fission products release to the coolant is directly connected to a severe accident in nuclear power plants. Fig. 4 shows the variation of the maximum wear depth in both the room and high temperature water with an increasing slip amplitude. It is apparent that the variation of the maximum wear depth, especially in the high temperature water condition, is very different from that of the wear
Fig. 5. Results of the evaluated average wear depth and protruded height at each test condition.
volume as shown in Fig. 3. The higher wear depth at 100 m of a slip amplitude appears at 30 Hz of an impact frequency in the room temperature water, but at 10 Hz in the high temperature water. From Figs. 3 and 4, the wear behavior at an impact frequency of 30 Hz in 300 ◦ C water is mainly concerned with the worn area expansion rather than the wear depth increase. However, it is possible to consider that this is due to an abnormal wear in a specific region which could generate a localized severe wear depth if the variation of the impact frequency and the test environment affects the mechanical properties, contact condition and wear debris behavior. In order to remove the abnormal effect, a variation of the average wear depth and protruded height are shown in Fig. 5 to compare the relative wear behavior. The protruded height could be regarded as the average thickness of the wear particle layer that adheres to the worn surface of the fuel rod. In the room temperature water, the average wear depth increased with an increasing slip amplitude and impact frequency, but the average height was not affected by the slip amplitude and decreased by increasing the impact frequency. This result indicates that the wear particles on the worn surface were dominantly removed by
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explain the above results by using the accumulation of the wear debris on the worn surface according to the slip amplitude. If a spring specimen that has a relatively thin thickness and long contact region could be easily deformed in a high temperature, the debris behavior on the worn surface could be expected to change with a dynamic response of a spring according to the impact frequency. However, it is difficult to consider the frequency response of a supporting spring in a high temperature water. Instead of this, the deformation properties of the tested spring in a high temperature water were observed. 3.2. Worn area observation To evaluate the wear mechanism on the worn surface with the variation of the slip amplitude and the impact frequency in the room temperature water, the worn surfaces were observed using an optical microscope and the results are shown in Fig. 7. In this figure, dark colors indicated the accumulated wear particles on the worn surface even though the wear tests were performed in a water environment. In the results of the sliding wear, the worn area was covered with wear particles regardless of the slip amplitude. For the impact frequencies of 5 and 10 Hz, wear particle layers were well-developed
Fig. 6. Effect of the impact frequency on the wear volume.
the impacting motion rather than the sliding one. However, in the high temperature water, the average wear depth and protruded height did not vary with an increasing impact frequency. When impact/sliding wear damage occurs in a water environment, it is expected that the wear volume will be increased because the wear particles are easily removed by the water fluctuations during a non-contacting time. Fig. 6 shows the variation of the wear volume with the impact frequency at each test condition. It is apparent that the wear volume in the room temperature water was rapidly increased with the impact frequency. However, in the high temperature water, the wear volume did not show a linear relation with the impact frequency. In the slip amplitudes of 80 and 100 m, a lower wear volume appeared at the impact frequency of 5 Hz. These small wear volumes compared with the sliding wear (i.e. impact frequency of 0 Hz) seems to be related to the formation of the wear particle layer and the variation of the spring characteristics. When the slip amplitude between the contact surfaces is small, the formation of the wear particle layer which restricts an excess wear is more dominant and could act as a load-bearing layer [10]. However, it is not enough to
Fig. 7. Variation of the wear scar with an increasing impact frequency and slip amplitude in the room temperature distilled water.
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at the slip amplitude of 50 m, but these layers were easily removed with an increasing slip amplitude. For the impact frequency of 30 Hz, the wear particle layer disappeared regardless of the slip amplitude. So, the generated wear particle layers in the room temperature water were rapidly fractured and disappeared by increasing the slip amplitude and the impact frequency. The expansion of the worn area in Fig. 7 is mainly due to the third body abrasion between the contacting surfaces when the wear particle is ejected from the worn surface. This result is in good agreement with the critical slip amplitude as shown in Fig. 3a. In the high temperature water, however, the results of the worn surface observation were quite different when compared with the room temperature results as shown in Fig. 8. In the sliding wear (Fig. 8a), well-developed wear particle layers also appeared, but the accumulation of the wear particles is concentrated at the worn area boundaries. So, the lower wear volume in a high temperature water is associated with these barriers which restrains any further wear process. But, the removal of these layers due to a crack initiation and fracture seems to be accelerated by an increasing slip amplitude. When an impact loading is applied, it is expected that wear particle layers will rapidly develop if the debris removal is restricted. For the impact frequency of 5 Hz, it is apparent that the formation of uniform wear particle layers is similar to the results of the sliding wear (Fig. 8b). But, with an increasing impact frequency, cracks on the wear particle layers were easily observed (Fig. 8c) and evidence of the removed particle layers is easily detectable (Fig. 8d). Therefore, the impact load could accelerate the crack formation in the generated particle layers and it is assumed that it will accelerate the fracture and removal of the wear particle layers with an increasing slip amplitude.
However, it is not enough to explain the lower wear volume in the high temperature water condition which was entirely due to the wear particle layers on the worn surface when compared with room temperature results. Because the wear particle layers could not accommodate the severe strain and deformation between the contact surfaces under an impacting/sliding wear. The spring characteristics could be changed with an increasing temperature and this could also vary the wear behavior between the contact surfaces. 3.3. The variation of the spring characteristic In the results of the sliding and impact/sliding wear tests, the wear scars had common elliptical shapes and their size (length and width) was depended on the test conditions. In the room temperature water, the length and width of the wear scar showed a good linearity with the increasing slip amplitude and impact frequency, but very scattered results at 300 ◦ C in water as shown in Fig. 9. In this figure, the slope, the ratio of the length to width, in the high temperature water condition has smaller value when compared with that in the room temperature condition. This means that the wear damages were less dominant in the direction of the length (i.e. slip direction) rather than the width direction. If the contact region of a spring specimen which is relatively long and thin could be easily deformed in a high temperature, the actual slip displacements between the contact surfaces were expected to have lower values when compared with the slip amplitudes applied at each test condition. In order to verify the possibility of a spring deformation, the load-displacement (P–δ) curves of the spring specimens were obtained and their results are shown in Fig. 10. With an increasing temperature, the slope of the P–δ curve (i.e. spring stiffness) was gradually decreased
Fig. 8. SEM results of the worn surface observation in a high temperature water.
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and the stiffness value at 300 ◦ C was decreased to about half of the one at room temperature. This result means that the spring specimen in a high temperature must accommodate more strain and deformation under the same applied normal load. If the spring specimen does not deform in a high temperature during a sliding and an impact/sliding wear, the wear scar after the high temperature tests should have a large size when compared with the room temperature results. But, the results were the opposite and it is thought that the deformed contact region of the spring specimen under a contact load results in a decrease of the actual slip displacement. Therefore, the wear volume of the fuel rods in a high temperature was decreased when compared with the results of the room temperature water condition.
4. Conclusions Sliding and impact/sliding wear tests were performed by using nuclear fuel rods for an evaluation of the spacer grid springs in room and high temperature distilled water. From these experimental results, the following conclusions are drawn.
Fig. 9. Results of the wear scar analysis using an optical microscope. The relationship between the width and the length of the wear scar could be changed by the test environment.
(1) With an increasing slip amplitude, the wear volume increased in both the room and high temperature water. In addition, the wear volume was rapidly increased with the impact frequency in the room temperature water. However, in the high temperature water, the wear volume did not show a linear relation with the impact frequency. (2) In the room temperature water, the formation of the loadbearing layer on the worn surfaces is closely related to the critical slip amplitude. (3) In the high temperature water, the impact load could accelerate the crack formation in the generated particle layers and it is assumed that it will accelerate the fracture and removal of the wear particle layers with an increasing slip amplitude. (4) The deformed contact region of the spring specimen under a contact load results in the decrease of the actual slip displacement. Therefore, the wear volume of the fuel rods in a high temperature was decreased when compared with the results of the room temperature water condition.
Acknowledgements
Fig. 10. Effect of the temperature on the load-displacement curve of the spring specimen.
This study has been carried out under the Nuclear R&D Program by Ministry of Science and Technology in Korea. The authors thank the research student, Mr. Ju-Sun Song from the Graduate School of Choongnam National University for conducting the wear volume measurements.
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