The Study Of Fretting Wear Behavior Of Cladding Tube And Heat Transfer Tube In High Temperature Pressurized Water

Nuclear power can be regarded as a clean,efficient,economical and safe energy,which is consistent with the target of the emission peak and carbon neutrality in our country.At present,the pressurized water reactor（PWR）is the main type of nuclear power plant in our country.In PWR,Zr alloy cladding tube is the first safety barrier to prevent the nuclear leakage;meanwhile,690TT heat transfer tube in steam generator is regarded as the second one.However,during PWR operation,fretting wear occurs inevitably for both cladding tube and heat transfer tube owing to the flow induced vibration（FIV）,causing thinning and perforation of them and then resulting in radioactive material leakage,which will seriously threaten the safe operation and efficient of PWR.Fretting wear is regarded as the main failure mode for cladding tube and heat transfer tube according to the running experience of PWR nuclear power plants.Since 1970s,Canada and other developed countries have carried out studies on fretting wear behavior of cladding tube and heat transfer tube,and plenty of non-public data have been obtained.However,the relevant studies in our country started relatively late,and most of fretting tests were conducted in room temperature condition.At present,fretting wear behavior of domestic cladding tube and heat transfer tube in high temperature pressurized（HTP）water is seldom studied.Thus,it is urgent to study the fretting wear behavior of Zr alloy cladding tube and 690TT heat transfer tube in HTP water,which can provide accurate data and theoretical supports for the safe operation and service life evaluation of them from the perspective of fretting wear.In this study,the fretting wear behaviour of Zr alloy cladding tube mated with Zr alloy dimple in simulated primary water of PWR was investigated in detail;meanwhile,the fretting wear behaviour of 690TT heat transfer tube mated with 405 SS antivibration bar in simulated secondary water of PWR was also studied.The fretting wear samples were prepared by HTP fretting wear system,and the curves of friction force versus displacement amplitude were drawn to estimate fretting regimes.White light interferometer and SPIP software were used to obtain the 3D morphology,the maximum wear depth and wear volume of worn area,and the wear coefficients were then calculated.The surface and cross-sectional morphologies and oxides of worn areas were analyzed by scanning electron microscope with energy dispersive spectroscopy（SEM/EDS）,focused ion beam（FIB）system,transmission electron microscope（TEM）,X-ray photoelectron spectroscopy（XPS）,X-ray diffractometer（XRD）and Raman spectrum.The main conclusions are as follows:（1）Running condition fretting map（RCFM）and material response fretting map（MRFM）of Zr alloy tube under grid-to-rod（GTR）configuration in simulated primary water of PWR were established.RCFM shows three fretting regimes,namely,partial slip regime（PSR）,mixed fretting regime（MFR）and gross slip regime（GSR）.According to MRFM,the wear mechanism under partial slip regime is adhesive wear with the character of delamination of local regions,both adhesive wear and cracking are found for mixed fretting regime,while abrasive wear and severe delamination for gross slip regime.Compared with the results at room temperature,the mixed fretting regime range increases obviously at high temperature,while friction coefficient is lower.In addition,the fretting wear behaviour of Zr alloy tube under different fretting regimes is simplely discussed.（2）The effect of mechanical parameters（displacement amplitude,normal force and frequency）on fretting wear behavior between Zr alloy cladding tube mated with Zr alloy simple in simulated primary water of PWR were studied.The results show that the wear degree of cladding tubes under PSR and MFR are slight,while the material loss is serious under GSR.However,the sum of wear depth and crack depth under MFR is deeper than the maximum wear depth under GSR,suggesting that the service life of cladding tube under MFR might be shorter than that under GSR.The maximum wear depth,wear volume,Archard wear coefficient and dissipated energy wear coefficient of Zr alloy cladding tube increase with the increasing of displacement amplitude.Moreover,abrasive wear and delamination are gradually aggravated under GSR.As normal force increased,the maximum wear depth,wear volume,Archard wear coefficient and dissipated energy wear coefficient increase initially and then decrease slightly,the degree of abrasive wear and delamination are aggravated firstly and then changed less.With the increasing of frequency,the maximum wear depth,wear volume,Archard wear coefficient and dissipated energy wear coefficient hardly change firstly and then increase sharply.Moreover,abrasive wear is aggravated and the delamination rate is accelerated.Meanwhile,the influence factors of normal force,amplitude and frequency on fretting wear of cladding tube are discussed.In addition,the degree map of fretting damage of Zr alloy cladding tube under different fretting regimes in HTP water is established.（3）The fretting wear behaviour of Zr alloy cladding tubes under partial slip regime in simulated primary water of PWR was investigated.The fretting regime remains unchanged with the increasing fretting time and hence both the wear volume and depth of the worn areas formed on Zr cladding tubes change slightly.The wear mechanism is adhesive wear,and delamination occurs at some local regions,resulting in slight damage.Third-body layer,oxide layer,tribologically transformed structure layer,and general deformation layer are observed form the cross-section of the worn area and their formation mechanisms are analysed in detail.Moreover,from the metal/oxide interface to surface,the main oxides transition from tetragonal-ZrO₂ to monoclinic-ZrO₂.Finally,the partial slip regime process and the microstructural evolution during fretting wear are discussed.（4）The fretting wear behaviour of Zr alloy cladding tube under mixed fretting regime in simulated primary water of PWR was studied.The main wear mechanism is adhesive wear,with characters of small-scale delamination at the center of worn area and serious delamination on the worn edge.A long crack throughout the worn area and other cracks propagated towards the substrate are observed.The cross-sectional microstructure of worn area can be divided into a thick third-body layer,thin inner oxide layer and thick tribologically transformed structure layer,and their formation mechanisms are analyzed in detail.Moreover,the second phase Zr（Nb,Fe）2 would firstly transition to amorphous oxide（Zr,Nb,Fe,O）,and then Fe atom diffuses quickly,and finally the amorphous oxide crystallized to form the oxides containing Zr and Nb.In addition,the mixed fretting regime process and the microstructural evolution during fretting wear are discussed.（5）The effect of fretting frequency on fretting wear behavior between Alloy 690TT heat transfer tube and 405 stainless steel plate in simulated secondary water of pressurized water reactor nuclear power plants was investigated.The wear volume and wear coefficient of 690TT tube increase initially and then decrease with increasing frequency,while the wear volume and wear coefficient of the corresponding 405 SS plate gradually increase.The wear mechanism is analyzed in detail.The friction coefficient and the total dissipated energy decease with increasing frequency,which is related to crack initiation and propagation.Moreover,the main wear mechanism of 690 TT tube changes from abrasive wear to oxidative wear and delamination as the stress increased.In addition,a predictive model of service life of 690TT tube is established.（6）The fretting wear-induced microstructural evolution of Alloy 690TT tubes submerged in a high temperature pressurized water was investigated.Two typical local regions characterized by significantly different worn morphologies are investigated in detail,which can be attributed to the stress field and material couple.A thin third-body layer and a thick Fe-rich layer are observed and their oxidation behaviour is analyzed.Moreover,the main oxide is spinel,whereas the amorphous band is only observed in the Fe-rich layer.The tribologically transformed structure is formed by the mechanism of discontinuous dynamic recrystallisation.Finally,the microstructural evolution of the two areas during fretting wear is discussed.