Ways to reduce friction and wear of fuel elements in nuclear power

. Friction forces affect the strength of the fuel assembly at all stages of its life cycle. The work is devoted to the issues of reducing friction and wear of fuel elements of a nuclear reactor. It presents methods and results of pre-reactor experimental studies of static and dynamic processes involving friction forces. Because of the conducted research, it was found that the transformation of the movable joints of the guide channels and spacing gratings into fixed ones by means of spot welding in "rigid" fuel assemblies, together with other measures, solved the problem of curvature of the fuel assemblies. It is also shown that during the assembly of fuel assemblies, the friction force can be reduced using ultrasound.


Introduction
The study of friction and wear in machines began in the middle Ages, and now scientific societies of tribologists work in all industrialized countries, since annual economic losses due to machine wear are calculated in huge amounts, despite the fact that highly effective scientific, engineering and industrial methods and means of combating wear have been created.The difficulty of combating wear is determined by the fact that the processes of friction and wear are nonlinear complex processes.Tribology, the science of friction and wear, is a complex one that describes processes using the concepts of mechanics, chemistry, physics, materials science, mathematics, similarity theory, etc.The intensive development of mechanical engineering continuously stimulates the activities of specialists in the field of tribotechnics, setting them new tasks.The development of space research and nuclear power requires ensuring the operability of rubbing interfaces in a wide temperature range, in vacuum, in aggressive environments, in liquids that do not have a lubricating effect, including liquid metals, in conditions of intense radiation.A feature of this area is the following: impossibility of using traditional lubricants in contact with molten liquid metals; high temperature of contact surfaces; low viscosity of liquid metal heat carriers; non-wettability of working surfaces (steels, cast iron) coated with oxide coatings with heat carriers; the possibility of intensive heat removal from the contact zones of rubbing pairs with liquid metal heat carriers; the concentration in the contact zone of friction pairs of fine particles and impurities (oxides of heat carriers, compounds of elements of structural materialsproducts of corrosion and erosion wear, accidental contamination) forming a dispersed system with pronounced surface properties that differ from the properties of the main volume of the coolant; an order of magnitude higher density compared to traditional reactor coolants (sodium, water); a large amount of surface tension, compared with traditional heat carriers.
Despite the well-known advances in the science of friction and wear, many issues related to increasing wear resistance and reducing friction losses are still far from being resolved.This is due to the complexity of simultaneously occurring phenomena in the contact zone of rubbing surfaces.Therefore, in order to ensure trouble-free operation of machines and mechanisms, diagnostics and operational control of tribological units, that is, issues of tribotechnics, are of particular importance.
In the nuclear power industry, in particular, in the internal devices and fuel assemblies of reactors, there are friction units operating in conditions different from traditional mechanical engineering [1][2][3][4][5][6].A number of factors characterizes this specificity.Such as the use of stainless steels and zirconium alloys in friction pairs, the inability to use lubricants, high temperature, the flow of coolant -water, radiation, the complexity of maintenance, etc [7][8][9][10][11][12][13][14][15][16].In the fuel assembly, there are several thousand nodes of connections of fuel rods and spacer grids with tension, in which, due to elastic deformation of the fuel rod and grid cells, the forces and reaction moments of the supports, which are spacer grids, act on the fuel rods [17][18][19][20][21][22].
The state of these joints can change from stationary to mobile at the stages of assembly and operation of the fuel assembly; at the same time, processes of mutual sliding of parts and friction occur.Friction forces can cause unacceptable deformations of spacing grids and wear of fuel element shells, so they need to be studied and taken into account when designing [23][24][25][26].Which indicates the relevance of this issue.
The article presents the methods and results of pre-reactor experimental studies of static and dynamic processes involving friction forces.These include the assembly of the fuel assembly at the manufacturer, hydrodynamic vibration of fuel rods and fretting corrosion in the nodes of the "fuel cell" of the spacing grid, the movement of the control and protection system controls in the guide channels of the fuel assembly, transverse bending of the fuel assembly, loading and unloading of the fuel assembly into the reactor, the growth of fuel rods under irradiation and thermomechanical interaction tvelov.

Materials and methods
The paper presents the methods and results of experimental studies of static and dynamic processes given in the literature.

Results
The sliding of fuel rods relative to spacing grids occurs during the assembly of the fuel assembly at the factory, during operation during thermomechanical non-synchronous elongation of fuel rods and guide channels, as well as during the vibration of fuel rods excited by the flow.To manage the life cycle of a fuel assembly from the design stage to the end of operation and, in particular, to substantiate the thermomechanical and vibration strength, knowledge about the laws of friction and wear processes of its parts is necessary, which can only be obtained experimentally.In the process of manufacturing the fuel assembly at the factory, the guide channels and fuel elements (fuel rods) are sequentially pushed into the cells of a series of fixed spacer grids located vertically on a horizontal assembly stand.At the same time, a varnish coating is used to reduce friction forces.In case of refusal of the varnish, the maximum values of the total friction force for all spacing gratings can significantly increase (figure 1).In the sliding friction zones, longitudinal risks and chips are formed on the surfaces of the fuel element shells, indicating the processes of plastic displacement and cutting.The refusal to assemble with varnish is economically advantageous, but at the same time a way is needed to reduce the friction forces and the depth of damage to the fuel element shells.Quasi-static frictional forces arise during the non-synchronous movement of fuel rods and guide channels due to the temperature and radiation creep of the material and the elongation of the fuel rod shells.The friction force is mainly a function of the tension between the fuel element and the cell of the spacing grid.As part of the program to create a "rigid" heatgenerating assembly with a welded frame in the early 2000s, the problem of frame strength with non-synchronous thermomechanical elongation of fuel rods and guide channels was solved.To assess the thermomechanical strength of the spacer grid on two-span layouts of the fuel assembly, tests were performed on pushing a bundle of fuel rods through spacer grids fixed by welding on the guide channels of the layout.The fuel element shells were in a state of delivery from the factory.Constructive solutions in the heat-generating assembly with a welded frame, aimed at ensuring thermomechanical strength, were chosen as conservative as possible.
The most well-known manifestation of friction forces during the operation of PWR is known as fretting corrosion of the fuel element in a contact pair with a spacer grid cell when a gap appears in the coupling node and excessive vibration, or debruising fretting corrosion in BWR, that is, wear of the fuel element shell by foreign objects.Transverse vibration arises from the interaction of fuel rods with the longitudinal-transverse flow of the coolant.
In addition to the problems noted above, in the process of improving the heat-generating assembly, the friction processes of the parts of the head of the heat-generating assembly were investigated when the upper shell was compressed, in the collet assembly of the removable head of the assembly itself to the guide channel.Successful changes in the design of the heads were carried out in order to eliminate the effect of "snacking" and consisted in increasing the gaps between the moving parts of the head, introducing guide shells covering the spring block, and locking collet bushings.
Friction forces were also investigated during bench modelling of the reactor core assembly and unloading of the fuel assembly from the reactor at increased speeds.
In order to reduce the friction forces during the assembly of the fuel assembly, studies were carried out on the effect of ultrasonic vibrations on the friction process of fuel element shells along the cells of spacing grids.
The decrease in force depends on the ratio of the thrust velocity and the wave velocity of ultrasound.The effect is provided by the additional force generated by the ultrasonic vibration emitter.
At nuclear power plants with PWR reactors in several countries at the end of the last century, there were cases of jamming of the absorbing rods of the protection control system into the guide channels of the fuel assembly during discharges caused by increased friction forces in curved fuel assemblies.
To find the causes of the curvature of the fuel assembly and the abnormal operation of the protection control system at the end of the last century and the beginning of the 2000s, a number of studies were conducted.Experimentally, it was found that the critical parameter determining the jamming of the absorbing rods is the curvature of the guide channels of the fuel assembly, i.e. the value inverse to the radius of curvature of the fuel assembly.The problem of jamming of absorbing rods was solved by eliminating the causes of accumulation of unacceptable parameters of the curvature of the fuel assembly during operation by increasing the individual bending stiffness of the fuel assembly of the new project with a welded frame and reducing the longitudinal force of the fuel assembly compression in the reactor.
The mechanism of friction during twisting of fuel rods during transverse bending of fuel assemblies and fuel rods explains the effects of increased damping and a decrease in the frequencies of resonant vibrations with an increase in the amplitude of vibrations.These effects are significant at large amplitudes of low-frequency oscillations, for example, arising from seismic effects.With an increase in the amplitude of the oscillations, the processes of slippage and friction of the fuel elements begin (for example, the slippage of fuel rods when turning in the cells of the DR, the movement of fuel pellets), and the number of nodes involved in slippage increases with an increase in the amplitude of the vibration of the supports.
One of the first reasons for the number of fuel failures of fuel assemblies PWR and BWR in numerous reliability analyses shows fretting corrosion, which is a consequence of the sliding friction of fuel rods relative to the DR in the medium of a coolant containing oxygen.It has been experimentally shown that the tested designs of the TVEL-DR VVER coupling units have a significant margin of vibration resistance for both steel and zirconium spacer gratings under normal operating conditions.With the amplitude of vibration accelerations of fuel element shells exceeding several times the design amplitude and the presence of gaps in the coupling of the fuel element sample with the DR samples, wear of fuel element shell samples and DR samples was obtained, similar to the wear of standard fuel assemblies operating under conditions of excessive vibrations.

Discussion
The transformation of the movable joints of the guide channels and spacing grids into fixed ones using spot welding in "rigid" fuel assemblies, together with other measures, solved the problem of curvature of the fuel assembly, but gave rise to the problem of thermomechanical longitudinal loads on the spacing grids from the friction forces of fuel rods in the spacing grids with their non-synchronous radiation growth with the guide channels under irradiation.The studies presented above show that the existing designs of "rigid" fuel assemblies contain a significant reserve of conservativeness, which can be used in improvements of the fuel assembly in subsequent projects.

Conclusion
The paper presented the methods and results of pre-reactor experimental studies of static and dynamic processes involving friction forces.The transformation of movable joints of guide channels and spacing grids into fixed ones using spot welding in "rigid" fuel assemblies, together with other measures, solved the problem of curvature of the fuel assemblies, but gave rise to the problem of thermomechanical longitudinal loads on the spacing grids from the friction forces of fuel rods in the spacing grids with their nonsynchronous radiation growth with the guide channels under irradiation.The results of studies on reducing friction forces using ultrasound during assembly of fuel assemblies can facilitate the assembly of fuel assemblies.Until now, some aspects of contact and friction in the junction of the fuel element-the spacing grid and their effects on the strength of the fuel assembly have not been studied, so it is necessary to develop these works.

Fig. 1 .
Fig. 1.Spacer grid of the fuel assembly of a nuclear reactor.The friction coefficients of zirconium alloys and the influence of structural, technological, and operational factors on them were unknown.Structural factors are the tension, the condition of the contact surfaces (oxidized, ground, anodized).Operational factors include vibration, sliding speed during thermomechanical elongation, etc.Constructive solutions in the heat-generating assembly with a welded frame, aimed at ensuring thermomechanical strength, were chosen as conservative as possible.The most well-known manifestation of friction forces during the operation of PWR is known as fretting corrosion of the fuel element in a contact pair with a spacer grid cell when a gap appears in the coupling node and excessive vibration, or debruising fretting corrosion in BWR, that is, wear of the fuel element shell by foreign objects.Transverse vibration arises from the interaction of fuel rods with the longitudinal-transverse flow of the coolant.In addition to the problems noted above, in the process of improving the heat-generating assembly, the friction processes of the parts of the head of the heat-generating assembly were investigated when the upper shell was compressed, in the collet assembly of the removable head of the assembly itself to the guide channel.Successful changes in the design of the heads were carried out in order to eliminate the effect of "snacking" and Conferences CIBTA-II-2023 https://doi.org/10.1051/bioconf/20237102018018 71 Conferences CIBTA-II-2023 https://doi.org/10.1051/bioconf/20237102018018 71