Friction and wear of elements of nuclear reactors

. Fuel elements (fuel rods) and spacer grids are the main elements of the fuel assembly of a nuclear reactor. Friction forces affect the strength of the fuel assembly at all stages of its life cycle. The paper presents 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's factory, hydrodynamic vibration of fuel rods and fretting corrosion in the "fuel cell" nodes of the spacing grid, 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


Introduction
The fuel element (fuel element) is the main structural element of the active zones of heterogeneous reactors, and there are practically no homogeneous reactors yet. More than 90% of thermal energy is released in fuel rods during the fission of U-235, Pu-239 and U-233. In the presence of U-238 or Th-232 reproduction materials in fuel rods, secondary nuclear fuel Pu-239 or U-233 is formed in them. As a rule, the fuel element consists of a fuel core, shell and end parts. Sometimes there are spacing elements on fuel rods, although the latter are more common on fuel assemblies (fuel assemblies) [1][2][3][4][5][6]. The core contains dividing or reproducing nuclides, the shell and the end elements form a sealed volume. The shell protects the fuel from erosion and corrosion, prevents fission products from entering the coolant. It transfers heat from the fuel element to the coolant, perceives loads from the fuel core and gas fission products. Another function of the fuel element is to localize and exclude the release of nuclear fuel and radioactive fission fragments into the coolant [7][8][9].
Fuel rods are classified according to various criteria: reactor type, coolant parameters, reactor purpose, design, etc.
Fuel assemblies (fuel assemblies) are a package (bundle) of fuel rods for loading into the core of most reactors. The exceptions are block fuel rods loaded into the fuel channels (TC) piece by piece, and ball fuel rods, which fill the entire volume of the core formed by the side and lower reflectors [10][11][12][13][14].
The main elements of the fuel assembly (fuel assemblies) are the fuel elements (fuel rods) in which energy is released. In the assembly, the fuel rods are assembled into a bundle and fastened together by means of spacing grids arranged in a certain step along the entire length of the assembly ( fig. 1). Depending on the type of fuel rods, the fuel assemblies are installed on supporting structures in the core (hull reactor) or in fuel channels (channel reactor) [1,4].
The functions of the fuel assemblies are fixing and spacing fuel rods, organizing the flow in the core (ensuring the required flow direction, its magnitude and distribution), facilitating transport and technological operations with nuclear fuel. In the internal devices and fuel assemblies of reactors, there are friction units operating in conditions other than traditional mechanical engineering. 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 b a E3S Web of Conferences 402, 05024 (2023) https://doi.org/10.1051/e3sconf/202340205024 TransSiberia 2023 maintenance, etc. 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. 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, therefore they need to be studied and taken into account when designing. Which indicates the relevance of this issue.
The following figure (fig. 2) shows the connection diagram of the fuel cell and the separation grid cell.

Main part
An important indicator of the normal operation of the assembly is the degree of its curvature. The curvature of the fuel assembly must not exceed the design value at any time during operation. The smaller the curvature of the assembly, the greater the rigidity of the fuel assembly structure. The rigidity of the FA structure is given by the welded joints of the spacer gratings and guide channels. 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.
The operating conditions of a nuclear reactor are such that under the influence of high temperature and neutron irradiation, all loaded joints in the assembly weaken due to the development of irreversible creep deformations. Under these conditions, relaxation of contact interactions between fuel rods and other cells occurs. The processes of relaxation of contact forces in the connection of the fuel element -the DR cell lead to the slipping of fuel elements in the fuel assembly, which reduces the rigidity of the assembly structure. 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. 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.
Because of the action of high temperature and neutron irradiation, there is a relaxation of the contact interaction between the fuel element and the cell of the spacing grid. As noted above, the relaxation of the contact interaction leads to the slipping of the fuel element in the cell of the spacing grid. These phenomena reduce the rigidity of the fuel assembly structure and lead to unacceptable deformation of the assembly.
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" heat-generating 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 twospan 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.
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 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.

Conclusions
The paper presented the methods and results of pre-reactor experimental studies of static and dynamic processes involving friction forces. It should be noted that studies of the processes of fretting corrosion of the fuel assembly, the mechanics of the contact of the fuel element with the spacing grids are conducted by all major fuel manufacturers in order to improve the design of the coupling unit of the fuel element with the spacing grids, and not for the purpose of understanding this unit from the standpoint of tribology. The science of friction and wear uses the methodology of a systematic approach, for example, considering the friction node with an analysis of input impacts and loads, the system structure (body, counter body, medium), useful outputs (for example, resource), as well as changes in parameters over time. 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.