Abstract
The representative C35M alloy among FeCrAl alloys was selected as the research object, and a small fuel rod model was established. Based on the user material (UMAT) subroutine, the radiation creep model of the C35M alloy was embedded in the subroutine. The thermomechanical coupling behavior of C35M alloy under neutron irradiation was calculated by the finite element software ABAQUS. Using Zr-2 alloy as a comparison, the evolution of the distribution of the temperature field, stress field, displacement field, and gap distance of the cladding over time of the alloys was analyzed. Results show that the temperature field and the stress field of the two alloys are basically the same. The temperature distribution is mainly affected by the coolant, while the stress field is related to the temperature and creep rate. During the simulation, the Zr-2 alloy mainly grows through irradiation, while C35M alloy shows the irradiation creep and has a little thermal expansion deformation. The gap closure rate of Zr-2 alloy is much faster than that of C35M alloy, which indicates that C35M alloy can prolong the accident response time. However, in order to adapt to the complex environment in the reactor, the material still needs to be optimized to improve its strength and creep rate.
Science Press
In the normal operation of nuclear reactors, the cladding is subjected to a complex working environment. The inner wall interacts with the fuel pellets, which is prone to cracking. The outer wall is subjected to high temperature, high pressure, neutron irradiation, and direct contact with the coolant, which is prone to chemical corrosio
FeCrAl alloys attract much attention due to their excellent resistance to high temperature water vapor oxidation, stable thermophysical properties, and good radiation resistanc
The materials of cladding tubes were cold-worked Zr-2 alloy and C35M wrought alloy in this research. The core block material was UO2. In the normal operation, the heat is transferred from the center of the UO2 pellet to the inner wall of the cladding, and then is spread along the radial directions of the cladding. Under the influence of temperature and neutron irradiation, the properties of the cladding changed during operation, affecting the performance and service life of fuel rods. Therefore, an effective model involving the physical parameters with temperature is important to analyze the thermal behavior of materials.
The heat transfer coefficient of UO2 fuel was calculated using the Lucita modified mode
(1) |
where k95 is the thermal conductivity of unirradiated UO2 at 95% theoretical density (W·
(2) |
(3) |
(4) |
(5) |
(6) |
where T is temperature (K); B is fuel consumption of the fuel system (in this research, B=50%); P is the porosity; s is the shape factor of the sphere (in this research, s=0.5).
The relationship between the thermal expansion coefficient of UO2 fuel and temperature can be expressed as follows:
(7) |
where K1=1.0×1
The density and Poisson's ratio of UO2 fuel are 10.96 g/c
The thermal conductivity of Zr-2 alloy (kZr, W·
T<670 K | (8) |
The Young's modulus of Zr-2 alloy (EZr, GPa) is also related to the temperatur
T<1090 K | (9) |
The irradiation growth and creep of the cladding tubes are induced by the neutron irradiation, which is affected by the fast neutron fluence rate. An empirical model is employed to evaluate the radiation growth (εg) performance of cladding in steady stat
(10) |
where ϕ is the fast neutron fluence rate (n·
The creep behavior of cladding is composed of thermal creep and radiation creep. The radiation creep of Zr-2 alloy is induced by neutron irradiation. The Hoop model was used to calculate the irradiation creep, which is related to the stress and fast neutron fluence rat
(11) |
where σM is the Mises stress (MPa); C0, C1, and C2 are the material constants of 9.881×1
The thermal creep rate of Zr-2 alloy can be calculated by
(12) |
where G is the shear modulus (MPa); Q is the activation en-ergy (270 kJ/mol); R is the gas constant (J·mo
Therefore, the creep rate of Zr-2 alloy can be described as follows:
(13) |
The density and Poisson's ratio of Zr-2 alloy are 6.55 g/c
The thermal conductivity of C35M alloy was tested by Oak Ridge National Laboratory (ORNL
(14) |
where kC35M is the thermal conductivity of C35M alloy (W·
The specific heat capacity of C35M alloy can be obtained by
(15) |
where CC35M is specific heat (J·k
The density of C35M alloy is 7.06 g/c
(16) |
By measuring the elastic modulus of different FeCrAl alloys, Thompson et a
(17) |
Yamamoto et a
(18) |
Terrani et a
(19) |
where is the thermal creep rate (
The radiation creep rate can be calculated by the following equatio
(20) |
Therefore, the creep rate of C35M alloy can be described as follows:
(21) |
The yield strength of Zr-2 and C35M alloys at different temperatures is shown in

Fig.1 Yield strength of Zr-2 and C35M alloys at different temperature
A small-scale fuel elemen

Fig.2 Schematic diagrams of fuel rod (a) and overall model (b) by finite element model
During the manufacturing process, there will be a gap between the fuel pellet and the cladding, and helium gas is added into the gap to improve the heat transfer and protect the structure of the fuel element. Under normal operation conditions, as the temperature of the fuel rod increases and the fission gas is released, the pressure in the gap is increased. In the simulation process, the internal pressure at the initial stage is set as 2 MPa, and it is increased with the procedure pro-ceeding, and finally turns to 10 MPa at the end of the simu-lation. The outer surface of the cladding is in direct contact with the coolant channel. The pressure on the outer surface of the cladding results from the coolant, which is set as 15.5 MPa and the cooling coefficient is 20 000 W·(
The temperature field distributions of fuel element at the end of the cladding tube after operation under neutron irradiation for 1200 d are shown in

Fig.3 Temperature field distributions of cladding tubes of Zr-2 (a) and C35M (b) alloys
In order to study the temperature changes of the cladding tubes of two alloys during operation, the data from the outer surface of the fuel pellets and from the inner and outer walls of the cladding tubes are used for analysis, as shown in

Fig.4 Temperature variations with operation time for cladding tubes of Zr-2 (a) and C35M (b) alloys
The gap and the wire power are important factors to affect the temperature field of the fuel rods. The instantaneous temperature rise at the beginning of the simulation is because the temperature at the middle area of the fuel pellet will rise to 1200 °C within 1 d, as shown in
The stress distribution of the cladding tube is important to evaluate the thermodynamic performance of materials and the safety of fuel elements. The size and distribution of stress on the cladding tube are affected by the temperature and irradiation. In this research, the thermomechanical coupling behavior of the Zr-2 and C35M alloys cladding tubes was studied. The stress field distribution of the two alloy cladding tubes after steady-state operation for 1200 d is shown in

Fig.5 Stress field distributions of cladding tubes of Zr-2 (a) and C35M (b) alloys
It can be seen from
In the performance simulation of fuel rod, the deformation behavior of the cladding tube and the evolution of the fuel-cladding gap during operation are critical, because they have a significant impact on the fuel temperature and the mechanical properties of the cladding tube. During the simulation, it is considered that the cladding deformation is caused by thermal expansion, thermal creep, radiation creep, and radiation growth. The displacement field distributions of these two alloy cladding tubes after steady-state operation for 1200 d are shown in

Fig.6 Displacement field distributions of cladding tubes of Zr-2 (a) and C35M (b) alloys
It can be observed from
In the displacement field, the deformation distribution of the cladding tubes of two alloys can also be observed. Through the analysis of the displacement field, the processing technique can be optimized for the severely deformed parts of the cladding tube. In order to further determine the pellet- cladding mechanical interaction (PCMI), the radial displacement data of the inner wall of cladding tube and the outer surface of fuel pellet were obtained, and the variation curves of gap distance is shown in

Fig.7 Gap distance variation with operation time between pellet and cladding tubes of Zr-2 (a) and C35M (b) alloys
It can be observed from

Fig.8 Microstructure of cladding tube failur
1) The temperature distributions and stress field distributions along the radial and axial directions of the C35M and Zr-2 alloy cladding tubes are consistent, showing a gradually decreasing gradient distribution from the inner wall to the outer wall of the cladding tubes. When the thermal conductivities of the two alloys are similar, the temperature is controlled by the cooling coefficient of the coolant. The stress depends on the material properties.
2) The irradiation growth is the main factor for uniform deformation of cladding tube under neutron irradiation. In contrast, the effect of irradiation creep is small. Compared with Zr-2 alloy cladding tube, the gap closure time of C35M alloy cladding tube is longer due to the lack of irradiation growth.
3) The gap distance in FeCrAl alloy cladding tube system should be further improved. In addition, the strength (creep rate and stress) of alloys should be ameliorated in order to prevent the cladding tube from cracking due to the failure of stress release.
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