PSI - Issue 60

A. Syed et al. / Procedia Structural Integrity 60 (2024) 195–202 A. Syed/ Structural Integrity Procedia 00 (2023) 000 – 000

197

3

during heating of the tubes. The internal pressure was varied from 3 to 70 bars at a heating rate of 5-19 ºC/s. The fuel clad balloons due to internal pressure and temperature and finally gets ruptured at a specific location along the length of the clad tubes. The variation of temperature and pressure profiles with time are recorded using on-line data acquisition system. 3. Finite element modelling of the fuel clad ballooning Axisymmetric modelling of the fuel clad has been carried out for simulating the ballooning of clad tubes under different operating conditions. 3.1. Material properties used in the model Zircaloy-4 material properties has been used for carrying out the finite element analysis. Elastic modulus of the material is taken to be 98 GPa and Poisson’s ratio is 0.3. Creep propert ies have been modelled in FE software using Norton’s creep rate equation as shown in Eq. (1). ̇ (t,T, σ )=A σ e T −C ( t ) (1) where A, n and C are the material parameters of Zircaloy-4 material. These parameters are evaluated using the strain at fracture of the fuel clad. T(t) is the variation of temperature (in K) with time. ‘σ’ is the von -Mises stress (MPa) on the material. Material parameters used in the analysis is given in Table 1.

Table 1: Material parameters for modelling of creep in FE analysis

Temperature, T (˚C)

n

C

A

(T<600)

2.45

28300 28300 28300 28300 28300 28300 28300 28300 28300 28300 28300 28300 28300

5.70E+07 5.70E+07 5.70E+07 5.70E+07 5.70E+07 5.70E+07 5.70E+07 5.70E+07 5.70E+07 5.70E+07 5.70E+07 5.70E+07 5.70E+07

(600

2.2

1.95 1.67 1.45 1.27 1.15 1.01

0.9 0.8 0.7 0.6 0.5

3.2. Description of FE model

Fuel clad of 220 MW reactor was modelled with inner diameter of 14.4 mm and outer diameter of 15.2 mm. Thickness of the fuel clad is 0.4 mm. Length of the clad tube is taken to be around 100 mm. The diameter of the clad tube near the central location is slightly reduced to model the effect of localized ballooning of the clad tubes. Axisymmetric model has been prepared due to the symmetric deformation of the clad tubes about its axis. 2D axisymmetric 8-noded element has been selected for the model. Length of element along the axial direction is taken equal to 1.16 mm. Aspect ratio for the element comes out to be 9 which is in the limit. Total element in the model comes out to be 90X3= 270 elements. Mesh convergence study is performed which results in 90 and 3 divisions