**2. Classification of fuel performance codes**

There are several available computer codes to analyze the thermal and mechanical behavior of fuel for different types of reactors viz., LWR, CANDU, VVER, etc. some of these codes are available in the public domain while some are proprietary and not available publicly.

The fuel rod behavior is determined by thermal, mechanical, and physical processes such as densification, swelling, fission gas generation, fission gas release, and irradiation damage. The fuel performance analysis code covers these aspects through thermal and mechanical components of fuel performance. The codes may be 1D, 2D, or 3D. However, experience shows that one-dimensional codes are most widely used for fuel analysis. The codes can be further classified as steady-state and/or transient codes. Examples of steady-state codes are FRAPCON, TRANSURANUS, COMETHE, etc. these codes calculate the radial temperature profile and fission gas release to the fuel plenum. Mechanical properties like creep deformation and irradiation growth can also be calculated using these codes. The transient codes like GRASS-SST [3] can calculate these parameters and additionally calculate cladding plastic stress-strain behavior, the effect of annealing, the behavior of oxide and hydrides during temperature ramps, phase changes, and large cladding deformation such as ballooning. The transient codes neglect long-term phenomena like creep deformation. Let us first discuss two computer codes for an understating of how we go about fuel characteristics quantification.

#### **2.1 GAPCON-THERMAL**

GAPCON-THERMAL-II (GT-II) [4] is an updated version of the older GAPCON-THERMAL-I (GT-I) [5] code that is widely used for calculating light water reactor fuel thermal performance. GT-I has been modified to improve upon the uncertainty in the calculation of power history and burn up. GT-II is an American National Standards Institute (ANSI) compliant Fortran-77 code. We can calculate the thermal behavior, fuel plenum conductance, temperature and pressure, and fuel stored energy using this code. There are models for power history, fission gas generation and release, fuel relocation, and densification in the code. For the power history simulation, the code uses constant power for each finite time step. At any time step other than the first for each axial node, the current fission gas release, relocation, and densification values are compared with the values used in the previous step. Relocation and densification displacement will not decrease if lesser values are subsequently calculated. The fission gas release algorithm depends upon
