**2.2 Fuel design analysis (FUDA)**

The computer code FUDA [6] is used for the design analysis of fuel for licensing application of CANDU type reactors in India. The code is used for fuel performance evaluation as well as to optimize the fuel design and fabrication parameters of Indian reactor fuel. The code is valid for the burnup of 50,000 MWD/Te of oxide fuel. Natural uranium and thorium oxide fuel can be analyzed using this code. There are models for computation of fuel temperature, thermal expansion, and clad stress parameters in the code.

The code uses the finite difference method for temperature and computation of thermal expansion. The clad stress, local stress, and ridge analysis is carried out by finite element technique. Fuel expansion is calculated by the two-zone model in which the stress in uranium oxide is ignored. Uranium oxide deformation is assumed to occur above a certain temperature as plastic, and below this temperature, the fuel element is assumed to behave as elastic solid with radial cracking. The extent of plasticity is governed by fuel temperature, stress due to cladding strength, and the coolant pressure in a time-dependent manner. Global clad stress and strain due to fuel thermal expansion, swelling and densification are calculated by models and correlation used in Notley [7]. The creep and stress relaxation in the time zone at constant power operation is calculated using semi-empirical formula considering a thermal and thermal creep including the effect of irradiation. Fuel sheath interfacial pressure is then calculated based on gas pressure and strains. Using global diametral changes, local deformation of the fuel element and sheath is calculated considering hourglass phenomena in the fuel element. The finite element method using asymmetrical 8-node isoparametric elements is used for calculating deformation, stress, and strain in the element and the clad.

Fuel gap conductance is calculated by the Ross and Stoute model [8] taking care of the physical gap existing in the fuel plenum. Plenum gap conductance consists of (a) conduction through solid-solid contact points (b) conduction through solid-gas contact points and, (c) radiation exchange between the element and clad. For plenum gap conductance, the URGAP model of K. Lassmann [9] has been used. The fission gas release is calculated using two methods (a) temperature-dependent release model and (b) physical model based on diffusion and grain growth mechanism.

To estimate the local flux perturbation, the Bessel function is used. The heat transfer from the clad surface to the coolant is calculated using Dittus-Boelter [10] equation. Using the fuel element-clad heat transfer coefficient, new temperature

distribution across the fuel and clad is calculated. The corresponding internal gas pressure is calculated using the new temperature distribution and when successive internal gas pressure is within �5% then the pressure and temperature results are assumed to converge and the iteration is stopped. For improving accuracy, the pellet is divided into 100 rings radially and the fuel temperature and pressure are calculated for each ring. The code is validated against the results of benchmarked codes ELESIM and ELESTRES.
