Many codes incorporate single rod models, which calculate thermal properties such as stored energy, radial temperature profiles, fission gas release to the gap and mechanical properties such as creep deformation and irradiation growth (NEA/CSNI/R(99)25, 2000). Examples of such codes are COMETHE, FRAPCON, METEOR, TOUTATIS, TRANSURANUS and ENIGMA (Bailly et al., 1999; Table 16.2). For LOCA analysis, it is important to calculate initial stored energy from normal operation conditions. Other parameters that need to be calculated are clad oxidation thickness, the internal gas pressure, and geometrical parameters including the axial clearance between rods and end fittings. It is important to calculate fission gas content in fuel grain boundaries, fuel porosities and fission gas movement between grains and grain boundaries for the analysis of fuel failure mechanisms in RIA transients. Under RIA conditions, pin failure may result if sufficient fuel swelling and grain swelling occur.

In order to calculate these properties fuel performance codes include a wide variety of models for calculating: radial power profiles, thermal conductivity and specific heats of

materials, gap conductance, hydrogen absorption, waterside corrosion, creep properties, mechanical properties, creep properties, stress-strain relationships, fuel densification, and fuel swelling.

In the future, these codes are likely to be called upon to model burn-ups of up to 65 MWd kgU 1 or higher. The FRAPCON code has recently been modified for burn-ups up to 65MWdkgU 1 (banning et al., 1997). Many of the current codes/models were originally developed and validated for more moderate burn-ups of 40 MWd kgU 1 and the applicability of these codes at higher burn-ups is under review. The models will also require review with regard to their application to MOX fuel.