[en] The future nuclear systems of generation IV (Gen-IV) consider the concept of 'dirty fuel' which is proliferation-resistant and recycles spent fuels. Not only fuel performance experiments but also simulations with computer codes for fuel performance are essential to understand peculiar characteristics of future fuel related with advanced fuel cycles. The performance of dry-processed fuel is quite different from that of the fresh UO₂ fuel because fission products are still remained after dry processing. To begin with conceptional study for fuel performance, we have newly constructed KAOS code systems based on the existing fuel performance code systems. The KAOS code system is written in a C computer language that calculates the steady-state response of dry process fuel in light water reactor during long-term burnup. The code also calculates the temperature, pressure, and deformation of a fuel rod as functions of time-dependent fuel rod power and coolant boundary conditions. In the future, the KAOS code will be extended to treat transient behavior of fuel rod such as a rod injection accident and locked rotor accident