**1. Introduction**

A good understanding of the factors governing the temperature distribution within a nuclear fuel element is important to predict the fuel temperature in all operating conditions of a nuclear reactor. The temperature distribution influences fuel performance in terms of solid-state reactions, e.g., grain-growth, densification, etc., and the temperature gradient results in fuel deformation or crack in low temperature zones. Oxide fuel is particularly disadvantageous due to its low density and low thermal conductivity [1]. Hence, a large temperature difference between the center and the surface of the rod is required for efficient heat extraction to make electric power generation economical. These constraints are at odds with each other. We intend to operate the reactor at the largest possible power density consistent with maintaining the fuel and coolant temperature below limits set by safety considerations. In accident conditions, we need to have enough margin so that the fuel does not lose integrity due to high temperature and poor heat transfer arrangement. Hence, the length of time and the fuel element that can be utilized in the reactor core is determined by the ability of the fuel element to withstand radiation damage and thermal and mechanical stresses experienced in the reactor environment and not so much on the depletion of fissile material. This is true for reactors utilizing enriched uranium as well as those using natural uranium as fissile fuel material.

Uranium metal is superior to oxide as far as density and conductivity is concerned, but the phase change at a low temperature of 600°C followed by a large volume change means that the fuel clad will be under severe stress. This has led to the investigation of other refractory compounds of uranium, such as uranium carbide, uranium nitride. For any type of fuel being used in the reactor, the fuel performance computer codes are needed to assure the continued safe operation of the reactor. With increasing demands of nuclear fuel efficiency, new fuel designs are being studied and the reliability of these new designs is in the interest of fuel manufacturers [1].

In this chapter, we will look into existing fuel analysis computer codes to develop an appreciation of the fuel characteristics. In the last section of this chapter, we will discuss a new code Fuel Characteristics Calculator (FCCAL) [2] and its suitability in the analysis of oxide fuel.
