In water reactors, PWR and BWR, the fuel itself is in the form of tubes about 10mm diameter, filled with UO2 fuel and clad in zircaloy sheathing. It is important to avoid melting of the sheath, which would release fission products into the coolant, and this has to extend to fault conditions as well as normal steady operation. The melting temperature for zircaloy is about 1800 degC, but as it is in contact with water at only about 320 degC in normal conditions, there is a good margin. The UO2 is a poor thermal conductor so there is an appreciable temperature gradient from the outer radius of the fuel pin to its centre. The melting temperature for UO2 is about 2800 degC. The actual fuel temperature varies both axially and radially in the core, due to the distribution of neutron flux. The maximum fuel temperature is about 650 degC with standard deviation 120degC for PWR’s and 800degC with standard deviation of 173degC for BWR’s, deduced from a sample of six PWR’s and 2 BWR’s. A wider survey might give different results. (Survey by Williams and Mueller, Oak Ridge National Lab) Thus during normal operation there is a good margin to fuel failure levels, the limits to operation depend mainly on behaviour in faults, particularly loss of coolant accidents (LOCA). Conditions in gas cooled reactors using stainless steel sheaths are somewhat different, but I assume you are interested in water reactors.
The temperature of a nuclear reactor is in the range of 300 degrees Celsius or 572 degrees F.
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