Modelling of the Uncertainty of Nuclear Fuel Thermal Behaviour Using the URANIE Framework

In the global framework of nuclear fuel behaviour simulation, the response of the models describing the physical phenomena occurring during the irradiation in reactor is mainly conditioned by the confidence in the calculated temperature of the fuel. Amongst all parameters influencing the temperature calculation in our fuel rod simulation code (METEOR V2), three sources of uncertainty have been identified as being the most sensitive: thermal conductivity of UO₂, radial distribution of power in the fuel pellet and local linear heat rate in the fuel rod. Because the validation of the models is based on measurement-simulation comparisons, a fourth parameter needs to be modelled: the inner diameter of the hole in the pellet that contains the thermocouple used to measure the temperature. Expert judgement and inverse methods have been used to model the uncertainty of these four parameters, either by distribution laws, or by defining a representative sample of the variables. Propagation of these uncertainties in the METEOR V2 code using the URANIE framework and a Monte-Carlo technique has been performed in the case of an irradiation experiment of 5000 hours. At each moment of the experiment, we get a temperature statistical distribution which results from the initial distributions of the uncertain parameters, and this distribution covers the measured values. This final result makes up a good validation of the whole modelling process which allows an accurate quantification of the uncertainty on fuel temperature calculation.