To determine the fission power in fuel targets irradiated in BR2, various methods are applied. The three most commonly used are the thermal balance method, the non-destructive gamma-spectrometry of the irradiated fuel rods, and the destructive post-irradiation measurements. A fourth method, called here the fluence-dosimetry method, combines the measurement of fluence dosimeters with the calculation of the flux chart inside the irradiation device, and in particular of the flux and the fission density in the fuel. The various factors influencing the accuracy of the latter method will be analysed.