[en] Reactivity measurements of fuel assemblies in a fixed core position are directly proportional to the k_∞ of the fuel assembly. The burnup, BU, of a fuel assembly corresponding to its k_∞ can be obtained from the group cross section library of the fuel assembly. Therefore, by determining the k_∞ of a fuel assembly through the reactivity measurements, its corresponding BU is determined. Previous methods determined the kt of a fuel assembly using two unused fuel assemblies of different but known kt to normalize the data [1]. Because there is only one type (. 23 fuel plates) of unused IRR1 fuel assemblies available, a new iterative method is introduced to determine their k_∞ and corresponding BU. The method iterates between calculations and experimental measurements until the calculations converge to agree with the experimental data. The accuracy of the results is a function of the experimental errors and the accuracy of the calculations. The MCNP4C Monte Carlo code [2] and the ORIGEN 2 code [3] are used to perform the accurate core calculations. Some initial calculations were made with the WIMS/EXT2 codes to help validate the input to the MCNP4C code