Method of burnup estimation from mass spectrometric data

Exposure of a natural uranium sample irradiated in a thermal reactor is estimated from isotopic ratios of uranium and plutonium measured by a mass spectrometer. The analysis was performed with a set of point burnup equations with effective cross sections as functions of the neutron temperature. Through the isotopic ratio-exposure relations obtained by the analysis, five isotopic ratios yield five exposure data with different accuracies. The error caused by the analytical method results in systematic deviations on the exposure data, which can be separated from the random deviations incurred in the measurements. By means of the theory of errors the exposure is estimated to be 7,530±380MWD/TU from these data, which indicates that the simple analytical model gives fair results in the present case. The error in the exposure is found to consist of two terms. One of which reduces to zero as the accuracy of the measurement improves, and is defined as the "measurement error". The other term, "the modelling error", originates from the error in the analytical model. The modelling error predominates when accurate measurements indicate significant discrepancies between the estimated exposures from different abundance ratios. The modelling error can be used as a measure to know whether further improvement in the analysis is necessary. (author)