Determination of radial gadolinium burnup in LWR fuel by secondary ion mass spectrometry (SIMS)

The ''International Experimental Programme on Gadolinia Fuel Evolution in PWR's (GAP)'' is devoted to the acquisition of experimental results in support of studies concerning the use of Gd2O3 as a burnable absorber in pressurized water reactors. One of the main tasks of the programme was to evaluate radial isotopic distributions of Gd in irradiated fuel samples. Mass spectrometry following radiochemical separations on samples taken by microdrilling is a laborious technique. Moreover, the lateral resolution is limited by the diameter of the drill. An alternative technique for the measurement of local isotopic compositions is Secondary Ion Mass Spectrometry (SIMS). This method is faster and provides greater lateral resolution. The major disadvantage of the SIMS technique is its limitation concerning quantitative analyses. It is therefore necessary to use average isotopic abundance values evaluated by dissolution and mass spectrometry for the normalization of the SIMS data. Alternatively, isotopic calibration standards with a chemical composition comparable to the fuel samples may be used. Samples of fuel with an initial 235U enrichment of ∼3.5% and Gd2O3 concentrations of ∼3 and ∼7 wt% have been analyzed. The burnup values lay between 2 and 6 GWd/tM. Several surface scans of the same sample showed a standard deviation of less than 10% (relative), provided that the residual isotopic abundances were higher than ∼1%. Results of radial Gd burnup calculations with the aid of the LWRWIMS code were in good agreement with SIMS evaluated isotopic distributions. (author). 4 refs, 9 figs