Validation of 134Cs, 137Cs and 154Eu single ratios as burnup monitors for ultra-high burnup UO2 fuel

A measurement station has been built for the non-destructive investigation of burnt fuel rod segments through high-resolution gamma spectrometry. Four UO2 pressurised water reactor fuel rod segments with different burnup levels between 50 and >100 GWd/t and 10 year cooling time have been experimentally characterised using gamma-ray spectrometry to determine 134Cs, 137Cs and 154Eu and their corresponding concentration ratios. Experimental errors of 2% (1σ) for the 134Cs/137Cs ratio were obtained for most of the segments. In parallel, pin cell depletion calculations have been performed for each segment using the deterministic code CASMO-4. Measured and calculated ratios have then been compared with the purpose of deriving and validating pin-averaged single-ratio burnup indicators for very high burnups. It is shown that the 134Cs/137Cs ratio, frequently used as a burnup monitor, is considerably less precise for values exceeding 50 GWd/t; discrepancies of 16% are found between measured and calculated values, increasing with burnup up to 23%. The ratios built with the 154Eu concentration show even much larger discrepancies, essentially because this isotope is rather poorly predicted as revealed by just using different basic cross section data.