To this end, Nagra, the Swiss implementer, undertook an analysis of the origin of all half-life data for all radionuclides deemed to be of relevance (i.e., all nuclides with half lives >60 days and important daughters, e.g., 90Y, 137mBa, etc.) in their repository design, to ensure that any potential for their change in the future was understood. This was not meant to assess minor changes as the accuracy of half-life measurement improves, but rather to ensure that, for the safety-relevant nuclides, the values used were based on data that had been verified by repeated measurements in more than one laboratory, using different methodologies. This study (McGinnes, 2006) indicated that, unlike 79Se, all other relevant radionuclide half-lives had been confirmed by multiple analyses.

2.7.6.3. Uncertainties

Uncertainties are an integral part of any scientific work and, in the case of radionuclide inventories, these can be assessed by performing cross-comparisons between inventories produced in different countries and analysing the differences via peer review. Other methods involve cross-comparison of codes (e.g., Andersson, 1999; McGinnes, 2002) to identify precisely where differences lie. This is especially useful in allocating uncertainty factors to simple codes based on their comparison with more accurate codes (see, e.g., Kolbe, 2004).

Finally, the best method is to compare the results of the calculations with actual measurements (validation), but this involves significant expense and, for some difficult- to-measure radionuclides, does not always lead to clear results (see comments in McGinnes, 2002).

However, to put matters in perspective, it should be noted that the conservatism inherent in the processes leading to the calculation of repository safety, combined with the vast improvements made in calculation and measurement techniques in the area of inventory and waste analysis, leads to the conclusion that, with due diligence, inventory uncertainties should not play a significant role in assessing repository performance over long time periods.

2.8. Conclusions

To conclude, it is considered to be essential that any person involved in inventory activities is aware of the impact of the many assumptions that have to be made to create an inventory that is suitably sized for safety assessments. This chapter has attempted to highlight some of the pitfalls that can be encountered during inventory development and has proposed a technique to allow the relative importance of waste types to be assessed and as a result, allow resources for more detailed waste characterisation activities to be better applied.

2.9. Acknowledgements

The author would like to acknowledge former colleagues at Nagra (to name but a few, H. Maxeiner, J. Schneider, P. Zuidema) who have assisted over the years to ensure suitable inventories were developed. Finally, a special mention for F. Bruno (formerly at ENEA, Italy) for a great time in Italy and recognition for the joint development of the relative importance of waste types for assessing.

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