Further Calculations using the QSS and RTM Computer Models

With the QSS Model, calculations have been undertaken in NRVB (Nirex Repository Vault Backfill) where a system can encounter positive temperature feedback when the pore water boils. A QSS calculation of two or more stages is required when this occurs. The QSS Model has also been used to undertake calculations in mudstone, a potential GDF host rock that has not been studied previously.
The RTM has recently undergone improvements to the structural response model, and an analysis of stability issues has led to much greater understanding of the role the equation of state plays. An ongoing study has identified a number of potential improvements to the RTM submodels, both in terms of solution methods, and the supplied equation of state. Many of these improvements have been implemented, and previous RTM calculations undertaken in earlier studies have therefore been repeated to make use of the latest input and model features. In addition, two completely new RTM calculations have been undertaken; one to complement the set of calculations with plutonium dioxide in NRVB, and another as the first RTM calculation with uranium dioxide in NRVB. In total, this report describes ten RTM calculations.
A further part of this study has been a detailed analysis of fission product inventories for example QSS and RTM calculations. Using methodologies developed in 2004-05 as a basis, a number of calculations have been undertaken for example QSS and rapid transients from the first set of calculations. The analysis shows that for the RTM calculations the conclusions of the 2004-05 study are correct. For a QSS calculation, a more detailed analysis is required than the original outline methodology to accurately predict the fission product inventory.
This study has not only extended the range of calculations that have been undertaken previously, but has provided much more insight into the range of phenomena that can occur, why the phenomenon occur, and what further investigations and developments may be beneficial for improving confidence in both models. In addition, it provides analysis and recommendations for predicted fission product inventories for both QSS and rapid transients. Results will provide a basis for future assessments.