Impact performance methodology for Robust Shielded Waste Packages
A review and update of the methodology for evaluating the release fractions (RFs) for Geological Disposal Facility (GDF) impact accident scenarios.
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A methodology for evaluating the release fractions (RFs) for Geological Disposal Facility (GDF) impact accident scenarios for robust shielded waste packages was developed by NDA in 2011. The methodology applies when gross structural integrity is demonstrably maintained during an impact accident scenario. It has been used to form the basis of Final stage Letter of Compliance (LoC) endorsement.
RWM has commissioned Arup, as subcontractors to Wood, to investigate the scientific underpinning of the methodology.
The work has 3 tasks:
- Task 1: Documentation and Evaluation of Methodology (208130/TR/001 - NWS-CR-23-008)
- Task 2: Update of methodology (208130/TR/002 - NWS-CR-23-009)
- Task 3: Consider the feasibility of a programme of research to further investigate Ductile Cast Iron Container (DCIC) performance (208130/TR/003 - NWS-CR-23-010)
Task 1 documents and critically evaluates the methodology
The RF methodology is broadly split into 5 steps. Each of these steps was evaluated by investigating the referenced sources and evaluating the scientific underpinning presented.
It is recognised that the methodology was based on information available at the time, and that a pragmatic approach was employed where there were gaps in the knowledge available to underpin the methodology. It has been found that the methodology does not take into account the appropriate physics of how particulates become airborne, it uses a scaling factor for drop height that is difficult to justify, scaling to sub-100 micron particulates is based on encapsulated wastes rather than unencapsulated waste, the calculation of package RF based on pressure difference simplifies the mechanism of release and further scientific underpinning is recommended over the particulate size distribution used to scale the package RF.
Many of the assumptions made could be revisited and revised. Some of the assumptions were found to be conservative, some were found to be unconservative and in some cases, further scientific underpinning is required to establish whether the assumption is either conservative or unconservative. The conclusion is that the methodology as it stands is “invalid”.
Task 2 updates the methodology using existing knowledge, with each stage having appropriate scientific underpinning
The revised methodology has 4 stages:
- Stage 1: Start with a baseline wasteform RF of 1.8x10-2, which is the wasteform RF for sub-10 micron particulates for a drop height of 9 m.
- Stage 2: Scale the baseline wasteform RF linearly from a drop height of 9 m to the required drop height.
- Stage 3: Obtain the proportion of sub-10 micron particulates from the particulate size distribution (PSD) and use this to scale the wasteform RF to take into account the type of wasteform stored. If the PSD is not available, the scale factor should be assumed to be 1.
- Stage 4: The package RF is calculated by assuming the release of particulates is caused by a pressure gradient between the inside and outside of the package, generating a flow of airborne particulates through an opening in the lid-body gap. A minimum internal gauge pressure of 0.5 bar should be assumed for the calculation.
Task 3 proposes and evaluates the feasibility of a programme of research to improve the scientific underpinning and reduce the conservatisms
Three proposals have been developed and are summarised as follows:
- To improve confidence in the baseline wasteform RF, small scale drop tests of different pulverised fly ash (PFA) powders in “unconfined” glass specimens from a drop height of 11 m are proposed;
- To improve confidence in scaling to height, small scale drop tests of PFA powder in “unconfined” glass specimens from various drop heights are proposed. These could be combined with the experimental programme to improve confidence in the baseline wasteform RF;
- The revised methodology from Task 2 assumes that the release of particulates is due to a pressure driven flow of airborne particulates through an opening in the lid-body gap and a minimum gauge pressure of 0.5 bar is used for the calculation of the release. The revised methodology assumes that any potential inertia driven release is very small compared to the pressure driven release. It is proposed to carry out analytical calculations to estimate the potential release due to an inertia driven mechanism. The work would be used to underpin the assumption that the inertia driven release is very small when compared to the pressure driven release and could subsequently be used to underpin a revised minimum pressure gradient requirement.