Welcome to the July 2011 edition of Quintessa Update, the electronic newsletter of Quintessa Limited, covering Quintessa's scientifically-based consultancy, contract research and software for supporting decisions related to nuclear power generation and the nuclear fuel cycle. This edition of Quintessa Update contains the following articles:
- Quintessa's Postclosure Safety Assessment of Canadian DGR Published
- Evidence Support Logic for Leak Mitigation
- FORGE: Treatment of Gas in Performance Assessment
- Long Term Cement Studies (LCS): Tournemire Analogue Modelling
- Biosphere Studies Supporting the NDA RWMD Disposal System Safety Case
- Coupled Modelling of Buffer Erosion and Copper Canister Corrosion
- 2-D Reactive Geochemical Transport Modelling of an EBS
Quintessa's Postclosure Safety Assessment of Canadian DGR Published
Quintessa and its subcontractors, Geofirma Engineering and SENES Consultants, have recently completed the
postclosure safety assessment of Ontario Power Generation's proposed Deep Geologic Repository (DGR) for Low and Intermediate Level Waste
at the Bruce nuclear site in the Municipality of Kincardine, Ontario. The work was undertaken on behalf of Canada's Nuclear Waste Management Organization, which is assisting Ontario Power Generation in seeking regulatory approval for DGR construction. The
suite of postclosure safety assessment documents totalling around 2,500 pages represent the culmination of more than four years' work by a team of over 20 consultants.
The impacts associated with a Normal Evolution Scenario and four Disruptive Scenarios were evaluated using the AMBER code, supported by the
FRAC3DVS-OPG and T2GGM codes for detailed groundwater and gas calculations.
The postclosure safety assessment forms part of the supporting documentation for the DGR's Environmental Impact Statement and Preliminary
Safety Report which were submitted for regulatory review in April 2011. The submission is necessary to secure an approved environmental
assessment and a site preparation and construction licence. Following the appointment of a Joint Review Panel, a public comment period
will be announced and a public hearing will be subsequently held by the Panel (anticipated to be in 2012), where stakeholders will have
the opportunity to present their feedback on the DGR.
For more information regarding the postclosure safety assessment, please contact Richard Little.
Evidence Support Logic for Leak Mitigation
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Quintessa is currently working with Jacobs Engineering (UK) to provide decision support to the Magnox Swarf Storage Silo decommissioning project at Sellafield.
The Magnox Swarf Storage Silo comprises a series of water-filled concrete waste silos, into which predominantly intermediate level wastes were consigned between 1964 and 1991. Decommissioning of the facility, including the retrieval of past disposals of Magnox fuel cladding, is one of the most complex and difficult challenges for Sellafield Limited.
Defining an approach to waste retrieval that minimises the potential for future leaks to occur during retrieval operations is a key component of the overall safety management strategy for decommissioning. There is also a need to define a suitable system for responding to and mitigating the impact of such leaks, should they occur. The aim of the present study is to support the Sellafield project team in reviewing and updating plans for mitigation of possible leaks to ground.
We are applying Evidence Support Logic, implemented using our TESLA software, to examine alternative approaches to mitigation and to assess the confidence with which they can be expected to achieve defined goals. This is a facilitated process, in which we work with experts at Sellafield, as well as external specialists, to develop a rationale for defining interactions between lines of evidence relevant to determining the preferred strategy. The intention is to build consensus by engaging different perspectives in structuring the complex issues involved and interpreting the available evidence.
For more information regarding ESL and our decision support services, please contact Alan Paulley.
FORGE: Treatment of Gas in Performance Assessment
FORGE is an EC research project considering issues associated with the generation, transport and fate of gases generated in radioactive waste disposal facilities. The Radoactive Wastwe Management Directorate (RWMD) of the UK Nuclear Decommissioning Authority (NDA) is participating in this project, and Quintessa is providing technical support to RWMD under Work Package 1 (Treatment of Gas in Performance Assessment). This work package is focussed on the complex issue of understanding how to characterise and represent gas migration in a manner suitable to support whole facility performance assessments.
To date Quintessa has been using the QPAC code to participate in benchmarking, sensitivity and upscaling activities looking at the behaviour of generated gas (along with water, water vapour and dissolved gas) during the resaturation of a spent-fuel (SF) facility in an argillite host rock. Other organisations - including ANDRA, ENSI, IRSN, SCK-CEN and LEI - are using other software in their input to the project. The benchmarks have examined :
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- a single SF disposal cell considering waste, engineered disturbed zone, bentonite plug, backfill and explicit representation of potential interface structures between the different materials ('cell' scale) and, more challengingly,
- a 'module' scale benchmark in which 100 of the disposal cells are modelled explicitly including all of the features in the smaller scale benchmark in addition to the larger drifts.
A whole repository benchmark model is also planned, and it is expected that understanding gained in how process and property upscaling is best handled at the 'cell' and 'module' scale will be applied to this larger scale model.
For more information please contact Alex Bond.
Long Term Cement Studies (LCS): Tournemire Analogue Modelling
The LCS project is a collaboration between JAEA, Nagra, NDA, Posiva and SKB, aimed at increasing our understanding of the interactions of cement with other engineered barrier materials and with potential host rocks. Quintessa (in association with Savage Earth Associates) has been working in collaboration with the NDA on LCS for a number of years, providing geochemical modelling support using QPAC.
The latest phase of the LCS project has involved using data from natural and industrial analogue sites such as Tournemire (France) as a basis for testing our understanding, data and models for cement and concrete based systems and cement-rock reactions. At Tournemire boreholes have been drilled into the base of a tunnel and subsequently filled with concrete. This has been in contact with a mudstone for a period of 15-20 years, and a volume fraction plot (white indicates porosity) of the simulation results after 15 years of alteration can be seen on the left. Work has been undertaken by IRSN in France to overcore these boreholes and analyse samples in the laboratory, an example of which can be seen below. The mineralogical alteration is clearly visible. Further information can be found on the project page on the IRSN website.
Each of the LCS modelling teams has applied their own geochemical models and codes to the Tournemire site. An important part of this modelling exercise is determining which processes are important within the system, such as mineral dissolution/growth kinetics, ion exchange and surface complexation. The modelling work by Quintessa has produced a good fit to the observations of the mineral characterisation studies, with ion exchange and surface complexation playing an important role in regulating the extent of the pH plume in the mudstone. Further details of the LCS Project can be found on the GTS website. For more information please contact Claire Watson.
Biosphere Studies Supporting the NDA RWMD Disposal System Safety Case
Over the past four years, Quintessa has been providing biosphere assessment support to the Nuclear Decommissioning Authority (NDA) Radioactive Waste Management Directorate (RWMD) safety case programme relating to geological disposal of the United Kingdom's higher activity radioactive wastes. The work has produced a substantial number published reports that are available via the NDA RWMD bibliography, covering topics including:
- Context and approach to future biosphere calculations;
- Biosphere calculations for different geological contexts;
- Representation of climate states and climate state transitions taking key FEPs into account;
- Geosphere-biosphere interface zone issues, catchment-scale modelling and distributed biosphere modelling;
- Representation of redox variations in the soil zone in the context of assessment modelling;
- Catchment modelling in support of post-closure performance assessment;
- Radiological screening;
- Identification of key radionuclides;
- The biosphere transport, distribution and radiological impact of Cl-36, Tc-99 and I-129 released from a geological disposal facility;
- The biosphere transport, distribution and radiological impact of Se-79 released from a geological disposal facility; and
- The biosphere transport, distribution and radiological impact of C-14 released from a geological disposal facility.
The work has been drawn together with more than 25 years' experience of biosphere studies in support of geological disposal in a recent Biosphere Status Report in support of the NDA RWMD's generic Disposal System Safety Case (DSSC). Further reports are in the process of being finalised, covering:
- Approaches to biosphere site characterisation;
- The biosphere transport, distribution and radiological impact of uranium series radionuclides from a geological disposal facility; and
- Biosphere assessment model enhancement.
For more information, please contact Russell Walke.
Coupled Modelling of Buffer Erosion and Copper Canister Corrosion
Bentonite buffer erosion as a consequence of intrusion of dilute glacial meltwaters has been identified as a process that could lead to increased corrosion of copper canisters. The enhanced hydraulic gradients that may exist during periods of glacial retreat can lead to advective fracture flow conditions and where such fractures intersect deposition holes there is the possibility for bentonite to form colloids in the dilute waters and be transported away. This erosion of the bentonite could lead to a lowering of the buffer density, and a reduction in buffer swelling potential and transport resistance, resulting in enhanced transport of corrosive agents to the canister surface than for 'normal evolution' scenarios.
Fully coupled 3-D models of the hydro-mechanical-chemical erosion and corrosion system in the EBS and neighbouring fracture were developed for SSM using Quintessa's general purpose coupled modelling code, QPAC. The models were constructed with the aim of demonstrating the effect of the couplings within the system and accordingly some processes were modelled in a simplified fashion. For example, the model represented the mechanical intrusion of bentonite gel into fractures and redistribution of bentonite within the deposition hole using a nonlinear diffusion equation rather than deploying a fully mechanistic model for each process. This allowed the geometry of regions of altered density that form in the bentonite after sustained periods of erosion to emerge from the model rather than being imposed. Cyclic periods of glaciation were simulated and results demonstrated that cumulative effects of multiple glaciations would be required before the formation of significant regions of low density or cavities is possible, and that the resulting corrosion profiles on the canister surface are uneven corresponding to the geometrically distributed corrodent supply.
For more details, please contact Steven Benbow.
2-D Reactive Geochemical Transport Modelling of an EBS
An essential activity in assessing the performance of Engineered Barrier Systems (EBSs) for disposal of radioactive waste is to demonstrate understanding of the long-term evolution of the construction materials and the neighbouring host rock, and to infer the consequences of any alteration to long-term performance. Typical EBS designs incorporate engineering materials such as metals (typically steel or copper), clay buffer materials such as bentonite or sand/bentonite mixtures, cement and concrete for tunnel seals and fracture grouting and processed (e.g. crushed) quantities of host rock. Their interactions with one-another, and with the natural host rock barrier, will dictate the long-term stability and efficacy of the EBS. In particular, chemical material alteration can impact upon transport properties and result in perturbation of diffusion coefficients and permeability due to changes in porosity and/or pore structure.
In a recent feasibility study for Andra, Quintessa developed 1-D and 2-D models of EBS interactions for their HLW disposal concept in COx clay host rock using the general purpose coupled modelling code, QPAC, and its reactive transport module. The models included iron - bentonite, bentonite - cement, bentonite - COx and cement - COx interfaces in a single 2-D simulation and included full coupling of transport and kinetic reaction processes in the clay/rock features, and treatment of redox evolution around iron surfaces undergoing corrosion. The models also included a novel treatment of transport and reaction in and around regions in which porosity had become clogged due to cement carbonation, through the addition of a custom porosity/reaction model.
For more details, please contact Steven Benbow.