Quintessa Update - October 2010

Quintessa Update October 2010 Header Image

Welcome to the October 2010 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:

Dounreay Site Closure Strategy

Dounreay in the north of Scotland is one of the most complex nuclear sites in the UK. Restoring the site involves the clean-up and demolition of the legacy of over 50 years work on fast reactor technology. In 2000 Dounreay was the first nuclear site in the UK to publish a site restoration strategy. Quintessa staff helped Dounreay to develop the original restoration strategy, and continue to assist with its refinement, primarily focusing on optimising the management of waste and contaminated land, and evaluating the long-term safety of the site. This has included:

Development of an Environmental Safety Case (ESC) for the Dounreay site that builds confidence that the restored site will meet safety and environmental targets, and provides inputs to improving the strategy;
Assistance with refining the site's restoration strategy taking into account stakeholder views as well as advising on the implications of regulatory developments, assessing the long-term safety of clean-up options, and seeking to optimise the programme of restoration activities;
Using mathematical models to improve the understanding of the Dounreay environment and the behaviour of radionuclides and other contaminants, to provide perspective on what aspects are important (e.g. the significance of coastal erosion), and to assess long-term health risks to hypothetical future users of the site; and
Technical and feasibility studies including the facilitation and documentation of internal workshop sessions to explore strategic management options, and participation in a number of stakeholder panel workshops.

Dounreay Site Restoration Limited (DSRL) has now begun a new phase of work that involves translating the End State vision into viable objectives for the closure of the site, and Quintessa has recently been awarded a contract to support this work over the next three years. The planned work will involve working closely with DSRL to:

Develop an understanding with the regulators and other stakeholders of the clean up objectives;
Put in place procedures and tools to enable DSRL to optimise its closure plans; and
Establish a well-founded case that measures currently being planned and implemented will result in the closed site achieving the required standards for safety in the long term.

The image (copyright DSRL and NDA) is an artist's impression of how Dounreay could look towards the end of the decommissioning phase when only stored wastes and closed disposal facilities remain and just prior to dismantling the DFR Sphere and ancillary buildings. For more information, please contact James Penfold.

Mathematical Modelling Services using QPAC

Mathematical modelling lies at the heart of Quintessa. Our core competence is in the formulation, development and application of innovative mathematical models and associated scientific software. Over the past five years, these services have been progressively focused around QPAC, Quintessa's in-house general-purpose modelling tool. QPAC differs from traditional computer codes, where the mathematical model is hard-wired, by adopting a flexible 'model as input' approach. This requires the user to input the equations to be solved along with the model parameter values and spatial discretisation. In this way complex coupled nonlinear models can be developed, where parameters can be time-dependent and variables can depend on each other, at a range of scales from detailed process models to high-level systems models. Having such a flexible tool at our disposal is invaluable for rapid prototyping exercises and investigating conceptual model uncertainty.

A number of modules have been developed for use with QPAC, which encapsulate commonly used physical processes such as multi-phase flow, radionuclide transport through porous media, heat transfer, geochemical and mechanical processes. These allow QPAC to operate in a mode more akin to a traditional modelling code, where the user simply specifies input parameter values, boundary conditions and the system discretisation. However, where QPAC differs from these traditional codes is that additional user-defined processes can easily be coupled to existing modules. Recently added features in QPAC 3.0 include:

QPAC Player, which provides a simple user interface to QPAC models. This allows clients and collaborators to reproduce results and perform their own sensitivity analysis, without requiring knowledge of the QPAC input language.
Enhanced tools for system discretisation, including the use of unstructured grids, which allow holes to be punched into Cartesian or polar grids or irregular edges to be specified.
Weak as well as strong couplings can now be included, by reloading results obtained from existing QPAC simulations.

QPAC is developed under Quintessa's ISO:9001, TickIT approved Quality Management System for software development. For more information, please download the QPAC Overview Report or contact Alex Bond.

Acceptable Release Rates to the Biosphere for the Disposal of Graphite

In 2008 a four-year European project was launched under the 7th EURATOM Framework Programme (FP7-211333) to address the 'Treatment and Disposal of Irradiated Graphite and other Carbonaceous Wastes (CARBOWASTE)'. The Radioactive Waste Management Directorate (RWMD) of the Nuclear Decommissioning Authority (NDA) is actively involved in the project, and commissioned Quintessa to undertake a modelling exercise to determine acceptable radionuclide release rates to the biosphere from disposed irradiated graphite. The intention of the study was not to assess particular features of an irradiated graphite waste disposal system, only to assess their required containment performance. As such, the cautious, scoping nature of the study is emphasised.

The study made generic and simplified assumptions for assessments of performance relating to treatment of waste graphite, engineered barrier systems (EBS), the geosphere and the biosphere. These assumptions were generally pessimistic, with factors which would improve the EBS and geosphere performance (e.g. sorption of radionuclides, path length to the biosphere) being considered in variant calculations. Consideration was given to groundwater and gaseous releases of radionuclides, with a focus upon C-14 and Cl-36, and also to human intrusion. A range of biosphere dose assessment models for C-14 and Cl-36 was considered, including those currently used by NDA RWMD, and those being investigated within the international cooperation programme BIOPROTA. Copies of the report are available from NDA RWMD upon request. For more information, please contact Laura Limer.

Understanding Controls on the Performance of Engineered Barrier Systems in Repositories for High-level Radioactive Waste and Spent Fuel

The Environment Agency recently issued a report prepared by Quintessa that explores the factors that might influence the long-term safety of an underground repository to dispose of high-level waste (HLW) and spent fuel (SF), focusing on the role of the engineered barrier system (EBS). The implications for waste form design, waste packaging and repository design, were examined by analysing our current understanding of processes that could influence the long-term performance of a repository.

The project carried out a literature review of disposal systems proposed by radioactive waste management programmes throughout the world, choosing some representative disposal concepts to illustrate the range of controls on performance. It also reviewed safety functions attributed to different components of the EBS, and identified groups of features, events and processes (FEPs) that describe these safety functions and threats to these safety functions. Simple computational models (see Figure) were used to explore the significance of each of these FEPs as controls on the performance of barrier components. Copies of the report and a two-page summary can be downloaded from the Environment Agency web site. For more information, please contact Richard Metcalfe at Quintessa or Claire Cailes at the Environment Agency.

Review of the Corrosion Performance of Selected Metals as Canister Materials for UK Spent Fuel and/or High-level Waste

The Radioactive Waste Management Directorate (RWMD) of the Nuclear Decommissioning Authority (NDA) has published independent research by Quintessa reviewing the performance of five potential metals that could be used in the manufacture of canisters for the disposal of high level waste (HLW) and spent fuel (SF) in a geological disposal facility. At this stage no site for a facility has been selected and Government is leading the search for a site based on voluntarism and partnership with potential host communities. The research therefore considers the use of the metals in a variety of geological environments and focuses particularly on the period following the eventual closure of a facility. The potential canister materials are copper, carbon steel, titanium alloys, nickel alloys and stainless steels. Lead was also considered, as a material to be used inside the canister. According to Cristiano Padovani, RWMD's Corrosion Specialist: "This research provides us with information that will be of use to us for the option studies we will carry out in the future when selecting an engineered barrier system (EBS) design to be used once a site has been selected." Copies of the report are available from NDA RWMD upon request. For more information, please contact Sarah Watson.

Innovation Rewarded

Quintessa takes pride in its ability to apply innovative techniques to address difficult real-world problems.

Recent projects have demonstrated the power of Statistical Modelling methods employing both ‘frequentist’ and Bayesian approaches to tackle the related issues of Uncertainty and Variability that are ubiquitous in model predictions for complex systems.

A paper describing work in this area for EDF Energy has been accepted for publication in the Journal of the Royal Statistical Society (Statistical Modelling of Graphite Brick Cracking in Advanced Gas-Cooled Reactors by Philip R Maul, Peter C Robinson and Paul Northrop, JRSS in press).

Another technique that we have applied in several projects is Genetic Algorithms. For many problems this powerful approach to optimisation makes searching through huge numbers of possible cases practicable. A particular area that we have developed is Portfolio Analysis, where the optimisation involves searching for a best combination of items rather than a single one. There is a large literature on this topic in the financial sector, but there are also wider applications to problems in the energy and environment sectors.

In addition to applying these approaches for our existing clients, we have recently entered several competitions advertised on the internet. Peter Robinson was announced as an Award Winner by Innocentive after submitting a solution to the challenge “Algorithms Optimized for Efficiency and Speed for Exploring and Testing Large Model Spaces”. Innocentive brings together companies with problems and innovators across the world in Open Innovation. Challenges cover a wide range of technical problems and innovators from across the world take part (200,000+ from 200 countries are registered). This success followed an earlier successful submission to Société de Calcul Mathématique in France for an efficient algorithm to calculate some high-dimensional integrals used in a novel approach to modelling extreme events (in this case high summer temperatures in Paris). For more information, please contact Peter Robinson.