N.P. Taylor

Re-evaluation of the use of low activation materials in waste management strategies for fusion

Abstract The world fusion programs have had a long goal that fusion power stations should produce only low level waste and thus not pose a burden for the future generations. However, the environmental impact of waste material is determined not only by the level of activation, but also the total volume of activated material. Since a tokamak power plant is large, the potential to generate a correspondingly large volume of activated material exists. The adoption of low activation materials, while important for reducing the radiotoxicity of the most active components, should be done as part of a strategy that also minimizes the volume of waste material that might be categorized as radioactive, even if lower in level. In this paper we examine different fusion blanket and shield designs in terms of their ability to limit the activation of the large vessel/ex-vessel components (e.g. vacuum vessel, magnets) and we identify the trends that allow improved in-vessel shielding to result in reduced vessel/ex-vessel activation. Recycling and clearance are options for reducing the volume of radioactive waste in a fusion power plant. Thus, the performance of typical fusion power plant designs with respect to recycling and clearance criteria are also assessed, to show the potential for improvement in waste volume reduction by careful selection of materials’ combinations. We discuss the impact of these results on fusion waste strategies and on the development of fusion power in the future.

Lessons learnt from ITER safety & licensing for DEMO and future nuclear fusion facilities

Abstract One of the strong motivations for pursuing the development of fusion energy is its potentially low environmental impact and very good safety performance. But this safety and environmental potential can only be fully realized by careful design choices. For DEMO and other fusion facilities that will require nuclear licensing, S&E objectives and criteria should be set at an early stage and taken into account when choosing basic design options and throughout the design process. Studies in recent decades of the safety of fusion power plant concepts give a useful basis on which to build the S&E approach and to assess the impact of design choices. The experience of licensing ITER is of particular value, even though there are some important differences between ITER and DEMO. The ITER project has developed a safety case, produced a preliminary safety report and had it examined by the French nuclear safety authorities, leading to the licence to construct the facility. The key technical issues that arose during this process are recalled, particularly those that may also have an impact on DEMO safety. These include issues related to postulated accident scenarios, environmental releases during operation, occupational radiation exposure, and radioactive waste.

Preliminary Safety Analysis of ITER

Abstract In order to support the licensing application for the ITER facility at Cadarache, a preliminary safety case has been prepared and submitted to the French nuclear safety authorities. This paper provides an overview of technical aspects of this case, which is based on an evolution of the safety approach developed and applied in earlier phases of the ITER project. The basis of the safety of ITER derives from the fundamental safety characteristics of fusion. The potential radiological hazards that arise are related to the tritium fuel and material activated by neutrons. The confinement of these materials is therefore the principal safety function, and it is reliably provided by robust barriers inherent in the design together with filtering and detritiation as a secondary level of confinement provision. A Defense in Depth approach is taken to ensure that off-normal events are minimized in their frequency, and that the consequences of accidents, even though extremely unlikely, are limited. A comprehensive set of analyses of postulated event sequences provides the demonstration that the consequences of enveloping scenarios are well within acceptable limits, and that even for hypothetical events involving two or more independent failures, the public and environmental impacts remain limited. An ALARA approach is taken to minimizing occupational radiation exposure, as well as other potential impacts of normal operation such as routine releases. Other hazards arising from internal and external risks are also considered, with design provisions, for example the Tokamak building is built on seismic isolation pads to minimise the effect of an earthquake.

The impact of materials selection on long-term activation in fusion power plants

Abstract Neutron-induced transmutation of materials in a D–T fusion power plant will give rise to the potential for long-term activation. To ensure that the attractive safety and environmental characteristics of fusion power are not degraded, careful design choices are necessary. An aim of optimising power plant design must be to minimise both the level of activation and the total volume of active material that might ultimately be categorised as waste requiring disposal. Materials selection is central to this optimisation. In this paper we assess the influence of materials choices for a power plant on the waste volume and the potential to clear (i.e. remove from regulatory control) and recycle material. Although the use of low activation materials in regions of high neutron flux is an important part of the strategy to minimise the level of activation, different choices may result from a strategy aimed at minimising the volume of active waste.

Results, conclusions, and implications of the SEAFP-2 programme

Abstract Fusion power stations inherently will have no actinides or fission products, extremely low levels of nuclear energy, and low levels of decay heat power. With appropriate design and material selection, these favourable inherent features could give rise to substantial safety and environmental advantages. Analyses performed within the SEAFP-2 project of the European fusion programme have shown that it should be possible to design commercial fusion power stations so that • the maximum doses to the public arising from the most severe conceivable accident driven by in-plant energies would be at the milliSievert level — well below the level at which evacuation would be considered; • after a few decades, most, perhaps all, of the activated material arising from the operation and decommissioning of the plant could be cleared or recycled, with little, or no, need for repository disposal; • the above goals can be achieved by using relatively well-developed and near-term low-activation martensitic steel as structural material. The results supporting these conclusions are summarised in this paper. The detailed lessons learnt will be input to a future European conceptual study of commercial fusion power stations.

European Fusion Power Plant Studies

Abstract The European Power Plant Conceptual Study (PPCS) reported in the summer of 2004. Several conceptual designs (“Models”) for commercial fusion power plants were developed, spanning a range from relatively near term to more substantial extrapolations. The parameters of the Models were chosen by systems analysis to be economically optimal, given the assigned constraints on plasma and technology performance. The conceptual designs were developed in some detail and analyses were made of their safety, environmental impacts and economic performance. The calculated cost of generating electricity from the Models is in the range of published estimates for the future costs from other sources. Even the near-term Models are economically viable. External costs are very low, for all the Models: similar to wind power and much less than for fossil fuels. Economic optimization of the designs did not jeopardize their safety and environmental performance. All the Models proved to have the attractive and substantial safety and environmental advantages found in earlier studies, now established with greater confidence.

A model of the availability of a fusion power plant

The availability of a fusion power plant is dependent on the frequency and duration of both planned and unplanned outages. Planned outages result from a maintenance plan for major component replacement, and are entirely predictable once divertor and blanket lifetimes are established. Unplanned outages result from failures of one or more of many diverse components, and can be estimated only by a probabilistic approach taking into account distributions of failure frequencies and repair times. A Monte Carlo model, PAMPAS, has been developed which will enable studies of new conceptual power plant designs once sufficient design detail and component reliability data are available. It can already be used for more generic studies and results are shown which give insight into the magnitude of the problem of achieving adequate plant availability. A simplified model of plant availability has been incorporated into the PROCESS systems code, in order to assess the economic impact of plant unavailability. Initial results show a strong influence on optimum plant parameters dependent on the speed of maintenance operations. This work is continuing as part of preparations for a European fusion power plant conceptual study.

Updated Modeling of Postulated Accident Scenarios in ITER

Abstract This paper gives an overview of the latest work on ITER accident analysis, describing the methodology and presenting some updated results. There are currently 25 ITER Reference Events, divided into two categories: incidents and accidents. Starting from the 2001 list of events, several new scenarios have been added, including fire events. Other former Reference Events have been updated and in some cases fully re-analyzed due to design modifications, such as changes in the confinement arrangements. The results demonstrate that the ITER General Safety Objectives are met and that the safety features of the ITER design are successful in minimizing the potential public and environmental consequences of off-normal events.

Structural load specification for ITER tokamak components

The substantial mechanical loads which can develop in multiple components are a major technical challenge associated with the design of the ITER tokamak. The various loads acting on ITER can be grouped into several types: inertial loads, associated with gravity and seismic events; pressure loads, particularly significant for the ITER pressure equipment; electromagnetic loads, which affect all conducting structures as a consequence of transient events inducing rapid magnetic field changes and which possibly involve currents flowing between the plasma and in-vessel components; thermal loads, which are extremely severe in the plasma facing components; assembly loads, typically due to preloads imposed during assembly.

UPDATED ACCIDENT CONSEQUENCE ANALYSES FOR ITER AT CADARACHE

Abstract Throughout the various phases of the ITER project, extensive safety analyses have been performed to ensure that potential hazards to the public, the environment, and personnel are minimized. This work, done before a location for ITER had been chosen, resulted in a very comprehensive assessment of ITER safety in terms of the impact at a “generic site”. By making good use of the favourable safety and environmental characteristics of fusion, a very good outcome was achieved. Now that the Cadarache site, in southern France, has been selected for ITER construction, it is necessary to reanalyze the impact of postulated accidental releases of tritium and activated material, taking into account the specific conditions of the site. These include regulatory requirements on dose limits and on assumptions to be made in analyses, as well as local environmental factors such as weather conditions, population demographics, and local food production and consumption patterns. This paper discusses the impact on the ITER safety case of new dispersion and dose calculations for accidental releases, taking into account these site-specific conditions. These indicate that doses arising from the release masses calculated for the most challenging accident scenario in previous generic-site studies will meet the new dose limits by a very large margin.