The nuclear generation - challenges for the future of nuclear power
Paul Mummery from The University of Manchester, UK, and Peter Flewitt from Magnox North Ltd, UK, members of the IOM3 Energy Materials Group, describe the future for nuclear energy.
With most of the UK nuclear reactors approaching the end of their working lives, a fission reactor new build programme is under consideration. The new build plant is based on proven technology incorporating robust safety systems. The Materials UK Energy Review (see pp38-39 this issue) sets out a range of material challenges for current plants in the UK and future developments. Here, UK-based experts describe these challenges and the progress in three specific areas – current plant, nuclear waste, and future fusion.
Professor Andrew H Sherry, Executive Director of the Dalton Nuclear Institute at The University of Manchester, says to maintain secure energy supply, British Energy Ltd has declared life extensions for advanced gas-cooled reactors (AGRs) that will prolong nuclear generating capacity for between five and 10 years. Extending light water reactor (LWR) plant life is being considered worldwide, with research aimed at technically underpinning 60-year lifetimes. In service degradation of structural materials is the main limiting factor to life extension and there are similarities with the challenges facing new build fission reactors.
Weld degradation is a concern in high temperature reactor applications. For non-stress relieved austenitic welds in AGR boilers operating above 500ºC, local residual stress and plastic strain can lead to reheat crack development. Research is focused on quantifying residual stress profiles by improving measurement and modelling techniques, predicting creep damage evolution under multiaxial stress states, and refining defect assessment procedures to quantify weld integrity.
In graphite-moderated reactors, the graphite core (see image below, left) acts as a structural component supporting fuel assemblies and control rods. During operation, graphite undergoes property changes due to temperature variations, fast neutron dose, and radiolytic oxidation or weight loss. New techniques are being used, including X-ray tomography and high resolution electron microscopy, to understand how microstructural changes influence structural properties (see image overleaf). Findings are being implemented into models that help elucidate the mechanisms that lead to graphite property changes.
Neutron irradiation of the ferritic steel components leads to atomic-scale degradation, including matrix damage, phase development and segregation. This embrittles the steel, increasing the ductile-to-brittle transition temperature. Three-dimensional atom-probe studies and high resolution electron microscopy are providing new insights into damage development and can help create physically-based multiscale models that relate atomic-scale damage to macroscopic fracture behaviour.
Irradiation of austenitic components leads to grain boundary chromium depletion which, combined with matrix hardening, tensile stress and modified water chemistry through radiolysis, can increase susceptibility to stress corrosion cracking (SCC). This challenges LWR life extension because of the limited availability of materials data and the need for a more detailed understanding of the key factors, including cold work, that influence SCC. Research is focusing on improving knowledge through near service-condition testing, high resolution electron microscopy of oxide structures at crack-tips and predictive model development.
Research in the UK and overseas is aimed at technically underpinning robust life extension arguments for nuclear plant by establishing reliable materials data, improved mechanistic understanding and predictive capabilities for these four primary degradation issues. International cooperation is enhancing developments by expanding materials databases, providing a forum to share understanding and enabling the review of predictive methods by peers. Moreover, the latter will also contribute to supporting new build plant.
Professor Bill Lee, Head of the Department of Materials at Imperial College London, notes that globally, we have been slow to deal with the waste legacy of military and civil nuclear programmes. Due to its early entry into the nuclear field, development of a wide range of reactors and significant military programmes, the UK has a complex array of waste types, many in large volumes. The Government instigated the Managing Radioactive Waste Safely (MRWS) programme, which involved setting up the Committee on Radioactive Waste Management (CoRWM) in 2004. In July 2006 CoRWM recommended geological disposal as the end point for long-term management of radwastes. The UK’s Nuclear Decommissioning Authority, established in 2005, is responsible for clean up of contaminated sites and for implementing geological disposal of higher activity wastes.
The MRWS White Paper published in June 2008 shows the process and stages leading to geological disposal and is at the first stage where communities are informed of the process and may express interest in hosting a geological disposal facility (GDF). The process of choosing, investigating and, if appropriate, building and filling such facilities will take many decades and is an engineering project on the scale of the Channel Tunnel.
There are many opportunities for materials scientists and mining engineers to collaborate and contribute to this flagship project for the UK. The UK has unique wastes for which suitable host matrices have yet to be found. This includes over 100t of plutonium – which could be reused in MOX (mixed uranium and plutonium oxide) fuel or immobilised in a ceramic host – and over 80,000t of irradiated graphite.
Research is needed to understand how cemented intermediate level wastes and vitrified high level wastes behave over extended periods, many decades in storage and then millennia in the disposal environment. While store atmospheres can be controlled, we cannot rely on this for the GDFs.
The reactions between the wasteforms, their buffer and backfill materials (cements and clays), the near field rock, engineered and far field geological environments must be understood and predicted. Modelling over such enormous length and timescales (over hundreds of thousands of years) is a great opportunity for the materials and geological theory and simulation communities. In addition, the UK needs to regain its world-leading position on experimental studies of radioactive materials using new and improved national facilities.
Key issues over long timescales associated with co-disposal of such wastes include interaction of corrosion products with the geosphere, radionuclide transport and retardation mechanisms via colloids, natural organics and microbes, gas generation, the alkali disturbed zone and the engineered disturbed zone. The predictability of such behaviour will be an important aspect of the safety for these facilities.
Professor Roy Faulkner of the Materials Department at Loughborough University, explains that a major breakthrough in the development of fusion as an energy source occurred in 2003 when an international agreement was reached to build an experimental Tokamak reactor at Cadarache in France. The large scale of the device, known as ITER (International Thermonuclear Experimental Reactor), will allow the possibility, for the first time, of making a fusion system that will produce more energy than it consumes. The system operates on the principle of confining a toroid shaped plasma in a huge magnetic field, so that the deuterium-tritium fusion reaction takes place on a large scale.
The plasma must be at temperatures approaching 100 million degrees Celsius. A magnetic field confines the plasma but eventually solid engineering materials have to maintain the integrity of the whole design. The energy transfer is huge at four megaWatts per square metre. Temperatures can rise to 600°C, and the blanket region is subject to radiation damage from 14.1MeV neutrons with a flux of 10-6dpa/s. The first wall of the system confronting the plasma consists of an arrangement of thin tungsten tiles, which act as an ablative screen. The real engineering begins underneath these tiles where the so-called blanket is placed to provide the framework of a heat exchanger and to give the necessary strength to support the whole assembly. The reactor is 25m high and 30m in diameter, meaning high strength, high temperature materials are required. Ferritic/martensitic steels, vanadium alloys and silicon carbide are being evaluated as potential blanket materials.
The main challenges are to find a material for the blanket that will withstand high temperatures, have low waste disposal problems and resist thermal fatigue because the system operates on a pulsed basis. It must also resist neutron irradiation damage at levels 10 times higher than for current fission reactors. In the latter, the higher energy of the neutrons compared with fission neutrons means large amounts of helium are generated leading to substantial embrittlement.
Ferritic martensitic steels have the advantage of low void swelling during neutron irradiation, which is a problem for designs that experience high neutron doses. Unfortunately, these steels suffer a ductile to brittle transition temperature increase of up to 250°C during irradiation (see graph above).
The waste disposal problem is likely to be overcome because the philosophy of reduced activation is being employed. This means that elements with a high radioactive half-life, such as cobalt, nickel, molybdenum, and niobium, are removed from the first wall material. For metallurgists developing ferritic/martensitic steels this philosophy presents further challenges. Oxide dispersion strengthening is also being considered to increase the high temperature strength of the steels.
Initially, ITER, and its prototype successor DEMO, will almost certainly use ferritic/martensitic steels for the blanket, although the first segments of ITER will use the 316 stainless steel. Research on SiC/SiC composites and vanadium alloys is at an earlier stage than the steels, but is of importance for future systems (see also pp14-15 this issue).
Further information: Paul Mummery