Metrofission: the future?
Alan Dinsdale and Lindsay Chapman from the Materials Division at the UK’s National Physical Laboratory discuss a measurement framework for future nuclear energy.
Friday 11 March 2011 will stay in minds around the globe for decades to come – the 9.0 Magnitude Tohuku earthquake struck 70 kilometres off the Japanese coast, approximately 32 kilometres below Earth’s surface. One of the five most powerful earthquakes ever recorded, the tsunami it caused resulted in waves more than 40 metres high that travelled up to 10 kilometres inland.
The subsequent nuclear disaster at the Fukushima Daiichi plant served to highlight the very real difficult ies of obtaining reliable measurements in such harsh environments. These environments will become even more extreme as nuclear technology develops and we migrate from current Generation II type reactors towards so-called Generation IV type reactors in perhaps 20 years’ time.
Within the EU, there are approximately 140 nuclear reactors, generating about 30% of the electricity and about 15% of the energy consumed in the EU. The average age of these reactors is about 25 years, and their overall expected lifespan is perhaps four decades. In spite of the Fukushima disaster, most countries in the EU are expected to build new nuclear reactors in the next 20–25 years. Many of these will represent incremental improvements on current nuclear technology (Generation III), but a number will look towards the new Generation IV designs that offer greater efficiency and safety.
Although several di¬fferent designs for Generation IV reactors are currently on the table, they will all have a number of features in common. Each will operate at relatively high temperatures (above 1,000°C), and this will require a new type of thermocouple that can withstand the higher neutron flux generated in these reactors. In addition to this, better characterisation of the heat transfer and thermal expansion properties of the containment materials used in the reactor will be essential. There will be a deliberate aim to keep decay and transmutation products within the fuel to limit its potential as a source of weapons-grade material. Furthermore, many Generation IV reactors will incorporate facilities for the reprocessing of spent nuclear fuel.
The National Physical Laboratory is one of 12 major European National Measurement Institutes collaborating to create the measurement techniques and compile the data necessary to support the development, construction and operation of future Generation IV nuclear technology. This Metrofission project is part of the European Metrology Research Programme (EMRP) of coordinated research and development that facilitates closer integration of national research programmes. The project began in September 2010 and will last for three years. It brings together expertise in radiation science, thermal metrology, materials, and multidisciplinary contributions from across Europe to enable the lifetime and efficiency of current generation nuclear power plants to be maximised. The project will also put in place a framework for a measurement infrastructure to support the development of future nuclear power in Europe.
Three key challenges were identified at the onset of the project:
- How can temperatures be measured, since existing thermocouple-based methods will not work in the temperatures and environments expected in these advanced nuclear reactors?
- What materials should be used in these reactors and what are their thermal properties?
- Are nuclear property data sufficiently accurate and what radionuclide and neutron metrology techniques will be needed in the future?
We will now focus on the second of these challenges. There are six main reactor technologies proposed for Generation IV reactors developed through the Generation IV International Forum (GIF), a collective representing 13 countries committed to the development of nuclear power. Each of these designs offers different combinations of nuclear fuels and coolants:
- gas-cooled fast reactor
- lead-cooled fast reactor
- molten salt reactor
- sodium-cooled fast reactor
- supercritical water-cooled reactor
- very high temperature gas reactor
Three of these reactor types are supported within the EU – the sodium-cooled fast reactor (Astrid), the gas-cooled fast reactor (Allegro) and the lead-cooled fast reactor (Myrrha). All these reactor types will require the development of new measurement techniques, and most of the work carried out in the Metrofission project will be applicable to whichever reactor type is selected. The exception to this is in the development of thermodynamic properties where work is focused on the sodium-cooled fast reactor.
One aim of the project is to obtain an in-depth understanding of the high temperature thermal behaviour (for example, heat capacity, thermal di¬ffusivity, thermal expansion and emissivity) of the structural and refractory materials to be used within the nuclear reactors. This applies to both the original material, following irradiation and also under accident conditions where the temperature could be thousands of degrees higher than normal.
Many of the partners in Europe have existing facilities such as dilatometers, differential scanning calorimeters, drop calorimeters and laser-flash facilities for accurate measurement of thermophysical properties, although most do not currently cover the full temperature range of interest. Clearly it is not possible to perform measurements on irradiated materials or nuclear fuels, except within special designated facilities that are available only within nuclear research institutes such as at JRC-ITU in Karlsruhe, Germany or the Commissariat à L’Energie Atomique in France. To allow this to happen, the Metrofission project’s focus is to develop transfer reference materials for high temperature thermal properties certified up to 2,000°C. These will be materials such as graphite, or a refractory alloy, based perhaps on tungsten or molybdenum, which have thermal properties similar to those used in fission reactors.
Another aim of the Metrofission project is the development of critically assessed data for the nuclear fuel and its interaction with the coolant, containment materials and the environment. The use of chemicalthermodynamic data is well established in the nuclear sector and the various compilations of data available thus far have been mainly concerned with the properties of stoichiometric crystalline materials and gas phase species. However, to model complex nuclear processes it is necessary to model the thermodynamic properties of phases over ranges of both temperature and composition. This requires use of the CALPHAD technique, where critically assessed thermodynamic data for component binary and ternary systems can be extrapolated to multicomponent systems and then software such as MTDATA used for calculations of the phase diagram. The benefits of this approach have already been demonstrated for a wide range of industrial problems, and the topic will underpin a special session on phase diagrams at the IOM3 Materials Congress 2012.
Work on thermodynamic properties within the Metrofission project is focussed on data to model potential interactions between the MOX fuel in the sodium cooled fast reactor, the liquid sodium coolant and the environment and will complement other activities within Europe such as the FUELBASE project.
These developments are part of ongoing work on materials within an EMRP Joint Research Project aimed at providing solutions to metrological challenges in relation to nuclear new build and specifically Generation IV reactors. While the risks of nuclear accidents remain very low, avoiding another Fukushima is a key priority for all working in the nuclear industry.
Calculated phase diagrams for the U-O and Pu-U systems from critically assessed thermodynamic data. Data generated within the Metrofission project will allow prediction of the complex chemistry associated with nuclear fuels during their use under a wide range of conditions.