Scientific and Technical Objectives
Improved temperature measurement for nuclear power plant applications
New temperature sensors and methods for in-situ measurements in nuclear power plants will be investigated and developed with an emphasis on extending the measurement range to higher temperatures and characterising, limiting, or completely eliminating the sensor drift in a high-temperature, high-neutron-flux environments. Reference fixed points above the copper point will be developed to perform post-irradiation studies of thermocouples. Suitable thermocouples from the Mo/Nb family will be identified, constructed, and studied with respect to their reference functions, stability, and thermoelectric homogeneity. The practicality of self-validation methods for thermocouples will be examined as a means to reduce or eliminate drift related problems. Practical primary acoustic thermometry, an inherently drift-less technique, will be developed through the construction of a demonstrator. If successful, this method could be used to assess the drift performance of more conventional sensors or as an in-situ temperature sensing method in its own right.
Thermal properties of advanced materials for nuclear design
Thermodynamic models will be developed and parameterised for a range of major and minor actinide containing systems relevant to nuclear fuels (both in reactor and during reprocessing) and coolants. These models will allow predictions related to the high-temperature phase equilibria, such as phase transformation temperatures, of these materials, which will partly be validated by comparison with experimental observations. These validated models will support the selection of the most appropriate materials for fuel and coolants of Generation IV Reactors.
Improved measurements and measurement techniques will be developed
to support high temperature characterisation of materials, for properties
relating to heat transfer. This includes specific heat capacity, thermal
diffusivity, thermal expansion, and emissivity. In some cases, these properties
can already be measured at high temperatures, however these measurements often
lack traceability to the SI above
800 °C. Therefore, reference
metrological methods will be implemented or improved for measurements up to
2000 °C (1500 °C in the case of specific heat of solids). Suitable
candidate materials to serve as "transfer reference materials" for high
temperature thermal properties will be identified and characterised. The
feasibility of applying a laser-flash technique to thermal diffusivity
measurements of molten salts up to 1000 °C will be investigated. These new
techniques will support the selection of the most appropriate materials to
safely construct Generation IV Reactors.
Nuclear data
Current nuclear databases of neutron cross-sections concentrate on thermal energies and much work is still required for higher energies. In this project, advantage is taken of the individual NMI neutron facilities (there are only a few world-wide) and the joint experience in neutron fluence measurements to improve cross-section measurements of high-energy neutrons. The objective is to establish an easy-to-use calibrated fluence standard and to demonstrate its potential for determination of cross sections of interest and the improvement of nuclear data in the form of reduced measurement uncertainties . In addition to the neutron data, alpha-particle emission probabilities of 238U with certified isotopic composition will be determined. This is the dominating fissile material in the fuel of new reactors and is a nuclide that has been highlighted in a recent review by the IAEA2 as an actinide that requires new and improved nuclear data. Furthermore, beta-particle spectrometry will be addressed using a unique, state-of-the-art cryogenic detector as a means to determine the shape of beta spectra that will subsequently verify and/or modify existing theoretical models. This might ultimately lead to improved calculations of decay heat produced by nuclear fission, a quantity important for safe reactor operation (e.g. the correct triggers for reactor shutdown and post irradiation handling of nuclear fuels).
Measurement techniques for radionuclides
The measurement techniques that are being developed within the project are aimed at performing on-site activity measurements, by the development of a portable Triple to Double Coincidence Ratio (TDCR) system. The fundamental theory of the TDCR method is that the detection efficiency is calculated using a model based on the detection of the triple- and double-coincidences of scintillation light emitted from a liquid scintillation solution (in which the radionuclide has been deposited). It is the only available detection method for beta particles (emitted from e.g. 3H and 241Pu) that deliver a result without preceding calibration and has a potential to reduce uncertainties by a factor of two.
A digital acquisition system is imperative for producing a portable TDCR but will be tested also on other detector systems in order to assess whether it can allow for pulse-shape discrimination (which can be used for discrimination between different types of radiation) and measurement at higher count-rates (which would be applicable to the nuclear industry). The digital system to be developed is a state of the art system with hardware that can acquire data at sampling rates of 109 s-1 and be implemented on a two channel (coincidence counting) and three channel (TDCR) detector system.
The JRP Summary can be found at: http://www.euramet.org/index.php?id=a169jrps
1 See 'Uncertainty and target accuracy assessment for innovative systems using recent covariance data evaluations' published by a working party of the OECD Nuclear Energy Agency in 2008.
2 A L Nichols, IAEA, Actinide Decay Data: Measurement requirements identified to date (IAEA – October 2008)