Metrology for new generation nuclear power plants - MetroFission

This EMRP (European Metrology Research Programme) project, MetroFission, has been looking at solving metrological problems related to a new generation of nuclear power plants. The proposed Generation IV power plants are designed to run safely, make efficient use of natural resources, minimize the waste and maintain proliferation resistance. In order to reach these goals, the reactor operation involves higher temperatures, high-energy neutron fluence, different types of fuel where the minor actinides are included etc. In this multidisciplinary project, which has 12 partners in 10 countries, the work has focused on improved temperature measurements, investigation of thermal properties of advanced materials, determination of new and relevant nuclear data and development of measurement techniques for radionuclides suitable for Generation IV power plants. The improved temperature measurement for nuclear power plant applications includes the development of a new Fe-C fixed point. Robust, repeatable and versatile cells have been constructed and compared with success among the project participants and their melting temperatures have been determined. Furthermore, the methodology of self-validating thermocouples has proven efficient at several fixed point temperatures (Au, Cu, Co-C, Fe-C) using different designs. A practical acoustic thermometer has been tested at high temperature (1000°C) with success thanks to the use of innovative signal processing methods. Mo/Nb thermocouples have been obtained with different sheath materials and tested with the aim to achieve for the first time a reference function determined with the best possible uncertainties. Following reviews of designs and technology proposed for fourth generation nuclear plants effort within this project, with regards to thermal properties of advanced materials for nuclear design, has concentrated on provision of thermodynamic data to (ASTRID). Data has been critically assessed to represent the potential interaction between the Na coolant and the nuclear fuel taken to be based on (U,Pu)O
₂
but incorporating minor actinides such as Np and Am. Data for the fission products and containment materials have also been reviewed and new data assessed to represent the interaction between them and the Na coolant, in order to permit calculation of phase and chemical equilibria for severe accident scenarios. The aim is to publish the data developed during this project within a revised version of “Thermochemical data for reactor materials and fission products”, last published in 1990. Reference metrological facilities and methods have been developed or improved for the measurements of normal spectral emissivity and specific heat up to 1500°C, and thermal diffusivity up to 2000°C. The following actions were notably performed: Development of two complementary facilities based on different metrological approaches (calorimetric and optical methods) by PTB and LNE for the measurement of emissivity of solid materials at high temperature. ; Modification of the LNE diffusivimeter with the development and integration of a specific inductive furnace, and the adaptation of the associated IR detection systems. These facilities have now been used for the measurement of thermophysical properties of some materials (Ni, MgO and ZrO
₂
), because they have thermal properties similar to those encountered in Generation IV reactors. Regarding neutron cross sections, the current nuclear databases concentrate on thermal energies, but new fast reactor designs will involve materials exposed to higher energy neutron fields. Advantage has been taken of National Measurement Institutes experience in neutron fluence measurements to improve high-energy cross-section measurements. A secondary fluence standard has been established and used to determine the cross sections of actual interest improving nuclear data standards in the form of reduced measurement uncertainties. The heavier actinides and their decay products have a role in the future development and adoption of nuclear power plants in whose fuels they will be present in controlled amounts. A list of specific needs for improved actinide decay data has recently been outlined in a review by the IAEA. Responding to those needs, an objective of the MetroFission project was to measure the alpha particle emission probabilities of
²³⁸
U with certified isotopic composition. The nuclear data from alpha spectrometry of
²³⁸
U sources has been successful, obtaining data that will be provided to the international community and that further lower the uncertainties of the nuclear data. The quality of the alpha spectrum measurement, in terms of energy resolution and peak tailing of the
²³⁸
U alpha spectra, was significantly better than in any previous measurement. Fission fragments containing more beta-decay isotopes will result from proposed fast reactors. Cryogenic detectors have been developed aimed at improving the measurement of beta spectra and verifying theoretical models. A beta spectrum shape of
⁶³
Ni has successfully been obtained. Improvements have been made in the source components and in the source production method in order to lower uncertainties. The exchange effect has been implemented in the calculation of theoretical beta spectra. The measurement of the beta spectrum of
⁶³
Ni has for the first time confirmed the exchange effect at very low energy. The energy threshold of the metallic magnetic calorimeter is 180 eV. At this energy, the enhancement of the beta emission due to the exchange effect is larger than 20 %. Nuclear power plants would benefit from on-site activity measurements of radionuclides including low-energy beta emitters that are abundant but can prove difficult to assay. Within this project, a primary liquid scintillation coincidence counting method called Triple to Double Coincidence Ratio (TDCR) has been modified for this purpose. Five partners have built miniature TDCR systems, miniaturising the instrument's detection chamber by using newer, smaller, more efficient photomultiplier tubes. The system also includes replacing the analogue electronic modules with digital acquisition systems and signal processing in order to significantly improve digital coincidence counting (DCC).