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WP 3 : Post-irradiation Examination, radiochemical investigations and back-end studies on selected fuel samples

The major objective of WP3 “Post-irradiation Examination, radiochemical investigations and back-end studies on selected fuel samples” is to address outstanding questions on the in-pile fuel behaviour and on the quantification of the actinides build-up and decay chains, and to address storage, disposal and reprocessing scenarios of (Th,Pu)O2 and inert PuO2 loaded CERCER fuels. The research will make use of fuel samples taken from recent 5th FP projects THORIUM CYCLE and OMICO and from the IFA-651 project. Results of this WP serve as input for theoretical investigations of WP4.

The destructive post-irradiation analysis of fuel samples is anticipated to be performed on the following matrix breakdown:

  • T3.1 Post-irradiation examination aims to provide an experimental data-base on the thermo-mechanical behavior of the fuel irradiated in LWR conditions.
    • SCK•CEN will contribute with destructive PIE (puncture, optical microscopy, EPMA, SEM) on 2 available FP5 fuel sample, i.e. homogeneous (Th,Pu)O2 and (U,Pu)O2 from the OMICO project. The emphasis of the analyses lies on: fission gas release; grain size evolution, porosity evolution and bubble growth and chemistry of the matrix & fission products: homogenisation, oxygen potential effects etc.). Fuel transport of small amounts of available OMICO fuels for additional PIE in other hot cell labs is foreseen.
    • ITU will cover non destructive and destructive PIE (including, profilometry puncture, optical microscopy, EPMA and SEM) on the homogeneous (Th,Pu)O2 fuel pin irradiated in the commercial light water reactor at Obrigheim (KWO) up to a burn-up of 39 GWd/tHM. The emphasis lies on fission gas release and possible restructuring of the fuel in terms of grains and fission gas bubbles, along with elemental distribution of key elements along the fuel radius.
    • PSI contributes with EPMA and SIMS or Laser-Ablation ICP-MS analysis on homogeneous (Th,Pu)O2 from OMICO and on homogeneous (Zr,Pu)O2 from IFA-651 (rod 2), the last already available at PSI. The exact mass-spectrometric technique to be used, i.e. SIMS or LA-ICP-MS, will be decided after evaluation of standard reference samples. For the OMICO fuel also destructive radiochemistry analysis (RCH) in T3.2 is foreseen, thus allowing a comparison of isotopic data from SIMS or LA-ICP-MS with those of RCH.
    • SCK•CEN contributes with the dissolution & radiochemical analysis of 1 available FP5 fuel sample, i.e. homogeneous (Th,Pu)O2 from the OMICO project. Radiochemical and chemical analyses techniques that will be applied include alpha- and gamma-spectrometry, Thermal Ionization Mass-Spectrometry (TIMS) and Inductively Coupled Plasma Mass-Spectrometry (ICP-MS).
    • PSI contributes with the dissolution & radiochemical analysis of 2 fuels: a neighbouring sample of the homogeneous (Th,Pu)O2 from the OMICO project and a homogeneous (Zr,Pu)O2 sample form IFA-651. Radiochemical and chemical analyses techniques that will be applied include gamma-spectrometry, High Pressure Liquid Chromatography-ICP-MS (HPLC-ICP-MS) and Multi-Collector-ICP-MS (MC-ICP-MS).
    • ITU contributes with the dissolution & radiochemical analysis of 1 available sample from the homogeneous (Th,Pu)O2 fuel, which was irradiated to high burn up at KWO Obrigheim.. Radiochemical and chemical analyses techniques that will be applied include alpha- and gamma-spectrometry, Thermal Ionization Mass-Spectrometry (TIMS) and/or Inductively Coupled Plasma Mass-Spectrometry (ICP-MS).
    • FZJ contributes with the dissolution & radiochemical analysis of a neighbouring sample of homogeneous (Th,Pu)O2 from the KWO project. Radiochemical and chemical analyses techniques that will be applied include alpha- and gamma-spectrometry as well as mass-spectrometry techniques (ICP-MS).
    • ITU will separate a suitable sample from the (Th,Pu)O2 fuel and prepare it for insertion in a suitable container that can be handled at the ITU Hot Cell facility. The transport organisation will be made by FZJ, who will contribute to leaching studies on this material from KWO. Parts and/or powdered material of an irradiated Th/Pu-MOX pellet will be immersed in aqueous phases, representative for clay and salt repositories, under an anaerobic atmosphere. Aliquots of the aqueous phase will be taken after specified time intervals to measure the leached fractions. Finally the residual MOX fuel will be dissolved to determine the total radionuclide inventory. The dissolution kinetics of the spent Th/Pu-MOX fuel will be compared with data obtained for Th/U-MOX.
  • T3.2 Radiochemical analysis aims to obtain a reliable experimental database to determine burn-up and to evaluate changes in the heavy nuclide content of the irradiated advanced nuclear fuels of the program. The nuclear database of key isotopes generated will be used for benchmark calculations in WP4 to address fuel performance and safety issues. Destructive radiochemical analyses on a selection of four fuels from 3 different irradiation programs are envisaged, i.e. homogeneous (Th,Pu)O2 from the KWO and from the OMICO project, homogeneous (U,Pu)O2 from the OMICO project and homogeneous (Zr,Pu)O2 from IFA-651. A selection of isotopes to be analysed includes Base Actinides (isotopes of Pu, Th and/or U), Minor Actinides (231Pa, 237Np, Am and Cm-isotopes) and a number of Fission Products (137Cs, 144Ce, 106Ru, 125Sb, 134Cs, 154Eu, 155Eu and Nd-isotopes). Radiochemical analysis should include the optimization of dissolution strategies of the advanced fuels in a hot cell environment, of the chemical separation techniques and of the radiochemical and chemical measurements. As the radiochemical analysis of the proposed experimental fuels is not routine for the participating laboratories, it is envisaged to assess the overall quality of the applied analyses methodologies, results and associated uncertainties by performing a cross check between the participating laboratories. Therefore for the two selected (Th,Pu)O2 fuels two neighbouring samples will be analyzed by two participating laboratories. This cross check is essential to create a reliable database for WP4 calculations.
    • SCK•CEN contributes with the dissolution & radiochemical analysis of 1 available FP5 fuel sample, i.e. homogeneous (Th,Pu)O2 from the OMICO project. Radiochemical and chemical analyses techniques that will be applied include alpha- and gamma-spectrometry, Thermal Ionization Mass-Spectrometry (TIMS) and Inductively Coupled Plasma Mass-Spectrometry (ICP-MS).
    • PSI contributes with the dissolution & radiochemical analysis of 2 fuels: a neighbouring sample of the homogeneous (Th,Pu)O2 from the OMICO project and a homogeneous (Zr,Pu)O2 sample form IFA-651. Radiochemical and chemical analyses techniques that will be applied include gamma-spectrometry, High Pressure Liquid Chromatography-ICP-MS (HPLC-ICP-MS) and Multi-Collector-ICP-MS (MC-ICP-MS).
    • ITU contributes with the dissolution & radiochemical analysis of 1 available sample from the homogeneous (Th,Pu)O2 fuel, which was irradiated to high burn up at KWO Obrigheim.. Radiochemical and chemical analyses techniques that will be applied include alpha- and gamma-spectrometry, Thermal Ionization Mass-Spectrometry (TIMS) and/or Inductively Coupled Plasma Mass-Spectrometry (ICP-MS).
    • FZJ contributes with the dissolution & radiochemical analysis of a neighbouring sample of homogeneous (Th,Pu)O2 from the KWO project. Radiochemical and chemical analyses techniques that will be applied include alpha- and gamma-spectrometry as well as mass-spectrometry techniques (ICP-MS).
  • T3.3 Fuel back-end studies: leaching experiments aim to assess the aptitude of the (Th,Pu)O2 fuel for disposal.
    • ITU will separate a suitable sample from the (Th,Pu)O2 fuel and prepare it for insertion in a suitable container that can be handled at the ITU Hot Cell facility. The transport organisation will be made by FZJ, who will contribute to leaching studies on this material from KWO. Parts and/or powdered material of an irradiated Th/Pu-MOX pellet will be immersed in aqueous phases, representative for clay and salt repositories, under an anaerobic atmosphere. Aliquots of the aqueous phase will be taken after specified time intervals to measure the leached fractions. Finally the residual MOX fuel will be dissolved to determine the total radionuclide inventory. The dissolution kinetics of the spent Th/Pu-MOX fuel will be compared with data obtained for Th/U-MOX.

The start of the analyses themselves is foreseen no earlier than at T0 + 6, and depends primarily on the arrangement and cost optimisation of fuel sample transport between the different laboratories. The PIE and radiochemical analyses will be performed during a period of approximately 12 months, leaching and dissolution + demonstration of reprocessing flow sheet performed at FZJ will take 24 months after arrival of samples.

According to current experience, the major unknown factor is the transport of samples. Such transports are difficult, but have been executed in the recent past. The envisaged transport container for sample transfer is operated by PSI, contractor in this consortium and is already validated in Switzerland, The Netherlands, Belgium (other countries, not participating in this WP are not listed here). A validation of the licence in Germany is to be applied for. The transport licence covers the fuel compositions envisaged in this project. Other transport containers are available, for hot cell activities, where this container cannot be used, one of these is now being licensed for the transport of the (Th,Pu)O2 fuel from Obrigheim to ITU and discussion on a further container is in progress.