Work Package 1
Safety of the fuel pin system
to assess the effectiveness of the fuel as first physical barrier
In a nutshell

ork Package 1 investigates irradiated fuel-coolant-cladding interactions, coupling experimental and modelling techniques, to assess the aftermaths associated with a clad breach for a thorough safety assessment of the fuel pin behavior.

Work scope
Task 1.1
Pb/LBE-JOG phases interaction:
characterization and modelling

The volatile (Cs, I, Te, Mo) migrate from the center of the fuel pin towards the pellet rim due to the strong temperature gradient, and accumulate in the space between the fuel and the cladding, forming the so-called JOG (Joint Oxide Gain) phase. In case of a breach of the cladding in a Pb/LBE-cooled reactor, the Pb/LBE coolant will come into contact with the JOG phase and elemental forms of volatile fission products. A comprehensive knowledge of the chemistry of the interaction is essential from safety perspective.

  • As a first step, the literature data available on the irradiation experiments performed in the Sodium-cooled Fast Reactor Phénix will be reviewed to assess the expected composition of the JOG as a function of temperature and oxygen potential. These data will be combined with the new results obtained in the framework of the European INSPYRE project into a comprehensive critical review of this system.
  • Subsequently, synthesis of “SIM”-JOG phase products and characterization by X-ray diffraction techniques will be performed, building on the knowledge and experience acquired at TU Delft.
  • The structural properties of the products formed following the interaction between Pb/LBE and the main phases of JOG will be investigated using X-ray diffraction (including high temperature) and neutron diffraction at the PEARL beamline in Delft. In addition X-ray Absorption spectroscopy (XAS) measurements will be performed at synchrotron facilities for determining the oxidation state. The thermodynamic properties of the interaction products (standard enthalpy of formation, melting temperatures, melting enthalpies, phase diagram equilibria points) will be measured using solution calorimetry and thermogravimetry-differential scanning calorimetry (TG-DSC).
  • The phase equilibria in the {lead iodide-bismuth iodide-cesium iodide} system will be moreover investigated at the TU Delft using DSC.
  • Using the input detailed above, a thermodynamic model of the key sub-systems in (Pb-Bi)-(Cs-I-Mo-Te)-O will be developed for the first time based on the CALPHAD methodology and the Thermocalc and FactSage softwares. Application calculations using this newly developed database will also be performed.

The potential contamination of the primary coolant with volatile fission products from the {Pb/LBE+JOG} phases, and their release into the environment is a subject of primary concern for the public.

The release of major fission products from the Pb/LBE+JOG mixture will hence be studied using the Knudsen Effusion Mass Spectrometry (KEMS) at JRC.

Samples will be synthesized at JRC by mixing Pb or LBE solvents with exactly defined additions of relevant fission products (I, Cs, Te) in elemental forms and of major compounds present in the JOG phase.

The retention capacity of Cs, I and Te fission products in Pb and LBE (Lead-Bismuth Eutectic) will be determined as a function of temperature using the KEMS technique.

KEMS measurements in isothermal conditions will be performed to determine the release kinetics (diffusion) of these fission products from liquid Pb and LBE for temperatures typical for normal operation and off-normal conditions.

Task 1.2
Retention capacity of Cs, I, Te fission products in JOG
and their diffusion in Pb and LBE using KEMS
Task 1.3
Pb/LBE-JOG/clad interaction tests
near clad melt temperature

Fuel Cladding Mechanical Interactions (FCMI) and Fuel Cladding Chemical Interactions (FCCI) occur during reactor operation at the interface with the cladding due to the release of the gaseous and solid fission products, resulting in a more corrosive environment. These phenomena can lead in turn to further/more frequent cladding failures.

Chalmers will study the interactions in the system formed by the coolant (LBE/Pb,) the fuel cladding and fission product compounds/SIM JOG phases in a range of temperatures starting at 500°C and going up to very high temperatures around the clad melting temperature near 1500°C. The SIM/JOG phases will be delivered by TU Delft or made locally at Chalmers, following TU Delft’s specifications.

Prior to the experiments, the reaction capsule will be designed and prepared. This involves material compatibility tests and verification of the temperature tolerance and capsule integrity from 500 to 1500°C. The capsule(s) will be designed and tested in Chalmers Materials and Corrosion engineering group.

Structural and chemical analyses before and after the experiments, as well as of the formed compounds will be performed with a Focus Ion Beam Scanning Electron Microscope (FIB-SEM) of type TESCAN GAIA3 which integrates a field emission SEM with a FIB as well as Electron Backscatter Diffraction (EBSD). Also, some sample analysis will be performed with Transmission Electron Microscopy (TEM), where BIB (Broad Ion Beam) will be used to produce high quality TEM samples or cross-sectional cuts of materials in order to study, e.g. interfaces and bulk matter. Other means of sample preparation or spectroscopy are readily available through Chalmers Materials Analysis Laboratory (CMAL). Inductive coupling plasma mass analyses (ICP-MS) or ICP-optical emission spectroscopy (ICP-OES) will be used for any samples which will require a prior sample preparation like dissolution and thus will be in aqueous form for quantitative analysis. If needed, radioactive tracers will be used, readily available in Nuclear Chemistry Department, fully equipped for radioactive work.

Next to the JOG-coolant interaction, research on radionuclides with a potential long-term radiological impact (Ba, Sr, U, Pu, minor actinides) is also essential for the safety assessment. U and Pu are of course the basis matrix of the mixed oxide fuel. Ba and Sr are classified as semi-volatile meaning that their release kinetics from the fuel matrix depend on the redox conditions. Their chemistry is closely related to that of molybdenum. The Mo/MoO2 redox couple indeed acts as an oxygen buffer with determines the chemical state of the other fission products (Cs, Ba, Sr) and therefore their release kinetics.

  • Investigations of the interaction products between UO2 and Pb and Bi have been reported in the literature. We will develop for the first time a CALPHAD model for the Pb-U-O and Bi-U-O systems based on these data. When needed, additional experiments using the same techniques as detailed in task 1.1 will be carried out to solve discrepancies.
  • Investigations of the interaction products between Pb/LBE and BaMoO4 and BaUO4 (the two major barium products expected to form at high burn up and oxygen potential) will be done using the same techniques as detailed in task 1.1.
Task 1.4
Pb/LBE interaction with radionuclides with long term radiological impact:
characterization and modelling
Task 1.5
Irradiated fuel-coolant interaction

Interaction tests between irradiated fast reactor fuel and Pb/LBE will be performed in hot cells at JRC-Karlsruhe using specifically designed containers. The fuel will be analyzed after interaction using Post Irradiation Experiment (PIE) techniques such as SEM/EDX (Energy Dispersive X-ray) and KEMS to determine the retention capacity of the fission products in the irradiated fuel after contact with Pb/LBE. Furthermore, Pb/LBE coolant will be analyzed using the same techniques to investigate release of fission products from fuel towards coolant upon their interaction.