Characterizing powders by gas adsorption analysis

SCK•CEN Mentor

Delville Rémi, rdelvill@sckcen.be, +32 (0)14 33 31 65

Expert group

Fuel Materials

SCK•CEN Co-mentor

Vanaken Koen , kvanaken@sckcen.be , +32 (0)14 33 31 40

Introduction

The nuclear fuel used in most fission reactors is based on uranium containing fuel assemblies where closed fuel rods, filled with small cylindrical UO2-pellets, are assembled into a grid structure. The fabrication of the pellets is a complex process where the properties (flowability, packing, surface area, bulk density) of the as-received fuel powder need to be improved first to facilitate the later compaction and sintering process. To improve the powder properties, it can be conditioned by wet or dry ball milling or by several other conditioning steps like heat treatment under controlled atmospheres. Once the fuel powder is properly conditioned, the powder is ready to be homogenised and blended with additives like pore former, lubricant and/or sinter aids. Now the powder is suitable to be compressed into small compacts with specified density by means of a pellet press. If the required density for the compact (so called ‘green pellet density’) cannot be obtained in one pressing cycle, an intermediate pre-compaction step to lower density is applied, followed by a granulation and spheroidisation step. This intermediate step will lead to spherical granulates with higher starting density which is now suitable for final pressing. At the final production stage, the compacted pellets are further densified and solidified in a high temperature furnace (also called sintering) and ground to a specific pellet diameter by means of a centerless grinder.

Objective

The fuel manufacturing lab of SCK•CEN is continuously striving for improvements in its pellet fabrication processes. High quality pellets are produced to meet various demands for fuel research.

Due to the complexity of the fuel pellet fabrication process described above, many parameters can have a distinct role in the final quality of the obtained pellets, which is essentially determined by the chemical purity, microstructure, thermal stability, pellet dimensions and pellet shape.

One parameter for the powder is particularly important: its specific surface area (SSA) (the total surface area per unit of mass). It will determine for the most part, how well a powder can be sintered (densification at high temperature) and its handling properties (flowability, compressability, agglomeration,…etc.). SSA is measured in a so-called BET (Brunauer-Emmet-Teller) analyser that relies the measurement of fine pressure drop caused by the adsorption of an atomic monolayer of a gas on the particle surface at low temperature (cooling by liquid nitrogen). BET analyser can in addition measure porosity in particles or solids (e.g. pellets) through multi-layer gas adsorption (gas molecules are then filling ‘holes’ in porous material).

SCK-CEN has recently acquired two BET analysers from Micromeritics (Gemini VII and Tristar II). They have slightly different working principles and options but should, in principle, provide consistent and reproducible results. It is proposed in this work to benchmark the performance of the two devices for the measurement of SSA using a range of standardized powders. Capabilities to measure porosity should also be investigated and compare to picnometry, an alternative technique available in the lab. Possible extension of the work could include the actual production of UO2 powders with different surface area to be measured by BET and that will serve as starting materials for other research projects.

All practical work will take place in the controlled area of the SCK•CEN where 2 brand new fuel laboratories have been reconfigured to accommodate all the infrastructure and safety precautions necessary for fuel pellet fabrication. The student will be able to work with modern, state-of the-art fabrication and characterisation equipment.

The minimum diploma level of the candidate needs to be

Academic bachelor , Master of industrial sciences , Master of sciences , Master of sciences in engineering

The candidate needs to have a background in

Chemistry , Physics

Estimated duration

3 months