Study of additives in nuclear fuel manufacturing

SCK•CEN Mentor

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

Expert group

Fuel Materials

SCK•CEN Co-mentor

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

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

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. Many of the pelletization process parameters need to be optimized in function of the initial powder used and the targeted end specifications.

Among these parameters, the addition of additives like pore former and lubricant to the fuel powder are of significant technological importance. Additives are usually carbonated molecules that are decomposed during high temperature sintering leaving porosities behind, allowing thereby a control of porosity. Some of these molecules may also act as lubricant, improving the flowability of the powder and the pressing thereof. Systematic study of their respective influence are scarce in the open literature and is mostly kept secret by fuel manufacturer. We proposed in this work to study systematically the influence of a set of additives on UO2 pellets manufacturing and their final microstructure and properties by varying the additives composition, quantity and processing.

The tests 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. Due to the very low activity and radiation doses of the fuel powder manipulated, the fuel lab is configured with classic fume hoods only, not requiring any closed glove-box. The student will be able to produce a set of high quality pellets whose properties and microstructure will be thoroughly analyzed. The work requires the skills of a careful and well-organized experimentalist willing to learn a wide range of experimental techniques.

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

Physics , Chemistry

Estimated duration

3 months