# Untersuchungen zu Sinterverhalten und Gefügeentwicklung der Monazitkeramiken LaPO$_{4}$, CePO$_{4}$ und PrPO$_{4}$ für die Endlagerung von Minoren Actinoiden

• Sintering behavior and microstructure development of the monazite ceramics LaPO$_{4}$, CePO$_{4}$ and PrPO$_{4}$ for final disposal of minor actinides

Schausten, Charlotte; Telle, Rainer (Thesis advisor); Roth, Georg (Thesis advisor)

Aachen (2020, 2021)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2020

Abstract

In the present work, the sintering behavior of the rare earth phosphates LaPO$_{4}$, CePO$_{4}$ and PrPO$_{4}$ with monazite structure was investigated and adapted sintering profiles were developed. The generation of dense and coarse-grained microstructures is the key point of the study for application as waste form of minor actinides in regard to chemical and mechanical stability in nuclear disposal. Both precipitated and solid-phase synthesized powders were first characterized and sintered according to conventional sintering profiles disclosed in the literature. Subsequently, the powders were preliminary examined in terms of shrinkage behavior and thereafter sintered with individually adapted sintering programs via Rate-Controlled Sintering (RCS). Depending on the synthesis methods and their effects on the powder properties, considerable temperature differences in the shrinkage range were measured. Based on dilatometric data, the RCS technology enabled a more effective use of the shrinkage mechanisms and thus an improvement of the densification process. The conventionally sintered samples showed insufficient densities and high values for open porosity. Furthermore, a strong intragranular porosity has been detected for the wet-chemical synthesized powders, which impedes further densification. The rate-controlled sintering led to maximum densities of 96 %TD, and grain sizes of up to 50 μm as well as a significant reduction of the intragranular pores concerning the precipitated powders. The solid-phase synthesized powders reached 91 %TD, with relatively fine grains and a very homogeneous particle size distribution. The obtained results are similar to literature data ranging between 92 and 98 %TD, depending on the synthesis and sintering method. In a commercially available precipitated powder, the metaphosphate CeP$_{3}$O$_{9}$ was detected. Its decomposition during sintering had a detrimental effect on the microstructure of the monazite. From a strictly microstructural point of view, the pure-phase precipitated RCS-sintered powder meets best the requirements related to waste form.