Synthesis and characterization of gadolinium phosphate neutron absorber

Hydrated gadolinium phosphate (GdPO 4·1H 2O) was synthesized by reacting high purity dissolved salts (gadolinium nitrates or chlorides) with phosphoric acid. The hydrated powders were shown to be extremely insoluble in water with a K sp measured to be between 2.07 E-14 and 4.76 E-13. Calcination to between 800 and 1000 °C resulted in the formation of GdPO 4 in a monazite (monoclinic) crystal structure. This was correlated with the first exothermic differential thermal analysis (DTA) peak (864.9–883.4 °C). The DTA also showed small peaks in the 1200–1250 °C range, that could be associated with a change from the monazite (monoclinic) crystal structure to the xenotime (tetragonal) crystal structure. However, calcination of a sample to 1400 °C, followed by relatively rapid cooling and XRD, showed the structure was still monazite (monoclinic). DTA results showed a melting point at 1899–1920 °C (endothermic peak). It was therefore concluded that the melting point probably was the melting of the monazite (monoclinic) phase, but may have been xenotime if a phase change at 1200–1250 °C was reversible and very rapid. The higher part of the melting range was achieved with material derived using the slightly higher purity nitrate salt. The results show that GdPO 4 is an excellent candidate for a chemically stable, water-insoluble neutron absorber for inclusion in spent nuclear fuel canisters.