Preliminary results on the leaching process of phosphate ceramics, potential hosts for actinide immobilization

A mixed zirconium–lanthanum phosphate La1/3Zr2(PO4)3, noted LaZrP, is studied as a potential host for actinides issued from high-level nuclear wastes. Chemical durability is evaluated and compared with two other phases: a monazite phase LaPO4, noted LaP, and a mixed oxide phase La0.1ZrO2.15, noted LaZrO. Leaching tests are performed and followed by solution and solid analyses. Static tests are performed with a low ratio between surface of ceramic and volume of solution, S/V (about 0.1 cm−1). For LaZrP, the phosphate release rate decreases from 10−2 to 10−3 g/m2/day, while zirconium and lanthanum releases remain very low (<10−5 g/m2/day). For the LaP phase, the phosphate release is about 10 times lower than for LaZrP and remains stationary, while the rate of the lanthanum release is below 10−6 g/m2/day. Tests performed at high S/V (about 20 cm−1) reveal that the lanthanum release rates after 2 weeks were, respectively, at 10−9, 10−6 and 10−4 g/m2/day for LaP, LaZrP and LaZrO phases. It is shown that NZP or monazite type ceramics are highly resistant to the leaching process.