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/m(2)/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/m(2)/day. Tests performed at high SIV (about 20 cm(-1)) reveal that the lanthanum release rates after 2 weeks were, respectively, at 10(-9), 10(-6) and 10(-1) g/m(2) /day for LaP, LaZrP and LaZrO phases. It is shown that NZP or monazite type ceramics are highly resistant to the leaching process. (C) 2001 Elsevier Science B.V. All rights reserved.