Role and properties of the gel formed during nuclear glass alteration: importance of gel formation conditions

A French SON 68 nuclear glass sample was experimentally altered to assess t he mechanisms limiting the glass alteration kinetics, especially during the transition phase between the initial rate ro and the final rate under sili con saturation conditions. A glass specimen was altered at the initial rate for one week to form a silicon-depleted nonprotective gel; the specimen wa s then leached under static conditions at a glass-surface-to-solution-volum e (SIV) ratio of 500 m(-1) and the alteration kinetics were compared with t hose of a pristine glass specimen altered under the same conditions. Unexpe ctedly, after static leaching the previously leached glass was 2.7 times as altered as the pristine specimen, and the steady-state silicon concentrati on was twice as high for the previously leached specimen. STEM characteriza tion of the alteration films showed that the initial non-protective gel con stituted a silicon pump with respect to the glass, and that the glass alter ation kinetics were limited only when a fraction of the gel became saturate d with silicon, and exhibited protective properties. This work also shows t hat silicon recondensation was uniform at micrometer scale: the silicon hyd rolyzed at the reaction interface then diffused before recondensing over a length comparable to the gel thickness. In addition to these findings, this investigation suggests a reinterpretation of the effect of the SIV ratio o n the glass alteration kinetics and on the steady-state dissolved silicon c oncentration. (C) 2001 Elsevier Science B.V. All rights reserved.