The Synroc system of immobilization has a simple mineralogy: two of the three components (perovskite and zirconolite) are present in natural environments and their stability may be assessed on a time-scale far in excess of the idealized 100 000-yr containment objective. Additional problems include that of the containment of Cs, perhaps as CsTi2NbO7 or as Cs-bearing members of the crichtonite group, freudenbergite or priderite. It is suggested that radwaste active pellets be diluted into an inert media within the canister to more closely approximate a rock rather than a concentrated and highly reactive mineral aggregate. Since the distribution of minerals in Synroc or alternatives is typical of kimberlites, the radwaste monoliths should perhaps be returned to the gaping diamond diatremes of the world.-R.A.H.
