Volkonskoite From a Natural Analogue of a Cementitious Repository

Abstract

Many repository designs-particularly those for low- and intermediate-level waste-involve use of very large quantities of cementitious materials. The interaction of such materials with groundwater results in a hyperalkaline leachate which will contain any radionuclides dissolved from the waste. The interaction of such leachate with surrounding rocks and the mobility of radionuclides in this complex system has to be considered in repository safety assessment. The very slow kinetics of many of the reactions involved, however, greatly limit the applicability of conventional laboratory studies.

The marbles travertines of Maqarin-Daba-Siwaqa sites of North and Central Jordan can serve as natural analogues of a cementitious repository. A similar hyperalkaline environment is still found in some of these areas, controlled by dissolution of natural portlandite and calcium silica hydroxide (CSH) mineral phases. The high pH groundwaters interact with the surrounding limestones and marls offering a close analogy of the situation likely to occur if a repository were constructed in a sedimentary site. The high pH seepages precipitate travertines as a result of the uptake of atmospheric CO₂.

The sites offer a good chance to investigate the interactions of the high pH water with rocks containing diagenetic enrichments in safety-relevant trace elements (Zn, Cr, U, Se, Ni, Sn, etc.). For example, the unusual smectite (volkonskoite) and other phases (similar to hydrated cement products) precipitated from the mobile alkaline water may act as sinks for such trace elements. Such studies can be used to qualitatively confirm the expected low mobility of important elements in this system. Coupled with hydrochemical studies, the geochemical models used to support performance assessment can be tested (validated).