Strontium-Basalt Reactions under Nuclear Waste Repository Conditions
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Science Publishing Company, by Elsevier BASIS FOR NUCLEAR WASTE MANAGEMENT
Inc.
389
Stephen V. Topp,editor STRONTIUM-BASALT REACTIONS UNDER NUCLEAR WASTE REPOSITORY CONDITIONS
SRIDHAR KOMARNENI Materials Research Laboratory, The Pennsylvania State University, Park, PA 16802
University
ABSTRACT Hydrothermal interactions of Sr as strontium chloride or hydroxide with basalts or basalt phases were investigated at 300%C for 4 and/or 12 weeks under a confining pressure of 30MPa. The nature and extent of Sr immobilization varied with the Sr source and with the basalt or basalt phase which immobilized up to 99.99% of the added Sr. The main mechanism of Sr fixation with basalts seems to be ion exchange Among the basalt phases, when SrCl 2 was the Sr source. basalt glass was the most effective for SrCI 2 immobilization However, by probably forming new strontium mineral phases. when Sr(OH) 2 reacted with basalts or basalt phases Sr was immobilized by the crystallization of strontium aluminosilicates such as SrAl 2 Si 2 0 8 . These results of Sr-nearfield interactions in a basalt repository indicate that basalt can be a good barrier for Sr migration from nuclear wast es.
INTRODUCTION Basalt is one of the candidate rocks for high-level nuclear waste isolation. The reactions of basalt with Cs and Sr from spent reactor fuel under simulated In the near-field repository conditions have been investigated earlier [1]. above study, reaction products of Cs with basalts were clearly identified but The identification of new not the reaction products of Sr with basalts. reaction products of radioactive waste elements with repository rocks is important because these phases will be the new "source terms" for nuclide release. Strontium is one of the main elements of concern and is released as Sr(OH) 2 from SrZrO3 of spent reactor fuel [2] and from SrTi0 3 of Synrock waste [3]. Strontium may be released as SrC1 2 from supercalcine ceramic or borosilicate These Sr sources may be immoglass when treated with a bittern brine [4,5]. bilized into new crystalline phases by reaction with basalt repository wall Therefore, the objective rock or backfill under repository conditions [6]. of the present study is to investigate the mechanisms of immobilization of the above Sr sources by basalts and basalt phases under simulated repository conditions. These experiments were of the closed system-type in which the total mass was conserved and the reactivity of the solid-solution was explored under hydrothermal conditions. MATERIALS AND METHODS Materials Two basalts and three basalt components or phases were used in the present study. The two basalts are the United States Geological Survey's standard Basalt Columbia River-i (BCR-l) and a Deep Drill Hole (DDH-3) basalt from a section.(A2120/3320) of a core at Hanford site in Washington state. The three
390 basalt components used include (a) labradorite from Saint John Lake, Quebec, Canada; (b) pyroxene (primarily diopside as identified by x-ray diffraction) from the Duluth gabbro complex, Minnesota; an
