WHAT’S IN RADIOACTIVE SLAG?–PHASE IDENTIFICATION AND PREDICTION OF URANIUM AND THORIUM FATE FOR USE IN SITE DECOMMISSIONING AND ENVIRONMENTAL RESTORATION

摘要

Slag containing radioisotopes of uranium and thorium was characterized to estimate a“realistic”release rate from the weathering of slag.The uranium and thorium bearing phases were identified,and the progression of slag weathering was investigated.The data from this research will be of use for the development of geochemical models for release of radioactivity from slag. rnSlag samples were collected from three industrial sites in the humid Eastern United States.The slag samples were characterized,physically and chemically,by a suite of analytical methods:petrographic microscopy,x-ray diffraction,x-ray fluorescence,scanning and transmission electron microscopy,and electron microprobe analysis. The microprobe was particularly useful for quantifying uranium and thorium depletion or enrichment on the weathered surfaces of the slags.To quantify the weathering of slag over the long term,a study of archaeological slags was undertaken.Samples were collected from Cyprus(Bronze age Cu slag),the Czech Republic(Pb and Ag slag~2 to 100 years old),Malaysia(50 to 100 year old Sn slag),and the United Kingdom(Sn slag~200 to 1500 years old,and Cu slag~300 years old).The electron microprobe information on the short-term leach rates and estimated“in-situ”leach rates from the archaeological slags of varying ages provide a measure of how the leach rates change as a function of time. rnThe Eastern United States slags were found to contain both crystalline and glass phases.Because they were formed under strongly reducing conditions with extremes of temperature and partial pressure of CO_2,the phases differ greatly from common igneous rocks,and their mineralogy more closely resembles that of meteorites.The most abundant crystalline phases containing uranium and thorium are calzirite,zirconolite,perovskite,pyrochlore, hibonite,and thorianite. rnIt is important to note that the uranium and thorium are present in discrete phases and are not distributed homogeneously throughout the slag.Some uranium and thorium are found in the glass,which weathers rapidly upon exposure to atmospheric conditions.However,the uranium and thorium are also incorporated in stable phases (e.g.,calzirtite,zirconolite,perovskite)that also make up the proposed high-level radioactive waste form SYNROC. Numerous studies provide experimentally determined solubility and leach rates for these phases,and these data provide insight into,and bounding values for,estimates of slag leach rates.With regard to weathering mechanisms (i.e.,dissolution and alteration),it was determined that the slags weather incongruently,i.e.,certain phases dissolve preferentially.Weathering occurs preferentially along grain boundaries and within particular grains,providing fluids more penetration and access to the interior of the slag fragments.It is possible that over time the slags will degrade chemically,while remaining semi-intact physically,similar to a saprolite.The data suggest that the thorium,and perhaps uranium,will remain in the resistant(refractory)phases,as long as they are stable. rnMicroprobe analysis on samples from one of the three sites indicates a release rate of 230 ng of uranium and thorium per cm~2,based on the depth of weathering and length of time the material was exposed to the elements.