Radioactive Disequilibria in Mineralised Drill Core Samples from the Bjoerklund Uranium Occurence, Northern Sweden

摘要

Forming the basis of a pilot study to investigate the natural mobility of uranium and its daughter products within the geological environment, a series of six samples, chosen from two mineralised fracture zones at depths of 100-130m within a homogeneous leucocratic granite, were examined mineralogically and isotopically to establish the presence or absence of radioactive equilibrium that may indicate recent rock/water interaction processes (i.e. during the last 0.5 m.y.). The results show clear evidence of radioactive disequilibrium, and hence recent rock/water interaction, in four of the six samples. Some is attributable mostly to solution-solid exp 234 U recoil gain (weakly mineralised zones adjacent to the main mineralisation) and others to solid-solution exp 234 U recoil loss (moderate to highly mineralised zones). The richer type of impregnation mineralisation indicates apparent radioactive equilibrium. The absence of significant exp 238 U loss in the samples helps to underline the reducing conditions prevalent within open fracture systems at these depths. This has meant that uraninite, now found in fractured rock at about 100m depth, has been chemically stable in its environment for most, if not all, of its 1750 m.y. existence, including the last 0.5 m.y. when it has been closest to the atmosphere. This could indicate that spent nuclear fuel, which essentially is crystalline UO sub 2 , and comparable to uraninite once the main fission products have decayed, would also have been stable in this environment for similar periods of time. (Atomindex citation 14:784183)