Nuclear waste elements slip through hydrothermally altered basalt

Abstract

Basalt that is altered hydrothermally in the laboratory does not bind the elements Pu, Np, U, Eu and Ba (and, by implications, Am, Sr and Ra) from flowing water as well as does freshly fractured basalt. The results suggest that the ability of a nuclear waste repository in basalt to retain disposed radioactivity will not be as great as measured for freshly-fractured basalt. Safety assessments of a repository in basalt must consider the enhanced mobility of many radioelements that now can be expected of rock surfaces in prolonged contact with water. The dominant secondary minerals of a tholeiitic basalt altered hydrothermally in the laboratory are stevensite, α-quartz and calcite. Gypsum, sericite, several zeolites and scapolite are also formed. Before the rock is hydrothermally altered, calcite, stevensite and minor amounts of quartz are identified on recent fracture surfaces of the rock as the products of natural alteration. The hydrothermally altered surfaces of the rock are more compatible, chemically, with groundwater sampled from the basalt than are surfaces of recent fissures, suggesting that the laboratory-altered surfaces approximate, chemically, old fracture surfaces of the rock that are now in contact with the groundwater.