MICROBIAL CHARACTERIZATION FOR THE SOURCE-TERM WASTE TEST PROGRAM (STTP) AT LOS ALAMOS

摘要

Microbes have been found to inhabit almost any subsurface environment,rnincluding areas once thought to be too hostile for any life forms to exist. The effects ofrnmicrobial activity on the performance of the proposed underground nuclear wasternrepository, the Waste Isolation Pilot Plant (WIPP) at Carlsbad, New Mexico are beingrnstudied at Los Alamos National Laboratory (LANL) as part of an ex situ large-scalernexperiment. Actual actinide-containing waste is being used to predict the effect ofrnpotential brine inundation in the repository in the distant future. The study conditions arernmeant to simulate what might exist should the underground repository be floodedrnhundreds of years after closure as a result of inadvertent drilling into brine pockets belowrnthe repository. The Department of Energy (DOE) selected LANL to conduct the ActinidernSource-Term Waste Test Program (STTP) to confirm the predictive capability ofrncomputer models being developed at Sandia National Laboratory. The project consists ofrn15 drum-scale test vessels containing heterogeneous wastes, 33 liter-scale vessels withrnhomogeneous wastes, and six pressurized containers. Testing of the vessel contents hasrnincluded assays of the actinide elements and measurements of the headspace gasesrngenerated by the transuranic waste immersed in brines that are chemically similar tornthose found in the salt formation in which the WIPP repository is located. The waste isrnrepresentative of inventories that currently are stored temporarily at several DOE sitesrnnationwide. In addition, the test containers were inoculated with a microbial populationrnobtained from the hypersaline brine environment near the WIPP site. We have focused onrnthree major activities with regard to this microbial population: 1) viability of thernorganisms in the liter-scale containers more than two years after inoculation, 2) microbialrncontribution to gas generation, and 3) the toxic effects of magnesium oxide, a materialrnbeing considered for use as a backfilled barrier at the repository. Our studies of recentrnbrine samples revealed little or no growth under the conditions chosen, but the absence ofrnviable, but nonculturable cells could not be ruled out. Microbial cells are surmised to bernpresent as a biofilm coating the surface of the sludge material in the test containers,rnwhich therefore were not sampled in the liquid column to any great extent. We utilizedrnfluorescent staining techniques that detect viable, respiring organisms. Low levels ofrnmicrobes were detected with these staining procedures in the liquid column of vesselsrnprepared with waste organics. Other experiments investigated the effect of radiolysis onrnthe generation of gases in the test containers. Large amounts of nitrous oxide detected in