Abstract
The safe disposal of nuclear waste necessitates a thorough understanding of the role microorganisms play on the fate of the waste materials over time. Despite significant volumetric contributions to nuclear waste inventories, little is known about the fate of plastics under the high pH and irradiating conditions of a geological repository. We conducted microbial enrichment experiments to assess the potential for additives in plasticised polyvinyl chloride (PVC) sheet, commonly used in the nuclear industry, to fuel nitrate reduction by a microbial community adapted to grow at pH 10. These additives include plasticisers and flame retardants, amongst others. PVC powder (no additives) served as an additive-free control, and samples of both materials were gamma irradiated (1 MGy) under hyperalkaline conditions, representative of conditioned nuclear waste, to assess the effect of irradiation on bioavailability. Plasticised PVC supported near-complete nitrate reduction, whether irradiated or not, although irradiated PVC sheet supported less nitrate reduction. No nitrate reduction was observed with non-irradiated PVC powder, although irradiated powder supported minor nitrate reduction. These results highlight the bioavailability of volumetrically significant plasticised PVC under conditions relevant to its geological disposal in cementitious intermediate level nuclear waste and highlight the critical need to constrain downstream effects on biogeochemical processes ultimately impacting on the safety case for disposal.
Highlights
The safe disposal of nuclear waste in a geological disposal facility (GDF) demands a thorough understanding of biogeochemical interactions between waste materials and surrounding geosphere
This study shows that plasticised polyvinyl chloride (PVC) sheet, a volumetrically significant component of low and intermediate level nuclear waste, can fuel nitrate reduction at pH 10
Gamma irradiation at the doses used here enhanced the bioavailability of the pure polymer (Figure 3A), irradiated PVC sheet was rendered less bioavailable, allowing for almost 30% less nitrate reduction than non-irradiated PVC sheet (Figure 3B)
Summary
The safe disposal of nuclear waste in a geological disposal facility (GDF) demands a thorough understanding of biogeochemical interactions between waste materials and surrounding geosphere. Key to this is understanding the role microorganisms may play in the degradation of organic materials within nuclear waste, and the implications of microbial processes on the chemical speciation, and mobility, of key radionuclides in groundwaters permeating the GDF. In the UK, halogenated plastics constitute the largest component of the organiccontaining waste inventory (NDA, 2014) This waste arises from the use of plastics in maintenance and decommissioning operations at nuclear power plants, and from reprocessing plants and laboratories. The bulk of the PVC in the UK National Inventory is expected to be flexible films and sheets of PVC derived from these activities (Smith et al, 2013)
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