Integrating Microbiology into the Drigg Post-Closure Radiological Safety Assessment
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Integrating Microbiology into the Drigg Post-Closure Radiological Safety Assessment I. Beadle, P. N. Humphreys, C. Pettit and J. Small BNFL Research & Technology, Risley, Warrington, WA3 6AS, UK. ABSTRACT BNFL owns and operates the UK’s principal solid Low Level Radioactive Waste disposal site at Drigg in Cumbria, north west England. Drigg has been receiving waste since 1959 with approximately 900,000 m3 of waste disposed of to date. Waste accepted for disposal at Drigg comes in a variety of forms including rubble, spoil, redundant equipment, scrap and process waste, and typically contains significant metallic and cellulosic components. The organic content of the waste means that microbial activity plays a significant role in the development of the repository environment. Consequently, microbial processes are integrated into many aspects of the Drigg Post-Closure Radiological Safety Assessment (PCRSA). This begins with the identification and screening of relevant features, events and processes, through supporting research, engineering designs and finally integration into radiological safety assessment modelling. This paper outlines how and where microbiology is integrated into the Drigg PCRSA and indicates areas of active research. INTRODUCTION BNFL owns and operates the UK's principal solid Low Level Radioactive Waste (LLW) disposal site at Drigg in Cumbria, north west England. Drigg is a near-surface disposal facility that has operated since 1959 with approximately 900,000 m3 of waste disposed of to date. Historically, waste was loose tipped into shallow trenches with a natural, low permeability layer forming the base. More recently, disposal has been via a containerized, compacted and grouted waste form to a concrete lined vault. Current and future vaults will extend the operational lifetime of the Drigg site into the middle of this century. Waste accepted for disposal at Drigg must not exceed 4 gigabequerels per tonne alpha and 12 gigabequerels per tonne other activity. These are upper limits and the bulk of the waste accepted falls substantially below this activity content. The waste comes in a variety of forms including rubble, spoil, redundant equipment, scrap and process waste, and typically contains significant metallic and cellulosic components. The presence of this degradable organic material in the Drigg waste stream means that microbial activity is a natural consequence of disposal operations at the site and that microbial activity plays a significant role in the development of the repository environment [1]. Consequently, microbial processes are integrated into the Drigg Post-Closure Radiological Safety Assessment (PCRSA) [2].
UNDERLYING ASSUMPTIONS There are three basic assumptions that underlie the integration of microbiology into the Drigg PCRSA: • • •
microorganisms are ubiquitous in the near-surface environment; if a microbial habitat is available then a microbial community will exploit it; and microbiology can be addressed at the processes rather than the species level.
The first two assumpti
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