Microbiology in nuclear waste disposal: interfaces and reaction fronts.

It is now generally acknowledged that microbial populations will be present within nuclear waste repositories and that the consequences of such activity on repository performance must be assessed. Various modelling approaches--based either on mass balance/thermodynamics or on kinetics--have been developed to provide scoping estimates of the possible development of these populations. Past work has focused on particular areas of the repository which can be considered relatively homogeneous and hence can be represented by some kind of 'box' or 'mixing tank'. In reality, however, waste repositories include a range of engineering materials (steel, concrete, etc.) which are emplaced at depth in a rock formation. Strong chemical gradients--of the type which may be exploited by lithoautotrophic microbial populations--are likely to be found at the contacts between different materials and at the interface between the engineered structures and the host rock. Over the long timescales considered, solute transport processes will cause the locations of strong chemical gradients to move, forming reaction fronts. The high-pH plume resulting from the leaching of cement/concrete in some repository types is a particularly important example of such a reaction front. Redox fronts, which may occur in different areas of all kinds of repositories, also play an important role and would be locations where microbial activity is likely to be significant. In this paper, the key microbial processes expected at (or around) interfaces and fronts will be discussed, with particular emphasis on the development of quantitative models. The applicability of the models used wil be tested by considering similar fronts which can be found in natural systems.

Hazards of laser smoke during endodontic therapy.

The purpose of this study was to evaluate the potential for spreading bacterial contamination from the root canal to the patient and the dental team via the smoke produced by the laser. Five extracted teeth were deliberately inoculated with a specific strain of Escherichia coli. The canals were subjected to an agron laser. The smoke plume was captured and cultured. All of the cultures were positive for growth of the E. coli used. It was concluded that the laser smoke does present a hazard of bacterial dissemination and that precautions must be taken to protect against spreading infections when using lasers in the root canal.

Factors affecting the negotiability of second mesiobuccal canals in maxillary molars.

Factors affecting the negotiability of MB2 canals were evaluated by studying 87 extracted maxillary molars that had undergone previous endodontic treatment in the endodontic technique laboratory. The mesiobuccal roots were resected and radiographed, after which a #08 file was used in an attempt to negotiate those roots with the potential for a second mesiobuccal canal. The file was then intentionally separated in the MB2 canals that could be negotiated. The roots were decalcified, cleared, and observed under a stereomicroscope. Several factors that could interfere with the total or partial negotiation of MB2 canals were identified and included accumulation of debris and sealer that blocked access to these canals, dentinal debris produced with the pathfinding instrument, the presence of anatomical variations, diffuse calcifications, and pulp stones.

Development and testing of radionuclide transport models for fractured rock: examples from the Nagra/JNC Radionuclide Migration Programme in the Grimsel Test Site, Switzerland.

The joint Swiss National Co-operative for the Disposal of Radioactive Waste (Nagra)/Japan Nuclear Cycle Development Institute (JNC) Radionuclide Migration Programme has now been on-going for over a decade in Nagra's Grimsel Test Site (GTS). The main aim of the programme has been the direct testing of radionuclide transport models in as realistic manner as possible. Although it will never be possible to fully test these models due to the large time and distance scales involved, tests of the model assumptions in scaled down but otherwise realistic conditions will contribute to developing confidence in the predictive power of the models. In this paper, the Nagra/JNC approach is highlighted with examples from a large programme of field, laboratory and natural analogue studies based around the GTS. The successes and failures are discussed as in the general approach to the thorough testing of predictive transport codes which will be used in repository performance assessment (PA). Some of the work is still on-going and this represents the first presentation of a unique set of results and conclusions.

Fukushima remediation: status and overview of future plans.

The 2011 accident at the Fukushima Dai-ichi Nuclear Power Plant, Japan, released large quantities of volatile radionuclides, requiring evacuation of a 20 km zone around the reactor site plus additional areas where fallout was particularly high. After decay of shorter-lived isotopes, off-site contamination is now dominated by (134/137)Cs, with ∼1800 km(2) having external gamma doses above 5 mSv y(-1). Although the significance for health of such radiation levels is low, there has been a Government decision that these areas will be cleaned up to reduce exposure and allow displaced residents to return home. After initial tests at 2 sites, a further 11 demonstration remediation projects have been carried out. This work is coordinated by the Japan Atomic Energy Agency (JAEA), with MCM providing support in quality assessment of radioactivity measurements, evaluating the success of different clean-up methods and developing guidelines for the next multi-year phase of large-scale remediation. This work provides a unique perspective on the progress of remediation, experience gained and issues that still need to be resolved - particularly associated with management of the huge quantities of waste generated. This knowledge base will also be important for the bigger challenge of on-site remediation, which will require decades to complete. Additionally, experience and tools may be transferable to cleaning nuclear legacy sites around the world, a problem that is often forgotten in the debate on national nuclear waste management.