Low-level Waste Disposal Sites Research Articles

Geochemical Study of Engineering Shield of Uranium, Strontium and Cesium at a very Low Level Waste Disposal Site

Advantages and disadvantages of natural shields in a very low level waste (VLLW) disposal area were studied with regard to the adsorption and fixation capability of the natural shields against radioactive elements Uranium, Strontium and Cesium, with the understanding of the hydrogeological and geochemical characteristics of the surrounding area. Necessity of establishing geochemical engineering shield in the disposal area is discussed based on the understanding of the inadequacy of the natural shields against Uranium and Strontium. A physical chemistry model involving three-fold of shields suitable for the disposal area is proposed with integrated consideration of the results from efficacy and endurance experiments on the natural shield.

Radiological and geochemical characteristics of an ultramafic massif (NE Portugal) regarding the site aptness to host a near surface repository for low and intermediate level radwaste

The search for suitable sites to install near-surface repositories for low to intermediate level radioactive wastes has increased in the recent years, reflecting both the continuous rise in radioactive materials applications by the society and the increasing awareness of public perception regarding radwaste management. International recommendations towards the application of higher safety standards to low and intermediate level waste disposal sites being developed by the International Atomic Energy Agency (IAEA), and the European Commission decision that each Member State must safely dispose its own radwaste in its territory, according to national radwaste management plans, has leading to new soundly scientific studies. In Portugal, possible locations for that purpose are under study since 2006, following the pre-requisites indicated by IAEA. One of these sites overlaps a significant portion of an ultramafic massif, corresponding to an allochthonous and metamorphosed ophiolite complex. In the surveyed area of this massif, the prevailing rock type consists of (serpentinised) peridotites, occasionally containing irregular bodies of (amphibolitised) gabbros, over which an irregular/ill-developed regolith exists. The activities of 40K, 137Cs, 210Pb, 226Ra and 232Th measured in samples representing three consecutive levels of the regolith upper portion in 13 stations confirm the heterogeneity put in evidence through in situ γ-ray surveys previously performed. The available dataset also demonstrates that radionuclide contents decrease prominently with depth, becoming closer to the bedrock values (up to ca. 41 Bq kg−1 for 40K in flaser-gabbros and near zero for all the remaining cases) but never reaching them. Therefore, the low/moderate radionuclide activity in the regolith upper portion reflect the contribution of sources that are independent of the lithological background; i.e. they indicate external and miscellaneous contributions to the “soil”-forming components, most of them coming from (dry or wet) atmospheric deposition. The chemical composition of the (serpentinised) peridotites and (amphibolitised) gabbros in the surveyed area, as well as their alteration products included in regolith, is homogeneous enough to be used as a faithful reference in monitoring studies of any kind. In addition, the significant 137Cs activities measured in some top “soil” samples (up to 92 Bq kg−1) strongly suggest that there are mineralogical specificities of the peridotite-derived regolith able to fix that radionuclide. This particular feature reinforces the existing criteria that favour the peridotites of Morais as a suitable host for a near surface repository of low to intermediate level radwaste.

Biotic, temporal and spatial variability of tritium concentrations in transpirate samples collected in the vicinity of a near-surface low-level nuclear waste disposal site and nearby research reactor

The results of a 21 month sampling program measuring tritium in tree transpirate with respect to local sources are reported. The aim was to assess the potential of tree transpirate to indicate the presence of sub-surface seepage plumes.Transpirate gathered from trees near low-level nuclear waste disposal trenches contained activity concentrations of 3H that were significantly higher (up to ∼700 Bq L−1) than local background levels (0–10 Bq L−1). The effects of the waste source declined rapidly with distance to be at background levels within 10s of metres. A research reactor 1.6 km south of the site contributed significant (p < 0.01) local fallout 3H but its influence did not reach as far as the disposal trenches.The elevated 3H levels in transpirate were, however, substantially lower than groundwater concentrations measured across the site (ranging from 0 to 91% with a median of 2%). Temporal patterns of tree transpirate 3H, together with local meteorological observations, indicate that soil water within the active root zones comprised a mixture of seepage and rainfall infiltration. The degree of mixing was variable given that the soil water activity concentrations were heterogeneous at a scale equivalent to the effective rooting volume of the trees. In addition, water taken up by roots was not well mixed within the trees. Based on correlation modelling, net rainfall less evaporation (a surrogate for infiltration) over a period of from 2 to 3 weeks prior to sampling seems to be the optimum predictor of transpirate 3H variability for any sampled tree at this site.The results demonstrate successful use of 3H in transpirate from trees to indicate the presence and general extent of sub-surface contamination at a low-level nuclear waste site.

Sorption of strontium and fractal scaling of the heterogeneous media in a candidate VLLW disposal site

Because of the deposit and accumulation from the debris flow, the heterogeneous geological characteris- tics is obvious for a candidate very low level waste (VLLW) disposal site, with the grain size ranging from tens of microns to 75 cm. Therefore, it is challenging to directly measure the sorption capacity of the media and the distri- bution coefficient of some radionuclides, such as strontium. We have studies the correlation of the particle mass content with different grade size and the sorption capacity, which is important in the modeling of radionuclide migration in the heterogeneous disposal site. A total of three deep pits and five shallow trenches were excavated, and 21 solid samples were collected for laboratory experiments. The grade and percentage of the different-sized particles were obtained, and the fractal dimension (D) of the media was calculated from the results of sieved experiments. Steady state sorp- tion time and sorption isotherm of strontium was deter- mined in the heterogeneous media, and sorption and distribution of strontium in the heterogeneous media were evaluated by the relationship between the mass percentage and distribution coefficient (Kd) of the fine-particle media, which was comprised of selected particles with a diameter less than 1 mm, and the correlation on the Kd and D was regressed fit. The results indicated that fractal dimension bounded from 2.39 to 2.62 in the media, and Kd values of strontium ranged between 119 and 126 in the fine-particle media, and corresponding value was 11 and 43 in the ori- ginal media. The correlation between Kd and D was approximately linear.

Locating a low-level waste disposal site

A model is developed to locate a waste disposal site for low-level (domestic and nontoxic industrial) waste. Account is taken of nuisance caused to population along routes and of equity considerations. Not only is equity as regards effects on different population centres considered, but also between carriers of waste from different towns giving rise to the concept of routing fairness. The resulting model problem is computationally complex and heuristic solution procedures are developed. These are applied to real data relating to the district of Algarve, Portugal, and results of numerical experiments are reported.

Modeling field-scale multiple tracer injection at a low-level waste disposal site in fractured rocks: Effect of multiscale heterogeneity and source term uncertainty on conceptual understanding of mass transfer processes

Multiple factors may affect the scale-up of laboratory multi-tracer injection into structured porous media to the field. Under transient flow conditions and with multiscale heterogeneities in the field, previous attempts to scale-up laboratory experiments have not answered definitely the questions about the governing mechanisms and the spatial extent of the influence of small-scale mass transfer processes such as matrix diffusion. The objective of this research is to investigate the effects of multiscale heterogeneity, mechanistic and site model conceptualization, and source term density effect on elucidating and interpreting tracer movement in the field. Tracer release and monitoring information previously obtained in a field campaign of multiple, conservative tracer injection under natural hydraulic gradients at a low-level waste disposal site in eastern Tennessee, United States, is used for the research. A suite of two-pore-domain, or fracture-matrix, groundwater flow and transport models are calibrated and used to conduct model parameter and prediction uncertainty analyses. These efforts are facilitated by a novel nested Latin-hypercube sampling technique. Our results verify, at field scale, a multiple-pore-domain, multiscale mechanistic conceptual model that was used previously to interpret only laboratory observations. The results also suggest that, integrated over the entire field site, mass flux rates attributable to small-scale mass transfer are comparable to that of field-scale solute transport. The uncertainty analyses show that fracture spacing is the most important model parameter and model prediction uncertainty is relatively higher at the interface between the preferred flow path and its parent bedrock. The comparisons of site conceptual models indicate that the effect of matrix diffusion may be confined to the immediate neighborhood of the preferential flow path. Finally, because the relatively large amount of tracer needed for field studies, it is likely that source term density effect may exaggerate or obscure the effect of matrix diffusion on the movement of tracers from the preferred flow path into the bedrock.

ATW neutronics design studies

The Accelerator Transmutation of Waste (ATW) concept has been proposed as a transuranics (TRU) (and long-lived fission product) incinerator for processing the 87,000 metric tonnes of Light Water Reactor (LWR) used fuel which will have been generated by the time the currently deployed fleet of commercial reactors in the US reach the end of their licensed lifetime. The ATW is proposed to separate the uranium from the transuranics and fission products in the LWR used fuel, to fission the transuranics, to send the LWR and ATW generated fission products to the geologic repository and to send the uranium to either a low level waste disposal site or to save it for future use. The heat liberated in fissioning the transuranics would be converted to electricity and sold to partially offset the cost of ATW construction and operations. Options for incineration of long-lived fission products are under evaluation. A six-year science-based program of ATW trade and system studies was initiated in the US FY 2000 to achieve two main purposes: (1) “to evaluate ATW within the framework of nonproliferation, waste management, and economic considerations,” and (2) “to evaluate the efficacy of the numerous technical options for ATW system configuration.” This paper summarizes the results from neutronics and thermal/hydraulics trade studies which were completed at Argonne National Laboratory during the first year of the program. Core designs were developed for Pb-Bi eutectic (LBE) cooled and Na cooled 840 MW th fast spectrum transmuter designs employing recycle. Additionally, neutronics analyses were performed at Argonne for a He cooled 600 MW th hybrid thermal and fast core design proposed by General Atomics Co. which runs critical for 3 4 and subcritical for 1 4 of its four year once-thru burn cycle. The mass flows and the ultimate loss of transuranic isotopes to the waste stream per unit of heat generated during transmutation have been calculated on a consistent basis and are compared. (Long-lived fission product incineration has not been considered in the studies reported here.)

Development of the Post-Closure Safety Case for the Low Level Waste Disposal Site at Drigg, United Kingdom

ABSTRACTThis paper provides an overview of the development of the post-closure safety case (PCSC) for the solid low level radioactive waste (LLW) disposal site at Drigg. The paper outlines the background to the site, the implementation of a systematic approach to understanding the site and undertaking a post-closure radiological safety assessment (PCRSA), communication activities, key factors in the PCSC and issues to be investigated further in the forward programme.The Drigg site is the UK's principal near-surface facility for disposal of LLW. Disposals commenced in 1959 to trenches whereas current practice involves disposal to concrete vaults. Disposals are carried out under an authorisation from the UK Environment Agency. Periodically the Environment Agency reviews the authorisation to ensure consistency with current regulatory requirements. British Nuclear Fuels plc (BNFL) will produce an updated PCSC in 2002; in preparation for this, BNFL published in March 2000 a status report on the development of the PCSC.BNFL and the Environment Agency conduct an information exchange process on the development of the PCSC. This process has led to notable improvements, as well as affording a mechanism for providing clarifications and building confidence in the PCSC. BNFL undertakes other communication activities including scientific publication, conference presentation, participation in international programmes (such as those of the IAEA and NEA) and liaison with local communities. Peer review of the PCSC is also being carried out.Central to the safety case is a systematic PCRSA, which promotes transparency and traceability and demonstrates a rigorous treatment of relevant uncertainties. The approach aligns with international best practice for near-surface disposal facilities and includes the assessment of features, events and processes and the use of tools such as conceptual model uncertainty forms. This formalisation helps to identify and understand the key factors for the PCSC. Additional qualitative or simple quantitative information is used to augment the PCRSA so as to provide a wider context to inform decision-making.

Results of an Aqueous Source Term Model for a Radiological Risk Assessment of the Drigg LLW Site, UK

A radionuclide source term model has been developed which simulates the biogeochemical evolution of the Drigg low level waste (LLW) disposal site. The DRINK (DRIgg Near field Kinetic) model provides data regarding radionuclide concentrations in groundwater over a period of 100,000 years, which are used as inputs to safety assessment calculations. The DRINK model considers the coupled interaction of the effects of fluid flow, microbiology, corrosion, chemical reaction, sorption and radioactive decay. The model simulates the development of a period of reducing conditions resulting from degradation of cellulose and steel wastes. Under these conditions U and Th remain as solubility controlling solids for periods over 30,000 years and provide an important source of daughter nuclides such as Ra. The fraction of 14C is followed through all reactions involving carbon. Less than 5% of 14C is present as mobile aqueous species.

Clastic dykes in over-consolidated tills: evidence for subglacial hydrofracturing at Killiney Bay, eastern Ireland

A swarm of vertical gravel-filled dykes up to 6 m high and several decimetres wide, cut through an over-consolidated till at Killiney Bay. The dykes are rooted in a gravel layer and many display plumes of clastic debris ejected into the overlying sediments — `burst-out structures'. Such features have not previously been described. These clastic dykes are interpreted as the infillings of hydrofractures which formed when water pressures in the basal gravel layer exceeded the overburden pressure and tensile shear strength of the capping till. The burst-out structures extend up to 7 m from the tops of the dykes and provide strong evidence for forceful upward flow. Evidence suggests that the hydrofractures formed subglacially, probably during a minor re-advance. Their presence in Late Devensian (26–13 ka BP) tills with an Irish Sea provenance (`Irish Sea till') may have important implications for the subglacial hydrology of the last Irish Sea ice sheet. These hydrofractures cannot form in unconsolidated glacimarine sediment and their presence precludes a glacimarine origin for these deposits. They greatly affect the geotechnical properties of Irish Sea tills, in particular providing very permeable routes through otherwise impermeable layers, with important consequences in situations elsewhere in the Irish Sea basin, where they have been used as aquicludes in landfill and low-level nuclear waste disposal sites.

Risk and impact tradeoffs in radioactive scrap metal management

Two management alternatives for radioactive scrap metal were evaluated: (1) recycling and reuse, and (2) disposal and replacement. The human health risks, environmental impacts, and sociopolitical issues potentially associated with these alternatives were assessed in an international context. For each alternative, the health risks from workplace and transportation accidents are greater in magnitude than the risks from potential exposure to radioactive materials or chemicals. The nonradiological risks are at least twice as high for disposal and replacement as they are for recycling, with the result that recycling has lower health risks overall. Environmental impacts from disposal and replacement of scrap metals are likely to be orders of magnitude higher than those for recycling. This is true of effects on land, water, and air quality, as well as for mineral and energy resources. In addition to risks and costs, issues affecting the choice between radioactive scrap metal management alternatives include low-level waste disposal site availability, public acceptability of recycling, potential impacts on metal markets, impacts of radioactivity on sensitive industrial uses of metal, and international equity issues.

Biological transport of radiocaesium in a semi-natural grassland ecosystem. 1. Soils, vegetation and invertebrates

An unused area of the British Nuclear Fuels plc low level waste disposal site at Drigg in Cumbria, together with a control site in Cheshire, have been used to investigate the behaviour of 137Cs in semi-natural grasslands over the period 1985–1988. Both sites showed significant inputs of 137Cs and 134Cs from the Chernobyl incident in 1986, estimated at up to 7330 Bq/m 2 at Drigg and less than 230 Bq/m 2 in Cheshire. The total deposit at Drigg was within the range of other observations in the area and could be explained without assuming any input from waste disposal operations on the site. Surface soil horizons showed the highest levels of 137Cs and 134Cs. During the study period, the dominant contribution to radiocaesium in soil and vegetation was from Chernobyl. Significant inter-specific variation in caesium concentrations of grasses was observed with an exponential decrease from June 1986 through to the summer of 1987, followed by a secondary peak in autumn 1987. Samples collected in the spring of 1988 showed 137Cs concentrations approaching pre-Chernobyl levels. Marked inter-specific and temporal differences in concentrations of radio-caesium were recorded for invertebrate populations. Radioactivity levels in herbivorous invertebrates were approximately proportional to levels in their diets with concentrations decreasing from the 1986 summer peak recorded after the input of Chernobyl radioactivity to the low levels observed during the summer of 1987. Herbivorous and predatory invertebrates showed similar concentrations of 137Cs but both groups were lower in radiocaesium than detritivorous species.

Monitoring the in-situ performance of a cement/bentonite cut-off wall at a low level waste disposal site

A cement/bentonite wall was emplaced around the north-western part of the Drigg Low Level Waste Disposal Site in Cumbria during 1988 as a low permeability barrier to limit the flow of water into and out of the disposal trenches. It is therefore important to monitor the cut-off wall and assess its performance. A monitoring programme was established based around a test cell arrangement adjacent to the main wall. Preliminary results from these cells suggest that the wall has an in-situ bulk permeability of 10 −8 m/s compared to 10 −9 m/s from laboratory measurements of samples taken from the wall when it was emplaced. Measurement of water table levels around the test cells suggest that the wall and associated trench capping are reducing the water present in perched aquifers in the trench area. Sorption studies suggest that the cut-off wall may also retard the transport of some actinides and fission products.

Importance of geologic characterization of potential low-level radioactive waste disposal sites

Using the example of the Geff Alternative Site in Wayne County, Illinois, for the disposal of low-level radioactive waste, this paper demonstrates, from a policy and public opinion perspective, the importance of accurately determining site stratigraphy. Complete and accurate characterization of geologic materials and determination of site stratigraphy at potential low-level waste disposal sites provides the frame-work for subsequent hydrologic and geochemical investigations. Proper geologic characterization is critical to determine the long-term site stability and the extent of interactions of groundwater between the site and its surroundings. Failure to adequately characterize site stratigraphy can lead to the incorrect evaluation of the geology of a site, which in turn may result in a lack of public confidence. A potential problem of lack of public confidence was alleviated as a result of the resolution and proper definition of the Geff Alternative Site stratigraphy. The integrity of the investigation was not questioned and public perception was not compromised.

Oxidation-Induced Geochemical Changes in Trench Leachates from the Maxey Flats Low-Level Radioactive Waste Disposal Site

A knowledge of extra-trench processes related to oxidation-induced geochemical changes that are likely to occur when iron-rich, anoxic trench waters encounter an oxidizing environment along a redox gradient is essential for modeling radionuclide transport at low-level waste (LLW) disposal sites. The results of laboratory oxidation experiments on several trench leachates from the Maxey Flats site show that, upon oxidation, a series of geochemical changes were initiated that resulted in a drastically different solute geochemistry, involving oxidation of ferrous iron and subsequent precipitation of ferric oxyhydroxide, changes in alkalinity and acidity, a drastic increase in redox potential (Eh), and generally relatively little change in the concentrations of 60Co, 137Cs, and 85Sr in solution. The observations made in this study have important geochemical implications for the modeling of LLW sites in that the source term as an input parameter cannot be assumed to be constant, both spatially and temporally. The acid-generating potential and buffering capacity of an anoxic source term are important geochemical controls that maintain a balance between acidity and alkalinity and largely determine the nature and extent of oxidation-induced geochemical changes likely to occur along a redox gradient. The presence of organic chelating agents can alter the source term geochemistry to such an extent that authigenic ferric oxyhydroxide, which represents a geochemical discontinuity at the redox interface along leachate migration paths, proves to be a relatively ineffective sink for radionuclides.

Biotic transport of radionuclides from a low-level radioactive waste site.

In the United States, concern for human exposures to radioactivity associated with the disposal of low-level radioactive waste has resulted in a series of regulatory guides, environmental assessments, management practices, and modeling tools. A large number of radionuclide transport processes and mechanisms that may contribute to human exposure have been modeled, using computer programs to make the required calculations. The objective of our work was to evaluate the relevance of potential biological transport processes in the assessment of potential impacts at low-level waste (LLW) disposal sites. As part of this effort, we developed an order-of-magnitude estimate for potential dose to man resulting from biological transport by burrowing animals and by plant translocation at a reference low-level waste site in the arid west. We also made comparative dose-to-man estimates for a more commonly considered human intrusion exposure scenario. Parameter values for defining a reference arid LLW disposal site and biotic transport processes are based on data reported in current literature. Estimates of waste volumes for the western United States are based on information described by the U.S. Nuclear Regulatory Commission in the Draft Environmental Impact Statement in support of 10 CFR Part 61. Our estimates of the dose-to-man resulting from biotic transport are of the same order of magnitude as those resulting from a more commonly evaluated human intrusion scenario. The previously assumed lack of potential importance of biotic transport at LLW sites in earlier assessment studies is not confirmed by our findings. Our results indicate that long-term biological transport processes have the potential to influence LLW site performance, and should be carefully evaluated as part of the impact assessment process.

Subsurface cobalt-60 migration from a low-level waste disposal site

After 28 years of continuous use, a low-level liquid radioactive waste infiltration pit at the Chalk River Nuclear Laboratories will soon be decommissioned. In a study undertaken to predict future releases of /sup 60/Co, ratios of sorbed to dissolved /sup 60/Co from 0.5 to greater than 130 mL x g/sup -1/ were found in the underlying aquifer. This ruled out the application of a simple K/sub d/ model for solution/sediment interactions. In subsequent, more detailed, studies a variety of techniques were used to characterize dissolved /sup 60/Co as predominantly (80%) hydrophilic weakly anionic organic complexes of intermediate molecular size. Both naturally occurring and contaminant organics appear to act as complexing ligands. Results of sequential desorption of sediment-bound radiocobalt show that at least three sorption mechanisms (ion exchange, sorption by iron oxide coatings, and ''fixation'') must be involved. On average, more than 80% of the sorbed /sup 60/Co is ''fixed''. Two possible scenarios of future release of sediment-bound /sup 60/Co are discussed. 20 references.

Geophysical Modeling of a Low-Level Nuclear Waste Disposal Site

ABSTRACTGeophysical measurements can, depending on the techniques employed, be used at low-level nuclear waste disposal sites to map the subsurface stratigraphy, identify interfaces, locate objects and targets, and aid in selecting locations for obtaining sample cores and placing monitoring wells. The geophysical techniques of ground penetrating radar and electrical resistivity have been applied in a complementary manner at nuclear waste disposal sites. The data obtained from the field surveys have been integrated in developing geophysical models of these sites.A two-layer electromagnetic geologic model has been developed that allows the prediction of radar attenuation and depth of penetration. Field tests conducted at a lowlevel nuclear waste disposal site show values consistent with the model.

Long-Term Monitoring of Closed Low-Level Waste and Uranium Mill Tailings Disposal Sites

The goals of radioactive waste disposal are reviewed and their impact on long term monitoring of closed low-level waste and uranium mill tailings disposal sites are described. It is shown that existing instruments are adequate for long-term monitoring and that advances in instrumentation can be accommodated as they become available. The important role of long-term monitoring in improving predictive models of system performance is described.

Natural Analogues—A Way to Increase Confidence in Predictions of Long-Term Performance of Radioactive Waste Disposal

ABSTRACTNo exact analogue to a radioactive waste disposal system exists but by studying natural analogues of the most important components or subsystems of a waste repository, confidence in long-term predictions can be increased. The U.S. Nuclear Regulatory Commission (NRC) is supporting research on igneous intrusions into proposed repository-type host rock (basalt, tuff, crystalline hard-rock, and salt), on uranium ore body analogues, on actinide solubility as a function of natural complexing agents in closed basin lakes and on field migration of radionuclide species at low-level waste and uranium mill tailings disposal sites. The NRC is studying analogues to ensure that important parameters which might be overlooked in laboratory tests and in modeling because of problems with scaling time, space and geologic complexity can be identified and their importance bounded.