Nuclear Fuel Waste Research Articles

Leaching of Whole, Defected Used Candu™ Fuel Elements in Saline Solutions Under Argon Pressure

ABSTRACTIn the context of the Canadian Nuclear Fuel Waste Management Program, experiments were performed in which whole, intentionally defected used CANDU™ fuel elements were exposed to deoxygenated saline solutions at 100°C for 484 days. Results suggest that both 90Sr and 99Tc are preferentially released from grain boundaries and that 85Kr release is a good measure of the extent of grain-boundary dissolution. Severe grain-boundary attack in particular areas of the fuel is also evident from scanning electron microscope observations. It is hypothesized that such localized attack is related to fuel porosity. Because of its poor corrosion performance in saline solutions, the Zircaloy sheath cannot be considered as a radionuclide release barrier.

Development of materials specifications and emplacement procedures for backfilling nuclear fuel waste vaults

Atomic Energy of Canada Limited and Ontario Hydro are conducting a research program to assess the environmental impact and safety of the concept of disposal of nuclear fuel waste in an underground vault in plutonic rock of the Canadian Shield. The Vault Sealing Program, one of the components of the Nuclear Fuel Waste Management Program, is concerned with the development of materials specifications and emplacement procedures for backfilling the vault. Backfilling materials would surround the nuclear waste containers and fill all vault openings to minimize leaching and movement of radionuclides.

Review of the role of nuclear analytical chemistry in the Canadian nuclear fuel waste management program

The Canadian Nuclear Fuel Waste Management Program involves both field and laboratory research, to assess the concept of disposal of nuclear fuel wastes at depths of up to 1000 m in plutonic rocks. This research program is multifaceted and requires data from a large number of disciplines, such as geology, geochemistry, hydrogeology, civil and mining engineering, chemistry, solid state science, environmental science, and meteorology. In many of these disciplines, nuclear analytical chemistry plays a major role. Examples of the application of nuclear analytical chemistry, including radiochemical separation and instrumental neutron activation analysis, will be discussed.

History of actinide and minor element mobility in an Archean granitic batholith in Manitoba, Canada

Surface and borehole core samples from the Lac du Bonnet granite, Manitoba, Canada, have been analysed for major element concentrations,Fe 3+/Fe (total) ratios, rare earth element (REE) content and actinide isotopic abundances. This work forms part of the geological investigations of the Canadian Nuclear Fuel Waste Management Program, performed by Atomic Energy of Canada Limited (AECL). The study attempts to understand the history of, and processes governing, mobilisation of elements and naturally occurring radionuclides during high- and low-temperature alteration events in fluid-bearing fractures in the granite. One surface sample and two core samples (from ∼ 150 m and 730 m) are each in contact with fractures in the granite and show evidence of alteration events that penetrated the rock matrix over distances of at least 3 cm. Loss of Ca and Na is seen in cores from a depth of ∼ 150 m from the highly altered, hematite-rich rock adjacent to sub-horizontal fracture zones at the Underground Research Laboratory (URL) of AECL, near Lac du Bonnet. In contrast, K, Fe,Fe 3+/Fe and U concentrations increase towards the fracture surface due to formation of illite and association of U with hematite and the illite. At the fracture surface, U continues to increase, but Fe and theFe 3+/Fe ratio decrease indicating Fe removal by reduction. The REE also show some enrichment in more altered rock at intermediate depths, but the total REE concentration is lower than in the surface and deep core samples. No clear trends are visible for parent and fracture-surface REE in surface and deep core samples, however. Disequilibrium values of 234U/ 238U and 230Th/ 234U ratios in surface and intermediate depth core samples indicate that U has been mobilised in recent geological time (the last Ma), but Th has remained relatively immobile. High Th/U and 230Th/ 234U ratios in surface samples are indicative of rapid leaching of U but little isotopic fractionation, probably within the last 10 5 a. Apparently unaltered rock, several centimetres distant from the fracture in surface and intermediate- depth samples, has lost appreciable U, but evidence from U-series disequilibrium studies suggests that this process occurred more than one million years ago, perhaps during deuteric or hydrothermal alteration. Core from a fracture at depth in the granite shows little hematite or clay formation and lacks evidence of REE and recent or ancient actinide mobilisation. The U-series results are correlated with the observed concentrations and isotope activity ratios of U in groundwaters sampled from the same or adjacent fractures. Analyses of samples of highly altered rubble recovered from centre portions of fracture zones at the URL show both excesses and deficiencies of 234U and 230Th in neighbouring locations, possibly due to the presence of a redox front whose position is controlled by modern groundwater composition. The implications of these results are discussed for the concept of disposal of nuclear fuel waste at depth in plutonic rock on the Canadian Shield.

The role of isotope geochemistry studies in the Canadian Nuclear Fuel Waste Management Program

The objective of the Canadian Nuclear Fuel Waste Management Program is to develop and assess a method for the safe disposal of used nuclear fuel wastes from Canadian nuclear power generating facilities. The concept that is being developed involves the disposal of these wastes in a cavity, mined at a depth of 500–1000 m in a plutonic rock formation. Both in the assessment of this concept, and in any possible future site selection, the geochemistry of the rock formation and of the groundwater plays a significant role. Isotope geochemistry studies involve the determination of the distribution and behaviour of stable and radioactive isotopes in the groundwater and in the rock matrix. This information can be used to shed light on the geochemical history of the rock formation under investigation, and increase the understanding of the geochemical processes that govern the evolution of these rock formations. The information can, in principle, also be used to date the various geological events and processes that have led to the distribution of radioactive isotopes in the rock and in the groundwater.

Casting Development for Nuclear Fuel Disposal Containers

AbstractIn the Canadian Nuclear Fuel Waste Management Program, metals and alloys with low melting points are being evaluated for their potential application as cast matrices within used-fuel immobilization containers. Since a high integrity matrix is essential to ensure adequate internal support to the container shell under pressure and temperature conditions anticipated within an underground disposal vault, an extensive casting development program has been undertaken at the Whiteshell Nuclear Research Establishment. The program includes theoretical solidification model development supplemented by casting experiments that employ both unidirectional and multidirectional cooling of small and halfscale prototype containers.Experimental data obtained from the lead casting experiments were compared with model predictions and excellent agreement was obtained. Multidirectional cooling decreases the total solidification time by a factor of two and a half, compared to unidirectional cooling. The techniques develop...

Casting Development for Nuclear Fuel Disposal Containers

In the Canadian Nuclear Fuel Waste Management Program, metals and alloys with low melting points are being evaluated for their potential application as cast matrices within used-fuel immobilization containers. Since a high integrity matrix is essential to ensure adequate internal support to the container shell under pressure and temperature conditions anticipated within an underground disposal vault, an extensive casting development program has been undertaken at the Whiteshell Nuclear Research Establishment. The program includes theoretical solidification model development supplemented by casting experiments that employ both unidirectional and multidirectional cooling of small and halfscale prototype containers.Experimental data obtained from the lead casting experiments were compared with model predictions and excellent agreement was obtained. Multidirectional cooling decreases the total solidification time by a factor of two and a half, compared to unidirectional cooling. The techniques developed have been used to cast three, void-free half-scale prototype containers, two smaller containers and a small container with a CANDU fuel bundle.

87Sr/ 86Sr values in groundwaters of the East Bull Lake pluton, Superior Province, Ontario, Canada

The East Bull Lake pluton is a 2.48 Ga-old, layered mafic intrusion located on the southern margin of the Superior Province, Precambrian Shield in Ontario, Canada. The groundwaters have been studied as part of a detailed geological mapping and geochemical study of the pluton for the Canadian Nuclear Fuel Waste Management Program. All groundwater samples are dilute—the most concentrated Na Cl type water contains 3.1 g/l total dissolved solids. Present day 87Sr/ 86Sr values of the rock samples varies from 0.705 to 0.714 and the groundwaters from 0.711 to 0.720. The Ca HCO 3 recharge waters show the greatest range in values, in contrast to the deeper Na HCO 3 and deep Na Cl waters where a narrow range of 0.712–0.713 is found. Fracture-filling laumontite, calcite and gypsum have the same isotopic signature as the deep water. The Ca HCO 3 water is the youngest component of the groundwater system and its spread in isotopic values reflects local conditions at the surface. The Na Cl water is considered to be the oldest component and its Sr isotopic ratio value indicates equilibration with silicate fracture mineral phases. The isotopic data suggest that laumontite, calcite, and gypsum fracture phases have either crystallized from this water or are currently controlling its isotopic composition by dissolution. No age can be assigned to the deep waters, except that the Na Cl component may date from the Early Paleozoic, based on geological arguments or, more likely, be 10 4–10 6 years old based on kinetic studies of rock-water interaction and other isotopic data. If seawater was one component of the Na Cl water history, there is no record of this in the 87Sr/ 86Sr ratios.

Xps and Xrd Studies of Uranium Rich Minerals from Cigar Lake, Saskatchewan

AbstractSamples of uranium ore from the 1,300 million year old Cigar Lake deposits in northern Saskatchewan have been investigated using X ray Photoelectron Spectroscopy (XPS) and X ray Diffraction (XRD). The XPS spectra showed the peaks for U, Pb, 0, C, Si, Ca and Mg. Analysis of highresolution XPS spectra showed that the oxidation state of uranium in these minerals is equal to, or lower than, that of U3O7. The XRD data confirm the presence of an oxidized phase, most likely α U3O7., in addition to UO2. The low U(VI)/U(IV) ratio in these minerals is consistent with the low uranium concentration found in groundwater in contact with these uranium minerals. The relevance of these results to the disposal concept of used nuclear fuel in the Canadian Nuclear Fuel Waste Management Program is discussed.

Contents and sulphur isotope composition of trace sulphate and sulphide in various rock types

The in vacuo Kiba extraction technique for trace sulphate and sulphide has been extended to a variety of rocks including some altered granites provided by the Canadian Nuclear Fuel Waste Management Programme. Although the technique requires considerable development, it shows promise for determining the redox conditions, temperature, and the sulphur isotope composition of dissolved species in fluids involved in initial mineralization and subsequent alteration processes. This may in turn be used to identify sources of sulphur in fluids and provide information about long term weathering of resistant rocks. Preliminary concentration and sulphur isotope data for altered granite samples from the Underground Research Laboratory, Whiteshell, Manitoba suggest that sulphur of other than igneous origin is minor or absent.

Time-dependent microcraking in plutonic rock due to heat from a nuclear-fuel waste vault : In: Research and Engineering Applications in Rock Masses (paper to the 26th US Symposium on Rock Mechanics, Rapid City, 26–28 June 1985)V1, P479–486. Publ Rotterdam: A. A. Balkema, 1985

Time-dependent microcraking in plutonic rock due to heat from a nuclear-fuel waste vault : In: Research and Engineering Applications in Rock Masses (paper to the 26th US Symposium on Rock Mechanics, Rapid City, 26–28 June 1985)V1, P479–486. Publ Rotterdam: A. A. Balkema, 1985

Multicomponent Systems Tests on Used Fuel and Fuel Recycle Waste Glass at 200 °C

AbstractExperiments were performed at 200 °C and 8.5 MPa on used fuel and a borosilicate waste glass in contact with clay, container metals, groundwater and granite selected as reference materials in the Canadian Nuclear Fuel Waste Management Program. The experimental design considered five parameters that determine a fractional factorial design 25-1. The parameters varied were: waste form (used fuel or Moly 99“W” glass), cation exchange capacity of the clay (calcium bentonite, 1360 meq/kg or illite, 160 meq/kg), ionic strength of the groundwater (Standard Canadian Shield Saline Solution, SCSSS, 1.4 mol/L or granitic groundwater GGW, 1 × 10−3 mol/L), surface area to volume ratio (SA/V) (120 or 12 m−1), and container material 3Inconel 625 or Ticode 12). The results show that (i) maximum release of 137Cs from used fuel and waste glass occurs in the presence of illite and GGW, (ii) maximum releases of 90Sr and the actinides from used fuel occur in the presence of bentonite and SCSSS, and (iii) maximum release of 90Sr from the waste glass occurs in the presence of illite and GGW.

Radionuclide migration over recent geologic time in a granitic pluton

Measurements of isotope activity ratios in the 238U decay series have been made for core samples obtained from boreholes up to 1-km depth in the Eye-Dashwa Lakes granite pluton, Atikokan, northwest Ontario, Canada. This work was done to determine the significance of recent radionuclide migration in plutonic rocks for the Canadian Nuclear Fuel Waste Management Programme. Most samples displayed secular equilibrium between 238U, 234U, 230Th and 226Ra, indicating that there has been no significant radionuclide migration in these samples over periods as long as the last 1 Ma. Disequilibrium was observed in some altered near-surface and deeper fracture-filling mineral samples with 234U deficiencies of up to 20%. A similar deficiency of 226Ra was found at ∼ 1-km depth in a gypsum-infilled core sample. These results indicate that U and Ra leaching by groundwater has occurred within periods 1 Ma and 8 ka ago, respectively. One instance of 226Ra excess was found at 94-m depth, indicating deposition of 226Ra in the last 8 ka. All cases of disequilibrium involved fracture-infilling minerals or altered granite cores from fracture zones, indicating that radionuclide migration occurred via fractures. Examples were found, however, of altered samples or mineral infillings that showed no detectable disequilibrium. These locations have been closed to migration in the last 1 Ma. Relative age estimates are made for some of the infilling minerals, based on their U-series isotopic characteristics.

Metal-Matrix Corrosion Studies in Canadian Shield Granitic Groundwaters

AbstractThe Canadian Nuclear Fuel Waste Management program is evaluating two basic supported-shell container concepts for used-fuel immobilization.One concept uses a compacted particulate material within the container to sunport a thin, corroston-resistant shell. The other concept, called “the metal-matrix concept”, uses a cast-in-place metal matrix for shell support.This paper describes corrosion studies on the candidate matrix materials, lead, zinc and aluminum-7 wt.7 Si (A1-7 wt.7 Si), in the temperature range 293 to 423 K and the dissolved oxygen content range 8 to 410 mg/L. Test specimens were analyzed for general and localized corrosion and the corrosion products were identified.The results showed that, unlike lead, zinc and Al-7 wt.% Si are susceptible to localized corrosion. Lead showed only low general corrosion rates of 2 to 83 pm/a compared with zinc (36 to 15800 μm/a) and Al-7 wt.% Si (6 to 209 μm/a) in the above temperature and oxygen content ranges. Maximum pit depths of about 0.9 mm on Al-7 wt.% Si and 0.6 mm on zinc were observed. These results indicate that, of the candidate matrix metals studied, lead has the greatest potential to retard radionuclide release from used-fuel disposal containers.

Uranium Recovery from a Nuclear Fuel Waste Form

Uranium Recovery from a Nuclear Fuel Waste Form

Estimations Of Changes in Microcrack-Population, Elastic Modulus, and Permeability, Due to Differential-Thermal Expansion in Plutonic Rock Surrounding A Nuclear Fuel Waste Disposal Vault

AbstractAtomic Energy of Canada Limited is studying the disposal of nuclear fuel-waste in a vault in plutonic rock. An important part of this study is to develop models to assess the rate of escape of waste components to the biosphere. In these models time-dependent changes in the system are important. One such change is the increase in temperature of the rock mass immediately surrounding the vault, and in the far-field. Differential- thermal expansion between rock constituents (crystals), and the expansion of trapped water, will produce microcracking and, hence, possibly changes in intact-rock elastic modulus and permeability. This paper describes the model devised to estimate the extent of microcracking in rock surrounding a vault, and the anticipated changes in rock elastic modulus and permeability. The model is applied to a possible vault, located at various depths in plutonic rock typical of the Canadian Shield. It estimates the dimensions of an envelope outside of which the influence of the vault on microcracking will be negligible.

A Review of Progress in the Canadian Nuclear Fuel Waste Management Program

AbstractThe Canadian Nuclear Fuel Waste Management Program is reviewed, illustrating the progress that has been made in assessing the concept of disposal of nuclear fuel waste in plutonic rock of the Canadian Shield. Research is being conducted into used fuel storage and transportation, fuel waste immobilization, site characterization and selection methods, and performance assessment modelling. Details of achievements in these areas are outlined, and results of the most recent interim assessment are discussed.

The Development of Durable Cementitious Materials for Use in a Nuclear Fuel Waste Disposal Facility

AbstractThis paper describes the work on cement paste development and short-term leaching tests in Standard Canadian Shield Saline Solution (SCSSS) in the presence of bentonite at 150°C. It has been found that:-supplementary cementing materials such as silica fume or fly ash could significantly improve the properties of sulphate resistant portland cement (SRPC), in particular, permeability to water and pore size distribution.-the addition of bentonite suppressed the normal tendency of the pH of groundwater to increase rapidly in the presence of cement.-the presence of bentonite increased the release of potassium ions from the cements.-– SRPC blended with 20% silica fume resulted in a groundwater pH lower than that of SRPC, with and without bentonite. Moreover, its cumulative fraction of release of potassium was significantly lower than that of SRPC.

Monitoring Hydrogeologlcal Conditions in Fractured Rock at the Site of Canada's Underground Research Laboratory

AbstractAtomic Energy of Canada Limited is constructing an Underground Research Laboratory (URL) at a depth of 250m in a plutonic rock body near Lac du Bonnet, Manitoba. The facility is being constructed to carry out a variety of in situ geotechnical experiments as part of the Canadian Nuclear Fuel Waste Management Program.A unique feature of the URL, in comparison to other similar facilities such as the Stripa Mine in Sweden, is that it is to be constructed below the ground water table in a previously undisturbed plutonic rock body. One of the main research objectives of the project is to develop and validate comprehensive three‐dimensional models of the hydrogeology of the rock mass encompassing the URL site. These models will be used, before excavation of the URL shaft begins, to predict the hydrogeological perturbation that will be created by the excavation of the shaft and the horizontal working levels below the ground water table. As a model‐validation exercise, these drawdown predictions will be compared with actual hydrogeological perturbations that will be monitored at the study area over the next several years by an extensive network of instrumented boreholes.Measurements made in an array of boreholes extending to depths of 1,000m on the 4.8 km2 study area have established that the permeability distribution in three major extensive subhorizontal fracture zones controls the movement of ground water within the rock mass. Several types of multiple‐interval completion systems have been installed in the boreholes to monitor the three‐dimensional, physico‐chemical hydrogeological conditions within the fractured rock mass. These include conventional piezometer nests and water‐table wells that have been installed in shallow holes (less than 30m deep), and multiple‐packer/ multiple‐standpipe piezometers and multiple‐interval casing systems installed in deeper holes (30 to 1,000m deep). An automated, electronic, piezometric pressure‐monitoring system has been designed to collect continuous measurements from 75 isolated hydrogeological monitoring positions within the rock mass. Another 200 positions are being monitored frequently using a variety of techniques. Piezometric data have been collected from this monitoring network to establish baseline conditions prior to any excavation into the rock mass. These data have also been used to determine the steady‐state, three‐dimensional ground water flow regimes that exist at the URL site under natural conditions.

Immobilization and geological disposal of nuclear fuel waste.

The Canadian Nuclear Fuel Waste Management Program is developing methods for the safe disposal of both used nuclear fuel and fuel recycle waste. The disposal strategy is based on interim storage of the used fuel, immobilization of either used fuel or recycle waste, and disposal, deep in a stable geological formation in the Canadian Shield. The disposal concept proposes a multibarrier system to inhibit the release of the radioactive waste from the disposal vault. The principal components of the multibarrier system are (i) the waste form in which the radionuclides are immobilized, (ii) engineered barriers including high integrity containers, buffers and backfills designed to retard the movement of groundwaters in the disposal vault, and (iii) the natural barrier provided by the massive geological formation itself. The research programs to investigate this concept are discussed briefly. Several different waste forms are being developed for the immobilization of high-level fuel recycle waste, including glass, glass-ceramics and crystalline materials. Dissolution of these materials in groundwater is the only likely scenario that could lead to radionuclide release. The factors that influence the aqueous dissolution behaviour of these materials are reviewed.