Research The Introduction of Microbial Nutrients into A Nuclear Waste Disposal Vault during Excavation and Operation SIMCHA STROES-GASCOYNE* AND MEL GASCOYNE Atomic Energy of Canada Limited, Whiteshell Laboratories, Pinawa, Manitoba R0E 1L0
AECL has developed a concept for permanent geological disposal of used nuclear fuel waste in Canada. This concept would involve disposal of the waste in corrosionresistant metal containers, surrounded by compacted claybased buffer and backfill materials, in a vault 500-1000 m deep in granitic rock of the Canadian Shield. Such a vault would not be a sterile environment. Microbial activity would be expected in those areas of a vault where effects of heat, moisture content, and radiation would not exclude microbial life and where sufficient nutrients would be present. Although the granitic rock environment is naturally nutrient-poor, a substantial amount of nutrients could be introduced from residues of explosives used in the excavation of a vault. Using standard rock leaching techniques, measurements of the concentrations of such residues were made in excavated rock, tunnel walls and mine service-water supplies at AECL’s Underground Research Laboratory. The effects of these residues on the bacterial population size in groundwater were also determined. Results showed that the largest potential nutrient addition (both N and C) to a vault would result from using untreated broken rock as part of the backfill. Nitrate in the residues could increase groundwater bacterial populations by several orders of magnitude.
Introduction Atomic Energy of Canada Limited (AECL) has developed a concept for the permanent disposal of nuclear fuel waste (1). The concept involves disposal in an engineered excavation (vault), at a depth of 500-1000 m in plutonic rock of the Canadian Shield. Fuel wastes would be isolated in corrosionresistant metal containers, which would be emplaced, surrounded by compacted buffer material, either in disposal rooms or in boreholes drilled in the floor of the rooms. After emplacement, the rooms would be backfilled with a mixture of 75 wt % crushed and graded host rock and 25 wt % glacial lake clay. Such a vault would not be sterile, and microbial activity could potentially affect the integrity of the multiple barrier system on which this concept is based (2, 3). The awareness that microbial activity could affect the performance of a system designed for subsurface disposal of nuclear waste gained acceptance in the early to middle 1980s. * Author to whom communication should be directed. Email:
[email protected]; Fax: 204-753-2455. S0013-936X(97)00496-3 CCC: $15.00 Published on Web 02/01/1998
1998 American Chemical Society
As a result, many countries considering radioactive waste disposal now started programs to study and quantify potential microbial effects, including microbially influenced corrosion, microbial effects on radionuclide transport and microbial gas production. Progress made in this field of research has been reviewed recently (2, 4). A large amount of important observations regarding the occurrence, viability, and survival of microorganisms in subsurface environments has been reported recently from geomicrobial investigations in deep aquifers under the auspices of the United States Department of Energy Deep Subsurface Science Program (e.g., refs 5-10). The emerging information is of utmost importance for the assessment of the potential effects of microbial growth and activity on geological disposal of nuclear waste. From a microbial point of view, subsurface environments are generally nutrientpoor, but the excavation of a vault for nuclear waste disposal may change this, with consequences for increased microbial activity and growth. This paper quantifies the potential for growth stimulation in the backfill of a Canadian nuclear fuel waste disposal vault as a result of the introduction of nutrients during vault construction and operation. Microorganisms can grow in any environment in the presence of liquid water if their nutrient and energy requirements are met and if that environment can be tolerated physiologically. The pre- and postclosure conditions in a Canadian nuclear fuel waste disposal vault [95 °C maximum temperature at the container skin; ∼12 MPa maximum pressure in the buffer/backfill and a maximum radiation dose rate of 52 Sv/h at the container skin (2)] may not be extreme enough to inhibit the survival of either introduced or naturally present organisms. Survival would be limited spatially because of the compacted nature of the buffer material surrounding the waste containers. Conditions would be less harsh further away from the waste containers, in the backfill, where the presence of nutrients and energy sources would then largely determine the extent and activity of the microbial population (2, 3). The quantity of nutrients and energy in a Canadian vault would be governed largely by the contribution from two sources, both introduced during the period of excavation and operation: the emplaced waste and surrounding barrier materials and other material introduced inadvertently (e.g., paint, oils, grease). Examination of the total nutrient and energy inventories of container materials, buffer, backfill, and associated groundwater and air that would be intentionally emplaced in a Canadian disposal vault showed that N and/or P were the growth-limiting nutrients, based on the assumption that all C present was available for microbial use (2). A maximum microbial population size was calculated on the basis of the limiting nutrient, and a sequence of reactions that could provide sufficient energy to sustain such a population was proposed. The analysis did not take into account the amount of nutrients that could inadvertently be introduced to a vault environment during the excavation and construction stages such as N and C from explosives and process water. Forsyth et al. (11) have recently shown that the main source of NO3- pollution in mine water and mine water effluent is the explosives used in the mining process. Nitrates can be introduced into water in the mine and at the waste rock disposal site. Nitrate contamination results from spillage during explosives transport and charging of the blastholes, from leaching of the explosives in blastholes containing water VOL. 32, NO. 3, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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and from undetonated explosives in the broken rock after a blast. In a nuclear fuel waste disposal vault, water would be ubiquitous in the environment and in construction processes. Nitrate contamination of water sources would, therefore, be present if explosives were used for excavation. In addition to spills and undetonated material in the broken rock, dissolved NO3- and/or gaseous N products could enter microfractures in tunnel walls (i.e., in the excavation damage zone) and in the broken rock. In a nuclear fuel waste disposal vault, this broken rock could be recycled and emplaced in the vault as part of the large amount of backfill needed to fill the vault (backfill would consist of 75 wt % crushed and graded host rock). The potential for development of significant microbial activity in a vault, therefore, would depend largely on the availability of nutrients (N, C, P) that are left in vault materials as residues of blasting operations. This paper describes the types of blasting residues, how they become incorporated in materials used in a nuclear waste vault, and the potential they create for microbial activity and growth in a vault. Estimates are given based on experience gained in excavating tunnels in Canada’s Underground Research Laboratory (URL), near Lac du Bonnet, southeastern Manitoba.
Materials and Methods for Excavation and Analysis Types and Compositions of Explosives. Modern commercial explosives generally contain a fuel and an oxidizer. Oxidizing agents are typically ammonium nitrate (NH4NO3), calcium nitrate [Ca(NO3)2], and sodium nitrate (NaNO3). Commonly used explosives can be divided into three groups: ANFO (ammonium nitrate and fuel oil), watergels/slurries, and emulsions. All contain significant amounts of N and C, but have different resistances to dissolution in water and, therefore, varying degrees of capacity to introduce N and C into the groundwater system (11). A basic ANFO mixture consists of 94% NH4NO3 and 6% fuel oil, giving a total of 33 wt % of N in two very watersoluble forms (NH4+ and NO3- ions). ANFO has no resistance to dissolution, and its N is readily soluble upon exposure to water. In addition, NH4NO3 is hygroscopic and will pick up moisture from the air if left exposed. If the ANFO absorbs too much water it may become desensitized, fail to detonate, and result in residual explosive in the broken rock (11). Typical watergels/slurries contain 20-30 wt % N, as water-soluble NH4+ and NO3- ions. Their resistance to dissolution is good due to a gelling gum that forms a barrier between the oxidizing agents and any external water. Emulsions also contain 2030 wt % N, in the form of NH4+ and NO3- ions, but are very resistant to dissolution because a thin film of oil surrounding the salt solution minimizes contact with external water sources (11). Commercial explosives currently used in mining and used in the excavation of the URL are (1) Amex II, a free-flowing mixture of granular NH4NO3 and diesel oil containing approximately 94% NH4NO3 by weight; (2) Lomex III, a freeflowing mixture of granular NH4NO3, emulsion explosive and polystyrene beads (as a filler) and containing half the amount of NH4NO3 of Amex II (about 47 wt %); (3) Primaflex, a reinforced detonating cord comprised of a mixture of PETN (pentaerythritol tetranitrate) and TNT (trinitrotoluene); (4) Xatec, a semigelatin dynamite (nitroglycerin-based) supplied in long, thin cartridges; and (5) Forcite 75%, a general-purpose ammonia gelatin dynamite (nitroglycerine-based). It also comes in cartridge form. Excavation Methods in the URL. AECL began construction of the URL in 1984, as part of the work done for the Canadian Nuclear Fuel Waste Management Program, to develop methods of site characterization and vault construction techniques. The URL is constructed in the Archean granitic rocks of the Lac du Bonnet Batholith and is located 318
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on the western edge of the Canadian Shield, 100 km northeast of Winnipeg, Manitoba. The URL shaft is 450 m deep and has two main levels developed for geotechnical investigations at 240 and 420 m below the surface (Figure 1). Two periods of excavation at the 240 m level of the URL (Figure 1) have been studied. In the first period (November, 1993, to March, 1994), about 120 m of access tunnel with a 3.3 m × 3.3 m cross-section was excavated (Excavation 1). In the second period (July-November, 1994), about 40 m of tunnel of similar dimensions, ending in a larger room for construction of a laboratory, was excavated (Excavation 2). These excavations have been studied to provide data on blasting residues and their incorporation in vault materials. Each blast round typically advanced the excavation by 3.2 m. The blastholes (3.6 m long and 41 mm diameter) were percussion-drilled using URL service-line water as the drilling fluid. Most central (production) blastholes were loaded with Amex II. Surrounding (cushion) blastholes were loaded with Lomex III, and primed with one cartridge of Forcite. The outer (perimeter) blastholes were loaded with two cartridges of Forcite and 4 m of Primaflex detonating cord. The bottom (lifter) blastholes were loaded only with cartridges of Forcite. Typically, about 27 000 L of water and 8 L of lubricating oil were used and discharged during the drilling of a blast round. After each blast, the newly exposed walls of the excavation were washed to remove fines and residues, and additional water (estimated at about 300 L/blast) was then used to wet the broken rock (known as “muck”). The total amount of water discharged during excavations was, therefore, 28 300 L/3.2 m tunnel advance, or about 8850 L/meter of tunnel advance. For an assessment of N contamination as a result of these particular excavations, only the more soluble explosives (e.g., ANFO) are taken into account. The total amount of NH4NO3 used in excavating 122 m of tunnel (Excavation 1) is estimated to be 3760 kg. Nutrient Residues. To determine the types and quantities of nutrients retained in vault materials, information was obtained from the URL excavation activities on the following. (1) Quantity and Composition of Blasting Materials. Although exact compositions of explosives are proprietary, about 10 kg of N and 1.5 kg of C were used per meter of tunnel excavated. (2) Chemical Reactions during a Blast. Explosives are compounds of C, H, O, and N. For optimal blasting, energy release is maximized at zero oxygen balance, the point at which there is sufficient oxygen to completely oxidize all the fuels in the mixture, but no excess to react with N in the mixture to form nitrogen oxides (12). Theoretically, at zero oxygen balance, the gases produced upon detonation of an ANFO explosive are H2O, CO2, and N2:
3NH4NO3 (94.5%) + CH2 (5.5%) f 7H2O + CO2 + 3N2 + ∼0.94 kcal/g (1) where CH2 represents the fuel oil component of ANFO. Other additives to explosives, such as antacids, absorbents, stabilizers, paper cartridges, paraffin coatings, and plastic borehole liners, are mostly consumed by the detonation to produce mainly CO2. In reality, small amounts of NO, NO2, CO, NH3, CH4, and other gases are generated by reactions, such as
2NH4NO3 (92.0%) + CH2 (8.0%) f 5H2O + CO + 2N2 + ∼0.82 kcal/g (2) 5NH4NO3 (96.6%) + CH2 (3.4%) f 11H2O + CO2 + 4N2 + 2NO + ∼0.61 kcal/g (3) It is the production of NH3 and NOx gases that create additional potential for entrainment of nutrients in vault
FIGURE 1. Schematic view of AECL’s Underground Research Laboratory, southeastern Manitoba showing location of Excavations 1 and 2, and the dry-drilled boreholes SL1-1 to SL1-4. materials, because these gases are capable of penetrating rock surfaces under the pressure of a blast and are readily soluble in mine waters. (3) Effectiveness of Consumption of Blasting Materials. The effectiveness with which blasting materials are consumed during a blast varies with the type and hardness of the rock, the skill of the blast-round designer, and the care taken in drilling, loading, wiring, and initiating the blast. Blastholes may fail to detonate for a variety of reasons. Advanced blast monitoring routinely shows that 10-20% of blastholes misfire (known as misholes) in a given blast (11), and losses could amount to between 5 and 15% of total ANFO used (13). In a particular mine, it was found that as much as 5% of total ANFO used per month ended up in the water system (11). For typical URL excavations, it has been estimated that 2-4% of the total ANFO used is spilled as a result of loading and
misholes. This percentage is low compared to normal mining operations, due to stringent handling procedures and careful blast design and implementation at the URL. (4) Fate of Gases Generated by a Blast. The quantity of gases generated by a blast can be calculated from eq 1. These gases are normally entirely removed by venting to the atmosphere. In the blasting operations at the URL, venting occurs through the existing tunnels at the 240 m level to the upper ventilation shaft (the vent raise, Figure 1). Some gases may be entrained by dissolution in seepage water, which enters the lower part of the vent raise at a flow rate of about 5 L/min, but because of the high airflow (∼10 m3/s), only a small amount of gas will be dissolved and returned to the URL system. Some of the vent raise water flows down the entire length of the vent raise, and so may pick up N-residues that have deposited on the walls. A secondary effect of gas VOL. 32, NO. 3, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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dissolution by seepage water is the production of carbonic acid (H2CO3) and nitrous and nitric acids (HNO2, HNO3) from the reaction of CO2 and NO2 with water. Shifts in the pH of vent raise water might therefore be expected and could be used to indicate the magnitude of gas dissolution during venting after a blast. For one of the blasting events during Excavation 1 (February 10, 1994), measurements were made over a 20-h period of the pH and NO3- and NO2- concentrations in water draining down the length of vent raise. The pH of vent raise water was determined at 5-15 min intervals using a flowcell, pH sensor, and autoprinter to record the results. Samples of vent raise water were taken over this period for NO2- and NO3- analysis using an automatic sampler and were analyzed by ion chromatography. (5) Residues in the Broken Rock. The rock broken by a blast provides a high surface area and microfractures that may absorb blasting residues, contaminated water and gases. It is difficult to calculate the total surface area exposed, and therefore, calculations are best done on a rock mass basis. Estimates of the types and quantity of residues that might be retained or emplaced in the excavated rock (muck) have been made by analyzing samples from the two periods of excavation at the URL. Muck samples were prepared for analysis by breaking with a hammer, crushing in a jawbreaker, powdering in a shatter-box, and sieving to various mesh sizes in the range 106-44 µm. Weighed portions of the dried powders [5 g in 15 mL water for NO2- and NO3analysis; 10 g in 30 mL water in glass bottles for dissolved organic carbon (DOC) analysis] were shaken for 5 min and centrifuged. The leachates were stored at 4 °C to minimize bacterial usage of NO3- or C and analyzed for anion content (mainly NO2- and NO3-) by ion chromatography, and for DOC content using an Astro 2001 C analyzer. Groundwaters were analyzed for anions and DOC in a similar manner. (6) Residues in Tunnel Walls. Blasting residues (gases and undetonated solids) may penetrate fractures and microfractures in the walls of an underground excavation, due to the pressure of the blast and infiltration of wash waters during mucking-out operations. To determine the amount of N and C that may have penetrated the excavation-damaged zone of tunnel walls, four 1-m-long cores (∼6-cm diameter) were drilled from tunnel walls at four locations in the URL (Figure 1), using only compressed air to cool the coring bit. (The use of water as coolant would have caused contamination and leaching of NO3-, C, and other soluble salts.) Cores SL1-1 and SL1-4 were drilled at the 240 and 420 levels of the URL, respectively, and are from relatively new tunnels excavated in the last two years. SL1-2 and SL1-3 were drilled at the 420 and 240 levels of the URL, respectively, and are from older tunnels excavated more than five years ago. The cores were sampled at ∼10 cm intervals in the laboratory, commencing at the rock face. Each sample was crushed and sieved to a mesh size of 45 mg/L (the environmental discharge limit). The volume of the holding pond was 800 000 L. Of this volume, approximately 10% was discharged at weekly intervals, thereby removing NO3- from the URL system. Over the full period of the excavation, holding pond NO3concentrations increased to a maximum of about 135 mg/L. The total amount of NH4NO3 used in the excavation was 3758 kg, containing 1315 kg of N. Measurement of NO3concentrations and volumes of holding pond water discharged over this period show that the total amount of NO3discharged from the holding pond was 159 kg (15). This corresponds to 36 kg of N and represents 2.7% of total N used in the excavation. This value compares favorably with
FIGURE 3. Variation of (a) NO3- and (b) DOC concentrations in URL service waters following underground blasting for Excavation 1. the figure from ref 11, where it was indicated that as much as 5% of total ANFO used entered the water system every month at a particular mine. The values for the URL show that 1.3 kg of NO3- (or 0.3 kg of N) was discharged to the surface environment per meter length of tunnel excavated. Figure 2 also shows that, during the time it was measured, the DOC concentration in the water discharged from the holding pond remained virtually constant between about 5 and 10 mg/L. Assuming FO is 6 wt % of the total amount of Amex II used, the amount of FO used was about 63 kg, which contained about 54 kg of C. If a similar percentage of FO entered the holding pond water as occurred for NH4NO3 (i.e., 2.7%), this would have caused a DOC increase of about 1.5 mg/L. Such a small amount is not clearly visible due to the variability of the DOC measurements. For a definite increase to be measured, the DOC would have to increase by about 5 mg/L, or ∼7% of the total amount of organic C (54 kg) used. Therefore, the amount of FO released to the holding pond was significantly less than ∼7% of the total amount used during this period.
Comparisons of NO3- and DOC concentrations in discharged holding pond water, mine service, and wall wash water during the first month of blasting are shown in Figure 3. Nitrate concentrations in the service and holding pond water are similar (Figure 3a) because the water source is the same. The wall wash samples showed a range of NO3concentrations varying from service water levels (∼40 mg/L) to concentrations as high as 2000 mg/L, with most values
