Publications

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The primary purpose of the preclosure radiological safety assessment (that this document supports) is to identify risk factors for disposal operations, to aid in design for the deep borehole field test (DBFT) engineering demonstration.

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Disposal of used nuclear fuel and vitrified high-level radioactive waste (UNF and HLW) in a mined geologic repository is the preferred alternative for the countries with the largest inventories of UNF and HLW. However, deep borehole disposal (DBD) may be especially well suited for countries with small nuclear power programs because DBD is relatively inexpensive and scalable; whereas the threshold costs to develop a mined geologic repository are high and do not scale with the inventory. Historically, options for countries with small nuclear power programs (programs that individually generate only a few percent of the world total mass of UNF and/or HLW) have been: (1) to return the UNF to the supplier, (2) to have the SNF reprocessed, with return and incountry disposal of the resulting vitrified HLW in a mined geologic repository, (3) to develop in-country, direct disposal of the UNF in a mined geologic repository or (4) to send the UNF to a hypothetical multi-national mined geologic repository for disposal. However, in-country DBD is likely to be least expensive, and technically achievable with existing technology. In-country DBD could also be a viable alternative for disposal of used fuel assemblies from decommissioned research reactors in developing countries.

This paper considers concepts for disposal of canistered high-level (radioactive) waste (HLW) in large diameter deep boreholes. Vitrified HLW pour canisters are limited in diameter to promote glass cooling, and constitute a large potential application for borehole disposal where diameter is constrained. The objective for disposal would be waste packages with diameter of 22 to 29 inches, which could encompass all existing and projected HLW glass inventory in the United States. Deep, large diameter boreholes of the sizes needed have been successfully drilled, and we identify other potentially effective designs. The depth of disposal boreholes would be site-specific, and need not be as deep as the 5 km being investigated in the Deep Borehole Field Test. For example, a 0.91 m (36 inch) diameter borehole drilled to 3 km could be used for disposal from 2.5 to 3 km (8, 200 to 9, 840 ft). The engineering feasibility of such boreholes is greater today than was concluded by earlier studies done in Sweden and the United States. Moreover, the disposal concept and generic safety case have evolved to a point where borehole construction need not be as elaborate as previously assumed. Each borehole in the example could accommodate approximately 100 waste packages containing canisters of vitrified HLW. Emplacement of the packages would be through a 32-inch (0.81 m) guidance casing, installed in two sections to reduce hoisting loads, and forming a continuous pathway from the surface to total depth. Above the disposal zone would be a nominal 1 km (3, 280-ft) seal interval, similar to previously published concepts. Following those concept studies, the seal system would consist of alternating lifts of swelling clay, backfill and cement. Above the seal zone the borehole would be plugged with cement in the conventional manner for oil-and-gas wells. The function of seals in deep borehole disposal is to maintain the pre-drilling hydrologic regime in the crystalline basement, where groundwater is increasingly saline, stagnant, and ancient. Seals would resist fluid movement and radionuclide transport during an early period of waste heating, but after cooling little fluid movement is expected. Thus, the function of seals could be less important with HLW that has low heat output, and sealing requirements could be limited. The safety case for deep borehole disposal relies on the prevalence of groundwater that is increasingly saline with depth, stagnant, and ancient, in crystalline basement rock that has low bulk permeability and is isolated from surface processes. The minimum depth for disposal depends on sitespecific factors, and may be less than the 2.5 km example. Rough-order-of-magnitude cost estimates show that deep borehole disposal of HLW would be cost-competitive with the lowest cost mine repository options. Thinner overburden, and shallower development of conditions favorable to waste isolation, could make drilling of large-diameter disposal boreholes even more cost effective. The dimensions of the disposal zone and seal zone would be site specific, and would be adjusted to ensure that both are situated in unaltered crystalline basement rock.

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The Deep Borehole Field Test (DBFT) is a planned multi-year project led by the US Department of Energy's Office of Nuclear Energy to drill two boreholes to 5 km total depth into crystalline basement in the continental US. The purpose of the first characterization borehole is to demonstrate the ability to characterize in situ formation fluids through sampling and perform downhole hydraulic testing to demonstrate groundwater from 3 to 5 km depth is old and isolated from the atmosphere. The purpose of the second larger-diameter borehole is to demonstrate safe surface and downhole handling procedures. This paper details many of the drilling, testing, and characterization activities planned in the first smaller-diameter characterization borehole.

The Deep Borehole Field Test will include demonstration of the emplacement and retrieval of test waste packages (containing no waste) in a 5 km deep borehole drilled into the crystalline basement. A conceptual design for packaging, surface handling and transfer equipment, and borehole emplacement was developed in anticipation of the demonstration project. Test packages are designed to withstand external pressure greater than 65 MPa, at temperature up to 170°C. Two packaging concepts were developed: 1) flasktype for granular waste, and 2) internal semi-flush type for waste that is pre-canistered in cylindrical containers. Oilfield casing materials and sealing connections would be selected giving a safety factor of 2.0 against yield. Packages would have threaded fittings top and bottom for attachment of impact limiters and latch fittings. Packages would be lowered one-at-a-time into the borehole on electric wireline. This offers important safety advantages over using drill pipe or coiled tubing to lower waste packages, because it avoids the possibility of dropping a heavy assembly in the borehole. An electromechanical latch would release each package, or reconnect for retrieval. Frequency of waste package delivery to a disposal site could be the effective limit on emplacement throughput. Packages would be delivered in a shielded Type B transportation cask and transferred to a shielded, doubleended transfer cask on site. The transfer cask would be upended over the borehole and secured to the wellhead. The transfer cask would become an integral part of the pressure control envelope for well pressure control. Blowout preventers can be incorporated as needed for regulatory compliance. Operational safety has been assessed with respect to normal operations, and off-normal events that could cause package breach in the borehole. Worker exposures can be limited by using standard industry practices for nuclear material handling. The waste packages would effectively be robust pressure vessels that will not breach if dropped during surface handling. The possibility of package breach in the borehole during emplacement can be effectively eliminated using impact limiters on every package.

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This report documents conceptual design development for the Deep Borehole Field Test (DBFT), including test packages (simulated waste packages, not containing waste) and a system for demonstrating emplacement and retrieval of those packages in the planned Field Test Borehole (FTB). For the DBFT to have demonstration value, it must be based on conceptualization of a deep borehole disposal (DBD) system. This document therefore identifies key options for a DBD system, describes an updated reference DBD concept, and derives a recommended concept for the DBFT demonstration.

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This report documents conceptual design development for the Deep Borehole Field Test (DBFT), including test packages (simulated waste packages, not containing waste) and a system for demonstrating emplacement and retrieval of those packages in the planned Field Test Borehole (FTB). For the DBFT to have demonstration value, it must be based on conceptualization of a deep borehole disposal (DBD) system. This document therefore identifies key options for a DBD system, describes an updated reference DBD concept, and derives a recommended concept for the DBFT demonstration. The objective of the DBFT is to confirm the safety and feasibility of the DBD concept for long-term isolation of radioactive waste. The conceptual design described in this report will demonstrate equipment and operations for safe waste handling and downhole emplacement of test packages, while contributing to an evaluation of the overall safety and practicality of the DBD concept. The DBFT also includes drilling and downhole characterization investigations that are described elsewhere (see Section 1). Importantly, no radioactive waste will be used in the DBFT, nor will the DBFT site be used for disposal of any type of waste. The foremost performance objective for conduct of the DBFT is to demonstrate safe operations in all aspects of the test.