[en] The effect that thermal loading has on the natural and engineered systems needs to be understood and demonstrated with reasonable assurance in the Viability Assessment and the License Application process for a potential underground high level waste repository at Yucca Mountain. Thermal loading can be defined in a number of ways but it basically is the amount of decay heat from the spent nuclear fuel produced per unit area and is related to the emplacement density of fuel. This paper provides an overview of the status of the development of the technical basis for a thermal loading decision for a potential repository at Yucca Mountain and emphasizes recent analyses conducted

[en] This experiment consisted of the construction, operation, and data analysis for a three-counter prototype station for a cosmic ray air shower array. Large-area (6 m²) scintillation counters were constructed and calibrated. A special calibration procedure was shown to be necessary for large-area (greater than or equal to 4 m²) cosmic ray counters. Three large-area (3 m²) counters were arranged at the vertices of a triangle 300 m on a side and shower data was obtained. Particle densities and relative timing among the counters was recorded. In order to analyze the data from so few counters, a theoretical model of the longitudinal and lateral electronic structure of nuclear primary air showers was developed. 427 showers were fully analyzed. These showers ranged in ground level size from 2.5 x 10⁵ to 2.5 x 10⁷ particles and in primary energy from 1.0 x 10¹⁵ to 2.5 x 10¹⁷ eV. From these data the primary integral spectrum is obtained: J(greater than W₀) = J₀/W₀/E₀/sup ^-GAMMA/ where W₀ is the primary energy in eV and E₀ is 1.0 x 10¹⁷ eV. Three choices of shower model parameters yielded spectral exponent values of 1.97 +- 0.13, 2.00 +- 0.15, and 1.95 + 0.16

[en] The ability to meet the overall performance requirements for the proposed Mined Geology Disposal System at Yucca Mountain, Nevada requires the two major subsystem (natural barriers and engineered barriers) to positively contribute to containment and radionuclide isolation. In addition to the postclosure performance the proposed repository must meet preclosure requirements of safety, retrievability, and operability. Cost and schedule were also considered. The thermal loading strategy chosen may significantly affect both the postclosure and preclosure performance of the proposed repository. Although the current Site Characterization Plan reference case is 57 kilowatts (kW)/acre, other thermal loading strategies (different areal mass loadings) have been proposed which possess both advantages and disadvantages. The objectives of the FY 1993 Thermal Loading Study were to (1) place bounds on the thermal loading which would establish the loading regime that is ''too hot'' and the loading regime that is ''too cold'', to (2) ''grade'' or evaluate the performance, as a function of thermal loading, of the repository to contain high level wastes against performance criteria and to (3) evaluate the performance of the various options with respect to cost, safety, and operability. Additionally, the effort was to (4) identify important uncertainties that need to be resolved by tests and/or analyses in order to complete a performance assessment on the effects of thermal loading. The FY 1993 Thermal Loading Study was conducted from December 1, 1992 to December 30, 1993 and this final report provides the findings of the study. Volume 1 contains the Introduction; Performance requirements; Input and assumptions; Near-field thermal analysis; Far-field thermal analysis; Cost analysis; Other considerations; System analysis; Additional thermal analysis; and Conclusions and recommendations. 71 refs., 54 figs

[en] The ability to meet the overall performance requirements for the proposed Mined Geologic Disposal System (MGDS) at Yucca Mountain, Nevada, requires the two major subsystems (natural barriers and engineered barriers) to positively contribute to containment and radionuclide migration retardation. In addition to the postclosure performance, the proposed repository must meet certain preclosure requirements of safety, retrievability, operability, and also must take into consideration cost and schedule. The thermal loading strategy chosen for such a repository may significantly affect both the postclosure and preclosure performance of the proposed repository. The FY93 Thermal Loading Systems Study was conducted to start developing the structured, detailed technical basis that will be ultimately required to make a thermal loading decision. Various thermal analyses were done and hydrothermal predictions made to evaluate effects on postclosure performance, operability, monitoring, cost, and worker safety. The results show that sufficient technical justification exists so that a recommendation can be made to narrow the range of thermal loading to between 24 and 100 MTU/acre. To emplace waste below 24 MTU/acre would require more area for the full 63,000 MTU of spent nuclear fuel than exists in the primary area plus the three most likely expansion areas of the Site Characterization Plan Conceptual Design of a Repository (SCP CDR). Compelling evidence was found that, above 100 MTU/acre, the conditions would be open-quotes too hotclose quotes with most of the thermal goals being violated

[en] We report on the construction and calibration of a 6 m² scintillation counter. The counter consisted of two 1.22times2.44times0.051 m pieces of clear Lucite which were optically joined along a common edge. Optically coupled on top of these were sixteen 0.61times0.61times0.019 m squares of NE 110 plastic scintillator. Light collection was achieved with eight photomultiplier tubes. This counter had an operational attenuation length of 4.8plus-or-minus0.9 m. Calibration of this counter with an anticoincidence counter to discriminate against multiple particle events is described. This anticoincidence counter permitted particles that passed through a small calibration area to be counted only if no other particles were passing through the remainder of the detector at the same time. Large area counters will often receive more than a single particle per event. Theoretical calculations for purely electromagnetic extensive air showers indicate that the 6 m² counter tested receives an average of 2.5 particles per event. The calibration data yield 1.79plus-or-minus0.08 particles per event. These results imply that if calibration on the response from a single particle of an extensive air shower is desired, it is necessary to use the anticoincidence counter method described for counters of more than a few square meters area

[en] This paper describes work performed by the Office of Civilian Radioactive Waste Management (OCRWM) Management and Operating (M ampersand O) Contractor. The work presented in this paper resulted from waste stream analyses conducted in support of the M ampersand O Phase 1 and 2 Repository Thermal Loading Studies. These analyses integrated system design with underground design. The Phase 1 study in particular examined the effects of various system configurations and operating concepts on the waste stream arriving at the proposed repository at Yucca Mountain

[en] The development of proton primary air showers is described in terms of a model based on a hadron core plus an electromagnetic cascade. The muon component is neglected. The model uses three parameters: a rate at which hadron core energy is converted into electromagnetic cascade energy, and a two-parameter sea-level shower-age function. By also assuming an interaction length for the primary nucleus the model is extended to nuclear primaries. Both models are applied over the energy range of 10¹³ to 10²¹ eV. Both models describe the size and age structure (neglecting muons) from a depth of 342 to 2052 g/cm²

[en] Heat generated as a result of emplacing spent nuclear fuel will significantly affect the pre- and post-closure performance of the Mined Geological Disposal System (MGDS) at the potential repository site in Yucca Mountain. Understanding thermo-hydrological behavior under repository thermal loads is essential in (a) planning and conducting the site characterization and testing program, (b) designing the repository and engineered barrier system, and (c) assessing performance. The greatest concern for hydrological performance is sources of water that would contact a waste package, accelerate its failure rate, and eventually transport radionuclides to the water table. The primary sources of liquid water are: (1) natural infiltration, (2) condensate generated under boiling conditions, and (3) condensate generated under sub-boiling conditions. Buoyant vapor flow, occurring either on a sub-repository scale or on a mountain scale, may affect the generation of the second and third sources of liquid water. A system of connected fractures facilitates repository-heat-driven gas and liquid flow as well as natural infiltration. With the use of repository-scale and sub-repository-scale models, we analyze thermo-hydrological behavior for Areal Mass Loadings (AMLs) of 24.2, 35.9, 55.3, 83.4, and 110.5 MTU/acre for a wide range of bulk permeability. We examine the temporal and spatial extent of the temperature and saturation changes during the first 100,000 yr. We also examine the sensitivity of mountain scale moisture redistribution to a range of AMLs and bulk permeabilities. In addition, we investigate how boiling and buoyant, gas-phase convection influence thermo-hydrological behavior in the vicinity of emplacement drifts containing spent nuclear fuel. The effort was done in support of a thermal loading systems study being performed to evaluate the impact of various thermal loads on the MGDS

[en] Because performance standards are not established for the Yucca Mountain Site (the Environmental Protection Agency (EPA) standards have been remanded), it is necessary to define surrogate or derived criteria to evaluate performance. The Site Characterization Plan (SCP) in 1988 attempted to define surrogate criteria that could be used to establish repository performance. Since that time, new knowledge has become available and some additional analyses of thermal loading have been performed. Thus it became clear that the thermal goals established in the SCP should be reevaluated. This paper reports on a two month effort undertaken to reevaluate the SCP thermal goals using an expert Working Group. Fifteen thermal goals identified in various sections of the SCP were evaluated by the Working Group. It was recommended that two goals be deleted: (1) to keep borehole wall temperature < 275 degrees C and keep the mid-drift temperature < 100 degrees C. It was also recommended that one goal be added to establish a thermal loading that would not degrade the Upper Paintbrush Tuff Formation (Lowermost Tiva Canyon; Yucca Mountain; Pah Canyon; and Uppermost Topopah Spring Members) (Vitric nonwelded) (PTn) barrier. Two other thermal goals and a process statement were reworded to afford compatibility with any emplacement mode, not just the vertical borehole. A recommendation was made to increase the conservatism of a goal to limit potential impact on the surface environment by limiting temperature rise to < 2 degrees C rather than < 6 degrees C. This revised set of goals was used in the Thermal Loading Systems Study

[en] Heat generated as a result of emplacing spent nuclear fuel will significantly affect the pre- and post-closure performance of the Mined Geological Disposal System (MGDS) at the potential repository site in Yucca Mountain. Understanding thermo-hydrological behavior under repository thermal loads is essential in (a) planning and conducting the site characterization and testing program, (b) designing the repository and engineered barrier system, and (c) assessing performance. The greatest concern for hydrological performance is source of water that would contact a waste package, accelerate its failure rate, and eventually transport radionuclides to the water table. The primary sources of liquid water are: (1) natural infiltration, (2) condensate generated under boiling conditions, and (3) condensate generated under sub-boiling conditions. Buoyant vapor flow, occurring either on a sub-repository scale or on a mountain scale, any affect the generation of the second and third sources of liquid water. A system of connected fractures facilitates repository-heat-driven gas and liquid flow as well as natural infiltration. With the use of repository-scale and sub-repository-scale models, the authors analyze thermo-hydrological behavior for Areal Mass Loadings (AMLs) of 24.2, 35.9, 55.3, 83.4, and 110.5 MTU/acre for a wide range of bulk permeability. They examine the temporal and spatial extent of the temperature and saturation changes during the first 100,000 yr. They also examine the sensitivity of mountain scale moisture redistribution to a range of AMLs and bulk permeabilities. In addition, they investigate how boiling and buoyant, gas-phase convection influence thermo-hydrological behavior in the vicinity of emplacement drifts containing spent nuclear fuel