Improving Operations and Long-Term Safety of the Waste Isolation Pilot Plant: Final Report (2001)
Chapter: Appendix B: Human Intrusion Scenarios
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Appendix B
Human Intrusion Scenarios
Oil, gas and other mineral resources are frequently found in association with salt beds, such as the Salado, where the WIPP is situated. The region around the WIPP has known a high rate of drilling activities in the past and future energy trends indicate that there will be incentives to explore the region again, once institutional controls are removed (starting 100 years after the closure of the repository). The risk of drilling directly into the repository and thus creating pathways for the release of radionuclides into the environment will then increase. Drilling through the repository could transport radioactive materials from the repository to the surface or bring water in contact with substances stored in the repository. The following two scenarios are possible sources of concern about the performance of the repository and have been taken into account in the performance assessment of the WIPP.
1. If there were an oilfield water-flooding operation in the vicinity of WIPP, a large amount of brine could flow from a leaky injection well and induce a hydraulic fracture in the anhydrite (or marker bed) directly above or below the WIPP repository (Box B.1). If, at some later time, another well were drilled through the repository and into this brine-filled fracture, the high-pressure brine in the fracture could flow through the borehole and flood the repository causing a release of radioactive materials. The scenario is known as the Hartman scenario.
2. Direct drilling into the WIPP repository could allow circulating drilling fluid to bring radioactive materials to the surface through a borehole as cuttings or spallings. The situation could be serious if the repository were flooded with high-pressure brines. The Sandia National Laboratories examined three possible flooding scenarios, designated as E1, E2, and E1E2, in their performance assessment. These scenarios are briefly explained in Box B.2, Box B.3, and Box B.4 and they are described in detail in the Compliance Certification Application (DOE, 1996).
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~ enlarge ~ Box B.1: The Hartman Scenario is a scenario in which water from a leaky injection well induces a hydraulic fracture in the anhydrite below or above the repository. If, at some later time, another well is drilled through the repository, the water in the fracture could flow through borehole into the repository. |
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~ enlarge ~ Box B.2: The E1 Scenario is any inadvertent penetration of a waste panel by a borehole that also penetrates a Castile brine reservoir. Sources of brine in the E1 scenario are the brine reservoir, the Salado, and under certain conditions, the units above the Salado. |
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~ enlarge ~ Box B.3: E2 is the simplest scenario for inadvertent human intrusion into a waste disposal panel. In this scenario, a panel is penetrated by a drill bit; cuttings, caving, spallings, and brine flow releases may occur in the borehole after it is plugged and abandoned. Cuttings will be discharged at the surface and may contain waste material if the borehole penetrates waste drums. Cavings, which include material eroded from the borehole wall during drilling, may also contain radionuclide waste from the repository horizon. Spallings include solid material carried into the borehole during rapid depressurization of the waste disposal region. The repository horizon could be pressurized by gas generation from degradation of the waste, organic materials and metal corrosion. Brine can be present in the Salado from natural sources or human activities associated with other drilling or production activities. Release to the biosphere is either to the surface or through the Culebra via a leaking casing. |
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~ enlarge ~ Box B.4: The E1E2 scenario involves multiple well penetrations of a waste panel, with one well penetrating a high-pressure brine panel below. Brine flows from a brine source through well E1 through the repository and is released through well E2. This flow path has the potential to bring large quantities of brine in direct contact with waste in the panel and to bring the contaminated brine to the overlying Salado or Culebra. |
