ABSTRACT: This paper describes a well blowout model, DRSPALL, which was developed for risk analysis of the Waste Isolation Pilot Plant, a nuclear waste disposal facility operated by the U.S. Department of Energy. DRSPALL (Direct Release SPALLing) was developed to quantify the solids release to the surface for the spallings mechanism. The model couples two primary domains: a wellbore that contains a mixture of drilling mud, solids, and gas; and a porous medium that represents the buried waste containing compressed gas from degradation processes. The code calculates coupled porous waste and wellbore transient mixed-phase compressible fluid flow before, during, and after the drill bit penetrates the waste. In addition, an effective stress model is applied to the porous medium to test for tensile failure of the solid. If tensile failure is detected, the failed solids are removed from the porous medium and added to the wellbore domain using a fluidized bed model. Model verification and validation are discussed and representative results are presented.
INTRODUCTION This paper describes the well blowout model, DRSPALL, developed as part of the performance assessment (PA) for the Waste Isolation Pilot Plant (WIPP). Located in southeast New Mexico, WIPP is an operational disposal facility for defense-related transuranic nuclear waste owned by the U.S. Department of Energy (DOE). The principal design concept of WIPP is to isolate nuclear waste from the accessible environment by entombing it in salt formations deep in the ground, a waste management approach endorsed by the National Academy of Sciences since 1957 [1]. The excavated region of WIPP that contains waste is located about 655m below the ground surface in bedded salt (see Figure 1). Salt is attractive for isolation of long-lived nuclear wastes because it exhibits a very low permeability, providing an effective barrier to transport of contaminated brine to the land surface. The salt also exhibits plastic properties and creep closure under stress, allowing it to literally entomb wastes that are placed underground for many years.