Establishment of Uncertainty Ranges and Probability Distributions of Actinide Solubilities for Performance Assessment in the Waste Isolation Pilot Plant

The Fracture-Matrix Transport (FMT) code developed at Sandia National Laboratories solves chemical equilibrium problems using the Pitzer activity coefficient model with a database containing actinide species. The code is capable of predicting actinide solubilities at 25 °C in various ionic-strength solutions from dilute groundwaters to high-ionic-strength brines. The code uses oxidation state analogies, i.e., Am(III) is used to predict solubilities of actinides in the +III oxidation state; Th(IV) is used to predict solubilities of actinides in the +IV state; Np(V) is utilized to predict solubilities of actinides in the +V state. This code has been qualified for predicting actinide solubilities for the Waste Isolation Pilot Plant (WIPP) Compliance Certification Application in 1996, and Compliance Re-Certification Applications in 2004 and 2009.We have established revised actinide-solubility uncertainty ranges and probability distributions for Performance Assessment (PA) by comparing actinide solubilities predicted by the FMT code with solubility data in various solutions from the open literature. The literature data used in this study include solubilities in simple solutions (NaCl, NaHCO3, Na2CO3, NaClO4, KCl, K2CO3, etc.), binary mixing solutions (NaCl+NaHCO3, NaCl+Na2CO3, KCl+K2CO3, etc.), ternary mixing solutions (NaCl+Na2CO3+KCl, NaHCO3+Na2CO3+NaClO4, etc.), and multi-component synthetic brines relevant to the WIPP.

Problems With the Spallings Model in the Compliance Certification Application of the Waste Isolation Pilot Plant

This paper questions the spallings model in the performance assessment supporting the compliance certification application (CCA) of the Waste Isolation Pilot Plant (WIPP). Spallings release is one of the dominant release pathways.

A Mechanistically-Based Model for Spallings Releases at the Waste Isolation Pilot Plant

SummaryThe Waste Isolation Pilot Plant (WIPP) is an underground facility operated by the US Department of Energy (DOE) for permanent disposal of transuranic waste. The compliance of the WIPP to the US Environmental Protection Agency regulations, as assessed by the performance assessment process, is demonstrated by comparison of potential releases to the regulatory limits. In the Compliance Certification Application the potential for solids release to the surface exists only for “disturbed” scenarios, whereby the repository is intersected by an exploratory borehole at some time during the regulatory period of 10,000 years. Releases for this scenario are dominated by three mechanisms associated with the hypothetical scenario. These mechanisms are “cuttings”; the removal of solid materials by the action of drilling, “cavings”; the removal of solids by erosion by the circulating mud, and “spallings”; the removal of solids by a gas-driven blow-out.