Waste Isolation Pilot Plant Performance Assessment Research Articles

Influence of Carbonate on Uranium Solubility in the WIPP

ABSTRACTIn the performance assessment (PA) for the Waste Isolation Pilot Plant (WIPP), the solubility of uranium (VI) was conservatively set at 10-3 M for all expected WIPP conditions, including the potential and likely effects of carbonate complexation [1]. Under WIPP-relevant conditions, long-term experiments were performed to establish the uranium (VI) solubility limits in WIPP-simulated brine over a broad range of pCH+ values [7.5-12.5] and to evaluate the contribution of carbonate complexation and hydrolysis to uranium (VI) speciation. Data obtained in carbonate-free ERDA-6 brine, a simulated WIPP brine, were reported earlier [2]. In the absence of carbonate, uranium solubility approached 10-7 M at the expected pCH+ in the WIPP (~ 9.5). In the presence of a significant amount of carbonate (millimole levels), recent experimental results showed that uranium (VI) concentrations will not exceed 10-4M. This measured solubility limit is an order of magnitude lower than the uranium solubility value currently used in the WIPP PA [3]. A small effect of borate complexation was found in the pCH+ range [7.5-10]. At pCH+ ≥ 10, hydrolysis overwhelmed carbonate effects, and no amphoteric effect was observed.

Environmental monitoring of radioactive and non-radioactive constituents in the vicinity of WIP

Abstract The Waste Isolation Pilot Plant (WIPP), a US Department of Energy (DOE) facility, is a deep geologic transuranic waste disposal site designed for the safe disposal of transuranic (TRU) wastes generated from the US defense program. Monitoring is a key component of the development and operation of any nuclear repository and is important to the WIPP performance assessment. Initial concerns over the release of radioactive and chemical contaminants from the WIPP led to various monitoring programs, including the independent, academic-based WIPP environmental monitoring (WIPP-EM) program conducted by the New Mexico State University (NMSU) Carlsbad Environmental Monitoring and Research Center (CEMRC) located in Carlsbad, NM. The mission of CEMRC is to develop and implement an independent health and environmental monitoring program in the vicinity of WIPP and make the results easily accessible to the public and all interested parties. Under the WIPP-EM program constituents monitored include: (1) selected radionuclides, elements, and ions of interest in air, soil, vegetation, drinking water, surface water and sediment from within a 100-mile radius of WIPP as well as in the air exiting the WIPP exhaust shaft, and (2) internally deposited radionuclides in the citizenry living within a 100-mile radius of WIPP. This article presents an evaluation of more than tens years of environmental monitoring data that informed the public that there is no evidence of increases in radiological contaminants in the region that could be attributed to releases from the WIPP. Such an extensive monitoring program and constant public engagement is an ideal model for all nuclear waste repositories anywhere in the world.

Solubility of Nd 3+ and UO 22+ in WIPP brine as oxidation-state invariant analogs for plutonium

A general status of the Waste Isolation Pilot Plant (WIPP), as it relates to plutonium chemistry and inventory, is given. We also report on the results of two redox-invariant studies, Nd 3+ and UO 2 2+, that are being investigated as analogs for Pu(III) and Pu(VI) chemistry under WIPP-relevant conditions. The WIPP repository is the only working repository for TRU waste in the world and continues to perform above expectations. Recertification of the WIPP was recently received in April 2006 and plutonium research to support these ongoing recertification activities continues. As oxidation-state analogs for plutonium, the solubility of uranium(VI) and neodymium(III) were established in long-term experiments as a function of pC H+ for two WIPP simulated brines that bracket the range in brine composition expected. The solubility of uranium(VI) measured at high pC H+ in the absence of carbonate was ∼2 × 10 −7 M in ERDA-6 brine. This is almost 100 times lower than the uranium solubility published in the literature for carbon dioxide-free systems and is almost four orders of magnitude lower than current assumptions in the WIPP Performance Assessment (PA). The Nd 3+ solubility determined was ∼1 × 10 −7 M. There was essentially no significant effect of carbonate on the solubility measured for Nd 3+. Taking into consideration experimental errors in our data, the neodymium solubilities determined in the present work are in good agreement with the solubilities calculated by WIPP PA.

Characterization of stochastic uncertainty in the 1996 performance assessment for the Waste Isolation Pilot Plant

The 1996 performance assessment (PA) for the Waste Isolation Pilot Plant (WIPP) maintains a separation between stochastic (i.e. aleatory) and subjective (i.e. epistemic) uncertainty, with stochastic uncertainty arising from the possible disruptions that could occur at the WIPP over the 10,000-yr regulatory period specified by the US Environmental Protection Agency (40 CFR 191, 40 CFR 194) and subjective uncertainty arising from an inability to uniquely characterize many of the inputs required in the 1996 WIPP PA. The characterization of stochastic uncertainty is discussed, including drilling intrusion time, drilling location, penetration of excavated/nonexcavated areas of the repository, penetration of pressurized brine beneath the repository, borehole plugging patterns, activity level of waste, and occurrence of potash mining. Additional topics discussed include sampling procedures, generation of individual 10,000-yr futures for the WIPP, construction of complementary cumulative distribution functions (CCDFs), mechanistic calculations carried out to support CCDF construction, the Kaplan/Garrick ordered triple representation for risk, and determination of scenarios and scenario probabilities.

Regulatory basis for the Waste Isolation Pilot Plant performance assessment

The Waste Isolation Pilot Plant (WIPP) is the first operational repository designed for the safe disposal of transuranic (TRU) radioactive waste from the defense programs of the US Department of Energy (DOE). The US Environmental Protection Agency (EPA) is responsible for certifications and regulation of the WIPP facility for the radioactive components of the waste. The EPA has promulgated general radioactive waste disposal standards at 40 CFR Part 191. and WIPP-specific criteria to implement and interpret the generic disposal standards at 40 CFR Part 194. In October 1996. the DOE submitted its Compliance Certification Application (CCA) to the EPA to demonstrate compliance with the disposal standards at Subparts B and C of 40 CFR Part 191. This paper summarizes the development of the overall legal framework for radioactive waste disposal at the WIPP, the parallel development of the WIPP performance assessment (PA), and how the EPA disposal standards and implementing criteria formed the basis for the CCA WIPP PA. The CCA resulted in a certification in May 1998 by the EPA of the WIPP'S compliance with the EPA's disposal standard, thus enabling the WIPP to begin radioactive waste disposal.

Characterization of subjective uncertainty in the 1996 performance assessment for the Waste Isolation Pilot Plant

The 1996 performance assessment (PA) for the Waste Isolation Pilot Plant (WIPP) maintains a separation between stochastic (i.e., aleatory) and subjective (i.e., epistemic) uncertainty, with stochastic uncertainty arising from the possible disruptions that could occur at the WIPP over the 10,000 yr regulatory period specified by the US Environmental Protection Agency (40 CFR 191,40 CFR 194) and subjective uncertainty arising from an inability to uniquely characterize many of the inputs required in the 1996 WIPP PA. The characterization of subjective uncertainty is discussed, including assignment of distributions, uncertain variables selected for inclusion in analysis, correlation control, sample size, statistical confidence on mean complementary cumulative distribution functions, generation of Latin hypercube samples, sensitivity analysis techniques, and scenarios involving stochastic and subjective uncertainty.

Conceptual structure of the 1996 performance assessment for the Waste Isolation Pilot Plant

The conceptual structure of the 1996 performance assessment (PA) for the Waste Isolation Pilot Plant (WIPP) is described. This structure involves three basic entities (EN1, EN2, EN3): (1) EN1, a probabilistic characterization of the likelihood of different futures occurring at the WIPP site over the next 10,000 yr, (2) EN2, a procedure for estimating the radionuclide releases to the accessible environment associated with each of the possible futures that could occur at the WIPP site over the next 10,000 yr, and (3) EN3, a probabilistic characterization of the uncertainty in the parameters used in the definition of EN1 and EN2. In the formal development of the 1996 WIPP PA, EN1 is characterized by a probability space (S{sub st}, P{sub st}, p{sub st}) for stochastic (i.e., aleatory) uncertainly; EN2 is characterized by a function {line_integral} that corresponds to the models and associated computer programs used to estimate radionuclide releases; and EN3 is characterized by a probability space (S{sub su}, P{sub su}, p{sub su}) for subjective (i.e., epistemic) uncertainty. A high-level overview of the 1996 WIPP PA and references to additional sources of information are given in the context of (S{sub st}, P{sub st}, p{sub st}), {line_integral} and (S{sub su}, P{sub su}, p{sub su}).

Representation of two-phase flow in the vicinity of the repository in the 1996 performance assessment for the Waste Isolation Pilot Plant

The following topics related to the representation of two-phase (i.e. gas and brine) flow in the vicinity of the repository in the 1996 performance assessment (PA) for the Waste Isolation Pilot Plant (WIPP) are discussed: (i) system of nonlinear partial differential equations used to model two-phase flow; (ii) incorporation of repository shafts into model; (iii) creep closure of repository; (iv) interbed fracturing; (v) gas generation; (vi) capillary action in waste; (vii) borehole model; (viii) numerical solution; and (ix) gas and brine flow across specified boundaries. Two-phase flow calculations are a central part of the 1996 WIPP PA and supply results that are subsequently used in the calculation of releases to the surface at the time of a drilling intrusion (i.e. spallings, direct brine releases) and long-term releases due to radionuclide transport by flowing groundwater.

Comments on an evaluation of the 1992 performance assessment for the Waste Isolation Pilot Plant

The following topics are discussed in a review of the 1992 performance assessment (PA) for the Waste Isolation Pilot Plant (WIPP): (1) inclusion of potash mining and other disruptions in the definition of possible futures at the WIPP, (2) definition of future drilling rates in the vicinity of the WIPP, (3) possible non-conservatisms in analysis assumptions, (4) characterization of uncertainty in solubilities, and (5) characterization of uncertainty in distribution coefficients. With respect to (1) and (2), the 1992 WIPP PA attempted to follow guidance in the US Environmental Protection Agency's (EPA's) standard for the geological disposal of radioactive waste, 40 CFR 191. Many of the reviewer's comments relate to the EPA's recent guidance in 40 CFR 194 on assessing compliance with 40 CFR 191, which was not published until early 1996 and which makes many changes from the guidance given in 40 CFR 191. The 1996 WIPP PA, which will support a compliance certification application to the EPA, will follow the guidance in 40 CFR 194. With respect to (3), (4) and (5), the WIPP PA tries to avoid both deliberate conservatism and deliberate non-conservatism. The 1992 WIPP PA attempted to use all the information available to it consistent with the requirements to incorporate uncertainty in PA results and to use ‘reasonable expectation’ as the criterion for assessing compliance with 40 CFR 191. Since 1992, many additional results related to solubilities and distribution coefficients have become available. The 1996 WIPP PA will incorporate these new results.

Conceptual Structure of Performance Assessments Conducted for the Waste Isolation Pilot Plant

ABSTRACTThe Waste Isolation Pilot Plant (WIPP) in southeastern New Mexico is being developed by the U.S. Department of Energy as a disposal facility for trans-uranic waste. In support of this project, Sandia National Laboratories is conducting an ongoing performance assessment (PA) for the WIPP. The ordered triple representation for risk proposed by Kaplan and Garrick is used to provide a clear conceptual structure for this PA. This presentation describes how the preceding representation provides a basis in the WIPP PA for (1) the definition of scenarios and the calculation of scenario probabilities and consequences, (2) the separation of subjective and stochastic uncertainties, (3) the construction of the complementary cumulative distribution functions required in comparisons with the U.S. Environmental Protection Agency's standard for the geologic disposal of radioactive waste (i.e., 40 CFR Part 191, Subpart B), and (4) the performance of uncertainty and sensitivity studies. Results obtained in a preliminary PA for the WIPP completed in December of 1991 are used for illustration.