United States Nuclear Regulatory Commission - Protecting People and the Environment

Radionuclide Release from Slag and Concrete Waste Materials – Part 3: Testing Protocol (NUREG/CR-7199, ANL-15/06)

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Publication Information

Manuscript Completed: March 2015
Date Published: December 2015

Prepared by:
W. L. Ebert

Argonne National Laboratory
9700 South Cass Avenue
Argonne, IL 60439

M. Fuhrmann, NRC Project Manager

NRC Job Code N6669

Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington DC 20555-0001

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Abstract

A protocol of simple laboratory tests designed to measure values needed to model the release and transport of contaminants from large piles of waste materials at surface sites for regulatory analyses is presented. In the absence of a specific site or performance model, a conceptual approach for modeling the release of contaminants from a waste pile was developed to identify the key information needed from laboratory testing and provide scientific bases for the recommended test methods. The time-dependent concentration of a contaminant released from the waste pile at the interface with the underlying regolith is expressed as a breakthrough curve (BTC) that depends on the release and transport of the contaminant. Each BTC can be thought of as the source term for a contaminant that is available at the bottom of the pile for transport to the surrounding environment. The system of discrete pieces of waste material forming the pile is modeled as a fractured matrix to express properties that depend on the surface areas of particular phases as average values for a representative volume or mass of the waste material. This allows standard approaches and existing models for contaminant transport to be applied to the waste piles. A probability distribution based on the values measured for many small field samples in laboratory tests can be used to represent the behavior of the entire waste pile. Two terms of primary importance to calculating the BTCs due to the weathering of various waste materials are (1) the effective contaminant release rate due to the dissolution or leaching of the host phases in the waste material that is the source of contaminants and (2) the effective retardation of contaminant transport due to several processes, including sorption onto various phases in the waste material, hold-up in stagnant water within tight fractures and voids, solubility limits and precipitation, etc.

An effective contaminant release rate is derived from test results to represent the release from all phases and an effective sorption term is measured to represent all processes retarding the transport of the contaminant. These are key terms in the advection-dispersion equation (ADE) used to model contaminant transport. Analogous to the distribution coefficient Κd that is used to quantify the overall effect of sorption to multiple phases on the contaminant concentration in solution, a term κr is used to quantify the steady-state release rate of contaminants from multiple host phases on a fractional mass basis. Most important, values of κr are expressed in terms of waste material mass rather than the surface area of the host phases. The objectives of the protocol of laboratory tests are provide values for these terms that represent the range of waste materials in a waste pile.

Simple batch tests are recommended for quantifying the effective contaminant release due to the dissolution or leaching of all host phases in the waste and the effective sorption/desorption behaviors of contaminants on the waste material. Simple modifications of standardized ASTM test methods are used to determine a representative pore water composition and range of contaminant concentrations (ASTM C1285), measure the contaminant release kinetics and rates, and the desorption rate (ASTM C1308), and measure the effective sorption properties (ASTM C1733). The variances in these property values throughout the waste pile due to heterogeneities in the waste material are estimated by testing samples collected from many locations in the pile to establish probability distributions of responses for each term that can be used in stochastic calculations.

While these tests provide parameter values for the isolated processes that can be used in reactive transport modeling, the release and sorption processes are coupled with each other and with transport terms in the ADE. An optional column test is recommended to calibrate the measured dissolution and sorption values for use in the ADE under well-controlled transport conditions to take the effects of couplings that are not explicitly represented in the ADE into account.

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