DUSTMS-D: DISPOSAL UNIT SOURCE TERM - MULTIPLE SPECIES - DISTRIBUTED FAILURE DATA INPUT GUIDE.

SULLIVAN, T.M.

Brookhaven National Laboratory

关键词: Leaching;    Diffusion;    Decay;    12 Management Of Radioactive Wastes, And Non-Radioactive Wastes From Nuclear Facilities;    Waste Forms;   

DOI  :  10.2172/881256
RP-ID  :  BNL--75554-2006
RP-ID  :  DE-AC02-98CH10886
RP-ID  :  881256

美国|英语

来源: UNT Digital Library

【 摘 要 】

Performance assessment of a low-level waste (LLW) disposal facility begins with an estimation of the rate at which radionuclides migrate out of the facility (i.e., the source term). The focus of this work is to develop a methodology for calculating the source term. In general, the source term is influenced by the radionuclide inventory, the wasteforms and containers used to dispose of the inventory, and the physical processes that lead to release from the facility (fluid flow, container degradation, wasteform leaching, and radionuclide transport). Many of these physical processes are influenced by the design of the disposal facility (e.g., how the engineered barriers control infiltration of water). The complexity of the problem and the absence of appropriate data prevent development of an entirely mechanistic representation of radionuclide release from a disposal facility. Typically, a number of assumptions, based on knowledge of the disposal system, are used to simplify the problem. This has been done and the resulting models have been incorporated into the computer code DUST-MS (Disposal Unit Source Term-Multiple Species). The DUST-MS computer code is designed to model water flow, container degradation, release of contaminants from the wasteform to the contacting solution and transport through the subsurface media. Water flow through the facility over time is modeled using tabular input. Container degradation models include three types of failure rates: (a) instantaneous (all containers in a control volume fail at once), (b) uniformly distributed failures (containers fail at a linear rate between a specified starting and ending time), and (c) gaussian failure rates (containers fail at a rate determined by a mean failure time, standard deviation and gaussian distribution). Wasteform release models include four release mechanisms: (a) rinse with partitioning (inventory is released instantly upon container failure subject to equilibrium partitioning (sorption) with the waste form), (b) diffusion release.(release from either a cylindrical, spherical, or rectangular wasteform), (c) dissolution release (uniform release over time due to dissolution of the wasteform surface), and (d) solubility limited release. The predicated wasteform releases are corrected for radioactive decay and ingrowth. A unique set of container failure and wasteform release parameters can be specified for each control volume with a container. Contaminant transport is modeled through a finite-difference solution of the advective transport equation with sources (wasteform release and ingrowth) and radioactive decay. Although DUST-MS simulates one-dimensional transport, it can be used to simulate migration down to an aquifer and then transport in the aquifer by running the code twice. A special subroutine allows the flux into the aquifer from the first simulation to be input as the flux at the upstream boundary in the aquifer. This document presents the models used to calculate release from a disposal facility, verification of the model, and instructions on the use of the DUST-MS code. In addition to DUST-MS, a preprocessor, DUSTINMS, which helps the code user create input decks for DUST-MS and a post-processor, GRAFMS, which takes selected output files and plots them on the computer terminal have been written. Use of these codes is also described. In using DUST-MS, as with all computer models, the validity of the predictions relies heavily on the validity of the input parameters. Often, the largest uncertainties arise from uncertainty in the input parameters. Therefore, it is crucial to document and support the use of these parameters. The DUST-MS code, because of its flexibility and ability to compute release rates quickly, is extremely useful for screening to determine the radionuclide released at the highest rate, parameter sensitivity analysis and, with proper choice of the input parameters, provide upper bounds to release rates.