2003 NGWA Midsouth Focus Conference September 2003 Neptune and Company, Inc. Modeling Uncertainty: Realism vs Conservatism in Radiological Performance Assessment John Tauxe, PhD, PE Paul K. Black, PhD Bruce M. Crowe, PhD Donald W. Lee, PhD, PE

2003 NGWA Midsouth Focus Conference September 2003 Presentation Outline What is Performance Assessment? Probabilistic PA modeling A Low-Level Radioactive Waste example Advantages of probabilistic modeling Modeling and uncertainty

2003 NGWA Midsouth Focus Conference September 2003 Quote of the Day “Reports that say that something hasn't happened are always interesting to me, because as we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns – the ones we don't know we don't know.” Secretary of Defense Donald H. Rumsfeld DoD News Briefing – 12 Feb 2002 Source:

2003 NGWA Midsouth Focus Conference September 2003 Performance Assessment For the DOE and its LLW sites, PAs are intended to establish “reasonable expectation” that performance objectives are not exceeded (e.g. DOE M 435.1), in order to authorize waste disposal. PAs are traditionally deterministic and conservative, yet any such analysis has inherent uncertainties in assumptions, parameter values, and in the models themselves PA DAS

2003 NGWA Midsouth Focus Conference September 2003 Determinisitic vs Probabilistic procon deterministic analysis May be appropriate for simple compliance demonstration Easy for decision makers and public Uncertainties are unspecified What is conservative may not be known probabilistic analysis Better represents state of knowledge Makes for better informed decisions Requires development of input distributions

2003 NGWA Midsouth Focus Conference September 2003 PAs and Uncertainty Sources of uncertainty in PA modeling include conceptual model assumptions and exposure scenarios, analytical and numerical models and their assumptions, and model input parameters in space and time (variability and knowledge uncertainty). ? ? ? ? ? ? ? ? ? ?

2003 NGWA Midsouth Focus Conference September 2003 Deterministic Modeling Deterministic models produce deterministic (single-valued) output with no uncertainty, are easy to compare to deterministic performance objectives, typically strive for conservatism*, and may be a good choice for simple demonstration of compliance. * What is conservative may not always be obvious, and conservatism can obscure model complexities.

2003 NGWA Midsouth Focus Conference September 2003 Probabilistic Modeling Probabilistic models strive to be realistic (not conservative), represent uncertainty using probability density functions for model parameters, propagate uncertainty through Monte Carlo simulation, and calculate model outputs as probability density functions.

2003 NGWA Midsouth Focus Conference September 2003 An Example from the Nevada Test Site Area 5 Radioactive Waste Management Site Photo courtesy NNSA/NSO

2003 NGWA Midsouth Focus Conference September 2003 The Area 5 RWMS The conceptual model of transport at the Area 5 Radioactive Waste Management Site at the Nevada Test Site includes: upward flux of water driven by high evapotranspiration potentials, diffusion in liquid and gaseous phases, biotic transport of contamination and materials in the near surface, and resuspension by wind. Processes are nonlinear and tightly coupled, so what makes for a conservative estimate?

2003 NGWA Midsouth Focus Conference September 2003 ground surface to groundwater unsaturated zone flow divide no-flux boundary Advective/Diffusive Transport What is conservative? Modeled processes: 1. advection of water down to distant water table up to “no-flux boundary” advection in water diffusion in water 2. diffusion in water below “no-flux boundary” (NFB) diffusion in air 3. diffusion in air phase throughout cap waste alluvium

2003 NGWA Midsouth Focus Conference September 2003 Plant-Induced Transport 1. Plant roots uptake contaminants during growth. 2. Contaminants are redistributed within the plants. 3. Contaminants are returned to soil upon senescence. Modeled processes: Again: What is conservative? cap waste

2003 NGWA Midsouth Focus Conference September 2003 Animal-Induced Transport Yet again: What is conservative? Modeled processes: 1. Animals excavate subsurface bulk materials and bring them to the surface. excavation 2. Burrows collapse, returning materials to the subsurface. collapse cap waste

2003 NGWA Midsouth Focus Conference September 2003 GoldSim at the NTS

2003 NGWA Midsouth Focus Conference September 2003 An example: These bulk density data need to be turned in to an input distribution. Stochastic Parameters Inventory Dimensions Material properties Biotic properties and rates of activities Human behavior Chemical properties

2003 NGWA Midsouth Focus Conference September 2003 Monte Carlo Simulation Select time stepping Select number of realizations Select seed Optional use of LHS

2003 NGWA Midsouth Focus Conference September 2003 Deterministic Results Comparison is easy, but is it honest? performance objective

2003 NGWA Midsouth Focus Conference September time (yr) Probabilistic Results Comparison is challenging, but more honest. performance objective

2003 NGWA Midsouth Focus Conference September 2003 Statistical Summaries median 25% 75% 5% 95% upper bound lower bound mean performance objective

2003 NGWA Midsouth Focus Conference September 2003 Advantages of Probabilistic Analysis More realistic (honest) answers More information for decision makers (not doing their job for them) Provides information for statistical comparisons with monitoring data and for value of information analysis (when to stop monitoring) Identification of sensitive parameters

2003 NGWA Midsouth Focus Conference September 2003 Sensitivity Analysis 1 Sensitivity analysis provides a ranking of sensitive parameters, enhancing appreciation for their significance. For example, dose may be driven by: 1. Cap thickness 2. Volume of materials excavated by ants 3. Inventory of 238 U 4. Plant/soil concentration ratio for 99 Tc

2003 NGWA Midsouth Focus Conference September 2003 Sensitivity Analysis 2 Using the MART* statistical technique, the range over which a parameter is sensitive can be evaluated. *Multiple Additive Regression Trees That’s cool! Cap Thickness (m) Sensitivity Index

2003 NGWA Midsouth Focus Conference September 2003 Value of Information Analysis Evaluate VOI from monitoring activities. Determine value of continued monitoring (this cannot be done with a deterministic model). Decide when monitoring no longer provides useful information (time to stop).

2003 NGWA Midsouth Focus Conference September 2003 Take Home Points Environmental modeling is most useful if done stochastically. Confirmation of performance assessment (through monitoring) requires statistical analysis. Probabilistic modeling provides a technical basis for deciding when to stop monitoring.

2003 NGWA Midsouth Focus Conference September 2003 Yucca Flat, Nevada: The world’s best radioactive waste disposal site. The holes have already been “dug”!