[en] A post-processing technique for determining relative system sensitivity to groups of parameters and system components is presented. It is assumed that an appropriate parametric model is used to simulate system behavior using Monte Carlo techniques and that a set of realizations of system output(s) is available. The objective of our technique is to analyze the input vectors and the corresponding output vectors (that is, post-process the results) to estimate the relative sensitivity of the output to input parameters (taken singly and as a group) and thereby rank them. This technique is different from the design of experimental techniques in that a partitioning of the parameter space is not required before the simulation. A tree structure (which looks similar to an event tree) is developed to better explain the technique. Each limb of the tree represents a particular combination of parameters or a combination of system components. For convenience and to distinguish it from the event tree, we call it the parameter tree. To construct the parameter tree, the samples of input parameter values are treated as either a '+' or a '-' based on whether or not the sampled parameter value is greater than or less than a specified branching criterion (e.g., mean, median, percentile of the population). The corresponding system outputs are also segregated into similar bins. Partitioning the first parameter into a '+' or a '-' bin creates the first level of the tree containing two branches. At the next level, realizations associated with each first-level branch are further partitioned into two bins using the branching criteria on the second parameter and so on until the tree is fully populated. Relative sensitivities are then inferred from the number of samples associated with each branch of the tree. The parameter tree approach is illustrated by applying it to a number of preliminary simulations of the proposed high-level radioactive waste repository at Yucca Mountain, NV. Using a Total System Performance Assessment Code called TPA, realizations are obtained and analyzed. In the examples presented, groups of five important parameters, one for each level of the tree, are used to identify branches of the tree and construct the bins. In the first example, the five important parameters are selected by more traditional sensitivity analysis techniques. This example shows that relatively few branches of the tree dominate system performance. In another example, the same realizations are used but the most important five-parameter set is determined in a stepwise manner (using the parameter tree technique) and it is found that these five parameters do not match the five of the first example. This important result shows that sensitivities based on individual parameters (i.e. one parameter at a time) may differ from sensitivities estimated based on joint sets of parameters (i.e. two or more parameters at a time). The technique is extended using subsystem outputs to define the branches of the tree. The subsystem outputs used in this example are the total cumulative radionuclide release (TCR) from the engineered barriers, unsaturated zone, and saturated zone over 10,000 yr. The technique is found to be successful in estimating the relative influence of each of these three subsystems on the overall system behavior

[en] To assess the potential radon hazard of a new home construction site and the steps (if any) that should be taken to mitigate that hazard, the soil pore gas radon source strength S (i.e., the number of radon atoms emitted into a unit volume of pore gas per unit time), the pore gas radon diffusion length L, and the soil porosity p must be known. Methods exist for measuring the steady-state soil pore gas radon concentration. The purposes of this paper are to analyze the kinetics of the radon concentration in a cavity in the soil, to determine the parameters that affect the kinetics, and to establish and analyze an in situ method for measuring S, L, and p

[en] It has been observed that the radon concentration in homes does not depend solely on the steady-state ²²²Rn concentration in the soil. An explanation for the lack of correlation between radon concentrations in the soil and in adjacent homes includes factors such as the construction of the homes, their heating systems, and the habits of their occupants. Another explanation, which is proposed in this paper, is that the steady-state concentration of radon in the pore gas does not fully characterize the soil as a radon hazard. Other soil properties, such as its diffusion length for radon and its porosity, may be important. In this paper, the authors have identified the soil properties important in radon transport into the basement of a home by mathematically modeling ventilated basement air enclosed in basement walls and surrounded by soil and by solving the model equations to determine an expression for the basement air radon concentration as a function of the properties of the soil and basement wall

[en] Measurements are presented of gold concentration in rock/soil samples by delayed neutron activation analysis using a device and method that are potentially field portable. The device consists of a polyethylene moderator and ²⁵²Cf as the source of neutrons for activating the samples and a high-purity germanium detector to measure the 412-keV gamma-ray emissions from activated gold. This information is used to extract the gold concentration in the sample. Two types of samples were investigated: (1) pure SiO₂ doped with a known amount of gold chloride and (2) US Geological Survey standards. The former types were used to evaluate optimum device performance and to calibrate the device and method. The latter types were used to show typical system performance for the intended application (field exploration for gold deposits). It was found that the device was capable of determining gold concentrations to ≅10 ppb with a turnaround time (the sum of irradiation, decay, and counting times) of approximately10 days. For samples where the gold concentration was much higher (i.e., gold ore), turnaround times are approximately2 days and could be shortened further by sacrificing accuracy (e.g., lessening irradiation, decay, and counting times) or by augmenting source strength

[en] There is little correlation between radon concentrations in soil and radon concentrations in homes. One explanation is that the soil radon concentration does not fully characterize the soil as a radon hazard. A mathematical model for the determination of important soil parameters for characterizing the flow of radon into a basement has been analyzed. We have identified important soil properties by mathematically modeling ventilated air enclosed in basement walls of thickness T (through which radon convects) and surrounded by soil of infinite extent (through which radon diffuses). The radon instantaneously mixed uniformly with the basement air and is lost from the basement air by ventilation (λ_v) and decay (λ). It was found that not only the soil pore gas radon concentration, C_s, but also the radon gas diffusion length, L₃, and the soil porosity, ε₃, are important to characterize the soil as a radon hazard. A model for determining the parameters C_s, L₃, and ε₃ has also been analyzed. It was found that it is possible to measure in situ these important soil parameters by monitoring the radon gas concentration time history of two cavities of different radii formed in the same soil

[en] The purpose of this note is to determine the relationship between the steady state radon concentration in a cavity in the soil and the soil gas radon concentration; and to determine the relationship between the activity of a charcoal canister placed within the cavity and the soil gas radon concentration. The radon concentration in a cavity is reduced from the infinite medium soil gas radon concentration by a factor which is a function of the ratio of the radon diffusion length to the cavity radius. If a charcoal canister is inserted in the cavity, then the charcoal canister will respond as though it were in an infinite volume of air with a radon concentration which is smaller by a factor of V/V_eff, than the radon concentration in the cavity in the absence of the canister, where V_eff is the effective volume of the canister. (author)

[en] The assessment of long-term isolation performance for a geologic repository requires the use of mathematical models that consider the probability and consequences of postulated disruptive scenarios. In the case of the proposed repository at Yucca Mountain, Nevada, volcanism is one of the important disruptive scenarios being considered in site evaluation. A stochastic modeling approach is developed for use in simulating the airborne release of radioactive particulates associated with the basaltic volcanism scenario. The modeling approach considers such factors as the eruption energetics, eruption duration, wind velocity, and particle properties to compute the activity areal density as a function of spatial location. Various components of the model are based on empirical relationships and data that are reported for observed and monitored cinder cone eruptions analogous to those that likely occurred in the Yucca Mountain region in the past. Illustrative applications of the stochastic model are presented for the cases of a single-event realization and a multiple-event average realization

[en] This paper describes an analytically based method for modeling the time-dependent radio nuclide areal densities of contaminated soil surface layers when the soil experiences simultaneous leaching, surface erosion and chain radioactive decay. The model is used to predict time-dependent radio nuclide areal densities in a volcanic ash blanket contaminated with spent nuclear fuel particles for the purpose of assessing the risks of radiation exposure from an extrusive volcanic event near a proposed high-level waste repository at Yucca Mountain. The method uses general analytical solutions (an expansion of the Bateman equations) for calculating serial decay, including non-radioactive decay loss terms, in order to calculate time-dependent radio nuclide areal densities in the ash blanket. In the presented example, 43 open-quotes keyclose quotes radionuclides are tracked and their concentrations in the blanket are displayed for a 10,000-y time period following the volcanic event. Although the analysis presented herein is for modeling contaminated volcanic ash blankets, the model would work equally well for modeling time-dependent radio nuclide contamination of land surfaces in, for example, site decommissioning. It is suggested that the general solutions for serial decay (with non-radioactive decay loss terms) can also be used to model the release of radionuclides from the waste packages under anticipated repository conditions. 16 refs., 13 figs., 2 tabs

[en] To develop capabilities for compliance determination, the Nuclear Regulatory Commission (NRC) conducts total system performance assessment (TSPA) for the proposed repository at Yucca Mountain (YM) in an iterative manner. Because the new Environmental Protection Agency (EPA) standard for YM may set a dose or risk limit, an auxiliary study was conducted to develop estimates of site-specific dose assessment parameters for future TSPAS. YM site-relevant data was obtained for irrigation, agriculture, resuspension, crop interception, and soil. A Monte Carlo based importance analysis was used to identify predominant parameters for the groundwater pathway. In this analysis, the GENII-S code generated individual annual total effective dose equivalents (TEDEs) for 20 nuclides and 43 sampled parameters based upon unit groundwater concentrations. Scatter plots and correlation results indicate the crop interception fraction, food transfer factors, consumption rates, and irrigation rate are correlated with TEDEs for specific nuclides. Influential parameter groups correspond to expected pathway readily to plants, such as ⁹⁹Tc, indicate crop ingestion pathway parameters are most highly correlated with the TEDE, and those that transfer to milk (⁵⁹Ni) or beef (⁷⁹Se, ¹²⁹I, ¹³⁵Cs, ¹³⁷Cs) show predominant correlations with animal product ingestion pathway parameters. Such relationships provide useful insight to important parameters and exposure pathways applicable to doses from specific nuclides