摘要:

AbstractUncertainty and sensitivity analysis techniques based on Latin hypercube sampling, partial correlation analysis, stepwise regression analysis, and examination of scatterplots are used in conjunction with the BRAGFLO model to examine two-phase flow (i.e., gas and brine) at the Waste Isolation Pilot Plant, which is being developed by the U.S. Department of Energy as a disposal facility for trans-uranic waste, to provide insights on factors that are potentially important in showing compliance with applicable regulations of the U.S. Environmental Protection Agency. Specific regulations include“Petitions to Allow Land Disposal of a Waste Prohibited Under Subpart C of Part 268”(40 CFR 268.6), which implements the Resource Conservation and Recovery Act and establishes maximum environmental concentrations for regulated chemicals such as volatile organic compounds (VOCs) and heavy metals, and“Environmental Standards for the Management and Disposal of Spent Nuclear Fuel, High-Level and Transuranic Radioactive Wastes”(40 CFR 191, Subpart B), which places a probabilistic limit on allowable radioactive releases from a disposal facility over a 10 000-yr time period. The primary topics investigated are (a) gas production due to corrosion of steel, (b) gas production due to microbial degradation of cellulosics, and (c) gas migration into anhydrite marker beds in the Salado Formation, which is the host unit into which the waste will be emplaced. Gas production and movement is of particular importance in establishing compliance with 40 CFR 268.6 because of its influence on the movement of VOCs. Important variables identified in the analysis include (a) initial brine saturation of the waste, (b) stoichiometric terms for corrosion of steel and microbial degradation of cellulosics, and (c) gas barrier pressure in the anhydrite marker beds.

ISSN:0029-5639    

DOI:10.13182/NSE96-A28545

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractA computational model of an open-cycle gas core nuclear rocket (GCR) is developed. The solution is divided into two distinct areas - thermal hydraulics and neutronics. To obtain the thermal-hydraulic solution, a computer code is written that solves the Navier-Stokes, energy, and species diffusion equations. The two-dimensional transport code TWODANT is used to obtain the neutronics solution. The thermal-hydraulic and neutronic models are coupled, and the solution proceeds in an iterative manner until a consistent power density profile is obtained.Various open-cycle GCR designs are evaluated. First, it is assumed that the fuel and propellant do not mix. In this ideal case, it is found that the limiting factor in determining thrust and specific impulse is the maximum allowable wall heat flux. Following this simplified study, the results from a complete thermal-hydraulic/neutronic solution are presented, and the use of alternate fuels and propellants is considered. Next, a parametric design study is conducted that examines the rocket performance of the open-cycle GCR as a function of various design and operational parameters. It is found that fuel containment is very adversely affected by high reactor power or rocket acceleration. Finally, some concepts are discussed that could help improve fuel containment.

ISSN:0029-5639    

DOI:10.13182/NSE96-A28546

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractImage restoration techniques are studied for Compton backscatter imaging as applied to identification of a land mine buried in soil. Mathematical methods are developed to restore images, which include artifacts due to photon noise, soil surface irregularity, and vertical motion of the imaging system. The image restoration is formulated as an inverse photon transport problem. The forward photon transport is modeled by using a two-collision response function. The inverse problem then is solved by applying an iterative minimization algorithm, resulting in an estimation of characteristic parameters of objects. Mathematical relations among detector responses are derived by experimentally analyzing the detector response characteristics when there are soil surface irregularity and vertical motion of the imaging system. These are used to remove the artifacts from the images. The method successfully restores the geometrical feature of the object under simulated battlefield imaging conditions.

ISSN:0029-5639    

DOI:10.13182/NSE96-A28547

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractAn autoregressive moving average model of neutron fluctuations with large measurement noise is developed from the Langevin stochastic equations with the noise equivalent source in the form of a vector Wiener process. The neutron field/detector interaction is explicitly treated, and delayed neutrons are included. The Kalman filter with nonzero covariance between input and output noise is applied in the derivations to reduce the state-space equations to the input-output form. Theoretical developments are verified using time series data from the prompt-neutron decay constant measurements at the zero-power reactor RB in Vinča. Model parameters are estimated by the maximum likelihood off-line algorithm and an adaptive pole estimation algorithm based on the recursive prediction error method with implemented regularization and stability control. The results show that subcriticality can be estimated from real data with high measurement noise using a shorter statistical sample than in standard methods based on the power spectral density or the Feynman variance-to-mean ratio method.

ISSN:0029-5639    

DOI:10.13182/NSE96-A28548

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractThe interpretation of the statistical error estimates produced by Monte Carlo transport codes is still somewhat of an art. Empirically, there are variance reduction techniques whose error estimates are almost always reliable, and there are variance reduction techniques whose error estimates are often unreliable. Unreliable error estimates usually result from inadequate large-score sampling from the score distributions’tail.Statisticians believe that more accurate confidence interval statements are possible if the general nature of the score distribution can be characterized. Here, the analytic score distribution for the exponential transform applied to a simple, spatially continuous Monte Carlo transport problem is provided. Anisotropic scattering and implicit capture are included in the theory. In large part, the analytic score distributions that are derived provide the basis for the ten new statistical quality checks in MCNP.

ISSN:0029-5639    

DOI:10.13182/NSE96-A28549

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractMethods are developed for solving the transport equation for radionuclides moving in porous rock by hydrodynamic dispersion and advection. The unique nature of the problem arises from the long time interval over which the solutions are required, e.g., 106yr, during which geological and climatic changes can radically alter the system properties, such as the retardation factor and the water velocity. In order to solve this problem, we have developed eigenfunction expansion methods which eliminate the spatial variable and thereby enable the time dependence to be incorporated explicitly. Various problems are considered, each based on two simple boundary conditions: (a) concentration is fixed at both ends of the layer and (b) a delta function impulsive source at one end. The convergence of the solutions is improved by a technique based on the Poisson sum formula which makes them readily tractable numerically over a wide range of practically interesting parameters.Some exact solutions are obtained for purely advective transport which are particularly useful as they are very general and lend themselves to a variety of analytical averaging techniques.Of considerable importance is the development of a stochastic averaging procedure to account for uncertainties in the parameters and onset of climatic changes. We have illustrated the effects of averaging by application to a single layer with a delta input and one climatic change (switchtime). The switchtime is regarded as a random variable and averaged over lognormal and uniform distributions. In the same way, we have considered the retardation factor as uniformly distributed between upper and lower bounds and give graphical results for the concentration as a function of time. Finally, we consider various developments of the method to multinuclide chains and multilayer systems.

ISSN:0029-5639    

DOI:10.13182/NSE96-A28550

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractA neutronic analysis is presented of three incinerator subcritical lattices, driven by accelerated protons and designed to transmute the minor actinides, the99Tc and the129I, of light water reactor (LWR) waste. A calculational methodology must first be established to enable a neutronic burnup analysis of fission cores driven by high-energy protons. The methodology is based on the following codes: HERMES, the Forschungszentrum Jülich adaptation of HETC, for high-energy interactions; MCNP3, for neutron interactions below 20 MeV of neutron energy; and KORIGEN, the Forschungszentrum Karlsruhe adaptation of ORIGEN, for burnup analysis. A result of applying the methodology is that the minor actinides, the99Tc, and the I29I, of LWR waste may be transmuted in subcritical cores, driven by the spallation neutrons emanating from the bombardment of the cores with 1600-MeV protons. Three core types are required. Core type I is fueled by the minor actinides and is a modification of the Brookhaven National Laboratory PHOENIX. With a proton current of 20 mA, the core incinerates the minor actinide waste of 14 LWRs. Core type II contains the 99Tc,129I, and plutonium waste of 19 LWRs. With a proton beam of 130 mA, the core incinerates the technetium and 60% of the iodine. With a fraction of the plutonium coming out of this core, the remaining 40% of129I is incinerated in core type III. All three cores run to 100 000 MWd/tonne or slightly higher; on the average, no core is a net consumer of grid electricity; all are cooled by sodium but remain subcritical with the loss of coolant.

ISSN:0029-5639    

DOI:10.13182/NSE96-A28551

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractGamma-ray buildup factors for a point isotropic source in stratified spherical shields have been studied experimentally. Energy absorption rates for an experimental setup of iron spheres in water have been measured using the thermoluminescent dosimeter (TLD)-200. The measured absorbed doses in the TLD have been converted to the absorbed doses in iron and water by using cavity ionization theory. The measured absorbed doses and energy absorption buildup factors in iron and water were then compared with those calculated using the BIGGI-4T and EGS4 codes.

ISSN:0029-5639    

DOI:10.13182/NSE96-A28552

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractCalculation modeling of detector activity in a VVER/pressurized water reactor pressure vessel surveillance program, necessary for neutron fluence assessments, is developed. The detector activity for certain detectors could be obtained by solving the neutron transport equation by the appropriate activity source taking into consideration the time-dependent local power distribution. The presented results demonstrate the importance of accounting for local power variations.

ISSN:0029-5639    

DOI:10.13182/NSE96-A28553

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractThe energy spectra and angular distributions of proton emission in a reaction ofnatFe(n,xp) at a neutron energy of 14.6 MeV are measured by the University of Science and Technology of China multitelescope system. The double-differential cross sections of 16 reaction angles from 25 to 164.5 deg are obtained in this measurement. The statistical errors are reduced largely because of the thick target used. The angular distributions show a slightly energy-dependent forward-backward asymmetry. The angle-integrated proton spectrum is compared with theoretical calculations and other results. The total proton-emission cross section is in fair agreement with the prediction and evaluation.

ISSN:0029-5639    

DOI:10.13182/NSE96-A28554

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor