摘要:

AbstractEven-order PNtheory has historically been viewed as a questionable approximation to transport theory. The main reason is that one obtains an odd number of unknowns and equations; this causes an ambiguity in the prescription of boundary conditions. We derive the one-group planar-geometry P2equations and associated boundary conditions using a simple, physically motivated variational principle. We also present numerical results comparing P2, P1, and SNcalculations. These results demonstrate that for most problems, the P2equations with variational boundary conditions are considerably more accurate than the P1equations with either the Marshak or the Federighi-Pomraning boundary conditions (both of which have also been derived variationally). Moreover, because the P2and P1equations can be written in diffusion form, the discretized P2equations require nearly the same computational effort to solve as the discretized P1equations. Our variational method can easily be extended to higher even-order PNapproximations.

ISSN:0029-5639    

DOI:10.13182/NSE93-A24040

出版商:Taylor&Francis    

年代:1993

数据来源: Taylor

摘要:

AbstractUncertainty and sensitivity analysis results obtained in a preliminary performance assessment for the Waste Isolation Pilot Plant (WIPP) in southeastern New Mexico are presented. The most appropriate conceptual model for performance assessment at the WIPP is believed to include gas generation due to corrosion and microbial action in the repository and a dual-porosity (matrix and fracture porosity) representation for solute transport in the Culebra Dolomite Member of the Rustler Formation. Under these assumptions, complementary cumulative distribution functions (CCDFs) summarizing radionuclide releases to the accessible environment due to both cuttings removal and groundwater transport fall substantially below the release limits promulgated by the U.S. Environmental Protection Agency (EPA). This is the case even when the current estimates of the uncertainty in analysis inputs are included. Performance assessment results are dominated by cuttings removal. The releases to the accessible environment due to groundwater transport make very small contributions to the total release. The variability in the distribution of CCDFs that must be considered in comparisons with the EPA release limits is dominated by the rate constant in the Poisson model for drilling intrusions. The variability in releases to the accessible environment due to cuttings removal is dominated by drill bit diameter. For a single borehole, whether or not a release due to groundwater transport from the repository to the Culebra occurs is controlled by Salado halite permeability, with no releases for small values (i.e.,<5×10−21m2). Further, releases that do reach the Culebra for larger values of halite permeability are small and usually do not reach the accessible environment. A potentially important scenario for the WIPP involves two or more boreholes through the same waste panel, of which at least one penetrates a pressurized brine pocket and at least one does not. For these scenarios, the uncertainty in release to the Culebra due to groundwater transport is dominated by borehole permeability, brine pocket pressure, and the solubilities for individual elements (i.e., americium, neptunium, plutonium, thorium, uranium). Once a release reaches the Culebra, matrix distribution coefficients for the individual elements are important, with releases to the Culebra often failing to reach the accessible environment over the 10 000-yr period specified in the EPA regulations.

ISSN:0029-5639    

DOI:10.13182/NSE93-A24041

出版商:Taylor&Francis    

年代:1993

数据来源: Taylor

摘要:

AbstractThe uncertainty of the235U,239Pu, and238U absolute delayed neutron yields vdis one of the principal sources of uncertainty in predicting the fission reactor reactivity scaleβeff. The current uncertainties in the dependence of vdon incident neutron energy is investigated for significance in the evaluation ofβeff. The uncertainty effects on the GODIVA, JEZEBEL, Zero Power Reactor, SNEAK, and Masurca benchmark facility calculations are analyzed using ENDF/B and JEF basic data. Different assumptions about the energy dependence result in variations of up to 5% in the reactor spectrum averaged values of vd, and these would result in variations of up to∼2% in the value ofβefffor a typical liquid-metal fast breeder reactor.

ISSN:0029-5639    

DOI:10.13182/NSE93-A24042

出版商:Taylor&Francis    

年代:1993

数据来源: Taylor

摘要:

AbstractAn-in-pile experimental technique to measure the decay constants and the relative abundances of the delayed neutron groups applicable for a given reactor system is presented. The method is based on a least-squares-fitting technique that simultaneously fits a series of transients produced by small reactivity perturbations to a reactor operating initially at delayed critical. The function that is least-squares fit is the analytic solution (written in terms of an arbitrary number of delayed neutron groups) as obtained by the point reactor model for the reactor response following a step change in reactivity. The application of the method does not require any knowledge of the size of the reactivity perturbations, and the method is independent of the detector efficiency. The results are based solely on the measurable quantities of relative power, time, and one measurable root of the Inhour equation.

ISSN:0029-5639    

DOI:10.13182/NSE92-78

出版商:Taylor&Francis    

年代:1993

数据来源: Taylor

摘要:

AbstractRelative cross-section measurements for the10B(n,α1γ)7Li reaction were made using the Oak Ridge Electron Linear Accelerator Laboratory neutron source. The cross sections were measured by observing the 478-keV photon using an intrinsic germanium detector. The neutron flux was monitored with a high-efficiency plastic scintillator. Monte Carlo calculations were used to provide multiple-scattering and neutron-attenuation corrections to the data. The measured cross sections differ as much as 40% from the ENDF/B-VI evaluation for incident neutron energies greater than 1.5 MeV.

ISSN:0029-5639    

DOI:10.13182/NSE93-A24044

出版商:Taylor&Francis    

年代:1993

数据来源: Taylor

ISSN:0029-5639    

DOI:10.13182/NSE93-A24045

出版商:Taylor&Francis    

年代:1993

数据来源: Taylor

ISSN:0029-5639    

DOI:10.13182/NSE93-A24046

出版商:Taylor&Francis    

年代:1993

数据来源: Taylor