摘要:

AbstractThe exact form of the differential scattering cross section for elastic scattering in an isotropic media with a Maxwellian velocity distribution is derived. Numerical examples show substantial neutron upscattering off heavy isotopes. Scattering resonances strongly affect neutron scattering, and it is found that for the lowest energy resonances of238U, the average energy of the scattered neutron can be higher than the initial neutron energy.

ISSN:0029-5639    

DOI:10.13182/NSE89-186

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractAn integral discrete ordinates method designed for use on modern, large-memory, vector and/or parallel processing supercomputers has been developed. The method is similar to conventional Sntechniques in that the medium is divided into spatial mesh cells and discrete directions are used. However, in place of an approximate differencing scheme, a nearly exact matrix representation of the streaming operator is determined. Although extremely large, this matrix can be stored on today’s large-memory computers for repeated use in the source iteration. Since the source iteration is cast in matrix form, it benefits enormously from vector and/or parallel processing, if available. Several electron transport test results are presented demonstrating a reduction in numerical diffusion and elimination of observable ray effects.

ISSN:0029-5639    

DOI:10.13182/NSE91-A23785

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractThe diffusion synthetic acceleration (DSA) algorithm effectively accelerates the iterative solution of transport problems with isotropic or mildly anisotropic scattering. However, DSA loses its effectiveness for transport problems that have strongly anisotropic scattering. Two generalizations of DSA are proposed, which, for highly anisotropic scattering problems, converge at least an order of magnitude (clock time) faster than the DSA method. These two methods are developed, the results of Fourier analyses that theoretically predict their efficiency are described, and numerical results that verify the theoretical predictions are presented.

ISSN:0029-5639    

DOI:10.13182/NSE91-A23786

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractA refinement of a simplified method, based on line-beam response functions, is presented for performing skyshine calculations for shielded and collimated gamma-ray sources. New coefficients for an empirical fit to the line-beam response function are provided and a prescription for making the response function continuous in energy and emission direction is introduced. For a shielded source, exponential attenuation and a buildup factor correction for scattered photons in the shield are used. Results of the new integral line-beam method of calculation are compared to a variety of benchmark experimental data and calculations and are found to give generally excellent agreement at a small fraction of the computational expense required by other skyshine methods.

ISSN:0029-5639    

DOI:10.13182/NSE91-A23787

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractThe boundary element method is used to generate energy-dependent matrix-type boundary conditions along core/reflector interfaces and along baffle-plate surfaces of pressurized water reactors. This method enables one to deal with all types of boundary geometries including convex and concave corners. The method is applicable to neutron diffusion problems with more than two energy groups and also can be used to model a reflector with or without a baffle plate. Excellent eigenvalue and flux shape results can be obtained when the boundary conditions generated by this technique are coupled with core-only finite difference calculations.

ISSN:0029-5639    

DOI:10.13182/NSE91-A23788

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractThe Doppler coefficient of reactivity is a crucial parameter in the evaluation of several transients in light water reactors (LWRs). It is relatively small in magnitude and cannot be measured directly in operating reactors. Doppler coefficients are presented for slightly idealized pressurized water reactor pin cells. These coefficients were calculated with the MCNP-3A continuous-energy Monte Carlo code using data taken directly from the ENDF/B-V nuclear data library. This combination represents the most rigorous analytical tool and the best nuclear data available. Consequently, these results comprise a set of numerical benchmarks that may be used to evaluate the accuracy of LWR lattice physics codes in predicting Doppler behavior at operating conditions. An example of one such evaluation, using the CELL-2 code, is included.

ISSN:0029-5639    

DOI:10.13182/NSE91-A23789

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractA set of efficiencies and response functions for 18 gamma rays in the range from 0.2 to 8.4 MeV has been experimentally determined via a (p,γ) coincidence method for a neutron capture detection setup. This consists of two cylindrical deuterated hexabenzene (C6D6) liquid scintillators placed symmetrically and normally with respect to the beam and operated in sum mode. A pulse-height weighting function is derived from this data set and applied to the measurement of neutron capture in the 1.15-keV resonance of56Fe relative to capture in the 5.2-eV resonance of109Ag. A value ofΓn= 62.9±2.1 meV has been obtained for the neutron width, in good agreement with the value ofΓn= 61.7±0.9 meV from transmission measurements. The extension of the validity of this weighting function to samples of different thickness and composition is discussed.

ISSN:0029-5639    

DOI:10.13182/NSE91-A23790

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractAn exact analytical solution of a reliability evolution equation is obtained for steady-state loading conditions. This solution is used as a Green’s function for more complicated loading. The importance of the dynamic aspects of failure conditions is revealed. The observable effects of an abrupt decrease in component reliability with a stepwise stress increase and the effects of apparent ceasing of deterioration with a stepwise stress reduction are explained. It is shown that deviations from the life fraction rule result from the combined dynamics of damage accumulation and failure processes rather than from requirements of nonlinearity of damage mechanisms with time.

ISSN:0029-5639    

DOI:10.13182/NSE91-A23791

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor