Showing papers in "Nuclear Science and Engineering in 1997"
TL;DR: A family of transport synthetic acceleration (TSA) methods for iteratively solving within-group scattering problems is presented in this paper, where a single iteration in these schemes consists of a transport sweep f...
Abstract: A family of transport synthetic acceleration (TSA) methods for iteratively solving within-group scattering problems is presented. A single iteration in these schemes consists of a transport sweep f...
64 citations
TL;DR: In this paper, a DUPIC fuel is made of spent pressurized water reactor fuel through a dry process, and the bundle design utilizes an advanced CANDU fuel bundle geometry that is mechanically compatible with the current fuel channel and refueling system.
Abstract: Physics studies have been performed for the compatibility of DUPIC fuel design to the existing CANDU-6. The DUPIC fuel is made of spent pressurized water reactor fuel through a dry process, and the bundle design utilizes an advanced CANDU fuel bundle geometry that is mechanically compatible with the current fuel channel and refueling system. The characteristics of a DUPIC core are compared to the current operation limits and performances of a natural uranium core. The refueling simulations have shown that the channel and bundle powers are well below the operation limits for the two- and four-bundle shift refueling schemes. The fuel performance parameters during the refueling operation reserve enough margin to the stress corrosion cracking threshold of natural uranium fuel. With the aid of burnable poison material in the fuel, the safety performance of a DUPIC core is made comparable to that of a natural uranium core. The details of DUPIC fuel design and analysis are described.
54 citations
TL;DR: In this paper, biases in the estimators of the variance and intercycle covariances in Monte Carlo eigenvalue calculations are analyzed and the relations among the real and apparent values of variances and interc...
Abstract: Biases in the estimators of the variance and intercycle covariances in Monte Carlo eigenvalue calculations are analyzed. The relations among the “real” and “apparent” values of variances and interc...
54 citations
TL;DR: In this paper, the potential role of accelerator-driven systems to burn long-lived wastes is also stressed, in particular with respect to thorium- or uranium-based fuel cycles.
Abstract: Accelerator-driven systems (ADSs) have been proposed for a variety of purposes. Different fuel cycles have also been associated with these systems. An analysis is made based exclusively on simple but well-established physics facts. Requirements related to the subcriticality level, the reactivity evolution during irradiation, and the energy needed to feed the accelerator are examined, in particular with respect to thorium- or uranium-based fuel cycles. Moreover, the somewhat forgotten aspect of the spallation neutron source importance is underlined to set up guidelines to optimize specific ADS design. The potential role of ADS to burn long-lived wastes is also stressed.
52 citations
TL;DR: Modern probability theory, incorporating results from information, decision, and group theory, is shown to provide straight and unique answers to questions about uncertainties and to deal easily with prior information and small samples.
Abstract: Long-standing problems of assigning uncertainties to scientific data became apparent in recent years when uncertainty information (covariance files) had to be added to applications-oriented large libraries of evaluated nuclear data such as ENDF and JEF. Questions arose about the best way to express uncertainties, the meaning of statistical and systematic errors, the origin of correlations and the construction of covariance matrices, the combination of uncertain data from different sources, the general usefulness of results that are strictly valid only for Gaussians or only for linear statistical models, and so forth. Conventional statistical theory is often unable to give unambiguous answers and tends to fail when statistics are poor, making prior information crucial. Modern probability theory, on the other hand, incorporating results from information, decision, and group theory, is shown to provide straight and unique answers to such questions and to deal easily with prior information and small samples.
39 citations
TL;DR: In this article, a new automated variance reduction method for the Monte Carlo simulation of multigroup neutron transport source-detector problems is described, based on a modified transport problem that can be solved by analog Monte Carlo with zero variance.
Abstract: A new automated variance reduction method for the Monte Carlo simulation of multigroup neutron transport source-detector problems is described. The method is based on a modified transport problem that can be solved by analog Monte Carlo with zero variance. The implementation of this modified problem is impractical, in part because it requires the exact solution of an adjoint transport problem. The new local importance function transform (LIFT) method is developed to overcome this difficulty by approximating the exact adjoint solution with a piecewise-continuous function containing parameters that are obtained from a deterministic adjoint calculation. The transport and collision processes of the transformed Monte Carlo problem bias source distribution, distance to collision, and selection of postcollision energy groups and directions. A companion paper provides numerical results that demonstrate the efficiency of the LIFT method.
38 citations
TL;DR: In this article, the multigroup simplified spherical harmonics equations with anisotropic scattering are derived from a variational principle that preserves nodal balance and discretized using a Ritz procedure with spatial trial functions that are complete polynomials within the nodes and on the interfaces.
Abstract: The multigroup simplified spherical harmonics equations with anisotropic scattering are derived from a variational principle that preserves nodal balance. The resulting equations are discretized using a Ritz procedure with spatial trial functions that are complete polynomials within the nodes and on the interfaces. The resulting equations are case in a response matrix form and incorporated as an option of the variational nodal spherical harmonics code VARIANT. Fixed source and multigroup eigenvalue calculations are performed on benchmark problems. The accuracy and computational efficiency of spherical harmonic and simplified spherical harmonic approximations are compared, and the compensating effects of spatial and angular truncation errors are examined. The results indicate that in most situations, simplified and standard spherical harmonics results of the same order are in close agreement, while the use of simplified spherical harmonics substantially reduces computing costs.
35 citations
TL;DR: In this article, the analytic function expansion nodal (AFEN) method is extended such that complex eigenmodes are treated within the framework of the original AFEN method for any type of geometry.
Abstract: The analytic function expansion nodal (AFEN) method has been successfully applied to two-group neutron diffusion problems. However, the current AFEN method cannot treat complex eigenmodes, which appear in the general multigroup equations. The AFEN method is extended such that complex eigenmodes are treated within the framework of the original AFEN method for any type of geometry. Also, a suite of new nodal codes based on the extended AFEN theory is developed for hexagonal-z geometry and applied to several benchmark problems. Numerical results obtained attest to their accuracy and applicability to practical problems.
32 citations
TL;DR: In this article, the performance of the local importance function transform (LIFT) method for several three-dimensional, linearly anisotropic-scattering, one-group, and multigroup transport problems is demonstrated.
Abstract: The performance of the local importance function transform (LIFT) method for several three-dimensional, linearly anisotropic-scattering, one-group, and multigroup transport problems is demonstrated. In these problems, the LIFT method is shown to be more efficient than the AVATAR scheme, which is one of the most efficient variance reduction techniques currently available in a production Monte Carlo code. For most of the problems considered, the LIFT method produces higher figures of merit than AVATAR, even when the LIFT method is used as a black box.
27 citations
TL;DR: In this article, a cylindrical channel with a 0.78mm inside diameter and 0. 78 mm in length was experimentally studied, and the measured critical mass fluxes were compared with three models appropriate for application to critical flow in small and short channels.
Abstract: Critical discharge of highly subcooled water through a cylindrical channel with a 0.78-mm inside diameter and 0. 78 mm in length was experimentally studied. The range of the initial water subcooling was 76 to 200 K, and the initial water pressure was in the range 0.5 to 5.2 MPa. The measured critical mass fluxes were compared with three models appropriate for application to critical flow in small and short channels. The experimental results confirm the significant effect of pressure losses on critical discharge rates in small channels. They indicate, however, that the frictional pressure losses in cracks may be considerably larger than losses predicted by the widely used correlations for rough channels. It is shown that models and correlations based on isentropic homogeneous equilibrium flow in the channel accurately predict the critical flow data, provided that the liquid initial stagnation pressure is adequately corrected for the channel entrance pressure loss.
26 citations
TL;DR: In this paper, a method is described for generating electron cross sections that are comparable with standard discrete ordinates codes without modification, and the cross sections are shown to provide accurate and efficient solutions to certain multidimensional electron-photon transport problems.
Abstract: A method is described for generating electron cross sections that are comparable with standard discrete ordinates codes without modification. There are many advantages of using an established discrete ordinates solver, e.g. immediately available adjoint capability. Coupled electron-photon transport capability is needed for many applications, including the modeling of the response of electronics components to space and man-made radiation environments. The cross sections have been successfully used in the DORT, TWODANT and TORT discrete ordinates codes. The cross sections are shown to provide accurate and efficient solutions to certain multidimensional electron-photon transport problems. The key to the method is a simultaneous solution of the continuous-slowing-down (CSD) portion and elastic-scattering portion of the scattering source by the Goudsmit-Saunderson theory. The resulting multigroup-Legendre cross sections are much smaller than the true scattering cross sections that they represent. Under certain conditions, the cross sections are guaranteed positive and converge with a low-order Legendre expansion.
TL;DR: A new importance map approach for Monte Carlo simulation that can be used in an adaptive fashion has been identified and developed based on using a mesh-based system of weight windows that are independent of any physical geometric cells.
Abstract: A new importance map approach for Monte Carlo simulation that can be used in an adaptive fashion has been identified and developed. It is based on using a mesh-based system of weight windows that a...
TL;DR: In this article, an adaptive Monte Carlo method that empirically produces exponential convergence on a simple continuous transport problem is described, which is the first time it has been shown that exponential convergence is possible using adaptive Monte-Carlo techniques.
Abstract: For more than a decade, it has been known that exponential convergence on discrete transport problems was possible using adaptive Monte Carlo techniques. An adaptive Monte Carlo method that empirically produces exponential convergence on a simple continuous transport problem is described.
TL;DR: In this paper, the authors examined the problem of obtaining valid confidence intervals based on estimates from sampled distributions using Monte Carlo particle transport simulation codes such as MCNP and proposed two new diagnostics to assess whether the conditions of central limit theorem-type results are satisfied: the relative variance of the variance determines whether the sample size is sufficiently large, and estimators of the slope of the right tail of the distribution are used to indicate the number of moments that exist.
Abstract: The problem of obtaining valid confidence intervals based on estimates from sampled distributions using Monte Carlo particle transport simulation codes such as MCNP is examined. Such intervals can cover the true parameter of interest at a lower than nominal rate if the sampled distribution is extremely right-skewed by large tallies. Modifications to the standard theory of confidence intervals are discussed and compared with some existing heuristics, including batched means normality tests. Two new types of diagnostics are introduced to assess whether the conditions of central limit theorem-type results are satisfied: the relative variance of the variance determines whether the sample size is sufficiently large, and estimators of the slope of the right tail of the distribution are used to indicate the number of moments that exist. A simulation study is conducted to quantify the relationship between various diagnostics and coverage rates and to find sample-based quantities useful in indicating when intervals are expected to be valid. Simulated tally distributions are chosen to emulate behavior seen in difficult particle transport problems. Measures of variation in the sample variance s{sup 2} are found to be much more effective than existing methods in predicting when coverage will be near nominal rates. Batched meansmore » tests are found to be overly conservative in this regard. A simple but pathological MCNP problem is presented as an example of false convergence using existing heuristics. The new methods readily detect the false convergence and show that the results of the problem, which are a factor of 4 too small, should not be used. Recommendations are made for applying these techniques in practice, using the statistical output currently produced by MCNP.« less
TL;DR: In this paper, the zinc ion was measured at 25, 50, 75, 185, 200, and 225°C through direct measurement of pH using pH sensors, especially of the yttria-stabilized zirconia membrane-ty...
Abstract: Hydrolysis constants of the zinc ion were measured at 25, 50, 75, 185, 200, and 225°C through the direct measurement of pH using pH sensors, especially of the yttria-stabilized zirconia membrane-ty...
TL;DR: In this article, the decay constants for 235 U, 237 Np, 241 Am, and 243 Am were measured at the Texas AM Waldo, Karam, and Meyer; and Tuttle.
Abstract: Delayed neutron yields and decay constants for 235 U, 237 Np, 241 Am, and 243 Am were measured at the Texas AM Waldo, Karam, and Meyer; and Tuttle. Very good agreement was obtained, especially for 235 U.
TL;DR: In this article, the authors employed the infinite-medium Green's function to obtain scalar and angular flux distributions in heterogeneous slab geometry with isotropic scattering for both multiplying and non-multiplying media.
Abstract: The Green`s function method (GFM) is employed to obtain scalar and angular flux distributions in heterogeneous slab geometry with isotropic scattering. All solutions utilize the infinite-medium Green`s function to obtain results in finite media. Past Green`s function analyses that do not resort to expansions of the angular flux in basis functions have been performed for nonmultiplying media only; in this paper, results are provided, for the first time, for both multiplying and nonmultiplying media using the GFM. Several source configurations are considered, including a beam source on the leftmost face, isotropic incidence on any face, and constant inhomogeneous volume sources in internal materials. Scalar and angular flux distributions compare favorably with those obtained using the F{sub N} method as well as the ONEDANT discrete ordinates code. In addition, the single and heterogeneous critical slab problems are investigated and solved using the GFM.
TL;DR: In this paper, the energy spectra of delayed neutrons for the isotope-separated, fission product precursors 87Br, 88Br, 89Br, 90 Br, 90Br, 137I, 138I, 139I, and 136Te were measured for an energy range up to t
Abstract: Measurement of the energy spectra of delayed neutrons for the isotope-separated, fission product precursors 87Br, 88Br, 89Br, 90Br, 137I, 138I, 139I, and 136Te are reported for an energy range up t...
TL;DR: In this article, a model-based parameter estimation method for nonlinear systems that does not require the linearization of the system equations and that can account for uncertainties in the monitored data as well as the parameters (e.g., random variations) is described.
Abstract: A model-based parameter estimation method for nonlinear systems that does not require the linearization of the system equations and that can account for uncertainties in the monitored data as well as the parameters (e.g., random variations) is described. The method is particularly suitable for fault diagnosis because of its capability to assign probabilities of occurrence to user-specified parameter magnitude intervals that may be associated with system faults. The method regards system evolution in time as transitions between these intervals as well as user-specified magnitude intervals of the dynamic variables. These transition rates are obtained on-line from the system model and the monitored dynamic variable data and constitute a Markov chain in discrete time. The method then compares predicted and observed data at a given time step to narrow the estimated parameter range in the next time step. Implementations using a second-order van der Pol oscillator and a third-order system describing temporal xenon oscillations in a hypothetical reactor indicate that the method is computationally efficient and can be used for multiparameter estimation with incomplete information on the system state.
TL;DR: In this paper, a generalized Fermi-Eyges-like formula was derived for a broader class of scattering kernels, which are described by the continuous slowing-down approximation (the Fokker-planck description is energy), but not sufficiently forward peaked in angle to allow an angular Fokkers-Planck representation.
Abstract: The problem of describing steady-state transport of a perpendicularly incident particle beam through a thin slab of material is considered. For a scattering kernel sufficiently peaked in momentum transfer to allow a Fokker-Planck description of the scattering process in both energy and angle, an approximate closed form solution to this problem was obtained almost 50 yr ago and is referred to as the Fermi-Eyges formula. It is shown that a Fermi-Eyges-like formula can be derived for a broader class of scattering kernels. This class consists of scattering described by the continuous slowing-down approximation (the Fokker-Planck description is energy), but not sufficiently forward peaked in angle to allow an angular Fokker-Planck representation. This generalized formula reduces to the classic Fermi-Eyges result for scattering operators with a valid Fokker-Planck limit and also describes problems that, while involving a forward-peaked scattering kernel, do not possess a Fokker-Planck description. A classic example of such a kernel is the Henyey-Greenstein kernel, and the Fermi-Eyges-like solution in this case exhibits more beam spreading than that predicted by the classic Fermi-Eyges formula. In particular, the scalar flux is non-Gaussian in the radial coordinate, as contrasted with the Gaussian Fermi-Eyges result.
TL;DR: In this paper, the authors extended the subgroup approach beyond the unresolved energy domain by partially taking into account correlation effects between the slowing-down source with the collision probability terms of the transport equation and obtained a pure subgroup solution of the self-shielding problem without relying on any form of equivalence in dilution.
Abstract: The subgroup method is used to compute self-shielded cross sections defined over coarse energy groups in the resolved energy domain. The validity of the subgroup approach was extended beyond the unresolved energy domain by partially taking into account correlation effects between the slowing-down source with the collision probability terms of the transport equation. This approach enables one to obtain a pure subgroup solution of the self-shielding problem without relying on any form of equivalence in dilution. Specific improvements are presented on existing subgroup methods: an N-term rational approximation for the fuel-to-fuel collision probability, a new Pade deflation technique for computing probability tables, and the introduction of a superhomogenization correction. The absorption rates obtained after self-shielding are compared with exact values obtained using an elastic slowing-down calculation where each resonance is modeled individually in the resolved energy domain.
TL;DR: In this paper, the authors investigated buoyancy-driven exchange flow behavior through breaches of a vacuum vessel (VV) by using a preliminary low-of-vacuum-event (LOVA) apparatus that simulated the tokamak VV of a fusion reactor.
Abstract: As one of thermofluid safety studies in the International Thermonuclear Experimental Reactor, buoyancy-driven exchange flow behavior through breaches of a vacuum vessel (VV) has been investigated quantitatively by using a preliminary low-of-vacuum-event (LOVA) apparatus that simulated the tokamak VV of a fusion reactor with a small-scaled model To carry out the present experiments under the atmospheric pressure condition, helium gas and air were provided as the working fluids The inside of the VV was initially filled with helium gas and the outside was atmosphere The breaches on the VV under the LO VA condition were simulated by opening six simulated breaches to which were set the different positions on the VV When the buoyancy-driven exchange flow through the breach occurred, helium gas went out from the inside of the VV through the breach to the outside and air flowed into the inside of the VV through the breach from the outside The exchange rate in the VV between helium gas and air was calculated from the measured weight change of the VV with time since the experiment has started Experimental parameters were breach position, breach number, breach length, breach size, and breach combination The present study clarifies that the relation between the exchange rate and the breach position of the VV depended on the magnitude of the potential energy from the ground level to the breach position, and then, the exchange rate decreased as the breach length increased and as the breach size decreased
TL;DR: The post-convergence automatic differentiation (PAD) technique results in better accuracy of the computed derivatives, as it eliminates part of the derivatives convergence error, and a large reduction in execution time, especially when many iterations are required to achieve convergence.
Abstract: A new approach for performing automatic differentiation (AD) of computer codes that embody an iterative procedure, based on differentiating a single additional iteration upon achieving convergence, is described and implemented. This post-convergence automatic differentiation (PAD) technique results in better accuracy of the computed derivatives, as it eliminates part of the derivatives convergence error, and a large reduction in execution time, especially when many iterations are required to achieve convergence. In addition, it provides a way to compute derivatives of the converged solution without having to repeat the entire iterative process every time new parameters are considered. These advantages are demonstrated and the PAD technique is validated via a set of three linear and nonlinear codes used to solve neutron transport and fluid flow problems. The PAD technique reduces the execution time over direct AD by a factor of up to 30 and improves the accuracy of the derivatives by up to two orders of magnitude. The PAD technique`s biggest disadvantage lies in the necessity to compute the iterative map`s Jacobian, which for large problems can be prohibitive. Methods are discussed to alleviate this difficulty.
TL;DR: In this article, the cross section for the 241Am(n,f) reaction has been measured using back-to-back type double fission chambers and a lead slowing-down spectrometer coupled to an electron linear accelerator.
Abstract: Making use of back-to-back type double fission chambers and a lead slowing-down spectrometer coupled to an electron linear accelerator, the cross section for the 241Am(n,f) reaction has been measur...
TL;DR: In this paper, the transverse intensity distribution of a beam emerging from a linear accelerator can be described by a two-dimensional Gaussian function with elliptical contours, and a method is presented that utilizes third-order optics to modify this Gaussian beam distribution and to produce at a given location along the beam direction a more uniform beam, which also is confined within a prescribed rectangular area.
Abstract: The transverse intensity distribution of a beam emerging from a linear accelerator can be described well by a two-dimensional Gaussian function with elliptical contours. A method is presented that utilizes third-order optics to modify this Gaussian beam distribution and to produce at a given location along the beam direction a more uniform beam, which also is confined within a prescribed rectangular area.
TL;DR: In this paper, the performance of a continuous neutron source using an electron accelerator was evaluated by computer simulation codes (EGS4 and MCNP) in terms of neutron yield, neutron flux distribution, neutron spectrum, and heat distribution.
Abstract: The performance of a continuous neutron source using an electron accelerator was evaluated by computer simulation codes (EGS4 and MCNP) in terms of neutron yield, neutron flux distribution, neutron spectrum, and heat distribution. Electrons with energies from 10 to 100 MeV were injected into a tungsten converter in order to generate photons by bremsstrahlung. When the photon irradiated a heavy water (DS{sub 2}O) target, neutrons were produced by photonuclear reaction in the D{sub 2}O target. This type of source was optimized for target geometry and electron energy from the point of neutron yield. The neutron spectrum was found to have two characteristic peaks, at the low-energy (thermal) region and the high-energy (million-electron-volt) region. The maximum photoneutrons per 1,000 MeV of electron energy was 0.56 at the electron energy of 30 MeV. In the case of irradiation by a 30-MeV, 33-mA continuous electron beam, the maximum thermal neutron flux was on the order of 10{sup 11} cm{sup {minus}2}{center_dot}s{sup {minus}1}.
TL;DR: In this article, a new optimization method is presented for determining the optimized PWR fuel-loading pattern in the maximization principle of the end-of-cycle (EOC) core reactivity.
Abstract: A new optimization method is presented for determining the optimized pressurized water reactor (PWR) fuel-loading pattern in the maximization principle of the end-of-cycle (EOC) core reactivity. The new method utilizes the point reactivity model in deriving the objective unction corresponding to the EOC core reactivity as a linear function of the fuel-loading binary variables. It also uses a mixed integer programming algorithm consisting of the branch and bound method and dual linear programming algorithm in order to maximize the EOC core reactivity. The utility of the new optimization method is discussed in terms of numerical examples for the fuel-loading-pattern optimization of the cycle 4 core of the Yong-gwang unit 2 PWR plant.
TL;DR: A weight-dependent capability is inserted into the direct statistical approach (DSA) to optimize splitting and Russian roulette parameters in Monte Carlo particle transport calculations.
Abstract: A weight-dependent capability is inserted into the direct statistical approach (DSA) to optimize splitting and Russian roulette (RR) parameters in Monte Carlo particle transport calculations. In the new model, splitting or RR is carried out on a progenitor arriving at a surface in such a way that the weight of the progeny is fixed (for the particular surface). Thus, the model is named the DSA weight line model. In the presence of weight-dependent games, all components of the second moment, and the time, are not separable. In the absence of weight-dependent games, the component of the second moment describing the weight-dependent splitting or RR is still not separable. Two approximations are examined to render this component separable under these circumstances. One of these approximations, named the noninteger approximation, looks promising. The new DSA model with the noninteger approximation is tested on four sample problems. Comparisons with the previous weight-independent DSA model and with the MCNP (version 4a) weight window generator are made.
TL;DR: In this article, the linear Boltzmann equation for the transport of neutral particles is investigated with the objective of generating a benchmark-quality calculation for the three-dimensional searchlight problem in a semi-infinite medium.
Abstract: The linear Boltzmann equation for the transport of neutral particles is investigated with the objective of generating a benchmark-quality calculation for the three-dimensional searchlight problem in a semi-infinite medium. The derivation assumes stationarity, one energy group, and isotropic scattering. The scalar flux (both surface and interior) and the current at the surface are the quantities of interest. The source considered is a pencil-beam incident at a point on the surface of a semi-infinite medium. The scalar flux will have two-dimensional variation only if the beam is normal; otherwise, it is three-dimensional. The solutions are obtained by using Fourier and Laplace transform models. The transformed transport equation is formulated so that it can be related to a one-dimensional pseudo problem, thus providing some analytical leverage for the inversions. The numerical inversions use standard numerical techniques such as Gauss-Legendre quadrature, summation of infinite series, H-function iteration and evaluation, and Euler-Knopp acceleration. The numerical evaluations of the scalar flux and current at the surface are relatively simple, and the interior scalar flux is relatively difficult to calculate because of the embedded two-dimensional Fourier transform inversion, Laplace transform inversion, and H-function evaluation. Comparisons of these numerical solutions to results from the MCNP probabilistic codemore » and the THREE-DANT discrete ordinates code are provided and help confirm proper operation of the analytical code.« less
TL;DR: Cross sections of the 58Ni(n,p)58Co, 51Co( n, p)59Fe, 51V(n),α)48Sc, and 59Co(N,2n)58 co reactions and isomer ratios for the 58 Ni(n-p) 58Co reaction were measured for some neutron energies between 9 and 9 neutron energies.
Abstract: Cross sections of the 58Ni(n,p)58Co, 51Co(n,p)59Fe, 51V(n,α)48Sc, and 59Co(n,2n)58Co reactions and isomer ratios for the 58Ni(n,p)58Co reaction were measured for some neutron energies between 9 and...