摘要:

AbstractThe ratio of ground-state transitions to excited-state transitions following neutron absorption in10B has been measured for the 20- to 1000-keV neutron energy region. Face-to-face silicon surface-barrier detectors were used to detect reactions and measure the total energy of the emitted alpha and lithium particles. The Oak Ridge Electron Linear Accelerator was used as a white neutron source, and time of flight was used to determine the neutron energy. The ratio varied from 0.064 at the lowest energies to 0.72 at 920 keV. The current measurements tend to be smaller than the currently accepted values by 10 to 30% in the 100- to 600-keV energy region.

ISSN:0029-5639    

DOI:10.13182/NSE91-A28510

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractA phenomenological scission point model including temperature-dependent shell effects is used to solve the energy partition problem as a function of mass asymmetry (A1/A2) for plutonium fission. Relevant fragment data such as average excitation energyĒand total kinetic energy TKE(A1/A2) are used as the basis for applying a temperature distribution model based on the Madland-Nix theory that includes the full mass number dependence of spectra, a realistic temperature distribution of fragments, a modified center-of-mass (CMS) spectrum ansatz, CMS anisotropy of neutron emission, and competition of neutron and gamma-ray emission. This new model describes neutron multiplicity, energy, and angular distribution of prompt fission neutrons. Calculated data for238Pu,240Pu, and242Pu spontaneous fission are presented and discussed in comparison with experimental data.

ISSN:0029-5639    

DOI:10.13182/NSE91-A28511

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractA model calculation of neutron activation cross sections is carried out with SINCROS-II for all stable isotopes of molybdenum. Results of the calculation are compared with experimental data, which were measured over the wide range from threshold to 20 MeV. The general agreement between the calculated and experimental cross sections is good, but some cross sections remain in significant disagreement. The calculational method is applicable to the estimation of production cross sections for the long-lived radioactive nuclides.

ISSN:0029-5639    

DOI:10.13182/NSE91-A28512

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractThe chi-square (χ2) values, which represent the degree of agreement between the calculated total, nonelastic, and differential elastic cross sections and their experimental values, are calculated for seven kinds of optical potentials: the phenomenological optimal optical potential (OOP) for a specific element, the global phenomenological optical potentials given by Becchetti and Greenlees (BGP) and by Varner et al. (CH86) for a large number of target nuclei, and the microscopic optical potentials based on conventional Skyrme force (SII and SIII), generalized Skyrme force (GS2), and modified Skyrme force (SKa). Fourteen natural elements (each containing one to four isotopes) are calculated with 12 to 20 neutron incident energies, which are in the 0.1- to 24-MeV energy region for each element. The calculated average total chi-square values areχ2OOP= 0.309,χ2BGP= 0.807,χ2CH86= 0.684χ2GS2= 0.600,χ2SKa= 0.646,χ2SII= 2.587, andχ2SIII= 1–368. The conclusion is that the microscopic optical potential based on generalized and modified Skyrme force (GS2 and SKa), which has an analytical formalism without any free parameters, is useful in nuclear data calculation and evaluation.

ISSN:0029-5639    

DOI:10.13182/NSE91-A28513

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractA new approximate method is developed to allow S(α,β) thermal collision contributions to next-event estimators in Monte Carlo calculations. The new technique is applicable to the more general problem of contributions to next-event estimators from any discrete probability distribution. The method is incorporated into the MCNP Monte Carlo radiation transport code.

ISSN:0029-5639    

DOI:10.13182/NSE91-A28514

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractThe effect on reactivity of changes in the coolant levels in the pressure tubes of a pressure-tube heavy water reactor is experimentally studied to clarify the effect of an axial coolant void fraction distribution. The coolant void fraction distribution is simulated by stepwise changes in the coolant levels in the Deuterium Critical Assembly (DCA). The reactivity is measured for a 25.0-cm-pitch square-lattice core with a positive coolant void reactivity. The reactivity changes resulting from changes in the coolant levels are measured as changes in the critical heavy water levels. The axial distribution of the thermal neutron flux is also measured by the copper activation method.In these measurements, the critical heavy water levels show a sinuous curve having a maximum and a minimum, and a positive reactivity larger than those of both the 0% and the 100% void uniform cores is introduced at certain coolant levels by stepwise changes in the coolant levels.An experimental analysis is performed with the coupled WIMS-ATR/CITATION code system, whose analytical method was established through DCA critical experiments. Agreement between experiment and analysis is fairly good. Furthermore, the peculiar reactivity behavior resulting from changes in the coolant levels is analyzed using a simplified model to take note of typical reactor physics parameters. It is clarified that this anomalous phenomenon is caused by the combined effect of the flattened S curve change in the thermal neutron absorption and the even flatter S curve change in the neutron leakage caused by the changes in the coolant levels. Useful information is obtained regarding reactivity behavior with an axial coolant void fraction distribution.

ISSN:0029-5639    

DOI:10.13182/NSE91-A28515

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractTo ensure that a space nuclear power system will operate safely and respond in a predictable and desired manner, the system’s controller design must account for changes in the system parameters over its lifetime. A model reference adaptive controller is applied to enable the actual space nuclear power system to follow a predictable and desired response of a reference model system, despite changes in the actual system’s operating parameters. Model reference adaptive control is well developed for linear systems and has been applied to simple, single-input, single-output (and the output’s derivative) systems. Model reference adaptive control is applied to a single-input, multiple-output nonlinear system but also shows the development for a multiple-input, multiple-output linear system. An algorithm is developed for linear systems to determine the constant gains in the model reference adaptive control algorithm and a method is developed that allows selective weighting of a desired state variable. Examples are presented to show that a model reference adaptive controller can ensure the load-following response of a nonlinear space nuclear power system and that the reference model can be complex enough to embody the physics of the plant. The results of the example cases show that a model reference adaptive controller can cause a selected nonlinear plant state variable to track the transient trajectory of the corresponding state variable of the reference model with local stability.

ISSN:0029-5639    

DOI:10.13182/NSE91-A28516

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractAxial flux shape control in large pressurized water reactors constitutes one of the most challenging control problems in the nuclear field. In commercial plants, the practical solutions are obtained at the expense of departure from the most economical operational conditions, often due to the difficulties in monitoring xenon-induced oscillations and inadequate control actions. The concept of inverse dynamics in control is introduced as an alternative approach for spatial control. The method is tested through computer simulations using a validated nonlinear model that successfully represents the limit-cycle behavior. Compared with the widely used half-cycling strategy or the proposed optimal control methods in the literature, the use of inverse dynamics for partial-length rod control yields desirable stability characteristics. The return to target axial offset exhibits a smooth transition without any residual flux oscillations between the upper and lower halves of the core. The proposed approach consists of a set of nonlinear algebraic equations for control with single-step solutions. Thus, it is easier to implement compared with iterative or integral techniques.

ISSN:0029-5639    

DOI:10.13182/NSE91-A28517

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractA one-dimensional method based on a combination of the“nodal equivalence theory”and response matrix homogenization methods was previously described for determining environment-insensitive equivalent few-group diffusion theory parameters for homogenized radial reflector nodes of a pressurized water reactor. This reflector model, called the NGET-RM model, yields equivalent nodal parameters that do not account for the two-dimensional structure of the baffle at core corners; this can lead to significant errors in computed two-dimensional core power distributions. A semi-empirical correction procedure is proposed for reducing the two-dimensional effects associated with this particular one-dimensional reflector model. Numerical two-group experiments are performed for a given reflector configuration (and soluble boron concentration) to determine optimal values for the two empirical factors defined by this model. It is shown that the resultant factors are rather insensitive to core configuration or core conditions and that their application yields improved two-group NGET-RM reflector parameters with which accurate nodal power distributions can be obtained. The results are also compared with those obtained with another one-dimensional environment-insensitive model that has an extra degree of freedom utilized here to reduce two-dimensional effects. Some practical aspects related to the application of the proposed correction procedure are briefly discussed.

ISSN:0029-5639    

DOI:10.13182/NSE91-A28518

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractNeutron capture cross sections for the197Au(n,γ)198Au reaction have been absolutely determined at neutron energies of 23 and 967 keV. These are the median energies of the two photoneutron sources, Sb-Be and Na-Be, applied in this study.Reaction rates were determined by gamma counting of the198Au activity using a pair of cylindrical NaI(Tl) detectors. The detection efficiency of the detectors was determined using 4πbeta-gamma coincidence counting techniques. The neutron emission rates of the photoneutron sources were determined by indirect comparison with the U.S. National Bureau of Standards NBS-I neutron standard source using the University of Michigan manganese bath. The values obtained for the197Au(n,γ)198Au cross section are 617.8±11.1 mb and 99.7±2.8 mb at 23 and 967 keV, respectively.

ISSN:0029-5639    

DOI:10.13182/NSE91-A28519

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor