Band 72, Heft 1-2

The usefulness of wavelet transform and wavelet filtering techniques for the improvement of the estimation of the decay ratio (DR), characterising the stability of BWRs, is discussed. There are two distinct areas investigated. The first concerns the improvement of the quality of the traditionally used auto-correlation function (ACF) for the estimation of the DR, by trend elimination and denoising. The subsequent estimation of the DR itself is made by traditional methods such as the peak-to-peak method or curve fitting. The second area is the estimation of the DR by the use of continuous wavelet transform. The possibility of estimating two different DRs in case of dual oscillations, and in particular the finding of the higher DR, is also investigated. It was found that wavelet pre-processing does not always improve the estimation of the ACF of a non-ideal signal, compared to other methods; but for signals containing various trends and data scatter in the ACF, it brings a noticeable improvement. As an extension of the discrete wavelet methods, the continuous wavelet transform appears to be a promising candidate to determine the critical DR even in the case of two oscillations being co-existent with different stability properties. The methods investigated or developed here were also tested on measured data from Swedish BWRs.

The paper presents analytical and experimental investigations of shear stress on the rods of a bundle with irregular arrays. Simplified, however, effective methodologies are developed for thermal hydraulic processes (shear stress on the bundle rods' surfaces) along the multi-rod bundles. A single-vortex transfer gradient model is recommended taking into account the calculation results and a number of referenced experimental works. Analytical calculation methodologies of shear stress are based on ordinary differential equations and implemented in the computer codes RKN and RKN-M. Concurrently with the analytical investigations, experimental works were performed to determine the shear stress on the rods in a bundle with irregular cells. The results of the calculations are compared with experimental data.

Reducing separated weapon grade (WG) plutonium is very crucial since this plutonium may be misused and/or released accidentally into the environment. Burning WG plutonium in nuclear reactors can help to solve this serious problem effectively. In this study, the incineration of WG plutonium mixed with thorium in a (DT) fusion driven high power density hybrid reactor was investigated for an operation period of 24 months. Furthermore, the effect of using different coolants on the burning of WG plutonium in the hybrid blanket was examined. Numerical calculations showed that WG plutonium can be denatured to reactor grade level within a very short time by using a high power density hybrid reactor. Moreover, addition of WG plutonium to thorium improved the neutronic performance of the reactor significantly.

A computer model was developed to perform neutronic and burn-up analysis for an assembly of a thorium reactor. The MCNP computer code was used to model the geometry of the assembly and to determine both, the power and flux distribution. A system of ordinary differential equations which represents all fuel isotopes was solved numerically to evaluate the time behavior of fuel composition and burn-up. The results of the present model were compared with the solutions of benchmark problems and satisfactory agreement was found.

The determination of the subcriticality level for source-driven systems is an important problem that can be handled by several deterministic methods developed for the interpretation of flux signals in pulsed experiments. The paper compares the performance of several methods for different physical characteristics of the system considered and for different durations of the source pulse. The effects of the subcriticality level and of the experimental uncertainty are also investigated. Furthermore, an analysis of the influence of spatial effects is carried out. The study gives some indications on the most suitable method in a specific experimental configuration pointing out at the situations where non-conservative evaluations of the reactivity can be expected, i.e. when the system effective multiplication constant is underestimated.

This paper describes the development of a dynamic model for the thermal-hydraulic analysis of MTR research reactor during a reactivity insertion accident (RIA). The model is formulated for coupling reactor kinetics with feedback reactivity and the reactor core thermal-hydraulics. Two channels are considered; the average and the hot channels to represent the reactor core. The developed computer program is compiled and executed on a personal computer using the FORTRAN language. The model is validated by safety-related benchmark calculations for MTR-TYPE reactors of IAEA 10 MW generic reactor for both slow and fast reactivity insertion transients. A good agreement is shown between the present model and the benchmark calculations. The model is used for simulating uncontrolled withdrawal of a control rod for the ETRR-2 reactor in transient with over power scram trip. The simulation results for the ETRR-2 are analyzed and discussed.

In the present work, prediction of the onset of flow instability (OFI) in Egypt's second Research Reactor (ETRR-2) under loss of flow accident (LOFA) conditions due to loss of off-site power has been performed based on the model developed in previous work. Calculations are performed for LOFA with Scram due to low flow signal at 90% of the nominal flow where the time length that covers this transient is 4.8 seconds. Both, the best-estimate and conservative calculations are performed under the specified operating conditions and geometrical data of ETRR-2 for both, exponential and ramp pressure gradient change. The OFI locus is predicted and plotted against the flow velocity, exit coolant temperature and bubble detachment parameter for several heat fluxes. The results are analyzed and assessed in order to obtain the safety margins for OFI phenomenon that measure how far the operating conditions are from the OFI locus. It is found that, the safety margins for OFI phenomenon in the best-estimate calculation are 2.62 and 2.35 for steady state and LOFA transient respectively, while its values in the conservative calculation are 1.68 and 1.47, respectively.

Case's eigenfunctions and their orthogonality relations can be improved assuming pure-quadratic anisotropic neutron scattering. As a result, the infinite medium Green function can be written with those eigenfunctions terms. Choosing the appropriate boundary conditions and using the method of singular eigenfunctions, the half-space values are obtained. Our numerical results for this problem are reported.

The H N method is used to calculate the time eigenvalues in one speed neutron transport theory for a slab and a sphere with given boundary conditions. The angular distribution of the method of elementary solutions and the orthogonality relations of the singular eigenfunctions are used. Calculations are performed considering both the angular and the incoming angular distributions at the boundaries of the given medium. Numerical values are compared with the data available in literature.

The U N method is used to solve the critical slab problem for reflecting boundary conditions in one-speed neutron transport theory. The isotropic scattering kernel with the combination of forward and backward scattering is chosen for the neutrons in a uniform finite slab. It is shown that the method converges rapidly with easily executable equations. The presented numerical results are compared with the results available in the literature.

H N and singular eigenfunction methods are used to determine the neutron distribution everywhere in a source-free half space with zero incident flux for a linearly anisotropic scattering kernel. The singular eigenfunction expansion of the method of elementary solutions is used. The orthogonality relations of the discrete and continuous eigenfunctions for linearly anisotropic scattering provides the determination of the expansion coefficients. Different expansions of the exit distribution are used: the expansion in powers of μ, the expansion in terms of Legendre polynomials and the expansion in powers of 1/(1 + μ). The results are compared to each other. In the second part of our work, the transport equation and the infinite medium Green function are used. The numerical results of the extrapolation length obtained for the different expansions is discussed.