Volnei Borges

Determination of the effective multiplication factor in a slab by the LTSN method

Abstract The aim of this work is to present a new approach based upon the application of the LTSN method to solve the eigenvalue problem in a slab, namely the problem of criticality. This procedure leads to the solution of a transcendental equation for the effective multiplication factor. We report numerical simulations considering the SN multigroup problem in heterogeneous slab.

Determination of the exposure build-up factor in a slab using the LTSN method

Abstract In this paper an exposure build-up factor formulation is reported by solving the photon transport equation in a heterogeneous slab by the LTSN method, assuming the Klein-Nishina scattering kernel as the scattering differential cross section as well as the multigroup model in the wavelength variable. Numerical simulations and comparisons with available results in the literature are presented for different compositions containing water, iron and lead.

Dynamic Stability of a Fluidized-Bed Nuclear Reactor

Recent advances in the study of a fluidized-bed nuclear reactor’s stability, due to short and long time transients, are discussed. The point-kinetic model, which considers flux variation in the axi...

A solution for the two-dimensional transport equation for photons and electrons in a rectangular domain by the Laplace transform technique

In the first part of this paper, we describe a review in the Laplace transform technique applied to SN two-dimensional transport equation in a rectangular domain considering Compton scattering. In Compton scattering the incident photon interacts with an electron of the irradiated medium, giving the electron kinetic energy, and also producing a scattered photon. In order to track that electron, in the second part of this paper, we apply the PN approximation to X,Y-geometry electron transport problems. In this derivation, the energy deposited by the free electron is taken into account. To reach this objective we solve two model problems: the first one for the transport of photons, assuming the Klein-Nishina scattering kernel as the scattering differential cross-section as well as the multigroup model in the energy variable; and the second one for the transport of free electrons considering the screened Rutherford scattering kernel. The results are presented under absorbed energy form in several points of interest. We report numerical simulations and comparisons with results obtained by using the Geant4 version 9.1 program which applies the Monte Carlos technique to low energy libraries for both two-dimensional problems.

The exposure buildup factor formulation in a slab and rectangle geometry by the LTSN method

In this paper, we report a formulation for the exposure buildup factor by solving the one-dimensional photon transport equation in a heterogeneous slab by the LTSN method, assuming the Klein-Nishina scattering kernel as the scattering differential cross-section as well as the multigroup model in the wavelength variable. We present numerical simulations and comparisons with available results in the literature for a multilayered slab composed of water, iron and lead. We also report an analytical solution to the exposure buildup factor by solving the photon transport equation in a rectangle applying the LTSN nodal method.

Estimating heat transfer coefficients for solidgas interfaces using the LandauTeller model

In this work, a new approach to estimate heat transfer coefficients for solidgas interfaces is proposed. The new formulation takes into account that the photon transfer process between solid walls and gases is not instantaneous, but involves electronic transitions, even when low frequency radiation is exchanged. The delay required to electron cloud transitions, called relaxation time, which plays a crucial role in the heat transfer process, is evaluated using the LandauTeller model.

Estudo da densidade mineral óssea em pacientes com lúpus eritematoso sistêmico antes e após uso de glicocorticóides

OBJECTIVE: To evaluate the bone mineral density variability in patients with systemic lupus erythematosus, before the beginning and at 30 and 60 days from the beginning of glucocorticoid therapy. MATERIALS AND METHODS: Fifteen premenopausal women with systemic lupus erythematosus were referred for bone densitometry before being submitted to glucocorticoid therapy. RESULTS: It was demonstrated that the bone mineral density in g/cm2 in the lumbar spine presented a significant decrease in the span between the first analysis previously to the beginning of the glucocorticoid therapy and the third analysis after 60 days under therapy. On the other hand no significant difference was found in the femoral neck. CONCLUSION: The authors conclude that, at short term, rather than only at long term as previously reported by other authors, corticotherapy in patients with systemic lupus erythematosus results in a decrease in the bone mineral density in the lumbar spine.