Katsuhiko Nakahara

Development of Molten Core Relocation Analysis module MCRA in the severe accident analysis code SAMPSON

IMPACT is a complex software system under development at the Nuclear Power Engineering Corporation, that includes the severe accident analysis code (SAMPSON). SAMPSON is an integration of twelve modules and will be capable of simulating hypothesized severe accidents in a nuclear power plant in the final phase of the IMPACT project. As one of these modules, the Molten Core Relocation Analysis (MCRA) module simulates the relocation behavior of a molten core during a severe accident. MCRA adopts a multi-phase, multi- component, multi-velocity field model to simulate severe accident phenomena mechanistically. Herein, we describe flow regimes, interfacial area modeling and physics models such as the momentum exchange and phase change models. Two separate effect analyses are presented as verification of MCRAs models following the model descriptions. First, the fluid dynamics of the multi-velocity field model was verified in the calculation of nitrogen gas bubbling through water in Leungs experiment. Second, data of the JRC-Ispra KROTOS-37 experiment were adopted for verification of the multi-velocity field model and vaporization/condensation model. Calculation results of both analyses compared well with the experimental data. Finally, MCRAs integrated function involving the melting/freezing model was demonstrated by calculating two-phase flow with a fuel rod and molten fuel.

Analysis of Core Degradation and Fission Products Release in Phebus FPT1 Test at IRSN by Detailed Severe Accidents Analysis Code, IMPACT/SAMPSON

The Phebus FPT1 test at IRSN, selected to be ISP-46, has been analyzed by the IMPACT/SAMPSON code, a detailed severe accidents (SAs) analysis code for an LWR. The results for the Bundle phase, involving an examination of bundle degradation and fission products release during SA conditions, were reported. Conclusions obtained from the analyses are as follows: (1) Temperature changes of fuel, cladding, and control rod were well predicted until about 12,000 s when almost all thermocouples measuring them failed. (2) Accumulated hydrogen generation due to Zr/steam reaction differed only about 3% from the test result. (3) Overall, good agreement was obtained for the fuel relocation and an accumulation of debris just below original spacer position was well predicted in the analysis. (4) Analysis of enhanced diffusion due to degraded fuel by Lewis et al.s method enabled simulation of release behaviors of Xe, Cs, I, and Te within the uncertainties in the test when the effective surface to volume ratio was changed in the evaluation of the UO2 oxidation. (5) Diffusion analysis through single-crystal grain could trace the release behavior of Mo, Sb, Tc, Ru, and Ba observed in the Phebus FPT1 test after optimizing their diffusion coefficients with the effective surface to volume ratio determined above.

Photo resonance excitation and ionization characteristics of atoms by pulsed laser irradiation

The authors report on obtaining generation of a CO2 laser with a phase- anisotropy three-mirror cavity operating in a linear orthogonal polarized double-mode regime. Power and frequency characteristics of a double-mode three-mirror laser are also studied. It was proved that in a double mode CO2 laser with an internal absorbing cell the saturation of the absorbing medium can be significantly reduced with the help of a three-mirror telescopic cavity. Narrow nonlinear resonances of high amplitude were obtained in a CO2/SF6 laser with the help of phase-anysotropy three-mirror telescopic cavity.

Spectroscopic Stability in Two-Photon Absorption by Atoms

When there exists no external field, physical results should not depend on the choice of quantization axis. This theorem is known as spectroscopic stability and is well established for one photon absorption and emission processes. In two photon absorption by a three level atom one can make the excitation of the top level forbidden if the absorption processes are restricted to one selection rule in ΔM (=+1, 0 or -1). The choice of selection rule assumes relative orientation between the polarization of the radiation field and an external field. If this relative orientation is changed the selection rule would also change and the top level would appear as a final result of two photon absorption . A question is what happens if the external field disappears in the latter case. Would the top level disappear although the selection rule stays unaltered? It is shown that the probability of having the top level disappears when the external field vanishes, confirming the spectroscopic stability and the degree of violation is obtained as a function of the strength of the external field.