Optimization of laser power balance is studied numerically for effective multi-step photoionization of degenerate atoms. To date, a large number of similar studies have been executed for simple nondegenerate atoms and, therefore, the emphasis in this study is placed on investigating the role of the level degeneracy. It is concluded that a monotonically increasing ]-value in the excitation ladder of levels is preferable, which is followed by some variations thereof. It should also be noted that a reasonably high ionization rate is attainable, in any case, provided that the Rabi-frequencies are optimized at the expense of the sizable laser powers required. The computer code employed here, which deals with the population evolution among numerous magnetic sublevels, has been tested against experiments in advance of the above numerical study.

No abstract

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 UO 2 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.

No abstract

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 UO 2 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.