Modified Rate Approach Research Articles

Strain Rate Calculation Approach in Environmental Fatigue Evaluations

Fatigue usage factor evaluations including the effects of reactor water environment have been performed in numerous nuclear plant license renewal efforts. A large number of these evaluations have used the environmental fatigue penalty factor, Fen, approach prescribed in various regulatory documents. The Fen equations require input of strain rate, but the prescribing documents do not provide methodology or criteria for the quantification of the strain rate to be input. As a result, numerous approaches have been offered and studied. This paper presents an approach used by Westinghouse to include strain rate in an automated calculation of Fen based on the modified rate approach (MRA) to integrated strain rate applications. The starting point of the approach is ASME Code Section III NB-3200 fatigue analysis. With environmental fatigue evaluations in new plant designs now emerging in ASME Code criteria, strain rate considerations remain part of the discussion. The intent of this paper is to provide further insight into this process.

205103 ひずみ速度変動条件下における炭素鋼のBWR環境中疲労評価(OS04 材料・機械・構造物における強度と寿命2,オーガナイズド・セッション)

Fatigue evaluation methods have been proposed based on environmental fatigue test results regarding parameters selected for simulating BWR and PWR conditions. Recently, effects of strain wave form have been discussed by comparing experimental fatigue life with predicted fatigue life evaluated by modified rate approach method. Environmental fatigue life of carbon, low-alloy and stainless steels under sine wave form was reported to be longer than that predicted in simulated BWR environment. In simulated PWR environment, however, it was reported that environmental fatigue life under sine wave form could be predicted appropriately. In this study, experimental fatigue lives of carbon steel under two kinds of strain rate changing conditions of sine wave and the wave obtained in thermal fatigue test in simulated BWR environment were compared with predicted fatigue lives. Modified rate approach method was indicated to be applicable to predict fatigue lives under these two kinds of strain rate changing conditions.

Mortality rate and confidence interval estimation in humanitarian emergencies

Surveys are conducted frequently in humanitarian emergencies to assess the health status of the population. Most often, they employ complex sample designs, such as cluster sampling. Mortality is an indicator commonly estimated in such surveys. Confidence limits provide information on the precision of the estimate and it is important to ensure that confidence limits for a mortality rate account for the survey design and utilise an acceptable methodology. This paper describes the calculation of confidence limits for mortality rates from surveys using complex sampling designs and a variety of software programmes and methods. It contains an example that makes use of the SAS, SPSS, and Epi Info software programmes. Of the three confidence interval methods examined--the ratio command approach, the modified rate approach, and the modified proportion approach--the paper recommends the ratio command approach to estimate mortality rates with confidence limits.

Molecular Evolution in Collapsing Prestellar Cores. II. The Effect of Grain‐Surface Reactions

The molecular evolution that occurs in collapsing prestellar cores is investigated. To model the dynamics, we adopt the Larson-Penston solution and analogs with slower rates of collapse. For the chemistry, we utilize the new standard model with the addition of deuterium fractionation and grain-surface reactions treated via the modified rate approach. The use of surface reactions distinguishes the present work from our previous model. We find that these reactions efficiently produce H2O, H2CO, CH3OH, N2, and NH3 ices. In addition, the surface chemistry influences the gas-phase abundances in a variety of ways. For example, formation of molecular nitrogen on grain surfaces followed by desorption into the gas enhances the abundance of this gas-phase species and its daughter products N2H+ and NH3. The current reaction network along with the Larson-Penston solution allows us to reproduce satisfactorily most of the molecular column densities and their radial distributions observed in L1544. The agreement tends to worsen with models that include strongly delayed collapse rates. Inferred radial distributions in terms of fractional abundances are somewhat harder to reproduce. In addition to our standard chemical model, we have also run a model with the UMIST gas-phase chemical network. The abundances of gas-phase sulphur-bearing molecules such as CS and CCS are significantly affected by uncertainties in the gas-phase chemical network. In all our models, the column density of N2H+ monotonically increases as the central density of the core increases during collapse from 3 × 104 to 3 × 107 cm-3. Thus, the abundance of this ion can be a probe of evolutionary stage. Molecular D/H ratios in assorted cores are best reproduced in the Larson-Penston picture with the conventional rate coefficients for fractionation reactions. If we adopt the newly measured and calculated rate coefficients, the D/H ratios, especially N2D+/N2H+, become significantly lower than the observed values.

Deuterium fractionation on interstellar grains studied with modified rate equations and a Monte Carlo approach

The formation of singly and doubly deuterated isotopomers of formaldehyde and of singly, doubly, and multiply deuterated isotopomers of methanol on interstellar grain surfaces has been studied with a semi-empirical modified rate approach and a Monte Carlo method in the temperature range 10– 20 K . Agreement between the results of the two methods is satisfactory for all major and many minor species throughout this range. If gas-phase fractionation can produce a high abundance of atomic deuterium, which then accretes onto grain surfaces, diffusive surface chemistry can produce large abundances of deuterated species, especially at low temperatures and high gas densities. Warming temperatures will then permit these surface species to evaporate into the gas, where they will remain abundant for a considerable period. We calculate that the doubly deuterated molecules CHD 2OH and CH 2DOD are particularly abundant and should be searched for in the gas phase of protostellar sources. For example, at 10 K and high density, these species can achieve up to 10–20% of the abundance of methanol.