Computational systems for describing multiphase process and system behavior must have constitutive relations which adequately describe physical processes. It is shown that nonequilibrium phase change for steady flow is an initial value, history dependent process which requires an accurate description of both the quantity of interfacial area and the interfacial heat transfer for proper description. In addition, proper selection of the initial condition for onset of nonequilibrium must be specified if the resultant constitutive relations are to prove accurate. Examples are given of application to both post-dryout heat transfer and flashing of initially subcooled fluids.

View full abstract

Measurements of reaction rates have been performed in three uranium-fueled zone-type cores of the FCA constructed for a series of experiments on a high conversion light water reactor (HCLWR). These cores possess central test zones of different fuel enrichments and moderator to fuel volume ratios. Radial and axial fission rates of ²³⁵U ²³⁹Pu, ²³⁸U and ²³⁷Np were measured in each test zone by means of the micro-fission counter traverse. A region where the fundamental mode spectrum is established in the test zone were determined by utilizing these fission rate distributions. Central reaction rate ratios relative to the ²³⁵U fission rate were obtained from the measurements by the micro-fission counters and metallic uranium foils to examine changes in the reaction rate ratios among the three cores.
The measured data were analyzed by the SRAC code system on the basis of the nuclear data file JENDL-2. The calculated fission rate distributions agree well with the experimental results for the all cases. The results of reaction rate ratios show that the calculations overpredict the experimental values of the ²³⁸U capture/²³⁵U fission and ²³⁸U fission/²³⁵U fission rate ratios in the three cores.

View full abstract

In the development of a knowledge based expert system, one of key issues is how to build the knowledge base (KB) in an efficient way with keeping the objectivity of KB. In order to solve this issue, an approach has been proposed to build a prototype KB systematically by a statistical method, factor analysis.
For the verification of this approach, factor analysis was applied to build a prototype KB for the JAERI expert system DISKET. To this end, alarm and process information was generated by a PWR simulator and the factor analysis was applied to this information to define taxonomy of accident hypotheses and to extract rules for each hypothesis. The prototype KB thus built was tested through inferring against several types of transients including double-failures. In each diagnosis, the transient type was well identified. Furthermore, newly introduced standards for rule extraction showed good effects on the enhancement of the performance of prototype KB.

View full abstract

Following Part (I) of the present paper, which presented the experimental results obtained on the void distribution and average void fraction shown by nearly fully-developed, vertically downward two-phase flow of air-water mixture, this Part (II) covers the flow regime transition criteria among the three basic flow regimes: bubbly, slug and annular flows. The annular flow further was divided into two subregions of falling film flow and annular drop flow. The general situation of the transition criteria is as follows: (1) bubbly-to-slug flow transition occurs when the local void fraction in the central region of the tube is 0.3; (2) slug-to-annular drop flow transition criterion is given as a case which equations giving average void fraction for the slug flow and the annular flow are simultaneously satisfied ; (3) slug-to-falling film flow transition occurs when the pressure difference between the crest of large wave and the bottom overcomes the surface tension; (4) the occurrence of liquid droplets from wave crests gives the transition criterion between the falling film flow and the annular drop flow.
These criteria were correlated to predict each flow regime boundary respectively considering flow mechanisms or from experimental results. The correlations obtained were compared with published flow regime maps for atmospheric air-water flow and showed satisfactory agreement.

View full abstract

A numerical analysis is presented on the diffusive separation behavior of a three-component gas mixture of ²³⁵UF₆, ²³⁸UF₆, and light gas in a gas centrifuge. The purpose of analysis was to examine the isotope separation performance of the centrifuge in the presence of light gases such as N2 and HF, which inevitably leak in from the atmosphere or accumulate from impurities contained in uranium hexafluoride.
An approximate basic equation is used, which is almost the same as derived by T. Kai for a ternary system from a generalized form of the Stefan-Maxwell equations, taking account of the pressure diffusion in a rapidly rotating cylinder. The method has previously been proposed for application to counter-current centrifuges, which are characterized by stable axial flow in two concentric streams, assuming that the gas mixture is in thermo-dynamical equilibrium between the light gas and UF6 mixture.
Calculations made using the above basic equation indicated that the separative power of the centrifuge is significantly lowered with increasing penetration of the light gas into the separative zone between the two concentric streams. The results of calculation are in fairly good agreement with previously reported theories over a wide range of light gas concentration.
Measurements made using an experimental centrifuge equipped with scoop substantiated the foregoing trends indicated from calculation up to about 1 ??, average concentration of light gas in the centrifuge cylinder. At higher concentrations, a more significant lowering of separative power by the presence of light gas was indicated from measurement than from calculation.

View full abstract

As a fundamental study on separating technetium from high-level liquid waste, the adsorption behavior of technetium for the active carbon was discussed in terms of its distribution coefficient. The adsorption of Tc on the active carbon previously treated with 4 M HNO₃ was also studied for the purpose of a recycle use of the active carbon.
Technetium was separated with Kd above 100 from HNO₃ below 0.5 M, and therefore this adsorption method was considered to be applicable to a partitioning process. The value of Kd was inversely proportional to nitrate ion concentration in the hydrogen ion concentration up to 0.5 M. The amount of Tc adsorbed on the active carbon can be expressed by Freundlich equation. When the Tc concentration was 0.015 M, corresponding to the estimated concentration in high-level liquid waste, the adsorbed amount for the active carbon was 0.60 meq/g, while it decreased to about 1/2 by the treatment with HNO₃. The change of chemical structure of the active carbon resulting from the treatment was also discussed.

View full abstract

Direct contact condensation of vapor occurs after injection of emergency core cooling (ECC) water during a loss-of-coolant accident (LOCA) in a light water reactor (LWR). Despite that vapor condensation may have large influences on system thermal-hydraulic responses, it is difficult to predict the condensation rate accurately during ECC water injection. This difficulty sometimes results in considerable discrepancy between predicted and measured system overall transients. For instance, analyses of PWR reflood tests with the TRAC-PF1/MOD1 code predicted non-physical large flow oscillations because the code largely overpredicted the condensation rate.
In an attempt to resolve this shortcoming of TRAC, a sensitivity study was carried out on the condensation heat transfer Stanton number, that is assumed to be a constant in TRAC, by analyzing the broken-loop hot leg responses to ECC injection in a combined injection test conducted at the Cylindrical Core Test Facility (CCTF). Based on this sensitivity study, it is recommended to use a Stanton number of 0.0035, which is about six times smaller than the value used in the current version of TRAC, to predict LWR system responses during an ECC water injection transient.

View full abstract