Search

Sodium fire is one of key issues in sodium-cooled fast reactor (SFR) plants. JAEA has developed sodium fire analysis codes, a zone model code SPHINCS and a field model code AQUA-SF, to evaluate the consequence of sodium fire events. This paper describes a PIRT (Phenomena Identification and Ranking Table) process for sodium fire events. Ranking table for important phenomena and an assessment matrix are completed. Because a sodium fire event in an SFR plant involves complex phenomena, the ranking table has been established through both element- and sequence-based phenomena analyses in addition to the engineering judgement. The assessment matrix confirms sufficiency of experimental data for validation of models corresponding to the identified important phenomena in the sodium fire analysis codes. As a part of comprehensive validation based on the assessment matrix, an experimental analysis for a large-scale sodium spray fire experiment Run-E1 is conducted by using the SPHINCS and AQUA-SF codes. The analytical results of the both codes show good agreements with the experimental data of gas pressure transient. Difference from the experimental result of the maximum pressure are about 5% smaller in the SPHINCS code and about 10% larger in the AQUA-SF code. General validity for the codes would be confirmed through the comprehensive validation based on the assessment matrix.

In case pressurized water or vapor leaks from a failed heat transfer tube in a steam generator of a sodium-cooled fast reactor, a high-velocity, high-temperature, and corrosive jet with sodium-water chemical reaction may cause tube failure propagation. In this study, a numerical analysis method to predict the occurrence of tube failure propagation by overheating rupture was developed to expand an application range of an existing computer code. This method consists of the elemental analysis models for a sodium-side temperature distribution formed by a reacting jet, water-side thermal hydraulics, heat transfer of a tube, and tube failure by internal pressure. To evaluate event progress at a shortest computation time, the elemental analysis models of this code were constructed from the experimental data, semi-theoretical correlations, or one-dimensional equations. Applicability of the method was investigated through the computation of an experiment on water vapor discharging in liquid sodium. In this experiment, one tube for water vapor discharging and the 91 target tubes were placed in a liquid sodium pool. Temperature of the target tubes increased by the effect of the reacting jet in this computation. Some of the target tubes near the initial water leak resulted in overheating rupture as with the experimental result. This computation demonstrated that the proposed method could predict the occurrence of overheating rupture and provide conservative results.

When pressurized water or vapor leaks from a failed heat transfer tube in a steam generator of sodium-cooled fast reactors, a high-velocity and high-temperature jet with sodium-water chemical reaction may cause wastage on the adjacent tubes. To evaluate the effect of the reacting jet to the adjacent tubes, a computational fluid dynamics code SERAPHIM, in which a compressible multicomponent multiphase flow with sodium-water chemical reaction is computed, has been developed. The multiphase flow under the tube failure accident is calculated by the multi-fluid model considering compressibility. The chemical reaction between liquid sodium and water vapor is considered under the assumption of an infinite reaction rate. The original SERAPHIM code is based on the finite difference method. In this study, unstructured mesh-based numerical method was developed and introduced into the SERAPHIM code to advance a numerical accuracy for a complex-shaped domain including multiple heat transfer tubes. Validity of the unstructured mesh-based SERAPHIM code was investigated through the analysis of an under-expanded jet experiment, which is a key phenomenon in the tube failure accident. The calculated pressure profile showed good agreement with the experimental data. Numerical analysis of water vapor discharging into liquid sodium was also performed. The calculated temperature field agreed with the existing experimental knowledge. It was demonstrated that the proposed numerical method could be applicable to evaluation of the sodium-water reaction phenomenon.

A plant dynamics analysis code named Super-COPD is being developed in Japan Atomic Energy Agency (JAEA) to offer a methodology for the design and safety assessments of future commercialized sodium-cooled fast reactors (SFRs). In this study, the friction loss coefficients in the whole core thermal-hydraulic model, which is based on flow network modeling, was modified to improve the prediction accuracy of the sodium temperature distribution in a fuel subassembly under the natural circulation conditions. Super-COPD with the modified whole core model was applied to analyses of experiments, that were performed by using JAEA's test facility PLANDTL and were simulated natural circulation decay heat removal operations in SFRs, as a part of the code validation study. The obtained numerical results of sodium temperature distributions in the core showed good agreement with the measured data. It implies that the modified whole core model can properly reproduce dominant thermal-hydraulic phenomena in the core region under natural circulation conditions, i.e., flow redistribution among fuel subassemblies as well as in a fuel subassembly and inter-subassembly heat transfer.

A vortex is considered as one of significant phenomena which may cause gas entrainment (GE) and/or vortex cavitation in sodium-cooled fast reactors. In our past studies, the vortex is assumed to be approximated by the well-known Burgers vortex model. However, the Burgers vortex model has a simple but unreal assumption that the axial velocity component is horizontally constant, while in real the free surface vortex has the axial velocity distribution which shows large gradient in radial direction near the vortex center. In this study, a new vortex model with realistic axial velocity distribution is proposed. This model is derived from the steady axisymmetric Navier-Stokes equation as well as the Burgers vortex model, but the realistic axial velocity distribution in radial direction is considered, which is defined to be zero at the vortex center and to approach asymptotically to zero at infinity. As the verification, the new vortex model is applied to the evaluation of a simple vortex experiment, and shows good agreements with the experimental data in terms of the circumferential velocity distribution and the free surface shape. In addition, it is confirmed that the Burgers vortex model fails to calculate accurate velocity distribution with the assumption of uniform axial velocity. However, the calculation accuracy of the Burgers vortex model can be enhanced close to that of the new vortex model in consideration of the effective axial velocity which is calculated as the average value only in the vicinity of the vortex center.

A new apparatus for in situ continuous measurements of atmospheric CO_2 concentration at Syowa Station, Antarctica was developed using a non-dispersive infrared CO_2 analyzer with a precision better than ±0.01ppm. Standard gases employed were CO_2 in air mixtures and their concentrations were determined against special gases with absolute accuracies around ±0.1ppm, which were prepared gravimetrically using an extremely precise balance.